For many purposes carbon-steel cutters are still found best, although where a large amount of material is to be removed at a rapid rate, it has given way to high-speed steels.

CARBON STEELS FOR DIFFERENT TOOLS

All users of tool steels should carefully study the different qualities of the steels they handle. Different uses requires different kinds of steel for best results, and for the purpose of designating different steels some makers have adopted the two terms "temper," and "quality," to distinguish between them.

In this case temper refers to the amount of carbon which is combined with the iron to make the metal into a steel. The quality means the absence of phosphorous, sulphur and other impurities, these depending on the ores and the methods of treatment.

Steel makers have various ways of designating carbon steels for different purposes. Some of these systems involve the use of numbers, that of the Latrobe Steel Company being given herewith. It will be noted that the numbers are based on 20 points of carbon per unit. The names given the different tempers are also of interest. Other makers use different numbers.

The temper list follows:

LATROBE TEMPER LIST OF CARBON TOOL STEELS No. 3 temper 0.60 to 0.69 per cent carbon No. 3-1/2 temper 0.70 to 0.79 per cent carbon No. 4 temper 0.80 to 0.89 per cent carbon No. 4-1/2 temper 0.90 to 0.99 pet cent carbon No. 5 temper 1.00 to 1.09 per cent carbon No. 5-1/2 temper 1.10 to 1.19 per cent carbon No. 6 temper 1.20 to 1.29 per cent carbon No. 6-1/2 temper 1.30 to 1.39 per cent carbon No. 7 temper 1.40 to 1.49 per cent carbon

USES OF THE VARIOUS TEMPERS OF CARBON TOOL STEEL

DIE TEMPER.—No. 3: All kinds of dies for deep stamping, pressing and drop forgings. Mining drills to harden only. Easily weldable.

SMITHS' TOOL TEMPER.—No. 3-1/2: Large punches, minting and rivet dies, nailmakers' tools, hammers, hot and cold sets, snaps and boilermakers' tools, various smiths' tools, large shear blades, double-handed chisels, caulking tools, heading dies, masons' tools and tools for general welding purposes.

SHEAR BLADE TEMPER.—No. 4: Punches, large taps, screwing dies, shear blades, table cutlery, circular and long saws, heading dies. Weldable.

GENERAL PURPOSE TEMPER.—No. 4-1/2: Taps, small punches, screwing dies, sawwebs, needles, etc., and for all general purposes. Weldable.

AXE TEMPER.—No. 5: Axes, chisels, small taps, miners' drills and jumpers to harden and temper, plane irons. Weldable with care.

CUTLERY TEMPER.—No. 5-1/2: Large milling cutters, reamers, pocket cutlery, wood tools, short saws, granite drills, paper and tobacco knives. Weldable with very great care.

TOOL TEMPER.—No. 6: Turning, planing, slotting, and shaping tools, twist drills, mill picks, scythes, circular cutters, engravers' tools, surgical cutlery, circular saws for cutting metals, bevel and other sections for turret lathes. Not weldable.

HARD TOOL TEMPER.—No. 6-1/2: Small twist drills, razors, small and intricate engravers' tools, surgical instruments, knives. Not weldable.

RAZOR TEMPER.—No. 7: Razors, barrel boring bits, special lathe tools for turning chilled rolls. Not weldable.

STEEL FOR CHISELS AND PUNCHES

The highest grades of carbon or tempering steels are to be recommended for tools which have to withstand shocks, such as for cold chisels or punches. These steels are, however, particularly useful where it is necessary to cut tempered or heat-treated steel which is more than ordinarily hard, for cutting chilled iron, etc. They are useful for boring, for rifle-barrel drilling, for fine finishing cuts, for drawing dies for brass and copper, for blanking dies for hard materials, for formed cutters on automatic screw machines and for roll-turning tools.

Steel of this kind, being very dense in structure, should be given more time in heating for forging and for hardening, than carbon steels of a lower grade. For forging it should be heated slowly and uniformly to a bright red and only light blows used as the heat dies out. Do not hammer at all at a black heat. Reheat slowly to a dark red for hardening and quench in warm water. Grind on a wet grindstone.

Where tools have to withstand shocks and vibration, as in pneumatic hammer work, in severe punching duty, hot or cold upsetting or similar work, tool steels containing vanadium or chrome-vanadium give excellent results. These are made particularly for work of this kind.

CHISELS-SHAPES AND HEAT TREATMENT[1]

[Footnote 1: Abstract of paper by HENRY FOWLER, chief mechanical engineer of the Midland Ry., England, before the Institution of Mechanical Engineers.]

In the chief mechanical engineer's department of the Midland Ry., after considerable experimenting, it was decided to order chisel steel to the following specifications: carbon, 0.75 to 0.85 per cent, the other constituents being normal. This gives a complete analysis as follows: carbon, 0.75 to 0.85; manganese, 0.30; silicon, 0.10; sulphur, 0.025; phosphorus, 0.025.

The analysis of a chisel which had given excellent service was as follows: carbon, 0.75; manganese, 0.38; silicon, 0.16; sulphur, 0.028; phosphorus, 0.026. The heat treatment is unknown.



At the same time that chisel steel was standardized, the form of the chisels themselves was revised, and a standard chart of these as used in the locomotive shops was drawn up. Figure 83 shows the most important forms, which are made to stock orders in the smithy and forwarded to the heat-treatment room where the hardening and tempering is carried out on batches of fifty. A standard system of treatment is employed, which to a very large extent does away with the personal element. Since the chemical composition is more or less constant, the chief variant is the section which causes the temperatures to be varied slightly. The chisels are carefully heated in a gas-fired furnace to a temperature of from 730 to 740 deg.C. (1,340 to 1,364 deg.F.) according to section. In practice, the first chisel, is heated to 730 deg.C.; and the second to 735 deg.C. (1,355 deg.F.); and a 1 in. half round chisel to 740 deg.C., because of their varying increasing thickness of section at the points. Upon attaining this steady temperature, the chisels are quenched to a depth of 3/8 to 1/2 in. from the point in water, and then the whole chisel is immersed and cooled off in a tank containing linseed oil.

The oil-tank is cooled by being immersed in a cold-water tank through which water is constantly circulated. After this treatment, the chisels have a dead hard point and a tough or sorbitic shaft. They are then tempered or the point "let down." This is done by immersing them in another oil-bath which has been raised to about 215 deg.C. (419 deg.F). The first result is, of course, to drop the temperature of the oil, which is gradually raised to its initial point. On approaching this temperature the chisels are taken out about every 2 deg.C. rise and tested with a file, and at a point between 215 and 220 deg.C. (428 deg.F.), when it is found that the desired temper has been reached, the chisels are removed, cleaned in sawdust, and allowed to cool in an iron tray.

No comparative tests of these chisels with those bought and treated by the old rule-of-thumb methods have been made, as no exact method of carrying out such tests mechanically, other than trying the hardness by the Brinell or scleroscope method, are known; any ordinary test depends so largely upon the dexterity of the operator. The universal opinion of foremen and those using the chisels as to the advantages of the ones receiving the standard treatment described is that a substantial improvement has been made. The chisels were not "normalized." Tests of chisels normalized at about 900 deg.C. (1,652 deg.F.) showed that they possessed no advantage.

Tools or pieces which have holes or deep depressions should be filled before heating unless it is necessary to have the holes hard on the inside. In that case the filling would keep the water away from the surface and no hardening would take place. Where filling is to be done, various materials are used by different hardeners. Fireclay and common putty seem to be favored by many.

Every mechanic who has had anything to do with the hardening of tools knows how necessary it is to take a cut from the surface of the bar that is to be hardened. The reason is that in the process of making the steel its outer surface has become decarbonized. This change makes it low-carbon steel, which will of course not harden. It is necessary to remove from 1/16 to 1/4 in. of diameter on bars ranging from 1/2 to 4 in.

This same decarbonization occurs if the steel is placed in the forge in such a way that unburned oxygen from the blast can get at it. The carbon is oxidized, or burned out, converting the outside of the steel into low-carbon steel. The way to avoid this is to use a deep fire. Lack of this precaution is the cause of much spoiled work, not only because of decarbonization of the outer surface of the metal, but because the cold blast striking the hot steel acts like boiling hot water poured into an ice-cold glass tumbler. The contraction sets up stresses that result in cracks when the piece is quenched.

PREVENTING DECARBONIZATION OF TOOL STEEL

It is especially important to prevent decarbonization in such tools as taps and form cutters, which must keep their shape after hardening and which cannot be ground away on the profile. For this reason it is well to put taps, reamers and the like into pieces of pipe in heating them. The pipe need be closed on one end only, as the air will not circulate readily unless there is an opening at both ends.

Even if used in connection with a blacksmith's forge the lead bath has an advantage for heating tools of complicated shapes, since it is easier to heat them uniformly and they are submerged and away from the air. The lead must be stirred frequently or the heat is not uniform in all parts of the lead bath. Covering the lead with powdered charcoal will largely prevent oxidization and waste of lead.

Such a bath is good for temperatures between 620 and 1,150 deg.F. At higher temperatures there is much waste of lead.

ANNEALING TO RELIEVE INTERNAL STRESSES

Work quenched from a high temperature and not afterward tempered will, if complex in shape, contain many internal stresses which may later cause it to break. They may be eased off by slight heating without materially lessening the hardness of the piece. One way to do this is to hold the piece over a fire and test it with a moistened finger. Another way is to dip the piece in boiling water after it has first been quenched in a cold bath. Such steps are not necessary with articles which will afterward be tempered and in which the strains are thus reduced.

In annealing steels the operation is similar to hardening, as far as heating is concerned. The critical temperatures are the proper ones for annealing as well as hardening. From this point on there is a difference, for annealing consists in cooling as slowly as possible. The slower the cooling the softer will be the steel.

Annealing may be done in the open air, in furnaces, in hot ashes or lime, in powdered charcoal, in burnt bone, in charred leather and in water. Open-air annealing will do as a crude measure in cases where it is desired to take the internal stresses out of a piece. Care must be taken in using this method that the piece is not exposed to drafts or placed on some cold substance that will chill it. Furnace annealing is much better and consists in heating the piece in a furnace to the critical temperature and then allowing the work and the furnace to cool together.

When lime or ashes are used as materials to keep air away from the steel and retain the heat, they should be first heated to make sure that they are dry. Powdered charcoal is used for high-grade annealing, the piece being packed in this substance in an iron box and both the work and the box raised to the critical temperature and then allowed to cool slowly. Machinery steel may be annealed in spent ground-bone that has been used in casehardening; but tool steel must never be annealed in this way, as it will be injured by the phosphorus contained in the bone. Charred leather is the best annealing material for high-carbon steel, because it prevents decarbonizing taking place.

DOUBLE ANNEALING

Water annealing consists in heating the piece, allowing it to cool in air until it loses its red heat and becomes black and then immediately quenching it in water. This plan works well for very low-carbon steel; but for high-carbon steel what is known as the "double annealing treatment" must be given, provided results are wanted quickly. The process consists in heating the steel quickly to 200 deg. or more above the upper critical, cooling in air down through the recalescence point, then reheating it to just above the critical point and again cooling slowly through the recalescence, then quenching in oil. This process retains in the steel a fine-grained structure combined with softness.

QUENCHING TOOL STEEL

To secure proper hardness, the cooling of quenching of steel is as important as its heating. Quenching baths vary in nature, there being a large number of ways to cool a piece of steel in contrast to the comparatively few ways of heating it.

Plain water, brine and oil are the three most common quenching materials. Of these three the brine will give the most hardness, and plain water and oil come next. The colder that any of these baths is when the piece is put into it the harder will be the steel; but this does not mean that it is a good plan to dip the heated steel into a tank of ice water, for the shock would be so great that the bar would probably fly to pieces. In fact, the quenching bath must be sometimes heated a bit to take off the edge of the shock.

Brine solutions will work uniformly, or give the same degree of hardness, until they reach a temperature of 150 deg.F. above which their grip relaxes and the metals quenched in them become softer. Plain water holds its grip up to a temperature of approximately 100 deg.F.; but oil baths, which are used to secure a slower rate of cooling, may be used up to 500 deg. or more. A compromise is sometimes effected by using a bath consisting of an inch or two of oil floating on the surface of water. As the hot steel passes through the oil, the shock is not as severe as if it were to be thrust directly into the water; and in addition, oil adheres to the tool and keeps the water from direct contact with the metal.

The old idea that mercury will harden steel more than any other quenching material has been exploded. A bath consisting of melted cyanide of potassium is useful for heating fine engraved dies and other articles that are required to come out free from scale. One must always be careful to provide a hood or exhaust system to get rid of the deadly fumes coming from the cyanide pot.

The one main thing to remember in hardening tool steel is to quench on a rising heat. This does not mean a rapid heating as a slow increase in temperature is much better in every way.

THE THEORY OF TEMPERING.—Steel that has been hardened is generally harder and more brittle than is necessary, and in order to bring it to the condition that meets our requirements a treatment called tempering is used. This increases the toughness of the steel, i.e., decrease the brittleness at the expense of a slight decrease in hardness.

There are several theories to explain this reaction, but generally it is only necessary to remember that in hardening we quench steel from the austenite phase, and, due to this rapid cooling, the normal change from austenite to the eutectoid composition does not have time to take place, and as a consequence the steel exists in a partially transformed, unstable and very hard condition at atmospheric temperatures. But owing to the internal rigidity which exists in cold metal the steel is unable to change into its more stable phase until atoms can rearrange themselves by the application of heat. The higher the heat, the greater the transformation into the softer phases. As the transformation takes place, a certain amount of heat of reaction, which under slow cooling would have been released in the critical range, is now released and helps to cause a further slight reaction.

If a piece of steel is heated to a certain temperature and held there, the tempering color, instead of remaining unchanged at this temperature, will advance in the tempering-color scale as it would with increasing temperature. This means that the tempering colors do not absolutely correspond to the temperatures of steels, but the variations are so slight that we can use them in actual practice. (See Table 23, page 158.)

TEMPERATURES TO USE.—As soon as the temperature of the steel reaches 100 deg.C. (212 deg.F.) the transformation begins, increasing in intensity as the temperature is raised, until finally when the lower critical range is reached, the steel has been all changed into the ordinary constituents of unhardened steels.

If a piece of polished steel is heated in an ordinary furnace, a thin film of oxides will form on its surface. The colors of this film change with temperature, and so, in tempering, they are generally used as an indication of the temperature of the steel. The steel should have at least one polished face so that this film of oxides may be seen.

An alternative method to the determination of temper by color is to temper by heating in an oil or salt bath. Oil baths can be used up to temperatures of 500 deg.F.; above this, fused-salt baths are required. The article to be tempered is put into the bath, brought up to and held at the required temperature for a certain length of time, and then cooled, either rapidly or slowly. This takes longer than the color method, but with low temperatures the results are more satisfactory, because the temperature of the bath can be controlled with a pyrometer. The tempering temperatures given in the following table are taken from a handbook issued by the Midvale Steel Company.

TABLE 23. TEMPERING TEMPERATURES FOR STEELS Temperature Temperature for 1 hr. for 8 min. - Color - Uses Deg. F. Deg. C. Deg. F. Deg. C. - - - - - 370 188 Faint yellow 460 238 Scrapers, brass-turning tools, reamers, taps, milling cutters, saw teeth. 390 199 Light straw 510 265 Twist drills, lathe tools, planer tools, finishing tools 410 210 Dark straw 560 293 Stone tools, hammer faces, chisels for hard work, boring cutters. 430 221 Brown 610 321 Trephining tools, stamps. 450 232 Purple 640 337 Cold chisels for ordinary work, carpenters' tools, picks, cold punches, shear blades, slicing tools, slotter tools. 490 254 Dark blue 660 343 Hot chisels, tools for hot work, springs. 510 265 Light blue 710 376 Springs, screw drivers.

It will be noted that two sets of temperatures are shown, one being specified for a time interval of 8 min. and the other for 1 hr. For the finest work the longer time is preferable, while for ordinary rough work 8 min. is sufficient, after the steel has reached the specified temperature.

The rate of cooling after tempering seems to be immaterial, and the piece can be cooled at any rate, providing that in large pieces it is sufficiently slow to prevent strains.

KNOWING WHAT TAKES PLACE.—How are we to know if we have given a piece of steel the very best possible treatment?

The best method is by microscopic examination of polished and etched sections, but this requires a certain expense for laboratory equipment and upkeep, which may prevent an ordinary commercial plant from attempting such a refinement. It is highly recommended that any firm that has any large amount of heat treatment to do, install such an equipment, which can be purchased for from $250 to $500. Its intelligent use will save its cost in a very short time.

The other method is by examination of fractures of small test bars. Steel heated to its correct temperatures will show the finest possible grain, whereas underheated steel has not had its grain structure refined sufficiently, and so will not be at its best. On the other hand, overheated steel will have a coarser structure, depending on the extent of overheating.

To determine the proper quenching temperature of any particular grade of steel it is only necessary to heat pieces to various temperatures not more than 20 deg.C. (36 deg.F.) apart, quench in water, break them, and examine the fractures. The temperature producing the finest grain should be used for annealing and hardening.

Similarly, to determine tempering temperatures, several pieces should be hardened, then tempered to various degrees, and cooled in air. Samples, say six, reheated to temperatures varying by 100 deg. from 300 to 800 deg.C. will show a considerable range of properties, and the drawing temperature of the piece giving the desired results can be used.

For drawing tempers up to 500 deg.F. oil baths of fresh cotton seed oil can be safely and satisfactorily used. For higher temperature a bath of some kind of fused salt is recommended.

HINTS FOR TOOL STEEL USERS

Do not hesitate to ask for information from the maker as to the best steel to use for a given purpose, mentioning in as much detail as possible the use for which it is intended.

Do not heat the steel to a higher degree than that fixed in the description of each class. Never heat the steel to more than a cherry red without forging it or giving it a definite heat treatment. Heating steel at even moderate temperature is liable to coarsen the grain which can only be restored by forging or by heat treating.

Let the forging begin as soon as the steel is hot enough and never let tool steel soak in the fire. Continue the hammering vigorously and constantly, using lighter blows as it cools off, and stopping when the heat becomes a very dull red or a faint brown.

Should welding be necessary care should be taken not to overheat in order to make an easy weld. Keep it below the sparkling point as this indicates that the steel is burnt.

Begin to forge as soon as the welds are put together, taking care to use gentle strokes at first increasing them as the higher heat falls, but not overdoing the hammering when the steel cools. The hammering should be extended beyond the welding point and should continue until the dull red or brown heat is reached.

PREVENTING CRACKS IN HARDENING

The blacksmith in the small shop, where equipment is usually very limited, often consisting of a forge, a small open hard-coal furnace, a barrel of water and a can of oil must have skill and experience. With this equipment the smith is expected to, and usually can, produce good results if proper care is taken.

In hardening carbon tool steel in water, too much cannot be said in favor of slow, careful heating, nor against overheating if cracks are to be avoided.

It is not wise to take the work from the hardening bath and leave it exposed to the air if there is any heat left in it, because it is more liable to crack than if left in the bath until cold. In heating, plenty of time is taken for the work to heat evenly clear through, thus avoiding strains caused by quick and improper heating, In quenching in water, contraction is much more rapid than was the expansion while heating, and strains begin the moment the work touches the water. If the piece has any considerable size and is taken from the bath before it is cold and allowed to come to the air, expansion starts again from the inside so rapidly that the chilled hardened surface cracks before the strains can be relieved.

Many are most successful with the hardening bath about blood warm. When the work that is being hardened is nearly cold, it is taken from the water and instantly put into a can of oil, where it is allowed to finish cooling. The heat in the body of the tool will come to the surface more slowly, thus relieving the strain and overcoming much of the danger of cracking.

Some contend that the temper should be drawn as soon as possible after hardening: but that if this cannot be done for some hours, the work should be left in the oil until the tempering can be done. It is claimed that forming dies and punch-press dies that are difficult to harden will seldom crack if treated in this way.

Small tools or pieces that are very troublesome because of peculiar shape should be made of steel which has been thoroughly annealed. It is often well to mill or turn off the outer skin of the bar, to remove metal which has been cold-worked. Then heat slowly just through the critical range and cool in the furnace, in order to produce a very fine grain. Tools machined from such stock, and hardened with the utmost care, will have the best chance to survive without warping, growth or cracking.

SHRINKING AND ENLARGING WORK

Steel can be shrunk or enlarged by proper heating and cooling. Pins for forced fits can be enlarged several thousandths of an inch by rapid heating to a dull red and quenching in water. The theory is that the metal is expanded in heating and that the sudden cooling sets the outer portion before the core can contract. In dipping the piece is not held under water till cold but is dipped, held a moment and removed. Then dipped again and again until cold.

Rings and drawing dies are also shrunk in a similar way. The rings are slowly heated to a cherry red, slipped on a rod and rolled in a shallow pan of water which cools only the outer edge. This holds the outside while the inner heated portion is forced inward, reducing the hole. This operation can be repeated a number of times with considerable success.

TEMPERING ROUND DIES

A number of circular dies of carbon tool steel for use in tool holders of turret lathes were required. No proper tempering oven was available, so the following method was adopted and proved quite successful.

After the dies had been hardened dead hard in water, they were cleaned up bright. A pair of ordinary smiths' tongs was made with jaws of heavy material and to fit nicely all around the outside of the die, leaving a 3/32-in. space when the jaws were closed around the die. The dies being all ready, the tongs were heated red hot, and the dies were picked up and held by the tongs. This tempered them from the outside in, left the teeth the temper required and the outside slightly softer. The dies held up the work successfully and were better than when tempered in the same bath.

THE EFFECT OF TEMPERING ON WATER-QUENCHED GAGES

The following information has been supplied by Automatic and Electric Furnaces, Ltd., 6, Queenstreet, London, S. W.:

Two gages of 3/4 in. diameter, 12 threads per inch, were heated in a Wild-Barfield furnace, using the pyroscopic detector, and were quenched in cold water. They were subsequently tempered in a salt bath at various increasing temperatures, the effective diameter of each thread and the scleroscope hardness being measured at each stage. The figures are in 10,000ths of an inch, and indicate the change + or - with reference to the original effective diameter of the gages. The results for the two gages have been averaged.

TABLE 24. CHANGES DUE TO QUENCHING After Tempering temperature, degrees Centigrade Thread quenching - 220 260 300 340 380 420 - 1 +25 +19 +17 +15 +13 +11 +11 2 +18 +12 +11 + 9 + 6 + 5 + 5 3 +12 + 6 + 5 + 3 0 0 0 4 +10 + 4 + 4 + 2 ... 0 - 1 5 + 9 + 4 + 4 + 2 0 0 0 6 + 9 + 4 + 3 + 2 0 0 0 7 +10 + 5 + 5 + 3 + 2 + 1 + 2 8 + 8 + 4 + 3 + 2 0 0 + 1 9 + 9 + 4 + 3 + 2 + 1 + 1 + 1 10 + 9 + 5 + 5 + 3 + 2 + 2 + 2 11 + 7 + 4 + 4 + 2 + 1 + 1 + 1 12 + 9 + 5 + 5 + 5 + 4 + 4 + 3 Scleroscope 80 70 70 62 56 53 52

Had these gages been formed with a plain cylindrical end projecting in front of the screw, the first two threads would have been prevented from increasing more than the rest. The gages would then have been fairly easily corrected by lapping after tempering at 220 deg.C. Practically no lapping would be required if they were tempered at 340 deg.C. There seems to be no advantage in going to a higher temperature than this. The same degree of hardness could have been obtained with considerably less distortion by quenching directly in fused salt. It is interesting to note that when the swelling after water quenching does not exceed 0.0012 in., practically the whole of it may be recovered by tempering at a sufficiently high temperature, but when the swelling exceeds this amount the steel assumes a permanently strained condition, and at the most only 0.0014 in. can be recovered by tempering.

TEMPERING COLORS ON CARBON STEELS

Opinions differ as to the temperature which is indicated by the various colors, or oxides, which appear on steel in tempering.

The figures shown are from five different sources and while the variations are not great, it is safer to take the average temperature shown in the last column.

TABLE 25. COLORS, TEMPERATURES, DEGREES FAHRENHEIT A B C D E Average - - - - - - Faint yellow 430 430 430 430 430 430 Light straw 475 460 450 ... 450 458 Dark straw 500 500 470 450 470 478 Purple (reddish) 525 530 520 530 510 523 Purple (bluish) ... 555 550 550 550 551 Blue 575 585 560 580 560 572 Gray blue ... 600 ... 600 610 603 Greenish blue ... 625 ... ... 630 627

TABLE 26. ANOTHER COLOR TABLE Degrees Fahrenheit High temperatures judged by color - 430 Very pale yellow 460 Straw-yellow 480 Dark yellow 500 Brown-yellow > Visible in full daylight 520 Brown-purple 540 Full purple 560 Full blue 600 Very dark blue / 752 Red heat, visible in the dark 885 Red heat, visible in the twilight 975 Red heat, visible in the daylight 1,292 Dark red 1,652 Cherry-red 1,832 Bright cherry-red 2,012 Orange-red 2,192 Orange-yellow 2,372 Yellow-white 2,552 White welding heat 2,732 Brilliant white 2,912 Dazzling white (bluish-white)

These differences might easily be due to the difference in the light at the time the colors were observed. It must also be remembered that even a thin coating of oil will make quite a difference and cause confusion. It is these possible sources of error, coupled with the ever present chance of human error, that makes it advisable to draw the temper of tools in an oil bath heated to the proper temperature as shown by an accurate high-temperature thermometer.

Another table, by Gilbert and Barker, runs to much higher temperatures. Beyond 2,200 deg., however, the eye is very uncertain.

TABLE 26. COLORS FOR TEMPERING TOOLS - Approximate color and Kind of tool temperature Yellow Thread chasers, hollow mills (solid type) twist drills 430 to 450 deg.F. centering tools, forming tools, cut-off tools, profile cutters, milling cutters, reamers, dies, etc. Straw-yellow Thread rolling dies, counterbores, countersinks. Shear 460 deg.F. blades, boring tools, engraving tools, etc. Brown-yellow Taps, Thread dies, cutters, reamers, etc. 500 deg.F. Light purple Taps, dies, rock drills, knives, punches, gages, etc. 530 deg.F. Dark purple Circular saws for metal, augers, dental and surgical 550 deg.F. instruments, cold chisels, axes. Pale blue Bone saws, chisels, needles, cutters, etc. 580 deg.F. Blue Hack saws, wood saws, springs, etc. 600 deg.F. -



CHAPTER X

HIGH-SPEED STEEL

For centuries the secret art of making tool steel was handed down from father to son. The manufacture of tool steel is still an art which, by the aid of science, has lost much of its secrecy; yet tool steel is today made by practical men skilled as melters, hammer-men, and rollers, each knowing his art. These practical men willingly accept guidance from the chemist and metallurgists.

A knowledge of conditions existing today in the manufacture of high-speed steel is essential to steel treaters. It is well for the manufacturer to have steel treaters understand some of his troubles and difficulties, so that they will better comprehend the necessity of certain trade customs and practices, and, realizing the manufacturer's desire to cooperate with them, will reciprocate.

The manufacturer of high-speed steel knows and appreciates the troubles and difficulties that may sometimes arise in the heat-treating of his product. His aim is to make a uniform steel that will best meet the requirements of the average machine shop on general work, and at the same time allow the widest variation in heat treatment to give desired results.

High speed steel is one of the most complex alloys known. A representative steel contains approximately 24 per cent of alloying metals, namely, tungsten, chromium, vanadium, silicon, manganese, and in addition there is often found cobalt, molybdenum, uranium, nickel, tin, copper and arsenic.

STANDARD ANALYSIS

The selection of a standard analysis by the manufacturer is the result of a series of compromises between various properties imparted to the steel by the addition of different elements and there is a wide range of chemical analyses of various brands. The steel, to be within the range of generally accepted analysis, should contain over 16 per cent and under 20 per cent tungsten; if of lower tungsten content it should carry proportionately more chromium and vanadium.

The combined action of tungsten and chromium in steel gives to it the remarkable property of maintaining its cutting edge at relatively high temperature. This property is commonly spoken of as "red-hardness." The percentages of tungsten and chromium present should bear a definite relationship to each other. Chromium imparts to steel a hardening property similar to that given by carbon, although to a less degree. The hardness imparted to steel by chromium is accompanied by brittleness. The chromium content should be between 3.5 and 5 per cent.

Vanadium was first introduced in high-speed steel as a "scavenger," thereby producing a more homogeneous product, of greater density and physical strength. It soon became evident that vanadium used in larger quantities than necessary as a scavenger imparted to the steel a much greater cutting efficiency. Recently, no less an authority than Prof. J. O. Arnold, of the University of Sheffield, England, stated that "high-speed steels containing vanadium have a mean efficiency of 108.9, as against a mean efficiency of 61.9 obtained from those without vanadium content." A wide range of vanadium content in steel, from 0.5 to 1.5 per cent, is permissible.

An ideal analysis for high-speed steel containing 18 per cent tungsten is a chromium content of approximately 3.85 per cent; vanadium, 0.85 to 1.10 per cent, and carbon, between 0.62 and 0.77 per cent.

DETRIMENTAL ELEMENTS.—Sulphur and phosphorus are two elements known to be detrimental to all steels. Sulphur causes "red-shortness" and phosphorus causes "cold-shortness." The detrimental effects of these two elements counteract each other to some extent but the content should be not over 0.02 sulphur and 0.025 phosphorus. The serious detrimental effect of small quantities of sulphur and phosphorus is due to their not being uniformly distributed, owing to their tendency to segregate.

The manganese and silicon contents are relatively unimportant in the percentages usually found in high-speed steel.

The detrimental effects of tin, copper and arsenic are not generally realized by the trade. Small quantities of these impurities are exceedingly harmful. These elements are very seldom determined in customers' chemical laboratories and it is somewhat difficult for public chemists to analyze for them.

In justice to the manufacturer, attention should be called to the variations in chemical analyses among the best of laboratories. Generally speaking, a steel works' laboratory will obtain results more nearly true and accurate than is possible with a customer's laboratory, or by a public chemist. This can reasonably be expected, for the steel works' chemist is a specialist, analyzing the same material for the same elements day in and day out.

The importance of the chemical laboratory to a tool-steel plant cannot be over-estimated. Every heat of steel is analyzed for each element, and check analyses obtained; also, every substance used in the mix is analyzed for all impurities. The importance of using pure base materials is known to all manufacturers despite chemical evidence that certain detrimental elements are removed in the process of manufacture.

The manufacture of high-speed steel represents the highest art in the making of steel by tool-steel practice. Some may say, on account of our increased knowledge of chemistry and metallurgy, that the making of such steel has ceased to be an art, but has become a science. It is, in fact an art; aided by science. The human element in its manufacture is a decided factor, as will be brought in the following remarks:

The heat treatment of steel in its broad aspect may be said to commence with the melting furnace and end with the hardening and tempering of the finished product. High-speed steel is melted by two general types of furnace, known as crucible and electric. Steel treaters, however, are more vitally interested in the changes that take place in the steel during the various processes of manufacture rather than a detailed description of those processes, which are more or less familiar to all.

In order that good high-speed steel may be furnished in finished bars, it must be of correct chemical analysis, properly melted and cast into solid ingots, free from blow-holes and surface defects. Sudden changes of temperature are to be guarded against at every stage of its manufacture and subsequent treatment. The ingots are relatively weak, and the tendency to crack due to cooling strains is great. For this reason the hot ingots are not allowed to cool quickly, but are placed in furnaces which are of about the same temperature and are allowed to cool gradually before being placed in stock. Good steel can be made only from good ingots.

Steel treaters should be more vitally interested in the important changes which take place in high-speed steel during the hammering operations than that of any other working the steel receives in the course of its manufacture.

QUALITY AND STRUCTURE

The quality of high-speed steel is dependent to a very great extent upon its structure. The making of the structure begins under the hammer, and the beneficial effects produced in this stage persist through the subsequent operations, provided they are properly carried out. The massive carbides and tungstides present in the ingot are broken down and uniformly distributed throughout the billet.

To accomplish this the reduction in area must be sufficient and the hammer blows should be heavy, so as to carry the compression into the center of the billet; otherwise, undesirable characteristics such as coarse structure and carbide envelopes will exist and cause the steel treater much trouble. Surface defects invisible in the ingot may be opened up under the hammering operation, in which event they are chipped from the hot billet.

Ingots are first hammered into billets. These billets are carefully inspected and all surface defects ground or chipped. The hammered billets are again slowly heated and receive a second hammering, known as "cogging." The billet resulting therefrom is known as a "cogged" billet and is of the proper size for the rolling mill or for the finishing hammer.

Although it is not considered good mill practice, some manufacturers who have a large rolling mill perform the very important cogging operation in the rolling mill instead of under the hammer. Cogging in a rolling mill does not break up and distribute the carbides and tungstides as efficiently as cogging under the hammer; another objection to cogging in the rolling mill is that there is no opportunity to chip surface defects developed as they can be under the trained eye of a hammer-man, thereby eliminating such defects in the finished billet.

The rolling of high-speed steel is an art known to very few. The various factors governing the proper rolling are so numerous that it is necessary for each individual rolling mill to work out a practice that gives the best results upon the particular analysis of steel it makes. Important elements entering into the rolling are the heating and finishing temperatures, draft, and speed of the mill. In all of these the element of time must be considered.

High-speed steel should be delivered from the rolling mill to the annealing department free from scale, for scale promotes the formation of a decarbonized surface. In preparation of bars for annealing, they are packed in tubes with a mixture of charcoal, lime, and other material. The tubes are sealed and placed in the annealing furnace and the temperature is gradually raised to about 1,650 deg.F., and held there for a sufficient length of time, depending upon the size of the bars. After very slow cooling the bars are removed from the tubes. They should then show a Brinnell number of between 235 and 275.

The inspection department ranks with the chemical and metallurgical departments in safeguarding the quality of the product. It inspects all finished material from the standpoint of surface defects, hardness, size and fracture. It rejects such steel as is judged not to meet the manufacturer's standard. The inspection and metallurgical departments work hand in hand, and if any department is not functioning properly it will soon become evident to the inspectors, enabling the management to remedy the trouble.

The successful manufacture of high-speed steel can only be obtained by those companies who have become specialists. The art and skill necessary in the successful working of such steel can be attained only by a man of natural ability in his chosen trade, and trained under the supervision of experts. To become an expert operator in any department of its manufacture, it is necessary that the operator work almost exclusively in the production of such steel.

As to the heat treatment, it is customary for the manufacturer to recommend to the user a procedure that will give to his steel a high degree of cutting efficiency. The recommendations of the manufacturer should be conservative, embracing fairly wide limits, as the tendency of the user is to adhere very closely to the manufacturer's recommendations. Unless one of the manufacturer's expert service men has made a detailed study of the customer's problem, the manufacturer is not justified in laying down set rules, for if the customer does a little experimenting he can probably modify the practice so as to produce results that are particularly well adapted to his line of work.

The purpose of heat-treating is to produce a tool that will cut so as to give maximum productive efficiency. This cutting efficiency depends upon the thermal stability of the complex hardenites existing in the hardened and tempered steel. The writer finds it extremely difficult to convey the meaning of the word "hardenite" to those that do not have a clear conception of the term. The complex hardenites in high-speed steel may be described as that form of solid solution which gives to it its cutting efficiency. The complex hardenites are produced by heating the steel to a very high temperature, near the melting point, which throws into solution carbides and tungstides, provided they have been properly broken up in the hammering process and uniformly distributed throughout the steel. By quenching the steel at correct temperature this solid solution is retained at atmospheric temperature.

It is not the intention to make any definite recommendations as to heat-treating of high-speed steel by the users. It is recognized that such steel can be heat-treated to give satisfactory results by different methods. It is, however, believed that the American practice of hardening and tempering is becoming more uniform. This is due largely to the exchange of opinions in meetings and elsewhere. The trend of American practice for hardening is toward the following:

First, slowly and carefully preheat the tool to a temperature of approximately 1,500 deg.F., taking care to prevent the formation of excessive scale.

Second, transfer to a furnace, the temperature of which is approximately 2,250 to 2,400 deg.F., and allow to remain in the furnace until the tool is heated uniformly to the above temperature.

Third, cool rapidly in oil, dry air blast, or lead bath.

Fourth, draw back to a temperature to meet the physical requirements of the tool, and allow to cool in air.

It was not very long ago that the desirability of drawing hardened high-speed steel to a temperature of 1,100 deg. was pointed out, and it is indeed encouraging to learn that comparatively few treaters have failed to make use of this fact. Many treaters at first contended that the steel would be soft after drawing to this temperature and it is only recently, since numerous actual tests have demonstrated its value, that the old prejudice has been eliminated.

High-speed steel should be delivered only in the annealed condition because annealing relieves the internal strains inevitable in the manufacture and puts it in vastly improved physical condition. The manufacturer's inspection after annealing also discloses defects not visible in the unannealed state.

The only true test for a brand of high-speed steel is the service that it gives by continued performance month in and month out under actual shop conditions. The average buyer is not justified in conducting a test, but can well continue to purchase his requirements from a reputable manufacturer of a brand that is nationally known. The manufacturer is always willing to cooperate with the trade in the conducting of a test and is much interested in the information received from a well conducted test. A test, to be valuable, should be conducted in a manner as nearly approaching actual working conditions in the plant in which the test is made as is practical. In conducting a test a few reputable brands should be allowed to enter. All tools entered should be of exactly the same size and shape. There is much difference of opinion as to the best practical method of conducting a test, and the decision as to how the test should be conducted should be left to the customer, who should cooperate with the manufacturers in devising a test which would give the best basis for conclusions as to how the particular brands would perform under actual shop conditions.

The value of the file test depends upon the quality of the file and the intelligence and experience of the person using it. The file test is not reliable, but in the hands of an experienced operator, gives some valuable information. Almost every steel treater knows of numerous instances where a lathe tool which could be touched with a file has shown wonderful results as to cutting efficiency.

Modern tool-steel practice has changed from that of the past, not by the use of labor-saving machinery, but by the use of scientific devices which aid and guide the skilled craftsman in producing a steel of higher quality and greater uniformity. It is upon the intelligence, experience, and skill of the individual that quality of tool steel depends.

HARDENING HIGH-SPEED STEELS

We will now take up the matter of hardening high-speed steels. The most ordinary tools used are for lathes and planers. The forging should be done at carbon-steel heat. Rough-grind while still hot and preheat to about carbon-steel hardening heat, then heat quickly in high-speed furnace to white heat, and quench in oil. If a very hard substance is to be cut, the point of tool may be quenched in kerosene or water and when nearly black, finish cooling in oil. Tempering must be done to suit the material to be cut. For cutting cast iron, brass castings, or hard steel, tempering should be done merely to take strains out of steel.

On ordinary machinery steel or nickel steel the temper can be drawn to a dark blue or up to 900 deg.F. If the tool is of a special form or character, the risk of melting or scaling the point cannot be taken. In these cases the tool should be packed, but if there is no packing equipment, a tool can be heated to as high heat as is safe without risk to cutting edges, and cyanide or prussiate of potash can be sprinkled over the face and then quenched in oil.

Some very adverse criticism may be heard on this point, but experience has proved that such tools will stand up very nicely and be perfectly free from scales or pipes. Where packing cannot be done, milling cutters, and tools to be hardened all over, can be placed in muffled furnace, brought to 2,220 deg. and quenched in oil. All such tools, however, must be preheated slowly to 1,400 to 1,500 deg. then placed in a high-speed furnace and brought up quickly. Do not soak high-speed steel at high heats. Quench in oil.

We must bear in mind that the heating furnace is likely to expand tools, therefore provision must be made to leave extra stock to take care of such expansion. Tools with shanks such as counter bores, taps, reamers, drills, etc., should be heated no further than they are wanted hard, and quench in oil. If a forge is not at hand and heating must be done, use a muffle furnace and cover small shanks with a paste from fire clay or ground asbestos. Hollow mills, spring threading dies, and large cutting tools with small shanks should have the holes thoroughly packed or covered with asbestos cement as far as they are wanted soft.

CUTTING-OFF STEEL FROM BAR

To cut a piece from an annealed bar, cut off with a hack saw, milling cutter or circular saw. Cut clear through the bar; do not nick or break. To cut a piece from an unannealed bar, cut right off with an abrasive saw; do not nick or break. If of large cross-section, cut off hot with a chisel by first slowly and uniformly heating the bar, at the point to be cut, to a good lemon heat, 1,800 to 1,850 deg.F. and cut right off while hot; do not nick or break. Allow the tool length and bar to cool before reheating for forging.

LATHE AND PLANER TOOLS

FORGING.—Gently warm the steel to remove any chill, is particularly desirable in the winter, then heat slowly and carefully to a scaling heat, that is a lemon heat (1,800 to 2,000 deg.F.), and forge uniformly. Reheat the tool for further forging directly the steel begins to stiffen under the hammer. Under no circumstances forge the steel when the temperature falls below a dark lemon to an orange color about 1,700 deg.F. Reheat as often as is necessary to finish forging the tool to shape. Allow the tool to cool after forging by burying the tool in dry ashes or lime. Do not place on the damp ground or in a draught of air.

The heating for forging should be done preferably in a pipe or muffle furnace but if this is not convenient use a good clean fire with plenty of fuel between the blast pipe and the tool. Never allow the tool to soak after the desired forging heat has been reached. Do not heat the tool further back than is necessary to shape the tool, but give the tool sufficient heat. See that the back of the tool is flatly dressed to provide proper support under the nose of the tool.

HARDENING HIGH-SPEED STEEL.—Slowly reheat the cutting edge of the tool to a cherry red, 1,400 deg.F., then force the blast so as to raise the temperature quickly to a full white heat, 2,200 to 2,250 deg.F., that is, until the tool starts to sweat at the cutting face. Cool the point of the tool in a dry air blast or preferably in oil, further cool in oil keeping the tool moving until the tool has become black hot.

To remove hardening strains reheat the tool to from 500 to 1,100 deg.F. Cool in oil or atmosphere. This second heat treatment adds to the toughness of the tool and therefore to its life.

GRINDING TOOLS.—Grind tools to remove all scale. Use a quick-cutting, dry, abrasive wheel. If using a wet wheel, be sure to use plenty of water. Do not under any circumstances force the tool against the wheel so as to draw the color, as this is likely to set up checks on the surface of the tool to its detriment.

FOR MILLING CUTTERS AND FORMED TOOLS

FORGING.—Forge as before.—ANNEALING.—Place the steel in a pipe, box or muffle. Arrange the steel so as to allow at least 1 in. of packing, consisting of dry powder ashes, powdered charcoal, mica, etc., between the pieces and the walls of the box or pipe. If using a pipe close the ends. Heat slowly and uniformly to a cherry red, 1,375 to 1,450 deg.F. according to size. Hold the steel at this temperature until the heat has thoroughly saturated through the metal, then allow the muffle box and tools to cool very slowly in a dying furnace or remove the muffle with its charge and bury in hot ashes or lime. The slower the cooling the softer the steel.

The heating requires from 2 to 10 hr. depending upon the size of the piece.

HARDENING AND TEMPERING.—It is preferable to use two furnaces when hardening milling cutters and special shape tools. One furnace should be maintained at a uniform temperature from 1,375 to 1,450 deg.F. while the other should be maintained at about 2,250 deg.F. Keep the tool to be hardened in the low temperature furnace until the tool has attained the full heat of this furnace. A short time should be allowed so as to be assured that the center of the tool is as hot as the outside. Then quickly remove the tool from this preheating furnace to the full heat furnace. Keep the tool in this furnace only as long as is necessary for the tool to attain the full temperature of this furnace. Then quickly remove and quench in oil or in a dry air blast. Remove before the tool is entirely cold and draw the temper in an oil bath by raising the temperature of the oil to from 500 to 750 deg.F. and allow this tool to remain, at this temperature, in the bath for at least 30 min., insuring uniformity of temper; then cool in the bath, atmosphere or oil.

If higher drawing temperatures are desired than those possible with oil, a salt bath can be used. A very excellent bath is made by mixing two parts by weight of crude potassium nitrate and three parts crude sodium nitrate. These will melt at about 450 deg.F. and can be used up to 1,000 deg.F. Before heating the steel in the salt bath, slowly preheat, preferably in oil. Reheating the hardened high-speed steel to 1,000 deg.F. will materially increase the life of lathe tools, but milling and form cutters, taps, dies, etc., should not be reheated higher than 500 to 650 deg.F., unless extreme hardness is required, when 1,100 to 1,000 deg.F., will give the hardest edge.

INSTRUCTIONS FOR WORKING HIGH-SPEED STEEL

Owing to the wide variations in the composition of high-speed steels by various makers, it is always advisable to follow the directions of each when using his brand of steel. In the absence of specific directions the following general suggestions from several makers will be found helpful.

The Ludlum Steel Company recommend the following:

CUTTING-OFF.—To cut a piece from an annealed bar, cut off with a hack saw, milling cutter or circular saw. Cut clear through the bar; do not nick or break. To cut a piece from an unannealed bar, cut right off with an abrasive saw; do not nick or break. If of large cross-section, cut off hot with a chisel by first slowly and uniformly heating the bar, at the point to be cut, to a good lemon heat, 1,800 deg.-1,850 deg.F. and cut right off while hot; do not nick or break. Allow the tool length and bar to cool before reheating for forging.

LATHE AND PLANER TOOLS

TO FORGE.—Gently warm the steel to remove any chill is particularly desirable in the winter. Then heat slowly and carefully to a scaling heat, that is a lemon heat (1,800 deg.-2,000 deg.F.), and forge uniformly. Reheat the tool for further forging directly the steel begins to stiffen under the hammer. Under no circumstances forge the steel when the temperature falls below a dark lemon to an orange color: about 1,700 deg.F. Reheat as often as is necessary to finish forging the tool to shape. Allow the tool to cool after forging by burying the tool in dry ashes or lime. Do not place on the damp ground or in a draught of air.

The heating for forging should be done preferably in a pipe or muffle furnace, but if this is not convenient use a good clean fire with plenty of fuel between the blast pipe and the tool. Never allow the tool to soak after the desired forging heat has been reached. Do not heat the tool further back than is necessary to shape the tool, but give the tool sufficient heat. See that the back of the tool is flatly dressed to provide proper support under the nose of the tool.

HARDENING.—Slowly reheat the cutting edge of the tool to a cherry red, 1,400 deg.F., then force the blast so as to raise the temperature quickly to a full white heat, 2,200 deg.-2,250 deg.F., that is, until the tool starts to sweat at the cutting face. Cool the point of the tool in a dry air blast or preferably in oil; further cool in oil, keeping the tool moving until the tool has become black hot.

To remove hardening strains reheat the tool to from 500 deg. to 1,100 deg.F. Cool in oil or atmosphere. This second heat treatment adds to the toughness of the tool and therefore to its life.

GRINDING.—Grind tools to remove all scale. Use a quick cutting, dry, abrasive wheel. If using a wet wheel, be sure to use plenty of water. Do not under any circumstances force the tool against the wheel so as to draw the color, as this is likely to set up checks on the surface of the tool to its detriment.

The Firth-Sterling Steel Company say:

INSTEAD OF PRINTING ANY RULES ON THE HARDENING AND TEMPERING OF FIRTH-STERLING STEELS WE WISH TO SAY TO OUR CUSTOMERS: TRUST THE STEEL TO THE SKILL AND THE JUDGEMENT OF YOUR TOOLSMITH AND TOOL TEMPERER.

The steel workers of today know by personal experience and by inheritance all the standard rules and theories on forging, hardening and tempering of all fine tool steels. They know the importance of slow, uniform heating, and the danger of overheating some steels, and underheating others.

The tempering of tools and dies is a science taught by heat, muscle and brains.

The tool temperer is the man to hold responsible for results. The tempering of tools has been his life work. He may find suggestions on the following pages interesting, but we are always ready to trust the treatment of our steels to the experienced man at the fire.

HEAT TREATMENT OF LATHE, PLANER AND SIMILAR TOOLS

FIRE.—For these tools a good fire is one made of hard foundry coke, broken in small pieces, in an ordinary blacksmith forge with a few bricks laid over the top to form a hollow fire. The bricks should be thoroughly heated before tools are heated. Hard coal may be used very successfully in place of hard coke and will give a higher heat. It is very easy to give Blue Chip the proper heat if care is used in making up the fire.

FORGING.—Heat slowly and uniformly to a good forging heat. Do not hammer the steel after it cools below a bright red. Avoid as much as possible heating the body of the tool, so as to retain the natural toughness in the neck of the tool.

HARDENING.—Heat the point of the tool to an extreme white heat (about 2,200 deg.F.) until the flux runs. This heat should be the highest possible short of melting the point. Care should be taken to confine the heat as near to the point as possible so as to leave the annealing and consequent toughness in the neck of the tool and where the tool is held in the tool post.

COOL in an air blast, the open air or in oil, depending upon the tools or the work they are to do.

For roughing tools temper need not be drawn except for work where the edge tends to crumble on account of being too hard.

For finishing tools draw the temper to suit the purpose for which they are to be used.

GRIND thoroughly on dry wheel (or wet wheel if care is used to prevent checking).

HEAT TREATMENT OF MILLING CUTTERS, DRILLS, REAMERS, ETC.

THE FIRE.—Gas and electric furnaces designed for high heats are now made for treating high-speed steels. We recommend them for treating all kinds of Blue Chip tools and particularly the above class. After tools reach a yellow heat in the forge fire they must not be allowed to touch the fuel or come in contact with the blast or surrounding air.

HEATING.—Tools of this kind should be heated to a mellow white heat, or as hot as possible without injuring the cutting edges (2,000 to 2,200 deg.F.). For most work the higher the heat the better the tool. Where furnaces are used, we recommend preheating the tools to a red heat in one furnace before putting them in a white hot furnace.

COOLING.—We recommend quenching all of the above tools in oil when taken from the fire. We have found fish oil, cottonseed oil, Houghton's No. 2 soluble oil and linseed oil satisfactory. The high heat is the important thing in hardening Blue Chip tools. If a white hot tool is allowed to cool in the open air it will be hard, but the air scales the tool.

DRAWING THE TEMPER.—Tools of this class should be drawn considerably more than water-hardening steel for the same purpose.

HEAT TREATMENT OF PUNCHES AND DIES, SHEARS, TAPS, ETC.

HEATING.—The degree to which tools of the above classes should be heated depends upon the shape, size and use for which they are intended. Generally, they should not be heated to quite as high a heat as lathe tools or milling cutters. They should have a high heat, but not enough to make the flux run on the steel (by pyrometer 1,900 to 2,100 deg.F.).

COOLING.—Depending on the tools, some should be dipped in oil all over, some only part way, and others allowed to cool down in the air naturally, or under air blast. In cooling, the toughness is retained by allowing some parts to cool slowly and quenching parts that should be hard.

DRAWING THE TEMPER.—As in cooling, some parts of these tools will require more drawing than others, but, on the whole, they must be drawn more than water hardening tools for the same purpose or to about 500 deg.F. all over, so that a good file will just "touch" the cutting or working parts.

BARIUM CHLORIDE PROCESS.—This is a process developed for treating certain classes of tools, such as taps, forming tools, etc. It is being successfully used in many large plants. Briefly the treatment is as follows:

In this treatment the tools are first preheated to a red heat, but small tools may be immersed without preheating. The barium chloride bath is kept at a temperature of from 2,000 to 2,100 deg.F., and tools are held in it long enough to reach the same temperature. They are then dipped in oil. The barium chloride which adheres to the tools is brushed off, leaving the tools as dean as before heating.

A CHROMIUM-COBALT STEEL

The Latrobe Steel Company make a high-speed steel without tungsten, its red-hardness properties depending on chromium and cobalt instead of tungsten. It is known as P. R. K-33 steel. It does not require the high temperature of the tungsten steels, hardening at 1,830 to 1,850 deg.F. instead of 2,200 deg. or even higher, as with the tungsten.

This steel is forged at 1,900 to 2,000 deg.F. and must not be worked at a lower temperature than 1,600 deg.F. It requires soaking in the fire more than the tungsten steels. It can be normalized by heating slowly and thoroughly to 1,475 deg.F., holding this for from 10 to 20 min. according to the size of the piece and cooling in the open air, protected from drafts.

A peculiarity of this steel is that it becomes non-magnetic at or above 1,960 deg.F. and the magnetic quality is not restored by cooling. Normalizing as above, however, restores the magnetic qualities. This enables the user to detect any tools which have been overheated, with a horseshoe magnet.

It is sometimes advantageous to dip tools, before heating for hardening, in ordinary fuel or quenching oil. The oil leaves a thin film of carbon which tends to prevent decarbonization, giving a very hard surface.

For other makes of high-speed steel used in lathe and planer tools the makers recommend that the tools be cut from the bar with a hack saw or else heated and cut with a chisel. The heating should be very slow until the steel reaches a red after which it can be heated more rapidly and should only be forged at a high heat. It can be forged at very high heats but care should be taken not to forge at a low heat. The heating should be uniform and penetrate clear to the center of the bar before forging is begun. Reheat as often as necessary to forge at the proper heat.

After forging cool in lime before attempting to harden. Do not attempt to harden with the forging heat as was sometimes done with the carbon tools.

For hardening forged tools, heat slowly up to a bright red and then rapidly until the point of the tool is almost at a melting heat. Cool in a blast of cold, dry air. For large sizes of steel, cool in linseed oil or in fish oil as is most convenient. If the tools are to be used for finishing cuts heat to a bright yellow and quench in oil. Grind for use on a sand wheel or grindstone in preference to an emery or an artificial abrasive wheel.

For hardening milling and similar cutters, preheat to a bright red, place the cutter on a round bar of suitable size, and revolve it quickly over a very hot fire. Heat as high as possible without melting the points of the teeth and cool in a cold blast of dry air or in fish oil.

Light fragile cutters, twist drills, taps and formed cutters may be heated almost white and then dipped in fish oil for hardening. Where possible it is better to give an even higher heat and cool in the blast of cold, dry air as previously recommended.

SUGGESTIONS FOR HANDLING HIGH-SPEED STEELS

The following suggestions for handling high-speed steels are given by a maker whose steel is probably typical of a number of different makes, so that they will be found useful in other cases as well. These include hints as to forging as well as hardening, together with a list of "dont's" which are often very useful. This applies to forging, hardening of lathe, slotting, planing and all similar tools.



HARDENING HIGH-SPEED STEEL

In forging use coke for fuel in the forge. Heat steel slowly and thoroughly to a lemon heat. Do not forge at a lower heat. Do not let the steel cool below a bright cherry red while forging. After the tool is dressed, reheat to forging heat to remove the forging strain, and lay on the floor until cold. Then have the tool rough ground on a dry emery wheel.



For built-up and bent tools special care should be taken that the forging heat does not go below a bright cherry. For tools 3/4 by 1-1/2 or larger where there is a big strain in forging, such as bending at angles of about 45 deg. and building the tools up, they should be heated to at least 1,700 deg.F. Slowly and without much blast. For a 3/4 by 1-1/2 tool it should take about 10 min. with the correct blast in a coke fire. Larger tools in proportion. They can then be bent readily, but no attempt should be made to forge the steel further without reheating to maintain the bright cherry red. This is essential, as otherwise the tools crack in hardening or while in use.



In hardening place the tool in a coke fire (hollow fire if possible) with a slow blast and heat gradually up to a white welding heat on the nose of the tool. Then dip the white hot part only into thin oil or hold in a strong cold air blast. When hardening in oil do not hold the tool in one place but keep it moving so that it cools as quickly as possible. It is not necessary to draw the temper after hardening these tools.



In grinding all tools should be ground as lightly as possible on a soft wet sandstone or on a wet emery wheel, and care should be taken not to create any surface cracks, which are invariably the result of grinding too forcibly. The foregoing illustrations, Figs. 84 to 91, with their captions, will be found helpful.

Special points of caution to be observed when hardening high-speed steel.

DON'T use a green coal fire; use coke, or build a hollow fire.

DON'T have the bed of the fire free from coal.

DON'T hurry the heating for forging. The heating has to be done very slowly and the forging heat has to be kept very high (a full lemon color) heat and the tool has to be continually brought back into the fire to keep the high heat up. When customers complain about seams and cracks, in 9 cases out of 10, this has been caused by too low a forging heat, and when the blacksmith complains about tools cracking, it is necessary to read this paragraph to him.

DON'T try to jam the tool into shape under a steam hammer with one or two blows; take easy blows and keep the heat high.

DON'T have the tool curved at the bottom; it must lie perfectly flat in the tool post.

DON'T harden from your forging heat; let the tool grow cold or fairly cold. After forging you can rough grind the tool dry, but not too forcibly.

DON'T, for hardening, get more than the nose white hot.

DON'T get the white heat on the surface only.

DON'T hurry your heating for hardening; let the heat soak thoroughly through the nose of the tool.

DON'T melt the nose of the tool.

DON'T, as a rule, dip the nose into water; this should be done only for extremely hard material. It is dangerous to put the nose into water for fear of cracking and when you do put the nose into water put just 1/2 in. only of the extreme white hot part into the water and don't keep it too long in the water; just a few seconds, and then harden in oil. We do not recommend water hardening.

DON'T grind too forcibly.

DON'T grind dry after hardening.

DON'T discolor the steel in grinding.

DON'T give too much clearance on tools for cutting cast iron.

DON'T start on cast iron with a razor edge on the tool. Take an oil stone and wipe three or four times over the razor edge.

DON'T use tool holder steel from bars without hardening the nose of each individual tool bit.

AIR-HARDENING STEELS.—These steels are recommended for boring, turning and planing where the cost of high-speed seems excessive. They are also recommended for hard wood knives, for roughing and finishing bronze and brass, and for hot bolt forging dies. This steel cannot be cut or punched cold but can be shaped and ground on abrasive wheels of various kinds.

It should be heated slowly and evenly for forging and kept as evenly heated at a bright red as possible. It should not be forged after it cools to a dark red.

After the tool is made, heat it again to a bright red and lay it down to cool in a dry place or it can be cooled in a cold, dry air blast. Water must be kept away from it while it is hot.



CHAPTER XI

FURNACES

There are so many standard furnaces now on the market that it is not necessary to go into details of their design and construction and only a few will be illustrated. Oil, gas and coal or coke are most common but there is a steady growth of the use of electric furnaces.



TYPICAL OIL-FIRED FURNACES.—Several types of standard oil-fired furnaces are shown herewith. Figure 92 is a lead pot furnace, Fig. 93 is a vertical furnace with a center column. This column reduces the cubical contents to be heated and also supports the cover.



A small tool furnace is shown in Fig. 94, which gives the construction and heat circulation. A larger furnace for high-speed steel is given in Fig. 95. The steel is supported above the heat, the lower flame passing beneath the support.

For hardening broaches and long reamers and taps, the furnace shown in Fig. 96 is used. Twelve jets are used, these coming in radially to produce a whirling motion.



Oil and gas furnaces may be divided into three types: the open heating chamber in which combustion takes place in the chamber and directly over the stock; the semimuffle heating chamber in which combustion takes place beneath the floor of the chamber from which the hot gases pass into the chamber through suitable openings; and the muffle heating chamber in which the heat entirely surrounds the chamber but does not enter it. The open furnace is used for forging, tool dressing and welding. The muffle furnace is used for hardening dies, taps, cutters and similar tools of either carbon or high-speed steel. The muffle furnace is for spring hardening, enameling, assaying and work where the gases of combustion may have an injurious effect on the material.



Furnaces of these types of oil-burning furnaces are shown in Figs. 97, 98, and 99; these being made by the Gilbert & Barker Manufacturing Company. The first has an air curtain formed by jets from the large pipe just below the opening, to protect the operator from heat.



Oil furnaces are also made for both high- and low-pressure air, each having its advocates. The same people also make gas-fired furnaces.

Several types of furnaces for various purposes are illustrated in Fig. 100 and 101. The first is a gas-fired hardening furnace of the surface-combustion type.

A large gas-fired annealing furnace of the Maxon system is shown in Fig. 101. This is large enough for a flat car to be run into as can be seen. It shows the arrangement of the burners, the track for the car and the way in which it fits into the furnace. These are from the designs of the Industrial Furnace Corporation.

Before deciding upon the use of gas or oil, all sides of the problem should be considered. Gas is perhaps the nearest ideal but is as a rule more expensive. The tables compiled by the Gilbert & Barker Manufacturing Company and shown herewith, may help in deciding the question.

TABLE 27.—SHOWING COMPARISON OF OIL FUEL WITH VARIOUS GASEOUS FUELS Heat units per thousand cubic feet Natural gas 1,000,000 Air gas (gas machine) 20 cp 815,500 Public illuminating gas, average 650,000 Water gas (from bituminous coal) 377,000 Water and producer gas, mixed 175,000 Producer gas 150,000

Since a gallon of fuel oil (7 lb.) contains 133,000 heat units, the following comparisons may evidently be made. At 5 cts. a gallon, the equivalent heat units in oil would equal:

Per thousand cubic feet Natural gas at $0.375 Air gas, 20 cp at 0.307 Public illuminating gas, average at 0.244 Water gas (from bituminous coal) at 0.142 Water and producer gas, mixed at 0.065 Producer gas at 0.057

Comparing oil and coal is not always simple as it depends on the work to be done and the construction of the furnaces. The variation rises from 75 to 200 gal. of oil to a ton of coal. For forging and similar work it is probably safe to consider 100 gal. of oil as equivalent to a ton of coal.

Then there is the saving of labor in handling both coal and ashes, the waiting for fires to come up, the banking of fires and the dirt and nuisance generally. The continuous operation possible with oil adds to the output.

When comparing oil and gas it is generally considered that 4-1/2 gal. of fuel oil will give heat equivalent to 1,000 cu. ft. of coal gas.

The pressure of oil and air used varies with the system installed. The low-pressure system maintains a pressure of about 8 oz. on the oil and draws in free air for combustion. Others use a pressure of several pounds, while gas burners use an average of perhaps 1-1/2 lb. of air to give best results.

The weights and volumes of solid fuels are: Anthracite coal, 55 to 65 lb. per cubic foot or 34 to 41 cubic feet per ton; bituminous coal, 50 to 55 lb. per cubic foot or 41 to 45 cubic feet per ton; coke, 28 lb. per cubic foot or 80 cubic feet per ton—the ton being calculated as 2,240 lb. in each case.

A novel carburizing furnace that is being used by a number of people, is built after the plan of a fireless cooker. The walls of the furnace are extra heavy, and the ports and flues are so arranged that when the load in the furnace and the furnace is thoroughly heated, the burners are shut off and all openings are tightly sealed. The carburization then goes on for several hours before the furnace is cooled below the effective carburizing range, securing an ideal diffusion of carbon between the case and the core of the steel being carburized. This is particularly adaptable where simple steel is used.

PROTECTIVE SCREENS FOR FURNACES

Workmen needlessly exposed to the flames, heat and glare from furnaces where high temperatures are maintained suffer in health as well as in bodily discomfort. This shows several types of shields designed for the maximum protection of the furnace worker.

Bad conditions are not necessary; in almost every case means of relief can be found by one earnestly seeking them. The larger forge shops have adopted flame shields for the majority of their furnaces. Years ago the industrial furnaces (particularly of the oil-burning variety) were without shields, but the later models are all shield-equipped. These shields are adapted to all of the more modern, heat-treating furnaces, as well as to those furnaces in use for working forges; and attention should be paid to their use on the former type since the heat-treating furnaces are constantly becoming more numerous as manufacturers find need of them in the many phases of munitions making or similar work.

The heat that the worker about these furnaces must face may be divided in general into two classes: there is first that heat due to the flame and hot gases that the blast in the furnaces forces out onto a man's body and face. In the majority of furnaces this is by far the most discomforting, and care must be taken to fend it and turn it behind a suitable shield. The second class is the radiant heat, discharged as light from the glowing interior of the furnace. This is the lesser of the two evils so far as general forging furnaces are concerned, but it becomes the predominating feature in furnaces of large door area such as in the usual case-hardening furnaces. Here the amount of heat discharged is often almost unbearable even for a moment. This heat can be taken care of by interposing suitable, opaque shields that will temporarily absorb it without being destroyed by it, or becoming incandescent. Should such shields be so constructed as to close off all of the heat, it might be impossible to work around the furnace for the removal of its contents, but they can be made movable, and in such a manner as to shield the major portion of the worker's body.

First taking up the question of flame shields, the illustration, Fig. 102, is a typical installation that shows the main features for application to a forging machine or drop-hammer, oil-burning furnace, or for an arched-over, coal furnace where the flame blows out the front. This shield consists of a frame covered with sheet metal and held by brackets about 6 in. in front of the furnace. It will be noted that slotted holes make this frame adjustable for height, and it should be lowered as far as possible when in use, so that the work may just pass under it and into the furnace openings.

Immediately below the furnace openings, and close to the furnace frame will be noted a blast pipe carrying air from the forge-shop fan. This has a row of small holes drilled in its upper side for the entire length, and these direct a curtain of cold air vertically across the furnace openings, forcing all of the flame, or a greater portion of it, to rise behind the shield. Since the shield extends above the furnace top there is no escape for this flame until it has passed high enough to be of no further discomfort to the workman.

In this case fan-blast air is used for cooling, and this is cheaper and more satisfactory because a great volume may be used. However, where high-pressure air is used for atomizing the oil at the burner, and nothing else is available, this may be employed—though naturally a comparatively small pipe will be needed, in which minute holes are drilled, else the volume of air used will be too great for the compressor economically to supply. Steam may also be employed for like service.



The latest shields of this type are all made double, as illustrated, with an inner sheet of metal an inch or two inside of the front. In the illustration, A, Fig. 102, this inner sheet is smaller, but some are now built the same size as the front and bolted to it with pipe spacers between. The advantage of the double sheet is that the inner one bears the brunt of the flame, and, if needs be, burns up before the outer; while, if due to a heavy fire it should be heated red at any point, the outer sheet will still be much cooler and act as an additional shield to the furnace man.

HEAVY FORGING PRACTICE.—In heavy forging practice where the metal is being worked at a welding heat, the amount of flame that will issue from an open-front furnace is so great that a plain, sheet-steel front will neither afford sufficient protection nor stand up in service. For such a place a water-cooled front is often used. The general type of this front is illustrated in Fig. 103, and appears to have found considerable favor, for numbers of its kind are scattered throughout the country.

In this case the shield is placed at a slight angle from the vertical, and along the top edge is a water pipe with a row of small holes through which sprays of water are thrown against it. This water runs down in a thin sheet over the shield, cooling it, and is collected in a trough connected with a run-off pipe at the bottom. The lower blast-pipe arrangement is similar to the one first described.

There are several serious objections to this form of shield that should lead to its replacement by a better type; the first is that with a very hot fire, portions in the center may become so rapidly heated that the steam generated will part the sheet of water and cause it to flow from that point in an inverted V, and that section will then quickly become red hot. Another feature is that after the water and fire are shut down for the night the heat of the furnace can be great enough to cause serious warping of the surface of the shield so that the water will no longer cover it in a thin, uniform sheet.

After rigging up a big furnace with a shield of this type several years ago, its most serious object was found in the increase of the water bill of the plant. This was already of large proportions, but it had suddenly jumped to the extent of several hundred dollars. Investigation soon disclosed the fact that this water shield was one of the main causes of the added cost of water. A little estimating of the amount of water that can flow through a 1/2-in. pipe under 30-lb. pressure, in the course of a day, will show that this amount at 10 cts. per 1,000 gal., can count up rather rapidly.

Figure 103 is a section through a portion of the furnace front and shield showing all of the principal parts. This shield consists essentially of a very thin tank, about 2-1/2 in. between walls, and filled with water. Like other shields it is fitted with an adjustment, that it may be raised and lowered as the work demands. The tank having an open top, the water as it absorbs heat from the flame will simply boil away in steam; and only a small amount will have to be added to make up for that which has evaporated. The water-feed pipe shown at F ends a short distance above the top of the tank so that just how much water is running in may readily be seen.

An overflow pipe is provided at O which aids in maintaining the water at the proper height, as a sufficient quantity can always be permitted to run in, to avoid any possibility of the shield ever boiling dry; at the same time the small excess can run off without danger of an overflow. The shield illustrated in Fig. 104 has been in constant use for over two years, giving greater satisfaction than any other of which the writer has known. It might also be noted that this shield was made with riveted joints, the shop not having a gas-welding outfit. To flange over the edges and then weld them with an acetylene torch would be a far more economical procedure, and would also insure a tight and permanent joint.

The water-cooled front shown in Fig. 105 is an absurd effort to accomplish the design of a furnace that will provide cool working conditions. This front was on a bolt-heating furnace using hard coal for fuel; and it may be seen that it takes the place of all of the brickwork that should be on that side. Had this been nothing more than a very narrow water-cooled frame, with brickwork below and supporting bricks above, put in like the tuyeres in a foundry cupola, the case would have been somewhat different, for then it would have absorbed a smaller proportion of the heat.

A blacksmith who knows how a piece of cold iron laid in a small welding furnace momentarily lowers the temperature, will appreciate the enormous amount of extra heat that must be maintained in the central portion of this furnace to make up for the constant chilling effect of the cold wall. Moreover, since there would have been serious trouble had steam generated in this front, a steady stream of water had to be run through it constantly to insure against an approach to the boiling point. This is illustrated because of its absurdity, and as a warning of something to avoid.

Water-cooled, tuyere openings, as mentioned above, which support brick side-walls of the furnace, have proved successful for coal furnaces used for forging machine and drop-hammer heating, since they permit a great amount of work to be handled through their openings without wearing away as would a brick arch. Great care should be exercised properly to design them so that a minimum amount of the cold tuyere will be in contact with the interior of the furnace, and all interior portions possible should be covered by the bricks. However, a discussion of these points will hardly come in the flame-shield class, although they can be made to do a great deal toward relieving the excessive heat to be borne by the furnace worker.

FLANGE SHIELDS FOR FURNACES.—Such portable flame shields as the one illustrated in Fig. 106 may prove serviceable before furnaces required for plate work, where the doors are often only opened for a moment at a time. This shield can be placed far enough in front of the furnace, that it will be possible to work under it or around it, in removing bulky work from the furnace, and yet it will afford the furnace tender some relief from the excessive glare that will come out the wide-opened door. To have this shield of light weight so that it may be readily pushed aside when not wanted, the frame may be made up of pipe and fittings, and a piece of thin sheet steel fastened in the panel by rings about the frame.

About the most disagreeable task in a heat-treating shop is the removal of the pots from the case-hardening furnaces; these must be handled at a bright red heat in order that their contents may be dumped into the quenching tank with a minimum-time contact with the air, and before they have cooled sufficiently to require reheating. Facing the heat before the large open doors of the majority of these furnaces, in a man-killing task even when the weather is moderately cool. The boxes soon become more or less distorted, and then even the best of lifting devices will not remove a hot pot without several minutes labor in front of the doors.

In Fig. 107 is shown a method of arranging a shield on one type of charging and removing truck. This shield cannot afford more than a partial protection to the body of the furnace tender, because he must be able to see around it, and in some cases even push it partly through the door of the furnace, but even small as it is it may still afford some welcome protection. The great advantage in this case of having the shield on the truck instead of stationary in front of the furnace, is that it still affords protection as long as the hot pot is being handled through the shop on its way to the quenching tank.

It might be interesting to many engaged in the heat-treating or case hardening of steel parts, to make a special note of the design of the truck that is illustrated in connection with the shield; the general form is shown although the actual details for the construction of such a truck are lacking; these being simple, may be readily worked out by anyone wishing to build one. This is considered to be one of the quickest and easiest operated devices for the removal of this class of work from the furnace. To be sure it may only be used where the floor of the furnace has been built level with the floor of the room, but many of the modern furnaces of this class are so designed.

The pack-hardening pots are cast with legs, from two to three inches high, to permit the circulation of the hot gases, and so heat more quickly. Between these legs and under the body of the pot, the two forward prongs of the truck are pushed, tilting the outer handle to make these prongs as low as possible. The handle is then lowered and, as it has a good leverage, the pot is easily raised from the floor, and the truck and its load rolled out.

HEATING OF MANGANESE STEEL.—Another form of heat-treating furnace is that which is used for the heating of manganese and other alloy steels, which after having been brought to the proper heat are drawn from the furnace into an immediate quenching tank. With manganese steel in particular, the parts are so fragile and easily damaged while hot that it is frequent practice to have a sloping platform immediately in front of the furnace door down which the castings may slide into a tank below the floor level. Such a furnace with a quenching tank in front of its door is shown in Fig. 108.

These tanks are covered with plates while charging the furnace and the cold castings are placed in a moderately cool furnace. Since some of these steels must not be charged into a furnace where the heat is extreme but should be brought up to their final heat gradually, there is little discomfort during the charging process. When quenching, however, from a temperature of 1,800 deg. to 1,900 deg., it is extremely unpleasant in front of the doors. The swinging shield is here adapted to give protection for this work. As will be noted it is hung a sufficient distance in front of the doors, that it may not interfere with the castings as they come from the furnace, and slide down into the tank.

To facilitate the work, and avoid the necessity of working with the bars outside the edges of the shield, the slot-like hole is cut in the center of the shield, and through this the bars or rakes for dragging out the castings are easily inserted and manipulated. The advantage of such a swinging shield is that it may be readily moved from side to side, or forward and back as occasion requires.

FURNACE DATA

In order to give definite information concerning furnaces, fuels etc., the following data is quoted from a paper by Seth A. Moulton and W. H. Lyman before the Steel Heat Treaters Society in September, 1920.

This considers a factory producing 30,000 lb. of automobile gears per 24 hr. The transmission gears will be of high-carbon steel and the differential of low-carbon steel, carburized. The heat-treating equipment required is:

1. Annealing furnaces 1,400 to 1,600 deg.F. 2. Carburizing furnaces 1,700 to 1,800 deg.F. 3. Hardening furnaces 1,450 to 1,550 deg.F. 4. Drawing furnaces 350 to 950 deg.F.

All of the forging blanks are annealed before machining, about three-quarters of the machined gears and parts are carburized, all the carburized gears are given a double treatment for core and case, all gears and parts are hardened and all parts are drawn.

The possible sources of heat supply and their values are as follows:—

1. Oil 140,000 B.t.u. per gallon 2. Natural gas 1,100 B.t.u. per cubic foot 3. City gas 650 B.t.u. per cubic foot 4. Water gas 300 B.t.u. per cubic foot 5. Producer gas 170 B.t.u. per cubic foot 6. Coal 12,000 B.t.u. per pound 7. Electric current 3,412 B.t.u. per kilowatt-hour

For the heat treatment specified only comparatively low temperatures are required. No difficulty will be experienced in attaining the desired maximum temperature of 1,800 deg.F. with any of the heating medium above enumerated; but it should be noted that the producer gas with a B.t.u. content of 170 per cubic foot and the electric current would require specially designed furnaces to obtain higher temperatures than 1800 deg.F.

TABLE 28.—COMPARATTVE OPERATING COSTS

Assuming Cost of oil- and gas-fired furnaces installed as $100.00 per square foot of hearth Cost of coal-fired furnace installed as 150.00 per square foot of hearth Cost of electric furnace 100 kw. capacity installed as 90.00 per kilowatt Cost of electric furnace 150 kw. capacity installed as 70.00 per kilowatt

Output 3,000 lb. charge, 8 hr. heat carburizing, 2 hr. heating only. Annual service 7,200 hr. Fixed charges including interest, depreciation, taxes, insurance and maintenance 15 per cent. Extra operating labor for coal-fired furnace 60 cts. per hour, one man four furnaces.

COST OF VARIOUS TYPES OF FURNACES - Class fuel Fuel per Unit fuel Installation Efficiency Fixed Cost per charge cost cost per cent charges charge - - 1 2 3 4 5 6 7 - - Carburizing - - 1 Oil 52.0 gal. $0.15 gal. $2,400.00 12.6 $.40 $8.20 2 Natural gas 4.4 M 0.50 M 2,400.00 18.8 0.40 2.60 3 City gas 8.3 M 0.80 M 2,400.00 17.0 0.40 7.04 4 Water gas 18.7 M 0.40 2,400.00 16.4 0.40 7.88 5 Producer gas 37.3 M 0.10 M 2,400.00 14.5 0.40 4.13 6 Coal 814.0 lb. 6.00 ton 3,600.00 9.4 0.60 3.98 7 Electricity 500.0 kw-hr. 0.015 kw. 9,000.00 53.0 1.50 9.00 - - Heating - - 1 Oil 30.8 gal. 0.15 gal. 2,400.00 21.4 0.10 4.72 2 Natural gas 2.61 M 0.50 M 2,400.00 32.0 0.10 1.40 3 City gas 4.9 M 0.80 M 2,400.00 28.8 0.10 4.02 4 Water gas 11.1 M 0.40 M 2,400.00 27.6 0.10 4.54 5 Producer gas 22.1 M 0.10 M 2,400.00 24.6 0.10 2.31 6 Coal 348.0 lb. 6.00 ton 3,600.00 22.0 0.15 1.38 7 Electricity 329.0 kw-hr. 0.015 kw. 10,500.00 81.75 0.44 5.38 -