HARDENING CARBON STEEL FOR TOOLS

For years the toolmaker had full sway in regard to make of steel wanted for shop tools, he generally made his own designs, hardened, tempered, ground and usually set up the machine where it was to be used and tested it.

Most of us remember the toolmaker during the sewing machine period when interchangeable tools were beginning to find their way; rather cautiously at first. The bicycle era was the real beginning of tool making from a manufacturing standpoint, when interchangeable tools for rapid production were called for and toolmakers were in great demand. Even then, jigs, and fixtures were of the toolmaker's own design, who practically built every part of it from start to finish.

The old way, however, had to be changed. Instead of the toolmaker starting his work from cutting off the stock in the old hack saw, a place for cutting off stock was provided. If, for instance, a forming tool was wanted, the toolmaker was given the master tool to make while an apprentice roughed out the cutter. The toolmaker, however, reserved the hardening process for himself. That was one of the particular operations that the old toolmaker refused to give up. It seemed preposterous to think for a minute that any one else could possibly do that particular job without spoiling the tools, or at least warp it out of shape (most of us did not grind holes in cutters 15 to 20 years ago); or a hundred or more things might happen unless the toolmaker did his own hardening and tempering.

That so many remarkably good tools were made at that time is still a wonder to many, when we consider that the large shop had from 30 to 40 different men, all using their own secret compounds, heating to suit eyesight, no matter if the day was bright or dark, and then tempering to color. But the day of the old toolmaker has changed. Now a tool is designed by a tool designer, O.K.'d, and then a print goes to the foreman of the tool department, who specifies the size and gets the steel from the cutting-off department. After finishing the machine work it goes to the hardening room, and this is the problem we shall now take up in detail.

The Modern Hardening Room.—A hardening room of today means a very different place from the dirty, dark smithshop in the corner with the open coal forge. There, when we wanted to be somewhat particular, we sometimes shoveled the coal cinders to one side and piled a great pile of charcoal on the forge. We now have a complete equipment; a gas- or oil-heating furnace, good running water, several sizes of lead pots, and an oil tank large enough to hold a barrel of oil. By running water, we mean a large tank with overflow pipes giving a constant supply. The ordinary hardening room equipment should consist of:

Gas or oil muffle furnace for hardening.
Gas or oil forge furnace.
A good size gas or oil furnace for annealing and case-hardening.
A gas or oil furnace to hold lead pots.
Oil tempering tank, gas- or oil-heated.
Pressure blower.
Large oil tank to hold at least a barrel of oil.
Big water tank with screen trays connected with large pipe from bottom with overflow.
Straightening press.
The furnace should be connected with pyrometers and tempering tank with a thermometer.

Beside all this you need a good man. It does not make much difference how completely the hardening department is fitted up, if you expect good work, a small percentage of loss and to be able to tackle anything that comes along, you must have a good man, one who understands the difference between low- and high-carbon steel, who knows when particular care must be exercised on particular work. In other words, a man who knows how his work should be done, and has the intelligence to follow directions on treatments of steel on which he has had no experience.

Jewelers' tools, especially for silversmith's work, probably have to stand the greatest punishment of any all-steel tools and to make a spoon die so hard that it will not sink under a blow from an 1,800-lb. hammer with a 4-ft. drop, and still not crack, demands careful treatment.

To harden such dies, first cover the impression on the die with paste made from bone dust or lampblack and oil. Place face down in an iron box partly filled with crushed charcoal, leaving back of die uncovered so that the heat can be seen at all times. Heat slowly in furnace to a good cherry red. The heat depends on the quality and the analysis of steel and the recommended actions of the steel maker should be carefully followed. When withdrawn from the fire the die should be quenched as shown in Fig. 80 with the face of die down and the back a short distance out of the water. When the back is black, immerse all over.

If such a tank is not at hand, it would pay to rig one up at once, although a barrel of brine may be used, or the back of the die may be first immersed to a depth of about 1/2 in. When the piece is immersed, hold die on an angle as in Fig. 81.

This is for the purpose of expelling all steam bubbles as they form in contact with hot steel. We are aware of the fact that a great many toolmakers in jewelry shops still cling to the overhead bath, as in Fig. 82, but more broken pieces and more dies with soft spots are due to this method than to all the others combined, as the water strikes one spot in force, contracting the surface so much faster than the rest of the die that the results are the same as if an uneven heating had been given the steel.

Take Time for Hardening.—Uneven heating and poor quenching has caused loss of many very valuable dies, and it certainly seems that when a firm spends from $75 to $450 in cutting a die that a few hours could be spared for proper hardening. But the usual feeling is that a tool must be hurried as soon as the hardener gets it, and if a burst die is the result from either uneven or overheated steel and quenching same without judgment, the steel gets the blame.

Give the steel a chance to heat properly, mix a little common sense with "your 30 years experience on the other fellows steel." Remember that high-carbon steel hardens at a lower heat than low-carbon steel, and quench when at the right heat in the two above ways, and 99 per cent of the trouble will vanish.

When a die flies to pieces in quenching, don't rush to the superintendent with a "poor-steel" story, but find out first why it broke so that the salesman who sold it will not be able to harden piece after piece from the same bar satisfactorily. If you find a "cold short," commonly called "a pipe," you can lay the blame on the steelmaker. If it is a case of overheating and quenching when too hot, you will find a coarse grain with many bright spots like crystals to the hardening depth. If uneven heating is the cause, you will find a wider margin of hardening depth on one side than on the other, or find the coarse grain from over-heating on one side while on the other you will find a close grain, which may be just right. If you find any other faults than a "pipe," or are not able to harden deep enough, then take the blame like a man and send for information. The different steel salesmen are good fellows and most of them know a thing or two about their own business.

For much work a cooling bath at from 50 to 75°F. is very good both for small hobs, dies, cutter plates or plungers. Some work will harden best in a barrel of brine, but in running cold water, splendid results will be obtained. Cutter plates should always be dipped corner first and if any have stripper holes, they should first be plugged with asbestos or fire clay cement.

In general it may be said that the best hardening temperature for carbon steel is the lowest temperature at which it will harden properly.

CARBON IN TOOL STEEL

Carbon tool steel, or "tool steel" as it is commonly called, usually contains from 80 to 125 points (or from 0.80 to 1.25 per cent) of carbon, and none of the alloys which go to make up the high speed steels. This was formerly known also as crucible or "cast" steel, or crucible cast steel, from the way in which it was made. This was before the days of steel castings. The advent of these caused so much confusion that the term was soon dropped. When we say "tool steel," we nearly always refer to carbon-tool steel, high-speed steel being usually designated by that name.

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:

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. 3½: 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. 4½: 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. 5½: 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. 6½: 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°C. (1,340 to 1,364°F.) according to section. In practice, the first chisel, is heated to 730°C.; and the second to 735°C. (1,355°F.); and a 1 in. half round chisel to 740°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°C. (419°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°C. rise and tested with a file, and at a point between 215 and 220°C. (428°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°C. (1,652°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 ¼ 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°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° 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°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°F.; but oil baths, which are used to secure a slower rate of cooling, may be used up to 500° 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°C. (212°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°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.

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°C. (36°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° from 300 to 800°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°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 ¾ 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.

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°C. Practically no lapping would be required if they were tempered at 340°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.

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°, however, the eye is very uncertain.