Metal process and article

Sept. 5, 1967

3,340,102

S. A. KUL1N ETAL

METAL PROCESS AND ARTICLE

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FIG. I

TENSILE YIELD STRENGTH

AND

ELONGATION

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0.2% YIELD STRENGTH

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ELONGATION IN 2 IN.

I. STANDARD HEAT TREATMENT

AUSTFORMED.....DEFORMED METASTABLE AUSTENITE TRANSFORMED

TO MARTENSITE AND TEMPERED

3. DEFORMED METASTABLE AUSTENITE TRANSFORMED TO BAINITE

4. DEFORMED METASTABLE AUSTENITE TRANSFORMED TO BAINITE STRAINED 2% AND TEMPERED

FIG.2

SAULA.KULIN DAVID KALISH PHILIP STARK

INVENTORS

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ATTORNEY

United States Patent Office Patented Sept. 5, 1967

3,340,102

METAL PROCESS AND ARTICLE Saul A. Kidin, Waltham, Philip Stark, Watertown, and David Kalish, Brighton, Mass., assignors to Manlabs, Inc., Cambridge, Mass., a corporation of Massachusetts

Filed May 15, 1962, Ser. No. 194,758

Claims. (Cl. 148—12.4)

The present invention relates to the processing of steel and to steel masses having a novel microstructure. More particularly, the invention relates to high strength steels and alloys and to the thermal-mechanical processes employed for their treatment.

In the prior art a number of processes have been proposed for improving the characteristics of high strength steels. The salient features of such high strength steel are determined primarily by the yield strength, the percentage of elongation or ductility, and the toughness. It is desirable for certain applications to obtain steels exhibiting yield strengths as high as possible. In so doing, however, the steel tends to become brittle. It is desirable, therefore, to provide a high strength steel which additionally exhibits improved ductility and toughness.

In Patent No. 2,934,463 issued Apr. 26, 1960, to D. J. Schmatz et al. for high strength steel, a method is disclosed for heat treating steel to obtain improved strength. The method was developed for use with steels which exhibit a temperature region below the pearlite transformation temperature and above the bainite transformation temperature in which the metastable austenite is comparatively stable, even under conditions of substantial working for a protracted period of time. In accordance with that method, steel compositions exhibiting a wide bay between the pearlite transformation zone and the bainite transformation zone have been very heavily worked in this selected temperature region while in a metastable austenitic state and have remained austenitic for sufficient time to permit all necessary working operations, even including a reheating operation. After this extensive working, the material is quenched to the martensitic transformation zone and permitted to transform to martensite. As particularly described in the claims, Schmatz et al. explicitly limits the process to a transformation from austenite to pure martensite without permitting any substantial transformation into bainite. In contrast with Schmatz et al., the present invention is concerned with the production of a bainitic structure without permitting any substantial transformation into martensite.

In Patent No. 2,717,846 issued to R. F. Harvey, Sept. 13, 1955, a method of surface hardening ferrous metals is disclosed. In that patent, Harvey uses a hardenable ferrous metal which is first austenitized by heating above its critical temperature, then quenched rapidly in a suitable heat abstracting medium maintained above the temperature range of martensite formation and held for a length of time sufficient to attain the temperature of the quenching medium but not long enough to permit transformation of the subcritical austenite at that temperature level, after which they are mechanically worked while above or less than about 100° F. below the temperature at which martensite starts to form on cooling, and finally the ferrous metals or steels are cooled to room temperature. He also found that this treatment results in a higher hardness and a more complete transformation of austenite to martensite. Thus it would appear that the Harvey patent is somewhat similar to that of Schmatz et al.

In contrast with Harvey’s production of martensite, bainite is produced in accordance with the present inven- tion. Harvey, too, explicitly precludes the formation of appreciable amounts of bainite.

In the March 1961 issue of the American Society for Metals Transactions Quarterly in an article by E. T. k Stephenson and Professor Morris Cohen, a process is disclosed for improving the strength of high strength steel by a combination of thermomechanical treatments consisting of hardening, tempering, straining and retempering. It is to be noted that here the specimen was austenitized at 1525° F. for 40 minutes and quenched in oil to form ¹⁰ martensite. The process outlined by Stephenson and Cohen is limited to the use of martensite. In contrast, the present invention is concerned with bainite.

While the prior art processes serve to increase the tensile strength of ferrous metals or steels, these processes, as noted above, failed to provide an improvement in ductility or toughness.

It is therefore an object of the invention to provide an improved process for treating high strength alloys 2Q which exhibit increased ductility and toughness while preserving extraordinarily high strengths.

A further object of the invention is to provide an improved high strength alloy product.

Yet another object of the invention is to provide an 25 improved high strength steel.

In accordance with the invention there is provided the process of forming and heat treating a steel product. The process includes heating a steel mass to a temperature sufficient to render the structure of the steel austenitic.

The chemical composition of the steel mass is such that it exhibits a metastable austenitic structure when cooled from a temperature at which austenite is stable into a selected temperature region below that at which austenite transforms into pearlite. The steel mass is cooled into the 35 selected temperature region. A substantial amount of mechanical deformation is performed upon the steel mass while maintaining the mass within the temperature region. The deformation is at least 10% at least locally. The deformed austenitic mass is allowed to transform into 40 a steel mass of substantially bainitic structure in a temperature region at which the isothermal transformation products would be predominantly bainitic. The bainitic mass is cooled. The cooled bainitic mass is then mechanically deformed to improve the strength of the steel 45 mass.

Other and further objects of the invention will become apparent from the following description of the invention, taken in connection with the accompanying drawings, and its scope will be pointed out in the appended claims. 50 In the drawings:

FIG. 1 is a TTT (time, temperature, transformation) diagram illustrating an .aspect of the invention; and FIG. 2 is a graph illustrating comparative strengths and ductility for various thermo-mechanical treatments of 55 a particular high strength steel.

Principles of operation

As noted above, the prior art techniques for improving the strength of metals and alloys are accompanied by 60 losses in ductility and toughness. While the strength of hardened (martensitic) or hardened and tempered steels may be increased by deformation treatments after heat treatment, with or without subsequent temperature treatments, losses of either or both ductility and toughness as 65 compared with these physical properties for the normal or standard heat treated steel are realized. Tn the present invention a sequence of thermo-mechanical treatments results in a substantial improvement in strength, of the order of 50%, in heat treatable steels accompanied by extra70 ordinary increases in ductility (over 100% ).

Referring now to the drawings, and with particular reference to FIG. 1, there is here illustrated a TTT (time,

3,340,102 temperature and transformation) diagram as typically used in physical metallurgy. The diagram is frequently employed to describe the transformations which may occur when steel is heated into the austenitic region and then cooled or cooled and held at a particular temperature. Three principal types of transformations may take place: peralitic, bainitic and martensitic. In certain steels it is possible to cool rapidly from the austenite region to a temperature where the austenite is metastable (the socalled bainite bay), and sufficient time is available prior to the occurrence of an isothermal transformation. During this period one may perform a wide variety of mechanical deformation treatments on the metastable austenite prior to its transformation. As noted above, the strength of steels has been increased in the prior art by mechanical deformation of metastable austenite in such a temperature region prior to cooling to form martensite. It is to be noted particularly that in the prior art processes great care is taken to limit the working temperatures and time in such a manner as to minimize the production of isothermal transformation products (bainite) from the metastable austenite prior to its transformation to martensite.

In the present invention a steel mass is austenitized at a high temperature and cooled to a temperature in or below the bainite bay. The austenite is subjected to me- 25 chanical deformation treatments such as rolling, forging, swaging, drawing, extrusion, etc. either during or preferably prior to its essentially complete isothermal transformation to bainite. Note that the metastable austenite is . __________________„„„ _iuc subjected to mechanical deformation and then directly 30 provement in strength may be of the order of 50%. transformed to bainite, substantially without the prior of ” subsequent formation of martensite. A small amount of martensite, for example under 5%, may be tolerated in the process of the present invention without substantially degrading its results. It has been found that in certain steels it is possible to deform the metastable austenite, transform it completely to bainite and cool to, for example, room temperature without increasing martensitic reaction products. An important aspect of the invention, however, is that the deformed metastable austenite is transformed substantially completely to bainite (the ferrite-carbide aggregate).

After the bainite has been formed and cooled, its strength may be increased by further mechanical deformation with treatments such as rolling, extrusion, swaging 45 and drawing, etc. The second deformation involves only a relatively small amount of working, for example up to a maximum of approximately 15%, at or in the vicinity of room temperature. A typical example involves 2% reduction by rolling at room temperature. Certain steel com- 50 positions may require the secondary deformation to be performed at sub zero temperatures or at temperatures above normal ambient. In working the steel above normal ambient, the high temperature range is limited by the possibility of overtempering. The final step in this sequence 55 of operations for obtaining maximum strength coupled wtih exceptional ductility and toughness involves an aging or tempering treatment. The tempering or aging is performed at a temperature above ambient and probably below 1000° F. The secondary deformation and tempering 60 may be repeated if necessary. In carrying out the invention a suitable steel composition is first austenitized by heating to a suitable temperature and then cooled into a selected temperature region for the metastable austenitic structure as indicated in FIG. 1. This temperature region 65 is preferably below 1100° F. The preferred region varies according to composition and fabrication techniques. When the steel mass has been cooled to the proper temperature it is worked to affect a substantial reduction of area and then allowed to transform or is cooled into a 70 temperature region in which rapid transformation to a substantially bainitic structure is possible. Precautions should be taken in working the metal to avoid excessive heating tending to reduce the effects of deformation or permit higher temperature transformation.

The bainitic steel thus formed is then cooled preferably to room temperature, the cooled bainite is then deformed mechanically by a small amount. The deformed bainite is then tempered or aged.

The sequence of operations may be summarized as follows:

(1) Austenitize the steel and cool to a temperature in the metastable austenitic region of the TTT diagram.

(2) Perform a mechanical deformation treatment involving a substantial amount of such deformation.

(3) Allow the deformed metastable austenite to trans,^is°thermally or on slow cooling into substantially bainitic products.

(4) After cooling to room temperature (in special cases, higher or lower temperatures may be employed), deform the bainitic products by a small amount.

(5) Age or temper the deformed bainitic (or isothermally formed reaction products) structure.

It has been found that the transformation of metastable austenite to bainite (step (3) only) subsequent to deformation of the austenite produces a steel mass having a lower strength than that obtained by transforming deformed metastable austenite directly to martensite. Even in this condition, however, the steel mass exhibits extraordinary ductility and toughness. By adding the steps (4) and (5) of a small mechanical deformation of the bainitic structure and temperating or aging, a significant improvement in the strength is obtained while retaining an unusually high amount, of ductility and toughness. The imReferring now to FIG. 2, there is here shown a graph illustrating the ductility and tensile yield strength of a particular type of high strength steel having treatments numbered from (1) through (4) inclusive.

Treatment #1 as shown on the graph is the standard heat treatment. Treatment #2 illustrates ausfoimed steel (the, Schmatz process), treatment #3 illustrates the ductility and tensile yield strength of deformed austenite which has been transformed to bainite, and treatment 40 #4 illustrates a steel mass processed in accordance with steps (1) through (5) inclusive above.

The invention has been practiced with H-ll hot work die steel having a composition including alloying elements of 5% chromium and .4% carbon, 1.25% molybj denum, .5% vanadium, 1% silicon and .3% manganese.

In thestandard heat treated condition, this steel exhibits a .2% yield strength of approximately 230,000 p.s.i. with an elongation in 2 inches of approximately 5%. When this steel is austenitized, cooled to the metastable austenite region, subjected to mechanical deformation, and then cooled to form martensite, a .2% yield strength of approximately 330,000-350,000 p.s.i. and an elongation in 2 inches, of approximately 5% is obtained under optimum conditions. Though this treatment produces a significant increase in strength, little improvement in toughness or ductility is realized.

In contrast, and in accordance with the present invention, austenitizing the steel, cooling to a low temperature in, the metastable austenite region, performing a mechanical deformation and producing a transformation to bainite, the .2% yield strength is found to be of the order of 250,000 p.s.i., and the elongation in 2 inches turns out to be of the order of 15%. A secondary mechanical deformation (2%) of the bainitic product followed by tempering or aging (600° F. for approximately one hour) results in a .2% yield strength of approximately 330,000350,000 p.s.i. with an elongation in 2 inches of approximately 12-14%. In this manner one obtains a substantial improvement in strength while retaining unusually high ductility and toughness.

It will be apparent that the bainite material (through step (3) only) exhibiting high ductility may be used without further processing where the tensile yield strength is not of prime importance. It is to be noted that appreciable plastic deformation occurs between the .2%

3,340,102 yield point and the ultimate strength of the steel. Thus the yield to ultimate ratio does not approach one as is found when one obtains strength increases by deformation and aging of martensitic structures. Furthermore, deformation and aging of martensitic structures decreases the ductility and toughness of the steel. When steel is processed in accordance with the method of the invention, high strength is coupled with ductility and toughness to a degree previously unobtainable. It will be apparent that the present invention presents an important step forward in the treatment of high strength steels.

While there have been heretofore presented what are considered to be the preferred embodiments of the invention, it will be apparent to one of ordinary skill m the art that all those changes and modifications which fall fairly within the scope of the invention shall be considered to be a part of the invention.