US3490957A - Heat treatment to eliminate the upper yield point in ferrous alloys - Google Patents

Description

1970 1D. P. KO'ISTINEIN 3,490,957 I HEAT TREATMENT TO ELIMINATE THE UPPER YIELD POINT IN FERROUS ALLOYS Filed .Maroh 23, 1967 FRACTURE REGION OF PLASTIC I I DEFORMATION I 33 5 I as g I I :5 I I 8 I I I I- I 5 I I I L I I o STRAIN UPPER YIELD POINT FRACTURE I m I I '53 I 5 II I II I I II 0 STRAIN INVENTOR. BY @zza/dfifiaz'szizzzz United States Patent 3,490,957 HEAT TREATMENT TO ELIMINATE THE UPPER YIELD POINT IN FERROUS ALLOYS Donald P. Koistinen, Birmingham, Mich., assignor to General Motors Corporation, Detroit, Mich., a corporation of Delaware Continuation-impart of application Ser. No. 558,676, June 20, 1966.. This application Mar. 23, 1967, Ser. No. 625,465

Int. Cl. C21d 9/00, 7/14 US. Cl. 148-12 9 Claims ABSTRACT OF THE DISCLOSURE Ferrous-based alloy specimens, as for example sheet steel, which have the property of yielding abruptly and discontinuously when mechanically deformed beyond their elastic limit, may be treated beneficially to temporarily eliminate this property without adversely affecting the ductility of the specimen by rapidly heating it to a temperature in the range from 320 C. to the lower critical temperature of the alloy, and then immediately quenching it in water or other suitable liquid.

This is a continuation-in-part application of my copending application S.N. 558,676 filed June 20, 1966.

This invention relates to the cold working of ferrous alloys and more particularly to a method of eliminating the upper yield point in siutable ferrous alloys to enhance their formability. As used herein, cold Working is intended to refer to drawing, stamping, rolling, and other similar processes which are used to plastically deform a metal below its recrystallization temperature.

A better appreciation of what my invention accomplishes will be obtained by referring to the drawings. Both figures are tension stress-strain curves illustrating the manner in which different ductile metals deform under an applied tensile stress.

FIGURE 1 depicts the tension stress-strain curve for a ductile metal such as aluminum or copper in which a suitably designed specimen is subjected to an increasing axial load until it fractures. The curve of FIGURE 1 will be readily recognized by those skilled in the art. The straight line portion from O to A represents the elastic region of deformation. The curved portion from A to the termination of the curve represents the manner in which the specimen deforms plastically under an increasing load until it eventually fractures. From the standpoint of my invention, it is significant to note that in FIGURE 1 the transition from the elastic region to the plastic region of deformation is smooth and continuous.

It is also known that there are metals and metal alloys, a very important example of which is low-carbon steel, which do not deform under tension as illustrated in FIG. URE 1. When suitably prepared specimens of these materials are subjected to an increasing axial load, the nature of the deformation is more accurately depicted as shown in FIGURE 2. In general, as the tensile stress on these materials is increased beyond the elastic limit there is no smooth and continuous deformation into the plastic region. On the contrary, as soon as a particular point is reached, which is called the upper yield point (identified in FIGURE 2), there is a sudden reduction in force per unit area because there is an abrupt plastic yielding of the metal. Further strain then occurs at a lower but fairly constant stress. After a period of deformation at fairly constant stress a point is reached (B in FIGURE 2) at which the deformation becomes more analogous to that of the plastic region of FIGURE 1. At this stage the stress must be increased for continued deformation until the specimen fractures. In the type of deformation represented by FIGURE 2 the strain under relatively constant stress is known as the yield point elongation and is so indicated. The value of the relatively constant stress is termed the lower yield point.

The observation of the upper yield point in metals such as low-carbon steel is of more than academic interest. For example, when a sheet of low-carbon steel, which has not been specially treated to eliminate the upper yield point, is drawn to form a body panel of an automobile all areas in which the deformation is less than about 6% are scarred by Liiders lines or stretcher strains. Liiders lines or stretcher strains are depressions in the metal surface which cannot be masked by paint. They are directly attributed to the presence of an upper yield point on stress-strain curve, i.e. to the heterogeneous transition from elastic to plastic deformation illustrated in FIGURE 2. At the upper yield point highly stressed portions of the sheet will preferentially yield. The deformation is not relatively uniform throughout the whole area. It occurs in specific regions and manifests itself as depressions in the surface of the material, the metal therein having been strained to point B in FIGURE 2. The remainder of the metal in the steel sheet has probably undergone essentially zero strain. If a deformation of the entire sheet is not continued until it has been uniformly strained beyond the point B, the depressions will remain in the surface of the drawn article as unsightly defects.

Attempts have been made in the prior art to eliminate the formation of stretcher strains or Liiders bands when low-carbon steel is drawn. They will be described in more detail below. However, these prior art techniques while at least temporarily eliminating stretcher strains have at the same time resulted either in a loss of ductility of the low-carbon steel or have led to a substantially weaker material.

Therefore, it is an object of my invention to provide a method of treating sheet steel and the like without decreasing the ductility of the material or its strength, such that it subsequently may be cold worked without introducing surface defects therein, as for example Liiders bands.

It is a more specific object of my invention to provide a method of treating suitable ferrous alloys to eliminate the heterogeneous transition from elastic to plastic deformation which produces an upper yield point in the tensile stress-strain curve of the alloy as shown in FIGURE 2.

It is a still more specific object of my invention to provide a method of eliminating, for a period of time, the upper yield point in those ferrous alloys which display this phenomenon, when they are plastically deformed without decreasing the ductility of the alloy or detracting from its strnegth.

It is also an object of my invention to provide ferrous alloy stock which has been treated to eliminate its upper yield point, but which treatment has not reduced the strength or ductility of the stock.

Iron alloys which may advantageously be treated by my method are those which contain relatively small amounts of solute atoms such as nitrogen or carbon which may adversely affect the deformation of such an alloy by producing an undesirable atomic arrangement within the metal, one consequence of which is an upper yield point in the tensile stress/strain curve. In accordance with my invention, the above specified objects and others may be accomplished by rapidly heating a suitable iron alloy workpiece to a temperature above approximately 320 C. but below the lower critical temperature of the alloy, suitably within a period of about 10 seconds and preferably within a period of about 6 seconds, and then immediately cooling the workpiece at a rate sufiicient to prevent the return of the upper yield point. Preferably, the alloy is cooled by quenching in water at room temperature. This treatment temporarily eliminates the upper yield point in the treated alloy. The yield point will slowly return again upon aging, the rate of return being primarily a function of the temperature at which the treated alloy is stored. However, in the period before the upper yield point returns, the treated material may be plastically deformed without forming Lilders bands or stretcher strains.

In another embodiment of my invention, as disclosed in my copending application S.N. 558,676, the iron alloy may be heated relatively slowly, i.e. without a limitation on the heating rate is specified in the above paragraph, to a temperature in the range from approximately 320 C. to the lower critical temperature of the alloy and then subjected to alternating electromagnetic radiation, preferably of a frequency from about 1 kilocycle to about 500 kilocycles per second for a brief period, usually from 1-5 seconds. The iron alloy member is then immediately cooled at a rate sufiicient to prevent the return of the upper yield point. The treated member may then be plastically deformed without forming Liiders bands.

To better understand how the above-stated objects are accomplished in accordance with the invention, a general discussion of the characteristics of metal in the solid state is required. It is well known that, in general, the atoms comprising the metal arrange themselves in relatively orderly geometric configurations in crystals. Moreover, metallurgical materials, which are normally subjected to forming processes, are polycrystalline, that is, they are comprised of a large number of crystals. However, the arrangement of atoms in these crystals is not completely orderly in a geometric sense, but rather there are defects in the configurations which are called dislocations. These dislocations are characterized by the fact that, unless obstructed, they have mobility and may be shifted throughout a crystal in response to an applied force. It is the mobility of these defects or dislocations which permits a metal to be deformed at stresses much lower than would be required to overcome the attractive forces between the atoms themselves. Moreover, it is the mobility of these dislocations which apparently permits the plastic deformation of a metal in the first place. Were it not for the presence of dislocations the metallic solid under stress would be elastically deformed until the interatomic attraction forces were overcome, at which point the metal would fracture having undergone little, if any, plastic deformation. Therefore, it may be seen that the presence and mobility of dislocations in the crystals comprising a metallic member are of extreme importance in the workability of the metal.

In general, the relatively free movement of dislocations in metals such as aluminum and copper account for the smooth and continuous transition from the elastic to plastic region of deformation which is depicted in FIG- URE 1. Accordingly, it is the lack of mobility of such dislocations which accounts for the heterogeneous transformation from the elastic to plastic region of deformation typified in FIGURE 2. In the case of low-carbon steel sheet, for example, relatively small amounts of carbon and nitrogen solute atoms are present. Low-carbon steels are usually considered to be those containing less than about 0.4% carbon. In general, sheet steel contains less than about 0.1% carbon and even less nitrogen by weight. Normally the solute atoms are not uniformly dispersed throughout the iron matrix but tend to congregate about dislocations. They form an atmosphere which retards the mobility of the dislocations. Since the metal may not be plastically deformed until the dislocations can .move, it is apparent that sufficient stress must be applied to tear the dislocations away from the atmosphere of solute atoms. Once this is accomplished the dislocations may move relatively freely and at a lower stress level until the metal undergoes strain hardening and increased stress is required for further deformation. Thus, this accounts for the phenomenon illustrated in FIGURE 2. The upper yield point is the stress which is required to tear the dislocations away from the solute atoms and commence plastic deformation. Once the dislocations have escaped deformation continues at a fairly constant stress level (the yield point elongation) until the well known strain hardening process occurs.

While this upper yield point phenomenon has been observed in many metals and alloys such as polycrystalline molybdenum, titanium, and in aluminum alloys, and in single crystals of iron, cadmium, zinc, alpha and beta brass and aluminum, an extremely important commercial example of this problem is in low-carbon sheet steel. In low-carbon sheet steel it is generally concluded that nitrogen is the most critical solute element, with regard to the upper yield point phenomenon, because of its relatively high diffusion rate in iron. It has been estimated that the presence of 0.001% nitrogen is suflicient in low-carbon steel to effect the upper yield point phenomenon.

In the prior art at least two different methods have been used to eliminate the upper yield point in low-carbon steel sheet. One technique involves adding elements such as aluminum, vanadium, titanium, columbium, or boron for the purpose of taking carbon and nitrogen out of solid solution in the form of stable carbides or nitrides. Steels to which such elements have been added are called killed steels. This technique has two disadvantages: The cost of the steel is significantly increased by the addition of the alloying elements. At the same time the strength of the steel is reduced because carbon and nitrogen are removed from solution.

A second industrial solution to the problem has been to temper roll the steel. This involves reducing the sheet in cross section about /2 to 4%. The cold rolling improves the surface of sheet metal and at the same time creates large numbers of new mobile dislocations with render the deformation characteristics of the steel more like the curve of FIGURE 1. However, such a treatment is not permanent because nitrogen and carbon diffuse to the newly created dislocations and form an atmosphere about them retarding their mobility. This diffusion occurs in a metal within a period of from a few hours to a few days. Moreover, the amount of cold rolling that is needed to create sufficient new dislocations reduces the ductility and thus the formability of the sheet steel. An object of my invention is to readily eliminate the upper yield point in lowcarbon steel without incurring the disadvantages of alloying or temper rolling.

As was disclosed in my copending application S.N. 558,676 the yield point in low-carbon steel may be eliminated by subjecting it to high frequency electromagnetic radiation at a temperature above 320 C. but below the lower critical temperature of the alloy for a very brief period of time and immediately quenching the material in water. At the time of that discovery, it was believed that the employment of induction-type heating was necessary to accomplish the objects of the invention. It was then known that the employment of ordinary furnace heating, which requires several minutes to bring the metal up to the specified temperature range, was not effective alone in eliminating the upper yield point. I have, however, now discovered that the upper yield point may be eliminated by rapidly heating ferrous alloys, displaying an upper yield point upon deformation, by any suitable heating means whereby the temperature of the alloy is increased to a level within the range from 320 C. to the lower critical temperature of the alloy within a period of about 10 seconds. In other words, the method of this invention is not as was earlier supposed limited to induction heating. Immediately upon attaining the specified temperature the alloy is rapidly cooled to about room temperature to prevent the return of the upper yield point. Preferably, the cooling is accomplished by quenching in water or other suitable quenching liquid at about room temperature.

By way of example of suitable heating means, I have found that such known methods as electrical resistance 5. heating, gas torch heating, or radiant heating may be employed. Of course, induction heating wherein the frequency of the alternating electromagnetic field is from about 1 kilocycle to 500 kilocycles per second may also be used. As a general rule, the specimen to be treated must be heated into the specified temperature range within a period of up to about 10 seconds to satisfactorily eliminate the upper yield point. Employment of longer heating periods typically results in retention of the upper yield point, or at best only a partial elimination thereof. Moreover, in general, the lower the treatment temperature in the above-defined range to which the alloy is heated, the more rapidly the required rate of heating. For example, in treating sheet steel which has been cold rolled after the final anneal I have found that optimum results are obtained by heating to a temperature of about 650 F. Within a period of less than about 6 seconds. However, if the steel is to be treated at a temperature of about 105 F., beneficial results will be realized so long as this temperature is reached within a heating period of about seconds. It has been noted in the practice of the invention that the more rapid the rate of heating the more completely the elimination of the upper yield point is achieved in accordance with my invention.

The only known exception to the requirement of extremely rapid heating is the special embodiment of this invention wherein the material to be treated is heated by any suitable means, without restriction of heating rate, into the specified temperature range. The material is then subjected to electromagnetic radiation in the frequency range of 1-500 kilocycles per second for a period of up to a few seconds. For some presently unknown reason, this impulse of induction heat into a preheated specimen immediately followed by rapid cooling will accomplish the objects of this invention.

The mechanism by which the rapid heating and immediate quenching, or the electromagnetic radiation treatment of a preheated specimen, effectively eliminates the upper yield point is not understood. However, apparently in some way the solute atoms are disspelled from the immediate neighborhood of the dislocations whereby plastic deformation of the ferrous alloy may proceed in a more homogeneous manner. It is known however that the iron alloy which has been heated in accordance with the invention must be rapidly cooled, preferably to about room temperature or below, so that the solute atoms do not recombine with the dislocations and so that the upper yield point is eliminated. In this regard, it has been my experience that the ferrous alloy workpiece must be quenched in water or other suitable liquid of similar viscosity. Quenching in oils such as are normally used in the heat treating art is not sufiicient to generate the benefits of my process.

When, in accordance with the invention, low-carbon steel or other suitable ferrous alloy is to be heated by exposure to alternating electromagnetic radiation, preferably a frequency between 1 kilocycle and 500 kilocycles per second, is employed. Some beneficial results may be obtained by utilizing alternate electromagnetic radiation outside this preferred frequency range. However, at frequencies below about 1 kilocycle per second the efficiency and uniformity of induction heating is lower, particularly in steel members of thin cross section such as sheet steel. This is known in the art of induction heating as is indicated at pages 186-187 of the Metals Handbook, 8th edition, vol. 2. Moreover, at frequencies above about 500 kilocycles per second it becomes more difficult to uniformly throughheat steel members of any substantial thickness. However, it is noted that in these cases the limitations are inherent in the art of induction heating rather than the characteristics of my method. When induction heating is used to raise the temperature of the ferrous alloy workpiece to a suitable temperature in the specified range, the yield point is generally eliminated by simply attaining such temperature and immediately water quenching the workpiece. However, as I have pointed out above, the alloy may be furnace heated to the temperature range in question and then briefly subjected to electromagnetic radiation. I have found that an exposure of 1-5 seconds in the specified temperature range is usually sufficient. Of course, this could vary upon size and geometry of the workpiece and in some cases it may be necessary to determine the preferred time experimentally.

Since the concentration of solute atoms such as nitrogen and carbon in the region of the dislocations appears to be the preferred thermodynamic configuration, it appears that no heat treatment or cold working could permanently eliminate the upper yield point. Only a chemical change such as the addition of the alloying elements which are used to kill steel could accomplish this. However, it is expected that my process could be carried out conveniently and readily prior to the expected forming operations. Depending on the application of the steel and the manner in which it is to be deformed, the benefits of my process remain for a period of some several hours to several days if stored at room temperature or several weeks if stored at sufiiciently low temperature.

This process may be successfully and advantageously applied to any ferrous alloy exhibiting the upper yield point phenomenon. As a practical consideration, however, it will preferably be conducted in connection with low-carbon steels, which are the ferrous-based alloys most economically formed by cold working. Moreover, it is expected that the most preferred application of this invention will be in connection with the forming of lowcarbon steel sheets and strip. These materials are used primarily in consumer goods, an application requiring materials that are serviceable under a wide variety of conditions, adaptable to low cost techniques of mass production and that present an attractive surface to enhance sales appeal in the finished article. To attain these characteristics under the most economical conditions for production, the bulk of the fiat rolled steel is of lowcarbon content0.l5% maximum. In producing sheets, rimmed steel is ordinarily used. In general, typical ladle analysis are approximately by weight 0.050.10% carbon, 0.25-0.50% manganese, 0.04% phosphorus maximum, and 0.05% sulfur maximum. These alloys also contain small amounts of nitrogen, for example, 0'.00l0.003% by weight, which is sufficient to effect the yield point phenomenon.

The following examples will better serve to illustrate the practice of my invention.

EXAMPLE I A large number of A" x 2" specimens were sheared from a sheet of annealed, rimmed, 18 gauge (0.48") sheet steel. The existence of a pronounced upper yield point in such material was demonstrated by the development of flutes or kinks when the steel specimen was bent around a rubber stopper of about 1" radius.

A Lepel high frequency (460 kilocycles) induction heater and six turns of a copper coil about a 3" diameter were adapted to heat the central 1" portion of the 2" long steel specimen. The preliminary heat treatments were done in air. It was determined that the specimen was at approximately 350 C. by using Tempilsticks, at which temperature it developed a characteristic silvery blue color. The induction heater was operated at a power level which heated the specimen to 350 C. in about 5 seconds.

At first, specimens were removed from the coil and allowed to cool in air until they could be handled. These specimens showed the normal amount of fiuting. In order to determine the effect of rapidly cooling a specimen from 350 C. subsequent specimens were quenched in water immediately after being removed from the coil. They could then be bent around the rubber stopper with perfectly uniform deformation and no fiuting. Further experimentation demonstrated that the deformation was 7 also uniform when a specimen was heated to temperatures as high as 900 C. and water quenched. Above that temperature the steel transformed to austenite.

EXAMPLE II Having demonstrated that the upper yield point can I be readily eliminated in simple bending, the effectiveness of the heat treatment for biaxial stretch during a drawing operation remained to be shown.

EXAMPLE III A coil was prepared to heat a narrow zone across an 11" wide sheet of steel with a Toccotron l5 kilowatt, 460 kilocycle induction unit. With a two-turn coil a sheet 12" x 11" of 20 gauge steel could be processed in about 40 seconds. A sheet was passed downward through the coil the rate of passage being controlled by hand to effect the silvery blue oxide layer characteristic of 350 C. As the sheet left the coil, it passed into a water quenching bath located immediately below.

A die assembly and punch was then selected for drawing large flat bottom cups. Ten-and-three quarter inch diameter circular blanks were cut from the 20 gauge sheet. The blanks were precisely clamped by a bolted holddown pad. The press was then operated in a consistent manner, cups being drawn from (1) annealed, rimmed steel (2) heat treated annealed, rimed steel in accordance with my process, and (3) annealed, killed steel. The bottom of the cups drawn from annealed, rimmed steel showed pronounced stretcher strains. Furthermore, noticeable buckling occurred just below the flange of the cups. In contrast, the bottom of the cups drawn from material from the same large sheet but heat treated in accordance with my process, as described above, showed no stretcher strains and the buckles just below the flange were markedly reduced. As would be expected, the cups drawn from killed steel were also free of stretcher strains and buckles.

EXAMPLE IV The ferrous metal specimens used in this and the following three examples were cut from commercial quality, temper-rolled sheet steel in the form of 11" long strips, 1" wide. A strip was connected to a low voltage (5 volts maximum, 60 cycle per second) high current source (400 amp maximum) for resistance heating. The specimen was gripped by an alligator clip at each end (the alligator clip being connected to the current source by suitable conductor wires) and held directly above a water tank into which it could be dropped by opening the clips. The temperature at the instant of release wa determined by reading the scale of infrared temperature sensing instrument. Five specimens were heated in this manner to temperatures from 385 C. to 582 C. in times of the order of six seconds and immediately quenched in water at room temperature. Tensile test of all specimens in a Weidemann-Baldwin testing machine showed that the upper yield point had been suppressed. Table I gives the physical properties of each of the five specimens treated.

*UTSU1timate tensile strength is determined by dividing the instantaneous maximum load by the cross sectional area before straining.

8 'EXAMPLE V Specimens of the commercial temper rolled sheet steel described above were heated by opposing gas torches to a temperature of approximately 350 C. in five seconds- The heated specimens were immediately quenched in water. The temperature in this case was estimated from the color of the oxide coating on the specimen. Again, the yield point was effectively suppressed while the ductility as measured by percentage elongation was not affected.

EXAMPLE VI Several specimens of commercial temper rolled sheet steel were heated in a tubular furnace which had been brought to the desired temperature. The specimens were left in the furnace for fifteen minutes to insure that they had reached the preset temperature. At the end of the fifteen minute heating period the specimens were immediately quenched in water at room temperature. In this series of experiments the upper yield point was not suppressed except for treatment temperatures above the transformation temperature of the alloy, 730 C. However, in this case the suppression of the upper yield point was accompanied by a significant loss in ductility which would not be suitable for sheet metal forming properties.

EXAMPLE VII A furnace was preheated to 2350" F. with two large iron blocks located therein in closely spaced relationship. When the furnace had reached about 2350" F. sheet metal specimens were placed between the iron blocks so as to be rapidly heated by radiation of heat. They reached an estimated temperature of about 650 F. in about six seconds and were then immediately quenched in water at room temperature. They were subjected to tensile testing and the stress-strain curve produced thereby indicated that the upper yield point had been eliminated.

In accordance with the invention, automobile body components such as fenders, tulip panels and the like have also been drawn. These parts were characterized by the complete absence of Liiders bands or stretcher strains despite the fact that the sheet material had not been killed.

As stated above, it is believed that my process will be extremely useful in the drawing of automobile body parts and the like from low-carbon sheet steel. Preferably, such material would be annealed and given a minimum temper roll or skin pass at the steel mill sufficient only to leave a fine, smooth surface thereon. The extent of deformation during the temper roll will not be as great as at present as the purpose therefor will not be to eliminate stretcher strains but simply to improve the surface. There will be no problem in having to use the material within a short period of time after it has been temper rolled as my process can be applied immediately before it is formed. By treating the steel in accordance with my process, there is no reduction in ductility as occurs after temper rolling. In fact, it has been observed that my process actually increases ductility. Also, the hardness of treated, formed and aged parts will be greater than the hardness of similar members formed using killed steel. This could, of course, permit reduction of weight in many articles of commerce. There will certainly be many otherapplications for my invention. For example, bumpers can be more readily formed without having stretcher strains therein which are difficult to cover by plating.

Thus, while my invention has been described in terms of certain specific embodiments, it is appreciated that other form could be adopted by those skilled in the art and my invention should be considered limited only by the scope of the following claims.

I claim:

1. A method of treating a ferrous alloy of composition such that solute atoms interact with dislocations in the alloy to produce an upper yield point, said method comprising rapidly heating said ferrous alloy to a temperature in the range from about 320 C. to the lower critical temperature of said alloy whereby said solute atoms are dissociated from said dislocations, and immediately rapidly cooling said alloy at a rate sufiicient to prevent the recombination of said solute atoms with said dislocations whereby said upper yield point is temporarily eliminated.

2. A method as in claim 1 whereby said alloy is cooled by quenching in a liquid.

3. A method as in claim 1 whereby said alloy is cooled by quenching in water.

4. A method of eliminating the upper yield point in a ferrous-based alloy comprising rapidly heating said alloy to a temperature in the range from about 320 C. to the lower critical temperature of said alloy within a period up to about 10 seconds and then immediately quenching said ferrous alloy in water.

5. A method of eliminating the upper yield point in low-carbon sheet steel comprising heating said sheet steel to a temperature in the range from about 320 C. to 350 C. within a period of up to about 6 seconds and then immediately quenching said sheet steel in water.

6. A method of eliminating the upper yield point in low-carbon sheet steel comprising heating said sheet steel to a temperature in the range from about 320 C. to around 565 C. within a period of up to about 10 seconds and then immediately quenching said steel in water.

7. A strong, ductile, ferrous metal alloy article which has been rapidly heated to a temperature above 320 C. but below the critical temperature within a period of about 10 seconds and subsequently cooled at a rate whereby the upper yield point has been temporarily eliminated.

8. In a method of deforming a ferrous alloy member of composition such that solute atoms interact with dislocations in the alloy to produce an upper yield point the improvement of rapidly heating said iron alloy member to a temperature in the range from about 320 C. to the lower critical temperature of said alloy within a period of about 10 seconds, immediately cooling said alloy at a rate sufficient to prevent the recombination of said solute atoms with said dislocations whereby said upper yield point is eliminated, and then deforming the alloy.

9. In a method of deforming a ferrous alloy member having an upper yield point the improvement of rapidly heating said iron alloy member to a temperature in the range from about 320 C. to the lower critical temperature of the alloy within a period of about 10 seconds, immediately quenching said alloy in water and subsequently deforming said alloy.

References Cited UNITED STATES PATENTS 1/1966 Tufts

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