Heat Treatment of Steel

From the 1924 edition of Machinery's Handbook
This is part 6 of 7.

Copyright: expired.

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Detailed table of contents
First section: Furnaces and Baths for Heating Steel
Previous section: Annealing
Next section: Application and Heat Treatment of S. A. E. Carbon and Alloy Steels

Casehardening

Casehardening is the process of hardening the surface of low-carbon steel or iron by carbonizing the surface. When parts must be casehardened in quantity, they should be packed in an iron box contained some carbonaceous material. The box and its contents are then heated for a certain length of time, depending upon the depth of hardened surface desired and the nature of the material. The heat for casehardening varies from 1600 degrees to 1800 degrees F., the temperature being governed, to some extent, by the requirements. The absorption of carbon begins when the steel reaches about 1300 degrees F. At the end of the carbonizing period, the box is withdrawn from the furnace and is allowed to become quite cold. The articles are then placed in a muffle furnace and are re-heated to about 1470 degrees F., after which they are quenched in cold water, tepid water or oil, the bath depending upon the purpose for which the parts are to be used. For ordinary purposes, clear cold water is satisfactory. To produce a very hard surface, use salt water. When a hard surface is not as important as a tough core, use an oil bath. The practice of allowing the box and its contents to cool, and then re-heating prior to quenching, is based on the old rule of hardening on a rising heat. This method gives more satisfactory results than that of dumping the parts into a tank of cold water at the end of the carbonizing period.

Different Methods of Casehardening. -- A committee of the American Society for Testing Materials recommended the following practice for casehardening carbon-steel parts. Four different conditions were considered, varying from the heat-treatment that would give the hardest surface and the least strength, to that which would give the greatest strength with the least hardness of surface: When a hard case is the the only requirement and lack of toughness or even brittleness is unimportant, the articles may be quenched by emptying the contents of the casehardening boxes directly into cold water or oil. In this way both the core and the case are coarsely crystallized and the strength is reduced. If the articles are allowed to cool to a temperature slightly exceeding the critical range of the casehardening, usually from 800 degrees to 825 degrees C. (1472 degrees to 1517 degrees F.), and then quenched, the core and case still remain crystalline, but the danger of distortion or cracking in the quenching bath is reduced and the strength is somewhat increased. The next recommended method is to increase the toughness and strength of the article and refine the case. The articles are allowed to cool slowly in the carbonizing pot to a temperature of about 650 degrees C. (1200 degrees F.), are then re-heated to a temperature slightly exceeding the lower critical point of the case, which usually is from 775 degrees to 825 degrees C. (1427 degrees to 1517 degrees F.) and are then quenched in water or oil. The should be removed from the quenching bath before the temperature has fallen below 100 degrees C. (212 degrees F.). By allowing them to cool slowly to a temperature of about 650 degrees C. (1200 degrees F.) and then re-heating to a temperature of about 900 degrees to 950 degrees C. (1652 to 1740 degrees F.), followed by quenching in oil, from which they are removed before they have dropped below a temperature of 100 degrees C. (212 degrees F.), then re-heating to about 800 degrees C. (1472 degrees F.) and again quenching in water or oil, both the case and the core will be thoroughly refined and their toughness greatly increased. In order to reduce the hardening stress created by quenching, the objects, as a final treatment, may be tempered by re-heating them to a temperature not exceeding 200 degrees C. (392 degrees F.).

Casehardening Packing Boxes. -- Hardening boxes are made of either cast or wrought iron. The box should not be too large, as pieces in the center may then not become sufficiently carbonized. For small articles, a box measuring 12 by 10 by 8 inches is large enough, and for such parts as bicycle axles, pedal pins, etc., the maximum size should be about 18 by 12 by 11 inches. The boxes should have a plate lid which must be luted or sealed with clay after the contents are packed.

Carbonizing Materials. -- The carbonizing materials in general use are charred leather, powdered bone, cyanide of potassium, wood and "animal" charcoal, prussiate of potash and other compositions consisting of mixtures of carbonaceous matter and certain cyanides or nitrates. For slight hardening, cyanides are often used. Charred leather gives good results, although poorly charred leather or that made from old boots, belting, etc., should not be used. A mixture preferred by some to charred leather consists of 60 parts of wood charcoal and 40 parts of barium carbonate. The casehardening compound employed at the Altoona Shops of the Pennsylvania Railroad is made in the following proportions: 11 pounds prussiate of potash; 30 pounds sal-soda; 20 pounds coarse salt; 6 bushels powdered charcoal (hickory preferred). These ingredients are mixed together, 30 quarts water being added.

Steels for Casehardening. -- The percentage of carbon in steels ordinarily used for parts to be casehardened varies, as a general rule, from 0.15 to 0.20 per cent. If the carbon exceeds 0.20 per cent, it tends to give a hard instead of a soft core. If the carbon content is too low, the steel may be difficult to machine; hence, steels containing as much as 0.20 to 0.25 per cent carbon are often used for casehardening. For general work, steel of the following composition will be found satisfactory: Carbon, 0.16 to 0.20 per cent; manganese, less than 0.35 per cent; silicon, not over 0.30 per cent. The sulphur and phosphorus should be as low as possible, not exceeding 0.1 per cent.

Degree and Depth of Hardened Surface. -- The percentage of carbon contained in the casehardened surface should vary according to requirements. A high-carbon case containing 1.1 per cent carbon gives a very hard wearing surface suitable for work that must withstand a fairly constant pressure, as shafts running bearings, etc., but for parts which must withstand repeated shocks, this amount of carbon would render them too brittle, and in such cases it is advisable not to exceed 0.90 to 1 per cent carbon. For most purposes, 0.90 per cent carbon is preferable. Recent investigations indicate that the percentage of carbon in the hardened crust varies with the depth of the latter; the deeper the penetration, the higher the carbon content. Crusts about 0.050 inch deep usually have from 0.85 to 0.90 per cent carbon on the surface. In many instances, a penetration of 0.40 inch is sufficient, but if the work is to be ground after casehardening, it is advisable to carbonize to a depth of about 1/16 inch. Too deep a carbonized case makes the work more brittle, partly because of the prolonged exposure to a high temperature and partly on account of the increase in the hardened section and the decrease in the softer and more ductile core; hence, parts to withstand bending stresses, like gear teeth, should not be carbonized too deeply. The penetration of the carbon increases with the temperature and with the time of exposure, but not in direct proportion to these two factors. Carbonization takes place rapidly until the crust is saturated with carbon, when there is a sudden diminution in the rate of carbonization, which varies according to the temperature.

Casehardening for Colors. -- For hardening and at the same time coloring such parts as wrenches, etc., the following mixture may be used: Mix 10 parts of charred bone, 6 parts of wood charcoal, 4 parts of charred leather and 1 part of powdered cyanide. The leather should be black, crisp and well pulverized, and the four ingredients well mixed. The object in charring the bone and leather is to remove all grease. The parts to be colored must be well polished and should not be handled with greasy hands. To obtain satisfactory work, these rules must be observed. If the colors obtained are too gaudy, the cyanide may be omitted, and if there is still too much color, leave out the charcoal. The parts to be colored and hardened should be packed in a piece of common gas pipe having a closed end. Pipe is preferable because the pieces can be dumped into the cooling water with little or no exposure to the air. The open end of the pipe can be places close to the surface of the water before the parts are removed, but with a box there would be more or less exposure. This class of work should be heated to a dark cherry-red and kept at that temperature for about four or five hours. If the temperature is too high, no colors will appear. The tank should be arranged with a compressed air pipe connecting with the water pipe at the bottom in such a way that a jet of air is forced upward, thus filling the tank with bubbles. There should also be a sieve or basket in the tank for receiving the work. After quenching, place the parts in boiling water for five minutes and then bury them in dry sawdust for half an hour. Another mixture recommended for coloring consists of 10 parts granulated bone, 2 parts bone black and 1 part granulated charred leather.

Gas Process of Casehardening. -- Owing to the growth of the armor plate industry, efforts were made to find a better and cheaper method of carbonizing the plate. The first method employed was to place an armor plate in a pit and cover it with a layer of charcoal; then another plate was laid onto it, after which the pit was covered and heated to a temperature high enough to cause the steel to absorb the carbon from the charcoal. The next method tried was to force a current of carbonaceous gas between the two plates, instead of using charcoal. This caused the carbon to penetrate in less time and was found to be more economical. Later the plates were heated by electricity, and the use of electricity and carbonaceous gas gave a more uniform carbon penetration. This process was followed by the development of a muffle carbonizing furnace. In this the work is placed in a revolving retort through which is forced a current of carbonaceous gas. This retort serves as a muffle and is surrounded with the flames of the heating gases. It is claimed that in this furnace small pieces can be carbonized much more quickly and at about one-half the cost, as compared with packing in iron boxes and baking. Some of the gases that have been experimented with are methane, ethylene, illuminating gas, carbon-monoxide, carbon-dioxide, and gases made from petroleum, naphtha and gasoline. Carbon-monoxide was found superior to other gaseous materials, but, while it is capable of rapid penetration, there is an oxidizing effect that might spoil small parts which cannot be ground afterwards. To overcome these bad effects of carbon-monoxide, a new process has been developed in which the work is packed with wood charcoal in a cylinder, and, when heated to the carbonizing temperature, a current of carbon-dioxide is injected into the cylinder.

To Clean Work after Casehardening. -- To clean work, especially if knurled, where dirt is likely to stick into crevices after casehardening, wash it in caustic soda (1 part soda to 10 parts water). In making this solution, the soda should be put into hot water gradually, and the mixture stirred until the soda is thoroughly dissolved. A still more effective method of cleaning is to dip the work into a mixture of 1 part sulphuric acid and 2 parts water. Leave the pieces in this mixture about three minutes; then wash them off immediately in a soda solution.

Pack-hardening. -- Pack-hardening, as the term is generally understood, consists in treating steel (generally tool steel) with some carbonaceous material and quenching it in oil. The terms "pack-hardening" and "casehardening" are often used interchangeably and the two processes are similar. The surface of the steel is supplied with additional carbon by the use of some carbonaceous material that will not be injurious. To do this, the steel is packed in sealed iron boxes with the carbonizing material. Bone should not be used for pack-hardening tool steel, as it contains a high percentage of phosphorus, which tends to make the steel weak and brittle. For steel that is to contain not more than 1.25 per cent carbon, charred leather is recommended. For obtaining a higher carbon content, use charred hoofs or horns or a mixture of the two. The leather, hoofs or horns can be used repeatedly by adding a quantity of new material each time. A mixture of charred leather and charcoal is also used for pack-hardening. The work should be so packed that it does not come in contact with the box. First place a layer of carbonizing material in the bottom and then a layer of work, no two pieces touching each other. When treating gages, or parts that are likely to spring, they should be so packed that there will be little liability of springing when they are drawn up through the packing material. The parts should not be dumped into the quenching bath, as it is better to handle the pieces separately. It is a good plan to attach a piece of iron wire to each part to facilitate removal from the box. If there are several layers of work, the wires should be so arranged that the various layers can be taken out in the proper order, beginning with the top row. The temperature for pack-hardening should be as low as is consistent with the desired results, and should be uniform throughout the box. To gage the heat, holes may be drilled through the cover at the center so that test wires (say, 3/16 inch in diameter) can extend to the bottom of the box. When the latter has been in the fire long enough to heat the contents to about a dull red (as near as can be judged) a wire is withdrawn; if it is red hot begin timing the heat; if not, wait and withdraw another later, the test being continued until one is withdrawn that has the desired heat. The length of time necessary for heating depends upon the depth of hardening surface desired. For ordinary snap-gages, from one and one-half to two hours after the steel is red hot is sufficient. Ordinary work requires a temperature of about 1475 degrees F. Pack-hardening minimizes the danger of cracking and warping.

Casehardening Alloy Steels. -- When nickel steels are heat-treated by casehardening, nickel seems to retard the process somewhat and the hardness of the "case" is somewhat lower than that obtainable in ordinary carbon steels. On the other hand, nickel tends to oppose the crystallization of the steel at high temperatures and to eliminate the consequent brittleness. With a 2 per cent nickel steel, the following temperatures are recommended: The steel should be quenched from a temperature of 1830 degrees F. It is then given a second heating to 1380 degrees F., and is again quenched, after cooling to about 1290 degrees F. A single quenching from 1290 degrees F. gives the greatest hardness in the case but not the greatest tenacity in the core. Quenching from 1380 degrees F. gives a somewhat higher tenacity but a slightly lower hardness in the case. A 6 per cent nickel steel should be quenched first from 1560 degrees F., and after re-heating, from 1245 degrees F. Since this high nickel percentage almost completely prevents the brittleness of the core, one quenching from about 1290 degrees F. is, in most cases, sufficient. Steels with from 1 to 1.2 per cent chromium are sometimes used when an especially hard case is required. This element aids the crystallization of the core and the double quenching is necessary. Chrome-nickel steels with a low chromium content require about the same heat treatment as pure nickel steels. A mixture of 60 parts wood charcoal and 40 parts of barium carbonate is recommended for carbonizing.

Casehardened Gears. -- There are four general classes of steel used for case-hardened gears, viz. , straight-carbon, nickel, chrome-vanadium, and chrome-nickel steel, and, in each of these classes, several modifications will be found in the market. On the whole, the steels containing chromium are preferable. Before being carbonized, the carbon content of each of the steels mentioned should be about 0.20 per cent; under no circumstances should it be more than 0.25 per cent, to avoid brittleness in the teeth. The carbon in the "case" should be increased to about 0.90 per cent, which can readily be done by using the proper carbonizing material and temperature for carbonizing. This temperature, in general, should be about 1600 degrees to 1650 degrees F. for the classes of steel mentioned. Lower temperatures do not give sufficient depth of case, unless the heating operation is much prolonged. Conversely, higher temperatures result in a case of excessive carbon content and a core of such large grain-size that it will not respond as readily to the subsequent heat-treatment. The proper heat-treatment, after case-carbonizing, is very important. The work is first allowed to cool in the box after carbonizing. It is then re-heated to 1550 degrees or 1625 degrees F. and quenched in a suitable medium to refine the core; next it is re-heated to 1350 degrees or 1425 degrees F. and is again quenched to harden the case; finally, it is drawn in oil to a temperature not over 400 degrees F., to further increase the strength and toughness of the material. The temperatures given are approximate, and more definite information concerning any particular steel should be obtained from the steel-maker. Casehardened gears have harder surfaces, as shown by the scleroscope test, than tempered gears.

Tempered Gears. -- Unlike casehardened gears, tempered gears are of uniform carbon content, and when hardened have a uniform hardness throughout the tooth section. The steels used for tempered gears are of three general classes, viz. , silico-manganese, chrome-vanadium and chrome-nickel steel, the last named, in different modifications, being the most generally used. The carbon content for the different classes varies from 0.40 to 0.60 per cent. The heat-treatment of all these steels consists simply in heating the gears slowly and uniformly to the hardening temperature, which is usually about 1500 degrees F., quenching in oil, and afterward drawing in an oil bath. Tempered alloy steel gears are preferred to casehardened gears for some purposes, especially where strength is the main consideration. They are particularly adapted for "clash gears".

Detailed table of contents
First section: Furnaces and Baths for Heating Steel
Previous section: Annealing
Next section: Application and Heat Treatment of S. A. E. Carbon and Alloy Steels

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