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Re+ NBV E N U A M ASSOCIATI01 of CllEllSTRY TEACBERS 4th.
Modern Heat Treating Practices1 CHARLES A. PETHYBRIDGE T h e New Britain Machine Company, New Britain, Connecticut
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HE title of this paper is in itself a tribute to the mdustrial . chemist. Had it not been for those working in the chemical laboratories throughout the industrial world for the last century, we would still be using the crude form of steel that was known a t that time. Any improvement in processing, any change in analysis, any betterment of equipment is largely due to the work of the industrial chemists. The metallurgist may call for the change in procedure or in fabrication and the metallurgist may and usually does receive the credit for the improvement in the product, but were it not for the industrial chemist working with refractories, the industrial chemist working with salts, the industrial chemist work'mg with gases, oils, refrigerants, etc., to provide the means for the change in processing-no improvement in the field of heat treating could be attained. There are, naturally, other branches of science which have played important parts in the improvement of heat treating methods but they have been of secondary importance, inasmuch as whatever material they have deemed necessary to promote the functions of heat treating has to be formulated and tested by the industrial chemist. The handbook of the American Societv of Metals defines heat treating as "an operation or combination of operations involving the heating and cooling of metal or alloy in the solid state for the purpose of obtaining certain desirable conditions or properties." These operations can be, for convenience, divided into three groups: annealing and normalizingz for structure refinement, hardening for strengthening and increasing wear resistance, and tempering for attaining the ultimate desired properties after hardening. There are few alloys of metals which do not respond in some manner to the application and removal of heat. Iron and its alloys respond in one manner, copper and its alloys in another, aluminum and its alloys in still another. Because of the immensity of the heat treating Abstract of an-addresspresented at the 223rd meeting of the N.E.A.C.T. at New Britain. Connecticut, November 6. 1943. Normalizing is a process similar to annealing, but, owing to a mare rapid cooling, yields a finer structure.
field and the innumerable combmations of elements which can be altered by the application of heat, this paper will attempt to cover but one field-the heat treating of steel, the alloy of iron and carbon. Of course, there are many elements which are used in conjunction with iron and carbon in the production of alloy steel, but in any and all steels iron and carbon must be alloyed. Any and all elements which are present in the alloy of iron and carbon are there either as impurities which enter during the fabrication, or as additions which are made during the melt for some expressed purpose such as increasing hardness, ductility, resistance to corrosion, etc. The effect of heat treatment of steel is to control the disposition of the carbon in the form of iron carbide, FeC, throughout the structure. TRANSFORMATION
The heat treatment of carbon steels is associated with the allotropy of iron. Alpha-iron or ferrite, which is stable below 900°C.and has little solubility for carbon, when heated through a critical temperature range changes to unstable gamma-iron which has a high solubility for carbon. The gamma-iron containing carbon in solution is called austenite. When the cycle is reversed and the heat is removed from the steel, the carbon tends to be thrown out of solution. However, the phase change from gamma- to alpha-iron requires time for completion and the change may be arrested by increasing the rate of heat removal. Thus, by arresting the phase change from austenite to ferrite, a supersaturated solid solution is obtained in which the finely divided carbon particles are held by force. This supersaturated solution is the martensite structure or fully hardened steel structure. It might well be said that the heat treatment of steel hinges much more on how the heat is removed from the mass than how the heat is applied to it. On any heat treatment which requires heating to a point above the critical temperature, the resulting structure is more dependent on the cooling rate than the heating rate. In annealing and normalizing, the cooling
a greater amount of carbon in the supersaturatedalphairon, or by lengthening the time required for the transformation from the gamma-iron to the alpha stage. There is a definite physical limit to the rate of heat removal, so the collective efforts of steel makers have been to formulate steels which would-because of their chemical make-up-allow a longer transformation time NEED OF HEAT TREATMENT from the gamma phase to the alpha phase of iron. To The ultimate use of heat treatment, of course, is to that end, additional elements other than carbon have improve as much as possible the physical properties been alloyed with the iron to promote hardenability by such as tensile strength, impact strength, ductility, and slowing up the required cooling rates. Almost all the eleresistance to fatigue. Because of the nature of the ments, with the exception of cobalt, when alloyed in steel properties it is impossible to improve them all by heat retard the rate of the austenite decomposition and thus treating, but by careful manipulation of the factors in- increase hardenability. This means that by the proper volved heat treating can be used to increase one prop- control of the elements the desired hardness can be oberty without decreasing the others to a dangerous tained without the use of a drastic quench. At first point. For example, any increase in tensile strength glance, this seems unimportant until the problem of heat is accompanied by an increase in hardness, and an in- removal from varying sections in the same piece of steel crease in hardness must always be accompanied by a is considered. Consider a piece of steel which has a loss in ductility. If the ductility of the steel were of base 2" thick by 4" wide by 8" long from which there more importance to the engineer than the increased extend four fins 4" long by 4" wide and '/P" thick. If tensile strength, then the part in process would be that piece were made of a 0.50 per cent straight carbon treated for ductility; if ductility were of secondary im- steel, to obtain a maximum hardness in the base would portance and increased tensile strength were desired, necessitate quenching in an ice cold brine solution. If then the method of heat treating would be applied to the same piece were made from a steel similar to S.A.E. 4650, which is a nickel-molybdenum steel, it could be build up the tensile strength. In fabrication problems, that is, problems of making quenched in oil at a temperature of 140°F. and the steel into parts, the control of the physical properties same hardness obtained. And if the same piece were is of extreme importance. The making of steel cart- made of a high chromium-vanadium steel, i t could be ridge cases bas been a classic example of the importance quenched in air a t room temperature and the same or of heat treatment to control the structural qualities so greater hardness obtained. The possibility of distorthese cases could be made as satisfactory as the pre- tion and cracking because of the slower and more univious brass cases. The machmability of steel is di- form heat removal is lessened when the steel with the rectly dependent on the heat treatment which precedes slowest required cooling rate is used. Heat treating today is concerned more and more with the machining operation. The increased use of alloy steel to obtain greater and greater physical properties modifying the qualities and properties of the steels has increased the problems of machining. The in- which are handled, because the demands made of the creased production demand has required faster turning, steels specified by the designers become more stringent. milling, and boring on steels which continue to grow Tensile strengths unheard of 10 years ago are routine tougher and tougher. Whereas the heat treating de- today, largely through the use of alloyed steels and the partment used to be the blacksmith shop with a couple proper beat treatment of them. Increased production of parts is more and more deof coal fires and a pail of water for a quench, the modern heat treating department is now equipped with the pendent on the processing of the steel prior to and durfinest possible equipment with furnace temperatures ing the machining operations. controlled, furnace atmospheres controlled, quenching CONTROLLED ATMOSPHERE temperatures controlled-all done automatically and The use of controlled atmosphere furnaces for the precisely. Heat treating has become a science. heating cycle is one of the outstandmg major developAs was said before, the ultimate use of heat treatment is to get the maximum physical properties from a given ments of recent years. As the temperature to which steel. The attainment of these physical properties the steel is heated is raised, the activity with which depends largely on the hardenability of the steel being oxygen attacks the steel increases. A bar of steel processed. Hardenability may be defined as the ability heated in an ordinary semimufflefurnace without any of a steel to hold carbon in the supersaturated solution protective atmosphere to a temperature of 1650°F. is of alpha-iron. The more carbon which can be held in covered by scale or iron oxide which not only causes this supersaturated solution the greater is the harden- loss of dimension but also alters the surface condition of the piece by causing decarburization. ability of the steel. By using a furnace in which the heating chamber is USE OF ALLOYS either a retort or a muffle and by maintaining a gas This hardenability can be attained in two ways: pressure within the retort slightly higher than the ateither by increasing the rate of heat removal to freeze mosphere, the control of the surface characteristics of rate from the gamma-iron or austenite stage is retarded to allow the formation of as much ferrite as is desired; in hardening, the cooling rate is accelerated in the attempt to hold as much carbon in the supersaturated solution as possible and thereby increase tensile strength and hardness.
the steel in process becomes almost micromatic. Sev- The immersed electrode furnace usually consists of two eral methods of obtaining these protective atmospheres or three electrodes (depending on the use of single- or are used, e. g., partially burned city gas, cracked an- three-phase power) spaced an inch or two apart and ' hydrous ammonia, and charcoal gas. immersed in the salt. The molten salt is a conductor. The time necessary for cleaning after heat treatment A low voltage is impressed across the electrodes, and is eliminated when the work is done in a proper atmo- alternating current then flows through the salt between sphere; the amount of stock left for grinding is reduced, the electrodes, which by virtue of its own resistance bewith a consequent speeding up of that operation; the comes heated. These immersed electrode furnaces can loss of tools and dies due to decarburization of the cut- be used at temperatures from 300' to 2400°F. and are ting edge is lessened, and the whole processing of the particularly efficient a t the higher temperatures. part or parts is hastened. The use of salt baths for the hardening of high speed steels was accelerated by the advent of molybdenum SALT BATHS high speed steels which have a marked tendency to The use of liquid baths for heating mediums has in- decarburize at the hardening temperature of 2250°F. creased greatly with the improvement of salt mixtures. The neutral salt bath provides maximumprotectionfrom Theseliquid baths offera form of atmospheric control in decarburization during the heating cycle and, because that the part processed is immersed in the bath, thus of the film of salt left on the part upon withdrawal from preventing any contact with the air and thereby pre- thesalt,eliminates oxidationof the part from the hardenventing scaling and oxidation. The heating is more uni- ing furnace to the quench. Liquid carburizing is done in baths consisting of neuform than in air heating, and because of this the tendency to warp is reduced. The rate of heating is more tral salts plus cyanide. Case depths can be obtained rapid and consequently the rate of production is greater from 0.002 inch to 0.150 inch, depending on the time and the unit production cost is lower than in dry heat- that the piece is held in the salt and the temperature a t which the carburizing is done. For case depths of 0.40 ing. Salt baths are produced from the carbonates, cya- inch and less, and parts of not too great size, this method nides, chlorides, nitrides, and nitrates of barium, cal- of liquid carburizing is much faster, cheaper, and more cium, potassium, sodium, and similar elements. The uniform than pack or gas carburizing. Tempering salts which melt a t 800°F. are in common above-mentioned salts are mixed according to the requirements and fused to give a stable bath. The baths use today doing double duty-tempering and descaling. can be neutral, nondecarburizing and noncarburizing, Scaly work which must be tempered comes out of the or they can be active and used either for uitriding or bright tempering absolutely clean. Use of this salt has eliminated thousands of hours of sand blasting which carburizing. These salt baths usually consist of an alloy pot ex- would have been necessary had the work been done in ternally heated by oil or gas, but the use of immersed an ordinary tempering furnace. electrode furnaces is displacing the externally fired pot. (Continued in the nest issue)
