STEEL AND OTHER METALS HEAT TREATMENT OF STEEL THE theory of the heat treatment of steel rests upon the influence of the rate of cooling on certain molecular changes in structure occurring at different temperatures in the solid state. These changes are of two classes, critical and progressive; the former occur periodically between certain narrow temperature limits, while the latter proceed gradually with the rise in temperature, each change producing alterations in the physical characteristics. By controlling the rate of cooling, these changes can be given a permanent set, and the physical characteristics can thus be made different from those in the metal in its normal state. The highest temperature that it is safe to submit a steel to for heat-treating is governed by the chemical composition of the steel. Pure carbon steel should be raised to about 1650 degrees Fahr., while some of the high-grade alloy steels may safely be raised to 1750 degrees Fahr., and the high-speed steels may be raised to just below the melting point. It is necessary to raise the metal to these points so that the active cooling process will have the desired effect of checking the crystallization of the structure. Methods of Heating Furnaces using solid fuel such as coal, coke, charcoal, etc., are the most numerous and have been used the longest. These furnaces consist of a grate to place the fuel on, an arch to reflect the heat and a plate to put the pieces on. The plate should be so arranged that the flames will not strike the pieces to be heated, and for that reason some use cast-iron or clay retorts which are open on the side toward the doors of the furnace. Liquid fuel furnaces, which have open fires and which use liquid fuels, are not very numerous at present, but their use is increasing, owing to the ease with which the fire is handled and the cleanliness as compared with a coal, coke or charcoal fire. Crude oil and kerosene are the fuels generally used in these furnaces, owing to their cheapness and the fact that they can be easily obtained. These fuels are usually stored in a tank near the furnaces and are pumped to them or flow by force of gravity. Heating in Liquids Furnaces using liquid for heating have a receptacle to hold the liquid, which is heated by coal, oil, gas or any other economical means; the liquid is kept at the highest temperature to which the piece should be heated. The piece should be heated slowly in an ordinary furnace to about 800 degrees, after which it should be immersed in the liquid bath and kept there long enough to attain the temperature of the bath and then removed to be annealed or hardened. The bath usually consists of lead, although antimony, cyanate of potassium, chloride of barium, a mixture of chloride of barium and chloride of potassium in the proportion of 3 to 2, mercury, common salt and metallic salts have been successfully used. This method gives good results, as no portion of the piece to be treated can reach a temperature above that of the liquid bath; a pyrometer attachment will indicate exactly when the piece has arrived at that temperature, and its surface cannot be acted upon chemically. The bath can be maintained easily at the proper temperature and the entire process is under perfect control. When lead is used it is liable to stick to the steel unless it is pure and retard the cooling of the spots where it adheres. Impurities, such as sulphur, are liable to be absorbed by the steel and thus affect its chemical composition. With high temperatures lead and cyanate of potassium throw off poisonous vapors which make them prohibitive, and even at comparatively low temperatures these vapors are detrimental to the health of the workmen in the hardening room. metallic salts, however, do not give off these poisonous vapors, and are much better to use for this purpose, but many times the fumes are unbearable. Gas as Fuel The Furnaces using gaseous fuel are very numerous and are so constructed that they can use either natural gas, artificial gas, or producer gas. They are very easy to regulate and if well built are capable of maintaining a constant temperature within a wide range. In first cost this style of furnace is greater than that of the solid fuel furnaces, but where natural or producer gas is used the cost of operating is so much less that the saving soon pays for the cost of installation. Illuminating gas, however, is more expensive than the solid fuels and is only used where high-grade work demands the best results from heat treatment. COOLING THE STEEL COOLING apparatus is divided into two classesening and the different appliances for annealing. baths for hard The baths for quenching are composed of a large variety of materials. Some of the more commonly used are as follows, being arranged according to their intensity on 0.85 per cent. carbon steel: Mercury; water with sulphuric acid added; nitrate of potassium; sal ammoniac; common salt; carbonate of lime; carbonate of magnesia; pure water; water containing soap, sugar, dextrine or alcohol; sweet milk; various oils; beef suet; tallow; wax. These baths, however, do not act under all conditions with the same relative intensity, as their conductivity and viscosity vary greatly with the temperature. With the exception of the oils and some of the greases, the quenching effect increases as the temperature of the bath lowers. Sperm and linseed oils, however, at all temperatures between 32 and 250 berees Fahr., act about the same as distilled water at 160 degrees. The baths for hardening which give the best results are those in which some means are provided for keeping the liquid at an even temperature. Where but few pieces are to be quenched, or a considerable time elapses between the quenching of pieces, the bath will retain an atmospheric temperature from its own natural radiation. Where a bath is in continuous use, for quenching a large number of pieces throughout the day, some means must be provided to keep the temperature of the bath at a low even temperature. The hot pieces from the heating furnace will raise the temperature of the bath many degrees, and the last piece quenched will not be nearly as hard as the first. Annealing The appliances for annealing are as numerous as the baths for quenching, and where a few years ago the ashes from the forge were all that were considered necessary for properly annealing a piece of steel, to-day many special preparations are being manufactured and sold for this purpose. The more common materials used for annealing are powdered charcoal, charred bone, charred leather, slacked lime, sawdust, sand, fire clay, magnesia or refractory earth. The piece to be annealed is usually packed in a cast-iron box, using some of these materials or combinations of them for the packing, the whole is then heated in a furnace to the proper temperature and set aside, with the cover left on, to cool gradually to the atmospheric temperature. For certain grades of steel these materials give good results; but for all kinds of steels and for all grades of annealing the slow-cooling furnace no doubt gives the best satisfaction, as the temperature can be easily raised to the right point, kept there as long as necessary, and then regulated to cool down as slowly as is desired. The gas, oil or electric furnaces are the easiest to handle and regulate. The Hardening Bath In hardening steels the influence of the bath depends upon its temperature, its mass and its nature; or to express this in another way, upon its specific heat, its conductivity, its volatility and its viscosity. With other things equal, the lower the temperature of the bath, the quicker will the metal cool and the more pronounced will be the hardening effect. Thus water at 60 degrees will make steel harder than water at 150 degrees, and when the bath is in constant use the first piece quenched will be harder than the tenth or twentieth, owing to the rise in temperature of the bath. Therefore if uniform results are to be obtained in using a water bath, it must either be of a very large volume or kept cool by some mechanical means. In other words, the bath must be maintained at a constant temperature. The mass of the bath can be made large so no great rise in temperature is made by the continuous cooling of pieces, or it can be made small and its rise in temperature used for hardening tools that are to remain fairly soft, as, if this temperature is properly regulated, the tool will not have to be re-heated and tempered later, and cracks and fissures are not as liable to occur. Another way of arriving at the same results would be to use the double bath for quenching, that is, to have one bath of some product similar to salt which fuses at 575 degrees Fahr. Quench the piece in that until it has reached its temperature, after which it can be quenched in a cold bath or cooled in the air. HIGH-SPEED STEELS THESE steels are made by alloying tungsten and chromium or molybdenum and chromium with steel. These compositions completely revolutionize the points of transformation. Chromium, which has a tendency to raise the critical temperature, when added to a tungsten steel, in the proportions of 1 or 2 per cent., reduces the critical temperature to below that of the atmosphere. Tungsten and molybdenum prolong the critical range of temperatures of the steel on slow cooling so that it begins at about 1300 degrees Fahr. and spreads out all the way down to 600 degrees. These steels are heated to 1850 degrees for the molybdenum and 2200 degrees for the tungsten, and cooled moderately fast, usually in an air blast, to give them the property known as "red-hardness." This treatment prevents the critical changes altogether and preserves the steel in what is known as the austenitic condition. austenitic condition is one of hardness and toughness. The One rule which has given good results in heat-treating these highspeed steels is to heat slowly to 1500 degrees Fahr., then heat fast to 2200 degrees; after which cool rapidly in an air blast to 1550 degrees; then cool either rapidly or slowly to the temperature of the air. CASE-HARDENING CASE-HARDENING, carbonizing, or, as it is called in Europe, “cementation," is largely used so that the outer shell can be made hard enough to resist wear and the core of the piece can be left soft enough to withstand the shock strains to which it is subjected. Several methods different from the old established one of packing the metal in a box filled with some carbonizing material, and then subjecting it to heat, have been devised in the last few years. Among them might be mentioned the Harveyizing process which is especially applicable to armor plate. The Harveyizing process uses a bed of charcoal over the work, the plates being pressed up against it in a pit or furnace and gas turned on so that the steel will be heated through the charcoal, thus allowing the carbon to soak in from the top. Factors Governing Carbonizing The result of the carbonizing operation is determined by five factors, which are as follows: First, the nature of the steel; second, the nature of the carbonizing material; third, the temperature of the carbonizing furnace; fourth, the time the piece is submitted to the carbonizing process; fifth, the heat treatment which follows carbonizing. The nature of the steel has no influence on the speed of penetration of the carbon, but has an influence on the final result of the operation. If steel is used that has a carbon content up to 0.56 per cent., the rate of penetration in carbonizing is constant; but the higher the carbon content is, in the core, the more brittle it becomes by prolonged annealing after carbonizing. Therefore it is necessary that the carbon content should be low in the core, and for this reason a preference is given to steels containing from 0.12 to 0.15 per cent. of carbon for carbonizing or case-hardening purposes. PENETRATION OF CARBON PER HOUR WITH TABLE I. The rate of penetration for ordinary carbonizing steel under the same conditions would have been 0.035 inch. Thus it will be seen that manganese, chromium, tungsten and molybdenum increase the rate of penetration. These seem to exist in the state of a double carbide and release a part of the cementite iron. Nickel, silicon, titanium and aluminum retard the rate of penetration 5 per cent. of silicon reducing it to zero - and these exist in the state of solution in the iron. The Carbonizing Materials The nature of the carbonizing materials has an influence on the speed of penetration and it is very essential that the materials be of a known chemical composition as this is the only way to obtain like results on the same steel at all times. These materials or cements are manufactured in many special and patented preparations. The following materials are used and compounded in these preparations, but many of them give as good results when used alone as when compounded with others in varying percentages: Powdered bone; wood charcoal; charred sugar; charred |