US3418178A - Bainitic steel of the 94xx type possessing high strength and fracture toughness - Google Patents

Description

United States Patent 3,418,178 BAINTTIC STEEL OF THE 94XX TYPE POSSESS- ING HIGH STRENGTH AND FRACTURE TOUGHNESS Saul A. Kulin, Waltham, and David Kalish, Cambridge, Mass., assignors to ManLabs, Inc., Cambridge, Mass., a corporation of Massachusetts No Drawing. Filed June 23, 1965, Ser. No. 466,417 17 Claims. (Cl. 14812.4)

ABSTRACT OF THE DISCLOSURE The specification describes a thermomechanical process for strain-tempering the bainitic form of so-called 94XX steels. The lower bainites of such steels are shown to have high strength and fracture toughness when strain-tempered at some temperature below the temperature of bainite formation. A process in which the reduction in cross-section is limited to between 5% to followed by a tempering step also provides such properties.

This invention relates to the processing of ultra-high strength steels to obtain combinations of high strength and impact energy, and more particularly to thermomechanical treatments of steels containing nickel and cobalt as the principal alloying elements.

A number of investigators have shown that thermomechanical treatments applied to ultra-high strength steels, can result in significant improvements in strength. H11 and 4350 bainites exhibit a large response to straintempering treatments. For both steels, small deformations (up to 3%) produce large strengthening effects. Yield strengths close to 400,0000 p.s.i. have been obtained by the application of strain-tempering treatments (50% deformation) to H11 bainite. Increasing the carbon content in 43XX steels enhances the strengthening response to strain-tempering.

Retained austenite determinations have ben made in H11 which had been subjected to various thermal and thermomechanical treatments. It was found that up to 35 percent austenite is retained when bainite is formed at 550 F. from either deformed or undeformed austenite. However, the deformation of the austenite prior to the bainitic trans-formation produces a retained austenite with decreased stability relative to subsequent refrigeration treatments. In addition, deformation of retained austenite, in a previously undeformed bainitic structure, converts a considerable portion of the austenite to martensite. For tempering temperatures below 1000 F., retained austenite has a significant effect on the yield strength of the mixed structure.

Previous work in the area of thermomechanical treatments applied to steels has yielded much information related to the strength changes resulting from the various deformation processes. However, there has been no systematic study of the influence of thermomechanical treatments and individual processing variables on the fracture toughness properties of ultra-high strength steels. In past investigations where fracture toughness was measured, the alloys had very low fracture toughness in the conventionally heat treated condition (pre-cracked Charpy impact energy generally 400 in.-lb./in. The extent of changes in toughness properties due to deformation processes were therefore made uncertain by the inherently brittle nature of the materials studied (H11 and 4350). In addition, in an alloy such as H11, the presence of retained austenite when tempering below 1050" F. further complicates the situation.

The steels to which the present invention relates were 3,418,178 Patented Dec. 24, 1968 selected on the basis of the following considerations: (a) the alloys possess high strength (yield strength 180,000 p.s.i.) and high fracture toughness (precracked Charpy impact strength 600 in.-lb./in. in the conventionally heat treated condition, (b) the compositions of the steels are such that they may be deformed in the metastable austenite condition and transformed to bainite and (c) retained austenite may be eliminated by refrigeration treatments and tempering at low temperatures. One type of steel that met these requirements nominally contained 9% nickel and 4% cobalt as the principal alloying elements. Two carbon contents, 0.25% and 0.45% C, were selected for study.

The steels were obtained from Republic Steel Corporation in the form of consumable-electrode vacuum melted 2-inch wide strip. The 0.4% carbon steel was from three heats, although most of the experiments were performed on the heat referred to as steel H (see Table 1). The strip from the heats L, H and H1 were about 0.280 inch thick while the heat H2 was obtained in 0.375 and 0.500 inch thicknesses. The compositions of all the steels are given in Table 1.

TABLE 1.STEEL COMIPOSITION [Element percent] Steel 0 Ni Co Mn Si Cr V Mo L 0. 26 8. 42 3. 0. 27 0. 04 0. 49 0. 11 0. 43 H 0. 44 7. 50 3. 78 0. 25 0. 02 0. 35 0. 09 0. 29 H1- 0. 41 8. 12 4. 70 0. 12 0. 05 0. 28 0. 10 0. 16 H2".-- 0.44 7.83 3.95 0.12 0.31 0.09 0.31

The process used in the present invention involves straining bainite. As an example, bainite is formed by austenizing for 30 minutes in salt at some temperature (T hot quenching into salt at the appropriate bainite-reaction temperature (T holding to the end of transformation (t and oil quenching to room temperature (RT). 1} means the time for bainite formation at temperature T in a temperature region at which the isothermal transformation products are predominantly bainitic. After formation of bainite, the specimen may be deformed or strained followed by tempering, or it may be single-tempered (e.g.: for one hour at some tempering temperature T followed by deformation or straining, and this deformation may be followed by retempering (e. g.: for one hour at some tempering temperature T which is not necessarily the same as temperature T1). Where it was desired to introduce subcooling to determine or to minimize the influence of retained austenite on the properties of the steel, refrigeration in liquid nitrogen was employed after the quench from T or T and also after the first tempering treatment when employed prior to deformation of the bainite.

The unnotched tensile properties were measured with flat tensile specimens 0.080 inch thick, 0.250 inch in the gage width, 2.0 inches in the gage length and 5.625 inches in overall length. A slight taper of 0.001 inch was introduced from each shoulder to the center of the gage length by hand polishing.

Tensile tests were performed on a Baldwin 60 BTE Universal Tensile Machine. Autographic stress-strain curves were recorded for each specimen using a microformer averaging extensometer. Yield strengths were determined graphically by dividing the load at 0.2% offset strain by the original cross-section area. Ultimate tensile strengths were determined by dividing the maximum load by the original cross-section area. Elongations were obtained by measuring the increase in gage length after fracture. Reductions in areas were obtained by measurements on the fractured specimens. Two to four tensile specimens were tested for each condition.

The precracked Charpy impact test was chosen as one means of evaluation of fracture toughness. This test has been found to be relatively easy to perform and evaluate. The parameter measured is the energy per unit area required to propagate an existing crack. The Charpy specimens used were 2.165 inches in length, 0.394 inch in width, and 0.080 inch in thickness. A standard Charpy V- notch is ground in each specimen after heat treatment, and then a precrack (about 0.015 inch deep and extending through the specimen thickness) was produced in a fatique apparatus specifically developed for the purpose.

The effect of bainite reaction temperature (T on mechanical properties developed by strain tempering (50% deformation) is illustrated in Table 2.

TABLE 2 tures at high strength levels is therefore obtained by utilizin g thermomechanical treatments.

As is noted above, it has been demonstrated for H11 that the ultimate tensile strength is independent of the retained austenite content whereas the yield strength may be reduced considerably by retained austenite. The foregoing data indicates that retained austenite is not playing a significant role in the yield strength data. Retained austenite in steel H is readily eliminated by straining according to the present invention.

The mechanical properties of steel L as a function of bainite reaction temperature T are given in Table 4. The changes in properties with T are complicated by the T =1,500 F.; tr=1 hour; T =room temperature; deformation=50%; and T:

Strength Percent Impact Increase T Stress, 10 Percent Reduction Energy in Yield F.) p.s.i. Elongation in Area in Strength lb./in. A Y.S., p.s.i.

same; at T =ro0m temperature, Yield Strength was almost 20 10 p.s.i. smaller, While Ultimate Tensile strength was between 5 and x10 smaller; other properties were closely similar.

It will be seen in Table 2 that the impact energy increases as the strength decreases for strain-tempering of the lower bainite (T up to 600 F.) However, when the transformation product becomes upper bainite both the strength and toughness decrease as T increases. The strength properties decrease as T increases. In addition, the increment in yield strength between zero and 50% deformation falls off as T increases.

The response of steel H bainite formed at 500 F., to strain-tempering is illustrated in Table 3, which shows the effect of the percent reduction in thickness on the mechanical properties of strain-tempered bainite. As the yield strength increases with strain-tempering the fracture toughness decreases. However, steel H has a high fracture toughness without deformation, and the rate of loss in toughness is such that a series of unique combinations of strength and impact energy are developed. For example the dramatic increase in yield strength within the initial 10% deformation is accompanied by a decrease in impact energy to a value well in excess of 1000 in.-lb./in.

TABLE 3 Mechanical properties of steel H lower bainite formed at 500 I as a function of percent reduction in thickness: at

T zi-oom temp. or 400 F.

Deformation Yield Percent Impact Percent Reduction Strength Percent Reduction Energy,

in Thickness Stress, 1O Elongation in area in.-lb./in.

p.s.i.

Average of curves for T :ro0m temp. and 400 F.; the ditference from 10% to 60% deformation was about 20x10 p.s.i. Ultimate Tensile Strength was more nearly linear with deformation, being about 265x10 p.s.i. at deformation =0%.

In comparing the foregoing strain-tempering data taken on steel H with information available concerning thermal treatments, it is seen that bainites processed according to the present invention have superior toughness at yield strength above the level of about 200,000 p.s.i. It should be noted that the range of strength values of unstrained bainite structures cannot be extended much over 220,000 p.s.i. by lowering T in this alloy because the Ms temperature is approached and mixed structures would be formed. The superior fracture toughness of bainitic struchigh Ms which result in mixed structures for T below 590' F. and also by the end of transformation occurring at less than bainite for a T of 700 F. The steel in this series was pretempered and retempered at room temperature and deformed 25% by rolling.

TABLE 4 Mechanical properties of steel L bainite as a function of bainite-reaction temperature T at T =1650 F; i121 hour; quench temperature, T and T are: room temperature; deformation=25%.

StrengthXlO p.s.i. Percent Impact T F.) Percent Reduction Energy Yield Ultimate Elongation in Area in.-lbs./,

Tensile in.

Unique properties for strain-tempered bainite processed according to the present invention stand out sharply for bainite formed from this steel at about 600 F.

The steels employed in the present invention may be referred to as 9-4-XX steels, where (a) The first digit (9) represents the percentage of nickel as an alloying element; and

(b) The second digit (4) represents the percentage of cobalt as an alloying element; and

(c) The third digits (XX) represent the amount of carbon present; XX may represent 0.25% or 0.45%, depending on whether the L series or the H series in Table 1 is intended. As is indicated in Table 1, a designation 9-4-25 or 9-4-45 will serve only to identify a particular steel series (L or H, respectively), but does not limit the composition to precisely the values represented by these digits, since the values of the alloy and carbon percentages can vary considerably. These designations, then, merely serve to identify alloy steels in which nickel and cobalt are the principal alloying elements and are present in approximately these percentages, and in which carbon is present in the approximate percentages 0.25 and 0.45.

As is noted above, the 9% nickel and 4% cobalt alloys of steel exhibit high fracture toughness at yield strengths up to about 250,000 p.s.i. in the heat-treated condition.

According to the present invention, it has been found that:

(A) The strength of 9-4-45 bainite can be substantially improved by strain-tempering. The largest increase in strength occurs with the first 5 to deformation.

(B) Above a yield strength of 200,000 p..s.i., 9-4-25 bainites have substantially higher impact strengths than 9-4-45 martensites which had been given thermal or strain-tempering treatments. In order to obtain a yield strength greater than 200,000 p.s.i. in 9-4-25 and as high as 300,000 p.s.i., strain-tempering treatments were employed, pointing out the advantages of thermomechanical treatment according to the present invention.

(C) Considering thermal and strain-tempering treatments that develop yield strengths in the range of 200,000 to 340,000 p.s.i., bainitic structures have approximately twice the precracked Charpy impact strength of the martensitic structures at equivalent strength levels. This relationship also holds true for the ultimate tensile strength range of 220,000 to 300,000 psi The bainitic yield strengths greater than 220,000 p.s.i, must be achieved by strain-tempering, thereby demonstrating the advantage of employing a thermomechanical treatment according to the present invention.

In practicing the invention, it is not required that the bainite be quenched to room temperature, or that tempering steps precede or follow the deformation of the bainite. Accordingly, the scope of the invention is intended to be measured by the claims that follow, and not to be limited but only to be exemplified by the foregoing specification.

We claim:

1. The method of producing a high strength steel containing approximately 9% nickel and 4% cobalt as the principal alloying elements, comprising the steps of heating a body of the steel capable of conversion to bainite to a temperature above its transformation temperature, quenching the body with corresponding transformation to a steel consisting esssentially of bainite, after formation of bainite quenching the steel to a lower temperature in the range of room temperatures, deforming the bainitic steel substantially at said lower temperature to produce a reduction in a cross-sectional dimension of the body of between about 5% and 10%, and thereafter tempering said body.

2. The method as claimed in claim 1 in which the bainitic body is tempered after quenching to the lower temperature and prior to deforming.

3. The method as claimed in claim 2 in which the bainitic body is again tempered following deformation.

4. The method as claimed in claim 6, in which the lower bainite is formed by austenizing the body in salt for a time interval at an austenizing temperature, hot quenching the body into salt at a bainite-reaction temperature in a temperature region at which the isothermal transformation products are predominantly lower bainitic, to the exclusion of martensite, and holding the body at said bainite-reaction temperature for a time interval sufiicient for lower bainite formation.

5. Method according to claim 4 in which said bainitereaction temperature is above about 500 F. and not more than about 650 F.

6. The method of producing steels of high strength from an alloy containing approximately 9% nickel and 4% cobalt as the principal alloying elements comprising the steps of transforming a body of such alloy from an austenizing temperature to a lower temperature such that a steel consisting essentially of lower bainite is formed, and strain-tempering said lower bainite at a temperature below that of bainite formation.

7. The method as claimed in claim 6 in which the deformation is limited to produce a plastic deformation not exceeding 10% of the undeformed value of any dimension thereof.

8. The method as claimed in claim 6 in which the deformation is done by rolling.

9. The method of producing a high strength steel containing approximately 9% nickel and 4% cobalt as the principal alloying elements, and containing approximately 0.45% carbon, comprising the steps of heating a body of the steel capable of conversion to bainite to a temperature above its transformation temperature, quenching the body to a transformation temperature between approximately 500 F. and 600 F. with corresponding transformation to a steel consisting essentially of lower bainite, after formation of said bainite quenching the steel to a lower temperature, and strain-tempering said bainitic steel.

10. The method as claimed in claim 9 in which deformation of said bainitic steel is limited to produce a reduction not exceeding 10% in any cross-sectional dimension of said body.

11. The method as claimed in claim 9 in which the transformation temperature is maintained at approximately 500 F.

12. The method of producing a high strength steel containing approximately 9% nickel and 4% cobalt as the principal alloying elements, and containing approximately 0.25% carbon, comprising the steps of heating a body of the steel capable of conversion to bainite to a temperature above its quenching the body to a transformation temperature between approximately 550 F. and not more than 700 F. with corresponding transformation to a steel consisting essentially of lower bainite, quenching said bainitic steel to a lower temperature, and mechanically deforming the bainitic steel to produce a reduction in a cross-sectional dimension of the body.

'13. The method as claimed in claim 12 in which the transformation temperature is maintained in excess of 590 F. and less than 700 F.

'14. The method as claimed in claim 12 including the steps of pretempering said bainitic steel substantially at said lower temperature prior to deformation thereof.

15. The method as claimed in claim 15 including the steps of pretempering said bainitic steel substantially at said lower temperature prior to deformation thereof, and retempering said bainitic steel at the same temperature following said deformation.

16. A steel product containing approximately 9% nickel and 4% cobalt as the principal alloying elements, produced by the process of claim 9 and having a straintempered lower bainitic structure.

17. A steel product containing approximately 9% nickel and 4% cobalt as the principal alloying elements, produced by the process of claim 12 and having a straintempered lower bainitic structure.

References Cited UNITED STATES PATENTS 3,053,703 9/1962 Breyer 148-12 3,240,634 3/1966 Nachtman l4812.4 X 3,252,840 5/1966 Tarwater 148--l2.3

OTHER REFERENCES Journal of Metals, September 1963, p. 692. Mechanical Working of Steel I, AIME, 1964, pp. 177 and 178.

L. DEWAYNE RUTLEDGE, Primary Examiner.

W. W. STALLARD, Assistant Examiner.

transformation temperature,

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,418,178 December 24 l Saul A. Kulin et a1.

It is certified that error appears in the above identified patent and that said Letters Patent are hereby corrected as shown below:

Column 1, line 41, "ben" should read been Column 6, line 41, the claim reference numeral "15" should read l2 Signed and sealed this 17th day of March 1970.

(SEAL) Attest:

Edward M. Fletcher, Jr. WILLIAM E. SCHUYLER, JR.

Attesting Officer Commissioner of Patents