RepMi
of the
NEW ENGLAND ASSOCIATION of CHEMISTRY TEACHERS CHARLES A. PETHYBRIDGE New Britain Machin.e Company, l\'etv Britain, Connecticu.t (umtinuedJrom page 154)
most of the quenching strains are removed and pieCtS can be quenched in this manner with very little likelihood of distortion and very slight dimensional changes.
AUSTEMPERING
The use of salts as a quenching medium has increased greatly. Austempering is a controlled time-temperature quench in which the work is quenched from its conventional hardening heat into molten salt held at a constant temperature (usually between 350° and 9OO oP.) and kept at this fixed temperature long enough to transform the austenite isothermally into the intermediate transformation products, usually Bainite. Because of the close control of the quenching temperatures and the time in the quench, the uniformity of .the product is unusual, and because of the nature and temperature of the quench, the ductility and toughness of the parts so treated are increased enormously. MARTEMPERING
The use of molten salts to increase the depth and uniformity of hardness is becoming better known. This process is kno\m as "Martempering. 1I In "Martempering" the desired structure is the same as would be obtained with a conventional quench, only the structure throughout the cross section of the piece is theoretically uniform. The temperatures for the several steels at which martensite 3 commences to form have been determined by experiment. After establishing the temperatures of the quench, obtained also by experiment, the salt bath is used as an oil quench would be used. The difference in procedure is that the piece, upon quenching in the salt bath at the temperature at which the martensite forms, is held at that temperauTe until the transformation takes place throughout the entire section. This, in effect, allows the use of a low alloy, shallow-hardening steel in place of a high alloy, deep-hardening steel. By allowing the transformation to take place throughout the cross section of the piece rather than getting the Quter portion hard and the interior progressively softer, as would happen in a conventional oil quench, a Martensite is a supersaturated solution of FeaC in a-iron formed by very rapid quenching and having a fine needle-like structure. A steel, the structure of which is martensitic, is in its hardest possible condition, will scratch glass, is brittle, and has almost no ductility. Its composition is not definitely kl1()Wll.
FIXTURE QUENCHING
Coupled with the controlled atmosphere furnaces is the use of quenching presses. Elimination of the necessity for grinding after hardening has increased the prac· tice of finish machining prior to heat treatment and then quenching the hardened piece in a press which forces the piece to hold its size. This means the "as hardened" part is held to very close tolerance during the heat treatment. The press consists of a segmented die which partiall}' collapses so that the piece may be located on it when the quenching starts. This die then opens hydraulically and forms the piece prior to the actual quenching to the dimensions to which it had been machined before it went into the furnace. The actual setting of the die to obtain the proper results is naturally a matter of trial and error, but once established, the pieces are heated and quenched and the tolerances are maintained uniformly. Straightening presses on which shafts are straightened while semiplastic before quenching saves much scrap and many work hours. Most of the straightening presses will straighten parts within 0.005 of an inch run-out. SUBZERO TREATMENT
Perhaps the most recent development in quenching practice, even later than martempering, is the subzero treatment of steel, especially high speed steels. An explanation of the term high speed steel might be helpful. The term is applied to those steels which because of their alloy content are capable of machining metal at speeds so great that they sometimes become red hot. They are so formulated that at the temperature of 1000° to 1050 0 P., they do not lose their original "as quenched" hardness and in some cases perform better at this heat than they do at atmospheric temperatures. They are of high alloy content, one 01 the better grades having an analysis of 18 per cent tungsten,
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4 per cent chromium, 2 per cent vanadium, and 9 per cent cobalt, and consequently, their transformation range is very wide. Many of these high speed steels will attain satisfactory hardness by quenching in air at room temperature. Recent research has determined that the transformation is not completed at room temperature but continues until a temperature of at least - 40 oP. is reached. Following this line of reasoning, if austenite is maintained in the structure in any great amount, when the "high speed steel is quenched out at room temperature, the maximum hardness is not obtained, and at the same time the stresses set up by the incomplete transformation tend to lessen the life of the piece. Consequently, to complete the transformation the high speed steel is hardened and quenched in the conventional manner, but instead of considering the quench completed when the piece gets to about room temperature, the steel is transferred from the quenching bath to either a dry ice pack or a unit similar to a "quick-freezing" unit where it is subjected to a temperature of -100°F. for a prescribed period. Hardnesses have uniformly increased in all cases-two to three points Rockwell "C" scale-and the life of the tool has been increased in many cases 500 per cent. This is not as yet a general practice throughont the trade. but from the excellent results obtained by those plants now practicing it, it soon will be. SURFACE HARDENING
The most widely publicized forms of heat treating today are flame hardening and induction hardening. More has been written about these two methods of heat treatment recently than most of the others combined. Briefly, both consist of rapidly heating the surface to be hardened, and quenching the heated surface immediately-thns utilizing the quenching effect of the internal mass of the object plus the effect of the external quench. Both processes are primarily used for heatingand quenching to adepthnotexceedingO.125inch although in special cases and with special alloys case depths up to 0.500 inch have been reached. Flame hardening consists of heating the surface by means of a torch. The gases used can be city gas and air, natural gas and air, or oxygen and acetylene. Usually oxygen and acetylene are used because of the tremendous heat value in comparison with the first two. The torches used for heating are designed for the particular function, and in many cases the hardening is done progressively, i. e., either the piece to be hardened moves at a constant speed under the fixed torch, and is heated and quenched progressively as it moves; or the work is stationary and the torch moves at a controlled rate of speed. Greater hardnesses than can be obtained by mass heating and quenching can be obtained in this manner. Distortion is held to a minimum, thus reducing stock . \kft for finishing. In many cases the dimensional effect ~ has been found to be so little that the only operation . following flame hardening is that of polishing. Flame
strengthening is a form of flame hardening, wherein the object is to increase the strength of the steel byestablishing stresses in the surface of the piece to counteract the expected stresses to be met in service. INDUCTION HARDENING
Hardening by the use of induction heating is done to develop a hard, wear-resisting surface at any desired spot. Thus a part does not have to be entirely hardened, which might endanger its toughness, ductility, machinability, and dimensions. The success of maintaining the above core properties while obtaining a hardened exterior is due to the extreme speed of high freqnency induction heating. Heating cycles are figured in seconds and fractions of seconds instead of hours, as ordinary heating time is considered. The heat starts at the surface of the piece and, as usually carried out, the maximum depth of hardness desired is not over 0.030 inch to 0.040 inch. In a heating cycle of less than 10 seconds the temperatnre of the metal one-quarter of an inch below the surface has not been appreciably altered. The higher the frequency .the faster is the rate of heating. Sets for induction hardening have frequencies between 9600 and 500,000 cycles, but with ultra high frequencies approaching 1,000,000 cycles a second heating times will be cut to fractions of seconds and depths of bardness figured precisely. The remarkable feature of induction heating for hardening is the precision with which it can be controlled. Heating cycles and quenching cycles are controlled electrically, and once a setup has been established and heating time and quench period determined, the millionth piece will have exactly the same properties as the first one. Inasmuch as the frequency determines the rate of heating and consequently affects the depth of hardening, induction heating machines of lower frequencies are used for deep heating instead of skin heating. Sets with frequencies between 1000 and 9600 cycles are used for deep heating of metal for forging, with the same proportional saving in heating time as is saved on heating for hardening. The heat treating practices just discussed are but a few of those going on night and day in every heat treating department in the country. The methods discussed have all been simple oues. Some procedures being followed today are quite complicated, perhaps more so than they need be with further experimentation, but all the processes have one purpose: the making of a better steel for a better tool for a better product. Notes
Dr. C. B. Gustafson, Chairman of the 6th Summer Conference, has announced that the invitation of Connecticut College, New London, Connecticut, has been accepted as a meeting place for the Conference during the last part of August.
