Heat-Treatment of Spring Steel. -- A number of experiments were made at the Baldwin Locomotive Works, to determine the effect of different heat-treatments on the transverse elastic limit and the modulus of elasticity of steels commonly used for locomotive springs. The points investigated were the effect of annealing, the comparative effect of quenching in water or oil, and the effect of re-heating the steel to various temperatures after complete cooling in water or oil. The steel used for the tests was basic open-hearth spring steel of the following composition: Carbon, 1.01 per cent; manganese, 0.38 per cent; phosphorus, 0.032 per cent; sulphur, 0.032 per cent; silicon, 0.13 per cent. This steel was found to reach its decalescence point at 1360 degrees F. Previous experiments had shown that, for annealing, it should be heated 40 degrees or 50 degrees above this temperature, and for hardening, from 50 degrees to 100 degrees above the point of decalescence. For the experiments, the following temperatures were used: For annealing, 1400 degrees F.; for quenching in oil, 1450 degrees F.; for quenching in water, 1425 degrees F. The results obtained are given in the table, "Results of Tests on Spring Steel".
As the table shows, the highest elastic limit obtainable, when heating to 1450 degrees and quenching in oil, was 187,400 pounds per square inch, which was obtained when the temper was not drawn after quenching. The higher the tempering temperature, the lower the elastic limit fell. When the steel was quenched at 1425 degrees F. in water, and the temper was not drawn, it was brittle and broke when deflected 0.175 inch. Drawing the temper to 600 degrees F., after hardening in water, gave an elastic limit of 219,800 pounds. When the temper was drawn to 1050 degrees F., the elastic limit dropped to 180,700 pounds, but the test piece did not break at 1.1 inch deflection. The tests show that the modulus of elasticity is practically constant, and apparently independent of heat-treatment. The conclusions are that steel of 1 per cent carbon, when quenched in cold water at its critical temperature or slightly above, is usually too hard and brittle for making springs or tools. The tests also show that the elastic limit of 1 per cent carbon steel can be made to vary from 78,500 to 240,800 pounds per square inch, by changes in the heat-treatment, and that very small changes in temperature when drawing the temper are sufficient to affect the elastic limit of the steel. Hence, to obtain good results, it is necessary to have means of heating the steel uniformly to the proper temperature, as well as cooling at the desired rate in a medium, the temperature and heat conductivity of which can be kept reasonably constant.
Local Hardening. -- One method of hardening locally is to cover the part that is to remain soft with a thin metal shield, so that it prevents the surface from being suddenly cooled by the direct action of the cooling medium. The steam or vapor which forms beneath the cover prevents the cooling medium from entering until the work has cooled sufficiently to prevent hardening; hence, a rather loose-fitting shield is desirable. The shield should be made of sheet iron or steel of about No. 29 gage (0.014 inch), for ordinary work. It is composed of one or more pieces, depending on the shape of the part, and, when several pieces are required, they can be bound together with wires or rivets. Of course, the surfaces to be hardened are left exposed. The heating should be done in a furnace or open-forge fire. A lead bath should not be used, because the hot lead beneath the shield will cause an explosion when the part is cooled. The quenching bath can be the same as when the shield is not used.
Local hardening is also effected by the application of a compound called "Enamelite" to the parts which are to remain soft. This compound, for tool steel, is in the form of a powder which is mixed with hot water to form a paste. It has the property of clinging to the steel and liberating hydrogen (the greatest known non-conductor) when the heated steel is plunged into the water. This causes the steel to retain its heat long enough to escape the chill, so that it remains soft where the enamelite has been applied.
Defects in Hardening. -- Uneven heating is the cause of most of the defects in hardening. Cracks of a circular form, from the corners or edges of a tool, indicate uneven heating in hardening. Cracks of a vertical nature and dark-colored fissures indicate that the steel has been burned and should be put on the scrap heap. Tools which have hard and soft places have been either unevenly heated, unevenly cooled, or "soaked", a term used to indicate prolonged heating. A tool not thoroughly moved about in the hardening fluid will show hard and soft places, and have a tendency to crack. Tools which are hardened simply by dropping them to the bottom of the tank, sometimes have soft places, owing to contact with the floor or sides of the tank. They should be thoroughly quenched before dropping. when a tool appears soft and will not harden, it probably has been decarbonized on the surface by too much heat or by soaking too long. The surface must be removed before the tool will harden properly. Tools are sometimes soft because the cooling bath is not large enough for the tools being hardened, and becomes too warm after a few pieces have been quenched.
Overheated Steel. -- Overheated steel that is not actually burned can be partly restored by heating to the proper heat, and allowing it to cool slowly in hot ashes or sand; when cold, the steel is hardened again at the proper hardening heat. Tools treated in this way are not as good as when treated at the proper heat throughout, but they are partially restored, and if the overheating originally took place in forging, the risk of cracking in hardening will be lessened by adopting the process mentioned. Care should be taken that the tuyere of the forge is well covered when heating tool steel; a tool coming in direct contact with the air blast will become surface burned, show soft places in hardening and wear badly in use.
Scale on Hardened Steel. -- The formation of scale on the surface of hardened steel is due to the contact of oxygen with the heated steel; hence, to prevent scale, the heated steel must not be exposed to the action of the air. When using an oven heating furnace, the flame should be so regulated that it is not visible in the heating chamber. The heated steel should be exposed to the air as little as possible, when transferring it from the furnace to the quenching bath. An old method of preventing scale and retaining a fine finish on dies is as follows: Fill the die impression with powdered boracic acid and place near the fire until the acid melts; then add a little more acid to insure covering all the surfaces. The die is then hardened in the usual way. If the boracic acid does not come entirely off in the quenching bath, immerse the work in boiling water. Dies hardened by this method are said to be as durable as those heated without the acid
Tempering
Tempering by the Color Method. -- Hardened steel can be tempered or made softer and less brittle by re-heating it to a certain temperature (depending on the nature of the steel and its intended use), and then cooling. When steel is tempered by the color method, the temper is gaged by the colors formed on the surface as the heat increases. First the surface is brightened to reveal the color changes, and then the steel is heated either by placing it upon a piece of red-hot metal, a gas-heated plate or in any other available way. As the temper increases, various colors appear on the brightened surface. First there is a faint yellow which blends into straw, then light brown, dark brown, purple, blue and dark blue, with various intermediate shades. The temperatures corresponding to the different colors and shades are given in the table on temperatures and colors for tempering. Turning and planing tools, chisels, etc., are commonly tempered by first heating the cutting end to a cherry-red, and then quenching the part to be hardened. When the tool is removed from the bath, the heat remaining in the unquenched part raises the temperature of the cooled cutting end until the desired color (which will show on a brightened surface) is obtained, after which the entire tool is quenched. The foregoing methods are convenient, especially when only a few tools are to be treated, but the color method of gaging temperatures is not dependable, as the color is affected, to some extent, by the composition of the metal. The modern method of tempering, especially in quantity, is to heat the hardened parts to the required temperature in a bath of molten lead, heated oil, or other liquids; the parts are then removed from the bath and quenched. The bath method makes it possible to heat the work uniformly, and to a given temperature with close limits.
High Temperatures judged by Color, and Colors for Tempering
Degrees Centi- grade Degrees Fahren- heit High Temperatures judged by Color
Degrees Centi- grade Degrees Fahren- heit Colors for Tempering
400 752 Red heat, visible in the dark
221.1 430 Very pale yellow
474 885 Red heat, visible in the twilight
226.7 440 Light yellow
525 975 Red heat, visible in the daylight
232.2 450 Pale straw-yellow
581 1077 Red heat, visible in the sunlight
237.8 460 Straw-yellow
700 1292 Dark red
243.3 470 Deep straw-yellow
800 1472 Dull cherry-red
248.9 480 Dark yellow
900 1652 Cherry-red
254.4 490 Yellow-brown
1000 1832 Bright cherry-red
260.0 500 Brown-yellow
1100 2012 Orange-red
265.6 510 Spotted red-brown
1200 2192 Orange-yellow
271.1 520 Brown-purple
1300 2372 Yellow-white
276.7 530 Light purple
1400 2552 White welding heat
282.2 540 Full purple
1500 2732 Brilliant white
287.8 550 Dark purple
1600 2912 Dazzling white (bluish-white)
293.3 560 Full blue
298.9 570 Dark blue
Tempering in Oil. -- Oil baths are extensively used for tempering tools (especially in quantity), the work being immersed in oil heated to the required temperature, which is indicated by a thermometer. It is important that the oil have a uniform temperature throughout and that the work be immersed long enough to acquire this temperature. Cold steel should not be plunged into a bath heated for tempering, owing to the danger of cracking it. The steel should either be pre-heated to about 300 degrees F., before placing it in the bath, or the latter should be at a comparatively low temperature before immersing the steel, and then be heated to the required degree. A temperature of from 650 degrees to 700 degrees F. can be obtained with heavy tempering oils; for higher temperatures, a lead bath is generally used. A tempering oil which has given satisfactory results in practice has the following characteristics: Composition, mineral oil, 94 per cent; saponifiable oil, 6 per cent; specific gravity 0.920; flash point, 550 degrees F.; fire test, 625 degrees F. The foregoing figures apply to new oil. When the oil has been used long enough to be rendered practically useless, an analysis shows the following changes: Composition, mineral oil, 30 per cent; saponifiable oil, 70 per cent; specific gravity, 0.950; flash point, 475 degrees F.; fire test, 550 degrees F. The great difference in the composition of new and old oil is due to the loss of mineral oil, resulting from the high heats to which tempering oil is frequently or constantly subjected; hence, the durability of the tempering bath can be increased by occasionally adding new mineral oil.
Flash Point and Fire Test. -- The distinction between the "flash point" and the "fire test" of an oil is as follows: The flash point is the temperature at which the amount of vapor given off is sufficient to form an inflammable or explosive mixture with the air over the surface of the oil, so that the gaseous mixture ignites and burns with a momentary flash when a flame is applied. As the temperature of the oil rises, more vapor is given off, and then the production of vapor is rapid enough to maintain a continuous flame, the oil takes fire and burns. The temperature at which this occurs is called the fire test, firing point or burning point of the oil.
Tempering in a Lead Bath. -- The lead bath is commonly used for heating steel preparatory to tempering, as well as for hardening. The bath is first heated to the temperature at which the steel should be tempered; the pre-heated work is then placed in the bath long enough to acquire this temperature, after which it is removed and cooled. As the melting temperature of pure lead is 618 degrees F., tin is commonly added to it to lower the temperature sufficiently for tempering. Reductions in temperature can be obtained by varying the proportions of lead and tin,
To Prevent Lead from Sticking to Steel. -- To prevent hot lead from sticking to parts heated in it, mix common whiting with wood alcohol, and paint the part that is to be heated. Water can be used instead of alcohol, but in that case the paint must be thoroughly dry, as otherwise the moisture will cause the lead to "fly". Another method is to make a thick paste according to the following formula: Pulverized charred leather, 1 pound; fine wheat flour, 1-1/2 pound; fine table salt, 2 pounds. Coat the tool with this paste and heat slowly until dry, then proceed to harden. Still another method is to heat the work to a blue color, or about 600 degrees F., and then dip it in a strong solution of salt water, prior to heating in the lead bath. The lead is sometimes removed from parts having fine projections or teeth, by using a stiff brush just before immersing in the cooling bath. This is necessary to prevent the formation of soft spots.
Pots for Lead Baths. -- Melting pots for molten lead baths, etc., should, preferably, be made from seamless drawn steel rather than from cast iron. Experience has shown that the seamless pots will sometimes withstand six months' continuous service, whereas cast iron pots will last, on average, only a few days, under like conditions. Cast steel melting pots, if properly made, are as durable as those made of seamless drawn steel.
Tempering in Sand. -- The sand bath is used for tempering certain classes of work. One method is to deposit the sand on an iron plate which is heated by suitable means as indicated in the accompanying illustration, Fig. 9. With this method of tempering, tools such as boiler punches, etc., can be given a varying temper by placing them endwise in the sand. As the temperature of the sand bath is higher toward the bottom, a tool can be so placed that the color of the lower end will be a deep dark blue when the middle portion is a very dark straw, and the working end or top a light straw color, the hardness gradually increasing from the bottom up. Tools to be heated by this method must be polished, as the temper is judged by the color. for tempering parts in quantity, sand tempering machines have been developed. One well-known design has a horizontal revolving cylinder containing rows of perforated pockets which become filled with sand in steady streams upon the work. The drum revolves at different rates of speed for different classes or work, usually making from 3 to 10 revolutions per minute. The heat is supplied by a gas burner. The machine is equipped with a thermometer, which does not indicate the actual temperature of the sand, but a somewhat lower temperature than would be required for the same tempering color, under other conditions. The thermometer reading, therefore, is relative and not a precise indication of the tempering temperature.
Tempering Furnaces. -- In tempering furnaces the only really important consideration is to insure that the furnace is so built as to heat the bath uniformly throughout. It is doubtful if there can be found a tempering furnace on the market that will fill this requirement entirely, although many give good results in general. It is never safe, however, to let any tools being tempered rest against the bottom or sides of the tank, as no matter how scientifically the furnace may be built these parts are, in most cases, hotter than the fluid itself. It is, of course, just as important not to let the thermometer rest against any of these parts in order to insure correct readings. After the pieces tempered are taken out of the oil bath, they should immediately be dipped in a tank of caustic soda (not registering over 8 or 9), and after that in a tank of hot water. This will remove all oil which might adhere to the tools.
Fig. 7 shows an ordinary type of tempering furnace. In this the flame does not strike the walls of the tank directly. The tools to be tempered are laid in a basket which is immersed in the oil. In Fig. 8 is shown a tempering furnace in which means are provided for preventing the tools to be tempered from coming in contact with the walls or bottom of the furnace proper. The basket holding the tools is immersed in the inner perforated oil tank. The same arrangement can, of course, be applied to the furnace shown in Fig. 7.