CHAPTER IVTreatment of Tool Steel

Fig. 54.—Hexagonal Head Bolt.

The upset metal should extend equally around the bolt. This will tend to prevent the head from forming unequally when the metal is being forged down on the heading tool. The head can be prevented from forming on one side by directing the blows toward the opposite side. Form the head by heating the upset end to a white heat, by inserting the opposite end in the heading tool, and by delivering upright blows on the heated end, unless others are required, thus forging down the upset metal to1⁄2inch thick. Remove it from the heading tool and forge the head into a hexagonal form. It will be necessary to insert the bolt in the heading tool several times to obtain the exact dimensions of the head, which should be7⁄8inch through its short diameter and1⁄2inch thick. The chamfered finish on the top of the head is produced by using a button head set while the bolt is held in the heading tool.

75. Square-cornered Angle.—Fig. 55. Upsetting, chamfering, and forging a square corner. Material required: 10 inches of 1 ×1⁄2-inch iron.

Upset the center by cooling 31⁄2inches of each end toconfine the operation to the required place. The center should be7⁄8inch thick, and all upset metal should be forged to one side; the opposite side and both edges should be straight. Draw both ends tapering from where the upsetting ceases to3⁄4×1⁄4inch at the ends; chamfer the edges of the drawn ends on the straight, flat side, beginning about 2 inches from the center and continuing to the ends. If the drawing and chamfering are properly done, each end will be 51⁄2inches from the center.

Fig. 55.—Upsetting for a Square Corner.

Heat and bend the stock at the upset center to a right angle, with the upset metal on the outer side to provide for the square corner. The bending should be done over the horn of the anvil to produce the quarter-round fillet on the inner side, and may be confined to the center by cooling both ends to where the upsetting begins.

As bends of this kind are somewhat difficult to make correctly, it would be a great advantage to provide a form which may be made to fit into the vise; then one end of the angle can be held securely with the form while the opposite end is bent over it. By any simple form it is impossible to make the outside corner perfectly sharp and square with one operation; it is therefore necessary toforge the outside corner sharp and square by delivering blows on both sides, somewhat in the manner shown inFig. 56, but good judgment must be used in doing this.

Fig. 56.—Square-cornered Angle.

The chamfering may be marred or entirely removed in forging the corner; if so, rechamfer, and if the ends are of unequal lengths, the longer one should be cut off equal with the other. Then all surfaces should be made straight and smooth with the flatter and the scale removed by occasionally dipping the flatter in water.

76. Fagot Welding.—Welding and forging to dimensions. Material required: convenient pieces of scrap iron and a bar of5⁄8-inch round stock from 24 to 30 inches long.

Temporarily weld several separate pieces of scrap on to the bar until sufficient metal is provided for a thorough welding and forging of a solid piece of square iron 31⁄2inches long and11⁄16inch square. The welding should be done so as not to show where the pieces were joined. Forge it perfectly square and smooth with the flatter. Cut one end off square with a sharp hot cutter, then cut it to the required length.

77. Round Weld.—Fig. 57. Scarfing, welding, and swaging. Material required: two pieces of7⁄16-inch round iron, 41⁄2inches long.

Upset one end to9⁄16inch, as shown ata. To form the scarf, deliver backing-up blows with the face of thehammer, as shown atb, and finish with blows delivered similarly with the ball. These backing-up blows will form the heel of the scarf. Draw out the point of the scarf with overhanging blows, as shown atc. The joining surface should be convex so that welding will proceed from the center. Scarf both pieces in the same manner, as atd.

Fig. 57.—Steps in Scarfing for a Round Weld.

Heat and weld according to instructions on welding and finish the work smoothly with swages; then cut to a length of 6 inches, having the weld in the center.

Properly formed scarfs will produce perfect welds provided they are heated to the welding temperature when they are joined, but those improperly formed generally produce imperfect welds, although the heat is right.

78. Flat Right-angled Weld.—Fig. 58. Material required: two pieces of iron3⁄4×3⁄8, 41⁄2inches long.

Upset one end1⁄8inch larger than its diameters, as ata. By using backing-up blows as in the previous exercise,form a heel on one side, as shown atb, then resting the straight side on the anvil, draw out the point with the ball of the hammer, as atc. In drawing this point, the metal will spread and form a wide fan-shaped end, but by resting the right sidedon the horn of the anvil and delivering blows on the left, the latter edge will be straightened, leaving all projecting metal on the right.

Fig. 58.—Steps in Scarfing for a Corner Weld.

Upset one end of the other piece to the same dimensions, allowing this upsetting to continue along the metal about 1 inch. Form a scarf on the left edge ate, with the ball of hammer, using blows similar to those shown atcand leaving the end square. Place them together to see if the points meet the heels; if not, make necessary alterations so they will.

Place the pieces in the fire, so that the side scarf will beremoved with the left hand and the end scarf with the right. When placing for welding, the right-hand piece should be laid on the anvil and the left-hand one placed in its proper position on top of it. The inside corner should form a quarter-round fillet, the outside should be sharp and square, and the longer end cut off to make them both equal. Smooth all surfaces with a flatter. SketchFshows the weld completed; the dotted lines indicate the location of the scarfs before welding.

79. T Weld.—Fig. 59. Scarfing and welding. Material required: two pieces of3⁄4×3⁄8-inch iron, 8 and 41⁄2inches long.

Fig. 59.—Steps in Scarfing for a T Weld.

Upset one end of the shorter piece1⁄8inch larger than its diameters, and form a scarf similar to the first one for the right-angled weld, but here allow it to form fan-shaped and project equally over each edge, as shown ata.

Upset the center of the long piece to1⁄8inch or more larger than its diameters, with the upset portion fully 1inch long, as atb. Form a scarf at this place with the ball of the hammer, allowing the metal to bend edgewise, as atc. Do not make this scarf quite so wide as the first one, as its edges should be entirely covered by scarfawithout leaving any openings. See that they fit properly before heating for welding.

Especial care should be taken to have a good fire. The long piece should be placed in the fire so as to be removed with the left hand, and the short one with the right. Place the short piece on the anvil, with the long piece, held in the left hand, on top of and overlapping it sufficiently to prevent any openings. When welded, the long piece should be perfectly straight, with the short one at a right angle to it. Finish the weld with the flatter while it is at a dull red heat. SketchDshows the T completed.

Fig. 60.—Chain Making.

80. Chain Making.—Fig. 60. Bending, scarfing, and welding links. Material required: 8 pieces of3⁄8-inch round iron, 6 inches long.

Heat and bend the center of each piece to a semicircle3⁄4inch inside diameter; make the ends of equal length and parallel from the semicircle, as ata. Take one of these bent pieces and form a scarf on one end by holding it on the edge of the anvil at an angle of 45 degrees, as shown atb, and delivering overhanging blows, as indicated by the dotted circle, which represents the hammer. Turn the link over, placing the other end in the same position as the first, and scarf. Bend both scarfs toward each other equally until they overlap sufficiently to prevent any opening being formed, as atc; this is called closing the scarf.

Heat and weld the link by delivering the first few blows on its sides while it is resting on the face of the anvil, then by delivering lighter ones, while it is hung on the horn. While striking the light blows, do not hold the link in a fixed position, but move it to receive the blows around the circumference. The finished dimensions are 2 ×3⁄4inches inside; a slight variation in length does not make any difference, but their ends and widths should be uniform.

Proceed with another piece in like manner, but after scarfing it insert the finished link and continue adding new ones, until there are five links all together. The three extra pieces are for use in the next three exercises.

81. Welded Ring.—Fig. 61. Bending, scarfing, and welding a ring of round iron. Material required: one piece of7⁄16-inch round iron, 8 inches long.

Fig. 61.—Steps in Making a Ring.

Heat, and bend over the horn of the anvil about 11⁄2inches of each end to an inside radius of no less than 1 inch, as atA. Then heat the straight portion to a uniform temperature and bend it by holding the piece in a vertical position on the anvil, and delivering upright blows, asshown atB; this should produce a form similar to that shown atC. Continue the bending by holding the work as atD. By carefully observing the effect of these blows, you will be able to determine how the work ought to be held to produce the complete ring. These blows are used here to give the same effect as leverage blows. If the position of the metal is changed when and where it should be, almost a perfect ring may be producedwithout holding it on the horn of the anvil. It is not the best method to hold the work on the horn, because blows delivered in this way have a tendency to produce oval sections where they hit. In forming this ring the ends should be left open about 1 inch.

The directions for scarfing and welding are somewhat similar to those given for links, except that the angle of the scarf should be nearly a right angle. After the welding is completed, the ring should be made perfectly round by placing it over a mandrel or the horn of the anvil. When the ring is welded and complete, connect it to the chain with one of the extra links.

82. Chain Swivel.—Fig. 62. Bending, scarfing, welding, and riveting. Material: about 2 feet of7⁄16-inch round iron. Norway iron is the best, and this length is the most convenient for the first operations.

Fig. 62.—Chain Swivel.

Fig. 63.—Tool for Welding a Swivel.

For making this swivel, a special mandrel (Fig. 63) should be provided, made of3⁄4-inch round, mild or tool steel, with a short offset of3⁄4inch; the gudgeon or pin which is shown atashould be 11⁄4inches long,7⁄16inch in diameter at the shoulder, and tapering to5⁄16inch at the end. Any convenient length of handle that will prevent burning the hand when welding, will do.

Bend about 21⁄2inches of the7⁄16-inch round stock to a right angle, as ata,Fig. 64; make the corner as square aspossible, by upsetting it before bending; or after bending, by using upright and backing-up blows. Flatten the bent portionbparallel with the bar, by first delivering the blows with the ball of the hammer to increase the width as much as possible, then finish it to3⁄16inch thick with the face of the hammer. The corner should be scarfed with the ball of the hammer and the rib worked out, as shown atc.

Fig. 64.—Steps in Making a Swivel.

Cut off the flat portion 2 inches from the bar, and form a thin scarf at the end ofb. Notice that this should be formed on the same side withc. Beginning with the scarf at the end, the flat portion should be bent or rolled up so that the scarfs will overlap considerably, as indicated in the end viewd. The special mandrel should now be inserted in the opening shown here, and all placed in a3⁄4-inch bottom swage, while the scarfs are hammered into close contact.

The long bar should now be cut off 41⁄2inches from the inside of the bend, and a fan-shaped scarf formed withthe ball of the hammer, as ate. This should be drawn thin on the end and sides. The center of the 41⁄2-inch length is next bent and the last scarf placed in position atfby again inserting the mandrel, placing it in the swage, and closing down the edges around the portions atf. It is then ready for welding. Figure62shows this in solid lines.

Fig. 65.—Making an Eye for a Swivel.

A good clean heat should be procured for welding; the mandrel should be quickly inserted, placed in the swage, and the welding done. This being completed, a small eye is to be made of3⁄8-inch round iron: first, by bending it in the form shown ata,Fig. 65; second, by inserting a punch in the opening and hammering the ends together, forming the eye, as shown atb; third, by welding these ends solidly together, as atc, and forging the whole to fit loosely in the swivel. The fitted end is now cut off square3⁄8inch longer than the depth of the hole in the swivel, heated, and, while the eye is held in the vise, it is quickly riveted into place with a small straight or ball peen hammer. The eye is shown in place by the broken lines inFig. 62. Connect this swivel to the chain with one of the extra links.

83. Chain Swivel.—Figs.66and67. Fullering, forging, bending, welding, and riveting. Material: a piece of 1 ×1⁄2-inch iron, 4 or more inches long.

Using top and bottom fullers, form two sets of depressions not deeper than1⁄4inch, on each edge and opposite to each other, the first pair to be 1 inch from the end, the second pair 1 inch from the first, as ata.

Fig. 66.—Steps in Making a Swivel.

Draw the 1-inch end to7⁄16inch round, leaving it slightly heavier where it was fullered to provide excess metal for further bending. The opposite end should now be cut off 1 inch from the fullered place and drawn to the same dimensions as the first end. Forge the central portion into a circular form and punch a3⁄8-inch hole in its center. Cut off all surplus material, making the ends 31⁄2inches long from the center of the hole, as atb.

Fig. 67.—The Completed Swivel.

Bend each end to a right angle close up to the eye and make the arms parallel and one inch apart, as atc. Drift the hole by driving the punchthrough between the parallel ends, thereby forming a slightly tapered hole. Scarf and weld the ends as you would a link. Make a small eye of3⁄8-inch round stock, proceeding in the manner explained in the previous exercise, also following the same instructions as to fitting, cutting, and riveting. Connect the link end of this swivel to the chain with one of the extra links. (SeeFig. 67.)

Fig. 68.—Steps in Making a Chain Grabhook.

84. Chain Grabhook.—Fig. 68. Forging, punching, and bending. Material: one piece of3⁄4×3⁄8-inch iron, 41⁄2inches long.

Form a depression as ata,1⁄4inch deep and3⁄4inch fromone end with overhanging blows. (The opposite edge should be kept perfectly straight during this and the following operations.) Forge the3⁄4-inch end into a circular-shaped eye3⁄8inch thick, and punch a1⁄4-inch hole, in the center, as atb. This hole should be drifted or expanded with a punch driven through from both sides alternately until the diameter becomes1⁄2inch.

By hanging this eye over the horn of the anvil so that the inner corners of the eye rest on the horn, by delivering blows opposite to those corners, and by changing its location so that blows will be delivered on all outside corners, the sectional form will be changed from square to octagon; by similar operations the form may be changed from octagon to round. During this change, light blows should be used in order to make the eye smooth. This stage is shown atcwith a sectional view of the eye.

Fig. 69.—The Completed Chain Grabhook.

Proceeding from the eye toward the opposite end, forge both edges round to correspond with the eye, leaving the metal3⁄4inch wide, 3 inches from the eye, as shown atd.

Draw the remaining section tapering from this extreme width to1⁄4inch, and forge the edges round as before. The hook should be3⁄16inch round at the end and 3 inches long from the widest point, as shown atE. Heat the middle portion; cool the point and the eye, and bend the hook edgewise over the horn of the anvil toward the straight side, until the point is opposite the depression first formed. The inside semicircle formed by bending should be1⁄2inchin diameter, the other inside lines straight and parallel. The extreme point should be slightly curved away from the eye, and all flat surfaces hammered smooth with light blows while the hook is at a dull red heat. Figure69shows the hook completed. Using the remaining extra link, connect the hook to the swivel.

Questions for Review

What forging operations are employed in making the staple and the draw spike? What hammer blows are used on them? What caution should be observed in heating the S hook for bending? What operations are employed in making the pipe hook? Which is the most difficult? Where was the most difficult forging encountered? How was the point drawn? What operations are employed in making the gate hook? Explain how the angle should be bent, and how the blows should be delivered to make it square. Why should the extreme corner of the angle be cooled off before bending the hook? What operations are employed in making the hasp? Which one is used first? Into what form is the metal to be forged in making the bolt? What is meant by chamfering? What kind of hammer blows should be used in chamfering? Why should the metal be upset for the round weld? What special hammer blows are to be used in forming the scarfs? Explain how the scarfs are formed for the right-angled weld. How should scarfs be placed in the fire? How should they be placed on the anvil? Explain how the scarfs are formed for the T weld. Describe the scarfing of a link. Describe the welding of a link. What is the effect of bending the ring over the horn of the anvil? What operations are used in making the chain swivel?

85. Selecting and Working Steel.—In making a tool, the differences in quality of steel should be considered, because steel suitable for a razor would not do for a cold chisel or any battering tool. (See sec.181.)

If the steel at hand is not exactly suitable, but the selection must be made from it, then that should be chosen which will most nearly meet the requirements, and tempering must be relied upon to make up the deficiency. In most large factories all grades of steel are kept on hand and are assorted in the stock room so that there need be no difficulty in making the proper selection.

The percentage of carbon in steel represents the amount of carbon it contains. A steel that is called a 75-point carbon steel is one that contains (.75) seventy-five one hundredths of one per cent, each point representing (.01) one one hundredth of one per cent.

Some steel makers use the word “temper” to indicate the amount of carbon, expecting the user of the steel to be familiar with the amounts of carbon each different temper represents. For instance, a razor-temper steel represents one that contains 1.50 per cent carbon and a tool-temper steel represents one containing about 1.25 per cent. The word “temper” as used in this connection should not be confused with the word as it is used in the art of tempering,where it indicates the operation of reducing the hardness of the metal in order to make it less brittle and more suitable for some particular use.

86. Uses of Different Grades of Steel.—As the percentage of carbon, and consequently the quality of steel, will vary somewhat with different makes, it is rather difficult to give a rule that will apply generally, but the following list of different grades of carbon will give a general idea of how steel should be selected, forged, and hardened.

Steel of 0.7 to 0.8 per cent carbon should be used for snaps, rivet sets, cupping tools, etc. This grade of steel should be forged at a light red heat. It can be welded easily and will harden at a light red heat.

Steel from 0.8 to 0.9 per cent carbon should be used for drop-forging dies, hammers, cold sets, track chisels, blacksmith’s tools, well drills, etc. It should be forged at a light red heat; it welds easily and hardens at a light red heat.

Steel from 0.9 to 1 per cent carbon should be used for large hand chisels, large punches, shear blades, dies, etc. Forging should be done at a light red heat. It welds readily and hardens at a bright red heat.

Steel from 1 to 1.1 per cent carbon should be used for hand chisels, punches, punch dies, small shear blades, etc. Forging should be done at a light red heat. It welds readily and hardens at a bright red heat.

Steel from 1.1 to 1.2 per cent carbon should be used for screw-cutting dies, large cutting and trimming dies, small punches, small hand chisels, large milling cutters, cups, cones, etc. Forging should be done at a light red heat. It welds readily when care is taken in heating, and hardens at a bright red heat.

Steel from 1.2 to 1.3 per cent carbon should be used for drills, taps, reamers, milling cutters, circular cutters, cutting and trimming dies, mill picks, engraving tools, twist drills, etc. Forging should be done at a bright red heat. Welding can be done when precaution is taken against overheating and burning. It hardens at a dull red heat.

Steel from 1.3 to 1.4 per cent carbon should be used for small drills, taps, cutters, boring tools, etc. Forging should be done at a bright red heat; welding can be done with care against overheating. It hardens at a dull red heat. This steel should be handled carefully.

Steel from 1.4 to 1.5 per cent carbon should be used for tools for working chilled castings or locomotive wheel tires, lathe and planer tools, razors, or any tools required to cut hard materials. Forging should be done at a dull red heat. Welding can scarcely be accomplished with this grade of stock. Hardening should be done at a dark red heat.

87. Injuries.—One of the most common injuries to steel comes from carelessness in the heating for forging. It is one of the important operations, for unless the metal is uniformly heated, violent strains are liable to occur, and, when hardened, the steel will show these strains by cracking. These defects are known as fire cracks.

The smith should always have plenty of fuel surrounding the metal while it is in the fire so that the cold-air blast will not come in direct contact with the metal. The air should be heated by passing through a bed of hot coals before it strikes the steel. It is always necessary to heat steel thoroughly to make it plastic, being careful not to overheat or burn any part of the metal. If it is overheatedor burned, it cannot be completely restored to its former state; the grain becomes coarse and the structure weak.

Never let steel lie in the fire to soak up heat after it is hot enough to work. If for any cause it cannot be worked when it is ready, it should be taken from the fire and left to cool, then reheated when it can be worked. By this precaution injury to the steel will be prevented.

If steel is heated so that the outer parts are hotter than the center, the metal will forge unevenly. The outer portion will be forged by the hammer blows, while the center remains almost in the original form. This will also cause an uneven grain, sure to produce cracks when the tool is hardened. Forging at too low a heat will injure the steel in the same manner as uneven heating.

After the steel has been properly heated, and forging has begun, the first blows should be struck rather heavily and followed by lighter ones as the heat vanishes. The forging should cease when the steel gets too cold, but it may be reheated as often as necessary to complete the work.

88. Annealing.—After the steel has been forged to the desired shape, it usually is necessary to do some finishing upon it before it can be hardened and tempered; in order to do this, it must be annealed or softened so that it can be machined or filed into shape.Annealing is the process of softening steel.It is done by heating the steel slowly to an even low red heat and placing it in an iron box containing unslaked lime or fine charcoal and leaving it there until perfectly cold. The object of this process is to retain the heat and prolong the cooling. The box is usually of cast iron, but sheet steel is equally good. Itshould be placed in a perfectly dry place and rest on bricks, if necessary, to avoid any dampness.

If an annealing box is not at hand, small steel forgings can be softened very satisfactorily by placing them between two boards, then completely covering all with dry ashes and leaving them there until entirely cold. Precaution should be taken here, also, to leave them in a dry place.

Another method, which is sometimes used, is calledwater annealing. Some mechanics claim to have had good results with it, while others condemn it entirely. By this method the article is heated to a dull red and allowed to cool partly, out of any direct current of air. When all redness has disappeared as it is held in a dark place, it is plunged into water and left there until perfectly cold.

The first method mentioned above is always the best; the second is nearly as good; and only when there is not sufficient time to allow the metal to cool slowly, should water annealing be attempted.

Such tools as cold chisels and lathe tools may be heated and laid in or on warm ashes until nearly cold, when they may be ground, hardened, and tempered. Quite frequently, if not generally, these tools are not treated in this manner, but it is no doubt the course to pursue to get the best results.

89. Hardening and Tempering.—When steel has been properly heated, forged, finished, or ground, the next two steps are hardening and tempering. These two processes are often understood as one, but they are entirely different in their results. The confusion arises because the two operations are sometimes performed withone heating of the steel as in hardening and tempering a cold chisel, or other similar tools.

As the steel has been subjected to severe strains during the heating and forging operations, its structure may have been somewhat altered. It can be restored to the proper crystalline structure by the hardening, scientifically known as refining. The hardening or refining heat is always lower than the forging heat, and should be only as high as is necessary to harden the steel to the required density by sudden cooling. Then this first operation of cooling will harden and refine the steel at the same time.

Extreme hardness is always accompanied by extreme brittleness, a quality undesirable in any cutting tool, and especially so in a tool required to withstand sudden shocks. As the hardness is reduced by subsequent heating, the toughness increases. This modification, called tempering, is accomplished by reheating the hardened portion of the tool until a sufficient toughness has been obtained, when the process is stopped by again plunging the tool into cold water. The heat for tempering may be supplied from the uncooled portion of the tool as in tempering a cold chisel, from the forge fire, from another hot piece of metal, or from a carefully heated furnace.

It has been found that the colored oxides formed on the surface of a piece of polished steel or iron represent a definite temperature in that metal. These colors have been used, therefore, to determine the desired temperature in tempering a tool. When we say “temper a tool to a light straw,” we mean that the hardened tool is to be heated again to a degree which will produce that color; namely, about 430 degrees Fahr. The colors as theyappear are light straw, dark straw, bronze, bronze with purple spots, purple, dark blue. The light color appears first. Do not allow the colors to pass too quickly, as will happen if the heat applied is too intense.

Fig. 70.—Hardening a Chisel.

There are two distinct methods of hardening and tempering. The one generally followed in tempering cold chisels, lathe and various other tools, requires only one heating. The tool is heated to a proper hardening temperature at the end, where hardness is desired, and also over an excess area to supply the heat for tempering. About 2 inches of the cutting end is heated; about 1 inch of this is plunged perpendicularly into water, as shown inFig. 70; it is then kept in motion perpendicularly between the places indicated ataandb, while the end is cooling. This will prevent a fixed cooling point and prevent a fracture that might possibly occur if it were held in one position while cooling. The portion betweenbandcshould retain sufficient heat to produce the necessary temper. When the end is perfectly cold it should be removed and immediately polished with sandstone or emery cloth to remove the scale of oxide so that the different colors may be more readily seen as they move frombtoward the point. The heat in the portion betweenbandcflows toward the point, causing the colors to appear as the heat extends. When the desired color covers the point, it should again be plunged intothe water and left there until entirely cold. In this method the first cooling is the hardening, and the second the tempering. A comparative color chart is appended to this chapter for guidance in obtaining the tempers for various tools.

Fig. 71.—Hardening A Reamer.

By the second method the steel is heated as in the first method, then it is cooled off entirely by immersing the tool exactly perpendicularly, as shown in hardening a reamer inFig. 71; after this it is polished. The temper is then drawn by holding the tool in contact with a piece of heated metal, cast iron preferably. InFig. 72the reamer is shown inside of a heated bushing, which is a more practical way than laying it on top of a heated flat plate. The bushing will impart sufficient heat to the tool to produce the desired color, when it should be again cooled. This method is used mostly for tempering plane bits, wood chisels, milling cutters, taps, reamers, and various other tools of a like nature.

Sometimes tools having sharp protruding edges, as milling cutters, taps, reamers, etc., are very liable to crack by the sudden cooling in water; this difficulty is avoided by using oil for hardening and tempering. Any so treated are called oil-tempered tools.

Fig. 72.—Tempering a Reamer.

The above methods of tempering are such as are ordinarilyused when only a common shop equipment is at hand, and the operator must depend entirely upon his judgment of the colors which represent the proper forging, annealing, hardening, and tempering heats. The degree of accuracy that has been attained in this practice is most surprising.

In large manufacturing establishments where many duplicate pieces are to be tempered, a more modern as well as scientific apparatus is employed to relieve the operator of dependence upon his discernment of colors. Here the steel is heated in a furnace, to which is attached a pyrometer that registers the exact degree of temperature. In this manner all pieces can be heated uniformly for any of the four required heats.

A.Natural Bar.B.Refined.C.Too hot.D.Burned.Fig. 73.—Sectional Views of Tool Steel, showing the Effects of Proper and Improper Treatment.

Fig. 73.—Sectional Views of Tool Steel, showing the Effects of Proper and Improper Treatment.

The views inFig. 73were photographed from the same grade or bar of steel to show the various granular structures produced by different heat treatments.Ashows the condition of the natural bar, which was broken to be photographed just as it was received from the steel makers.The lower left side shows where it was nicked with the cutter to be broken.Bshows the structure when proper conditions of heating and hardening have been maintained. Notice how much finer the structure here appears to be; this effect was caused by, and previously referred to as, the refining heat of steel. A similar condition should be produced with any tool steel under correct treatment.Cshows a much coarser structure; it was heated too hot and hardened in the same manner. If a tool were made thus, its weakness would be hardly noticeable at the time, but the structure shows that it is considerably weaker.Dshows the condition of the stock after being burned. It has produced from a quality of steel that was valuable, a metal worthless for any kind of tool.

90. Casehardening.—Another method of hardening, called casehardening, is used for wrought iron and low carbon or soft steel parts which are to be subjected to considerable friction. Neither of these metals could be hardened by the other methods mentioned. This process adds carbon to the exterior surfaces only, and for that reason is called casehardening, as the outside is made extremely hard, while the inner portion or core remains in a condition like that produced by sudden cooling, thus providing a hard wearing surface and great strength at the same time. It is similar to the old cementation process of steel making, but is not prolonged sufficiently to allow the hardening to continue through the entire structure.

The articles to be hardened are packed in a box somewhat similar to an annealing box. This should be partly filled with charred leather, ground bone, or wood or bone charcoal, all of which are highly carbonaceous materials;then the articles are placed in and entirely surrounded with a thin coating of cyanide of potassium, especially if iron is being hardened. The remaining space in the box is filled with the leather, bone, or pieces of charcoal. The box should be provided with a lid that will drop loosely between the outer projecting rims. The outer edges of this lid should be luted with clay to keep it as air-tight as possible. If a few small holes are provided in the center of the lid, test wires can be inserted; by removing a wire and cooling it, the progress of the operation may be known. These wires should be inserted before the box is placed in the furnace. The box and its contents are then placed in a suitable furnace and kept thoroughly heated from 6 to 15 hours, depending upon the depth of hardness required. Then it is withdrawn, the lid removed, and the articles quickly plunged into a large tank of water. This will complete the hardening.

When a number of very small articles are to be hardened, it is advisable to connect them with strong bailing wire before they are placed in the box so that they can all be removed at once. Beside holding the articles together, the wire will provide a means of testing the depth and quality of the process.

If only a thin coating of hardness is needed, or the labor and expense are excessive, the following method may be used: The article is heated thoroughly and evenly to about a bright red and thoroughly sprinkled with, or rolled in, cyanide of potassium. Then it is reheated so that the cyanide may penetrate as deeply as possible, after which it is quickly chilled in cold water. This is a good method of hardening small tack hammers made of soft steel, set screws, nuts, and very small tools.

Temperature and Color Chart to be Used in Tempering

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