US3108870A - High-strength age-hardenable stainless steel - Google Patents

Description

United States Patent 3,108,870 I-HGH-STRENGTH AGE-HARDENABLE STAlNLEEiS STEEL Richard R. Brady, Monroeville, and Kenneth G. Brickner,

Pittsburgh, Pa, assignors to United States Steel (Zorporation, a corporation of New Jersey No Drawing. Filed June 21, 1960, Ser- No. 37,592 7 Claims. till. 75-124) This invention relates to improvements in age-hardenable stainless steels and more particularly to ,agehardenable stainless steels having improved tensile properties.

In the aircraft, missile and chemical industries, as well as others, there is a definite need for stainless steels, particularly in sheet form, having better ductile properties and higher strengths at both room and elevated temperatures than presently available steels possess. Some present ly available steels have room temperature tensile strengths approaching the required 225,000 p.s.i. minimum and adequate resistance to corrosion and stress-corrosion cracking. However, such steels do not have the required strengths of 185,000 p.s.i. at temperatures of 800 F. and higher. In addition, the steels should have an elongation in excess of 20% in 2 inches in the annealed condition whereby they are readily formable. Further such steels should be susceptible to hardening at relatively low temperatures to minimize distortion.

It is accordingly an object of the present invention to provide an age-hardenable stainless steel having high strength in the hardened condition and good ductility in the annealed condition.

It is another object to provide an age-hardenable stainless steel having high strength at elevated temperatures.

It is a further object to provide an age-hardenable stainless steel which combines the foregoing properties along with the properties of being conditioned for trans formation by a relatively low temperature heat treatment.

The stainless steel of our invention, which developssuperior mechanical properties consists essentially of iron, carbon, silicon, chromium, nickel, molybdenum, aluminum, and vanadium and/or other carbide forming elements, such as tungsten, tantalum, titanium, columbium and Zirconium that are stronger carbide formers than molybdenum. The steel is substantially austenitic after air cooling after annealing at temperatures of the order of 1950 to 2050 F. ductile and may be easily formed by conventional fabrieating operations into the desired shape. The composition is also balanced so that after a relatively low temperature conditioning treatment in the range of 1750 to 1850 F. following the annealing and cooling treatment, the austenite will transform to martensite upon cooling to '100 F. Furthermore, the aluminum and the vanadium and/or other stronger than molybdenum carbide formers are controlled so that on aging at temperatures as high as 1000 F., intermetallic compounds are pre cipitated to develop a minimum room temperature tensile strength of 225,000 p.s.i.

The steel has adequate resistance to atmospheric corrosion, due to the chromium content and. adequate re-.

sistance to elevated temperature oxidation due to the chromium and silicon content. The molybdenum in addition to its known beneficial effect on high temperature strength also improves resistance to stress-corrosion cracking. The vanadium and/or other strong carbide formers, in addition to their function as precipitationhardening elements, impart to the martensite formed during the austenite-to-martensite transformation a resistance to softening during elevated-temperature service and contribute to the good ductility of the steel at elevated temperatures by preventing the relatively high carbon content of the steel from embrittling the steel. The austenitizing In this condition, the steel is veryv 3i,i08,870 Patented Get. 29, 1963 elements carbon and nickel are present in amounts which so balance the ferritizing elements chromium, molybdenum, silicon, aluminum, and vanadium and/ or theother strong carbide formers that the M temperature of the steel in the annealed condition is below about F. and after suitable conditioning treatment the afore-mentioned austeniteto-martensite transformation will occur upon cooling to about -100 F. The carbon content of the steel is maintained at a relatively high level in order that a high carbon martensitic matrix having good elevated temperature strength will be formed. I This relatively high carbon content prov-ides some carbon for combining with the vanadium and/ or other carbide formers to form an intermetallic compound which is believed to precipitate as a critical dispersion on aging.

The broadest chemical composition limits of the steel of this invention are as follows:

forming element of the class consisting of vanadium, tungsten, tantalum, columbium, titanium, and Zirconium or mixtures thereof. These elements may be present in the following amounts:

Percent Vanadium Up to 1.25

Tungsten Up to 4.50

. Tantalum Up to 4.40 Columb-ium Up to 2.25

Titanium Up to 1.25

Zirconium Up to 2.25

with the balance iron and other elements in amounts which do not adversely affect the properties.

Within such range the austenite and ferrite formers must be properly balanced to achieve optimum results.

" Thus the ferriteformers, such as chromium, silicon,

aluminum, molybdenum and vanadium or the other ferrite formers are on the high side of their respective ranges, the austenite formers nickel and carbon should likewise be on the high side. Correspondingly, if the ferrite formers are atthe lower end of the permitted range, the austenite formers should likewise be at the low end. This insures a composition which is sufiiciently stable to remain austenitic after cooling to room temperature from the austenitizing temperature and also thaton conditioning at the recommended temperature, a certain amount 'of carbide will precipitate out, rendering the steel sufficiently metastable so that it will transform on cooling below about 50 F.

The desired balance of ferrite and austenite formers forming element of the class. consisting of vanadium,

tungsten, tantalum, colunibium, titanium, and zirconium or mixtures thereof equivalent to the following amounts:

I Percent Vanadium Up to .30 Tungsten Up to 1.40 Tantalum Up to 1.30 Columbium Up to .55 Titanium Up to .30 Zirconium Up to .55

with the balance iron and other elements in amounts which do not substantially affect the properties.

In the preferred embodiment, the steel contains:

with the balance iron and residual impurities.

It is essential that the carbon be above 0.18% and preferably above 20%, that the steel contain at least .05 and preferably above about .10% of a stronger than molybdenum carbide former, that the aluminum be above 0.85% mid preferably above 1.0%, and that austenitizing elements properly balance the -ferritizing elements;

that is, if the ferritizing elements chromium and molybdenum are on the high side of the composition range, the austenitizing elements carbon and nickel must also be on the high side of the composition range.

The composition of a number of steels tested during our investigations are given in the following Table I:

4 TABLE 11 Room Temperature T ensiie"-Properties in the 950 F. Aged Condition 5 Yield Tensile Elongation Steel strength strength, in 2 inches,

(0.2% onset) p.s.i. percent 35 B Steels of this invention.

N0rE.The results are the average of two tests on longitudinal specimens which were austenitized for 5 minutes at 1950 F., conditioned for minutes at 1800 F., refrigerated for 16 hours at 100 Rand aged. for 2 hours at 950 F.

TABLE I Compositions of Steels InvestigatedPercent of Mo v Al N Ti Cb-Ta W Zr Steel C lVIn P S Si Ni 0. 62 0. 80 5. 23 0. 0. 66 0.004 0. 023 0. 93 6. 7'8 0. 25 0. 65 0. 004 0. 022 0. 80 5. 47 0. 24 0. 63 0. 008 0. 021 0. SO 6. 17 0. 24 0. 64 0. 007 0. 020 0. 80 6. 91 0. 18 0. 68 0. 008 0. 020 0. 79 5. 43 0. 24 0. 66 0. 009 0. 020 0. 86 6. 88 0. 23 0. 58 0. 008 0. 018 0. 79 6. 27 I 0. 19 0. 58 0. 008 0. 030 0. 79 7.02 0. 19 0. 63 O. 005 0.024 0. 83 7. 00 0. 24 0. 62 0. 010 0. 022 0. 74 5. 47 0. 22 0. 61 0. 013 0. 022 0. 79 6. 23 0. 23 0. 58 0. 012 0. 023 0. 76 5. 46 0. 21 0. 62 0. 011 0. 023 0. 79 7. 0S 0. 21 0. 60 0. 012 0. 023 0. 8O 6. 0. 19 0. 61 0. 013 0. 018 0. 78 7. 10 0. 21 0. 59 0. 015 0. 021 0. 81 6. 39 0. 24 0. 62 0. 010 0. 024 0. 80 7. 11 0. 21 0. 59 0. 011 0. 023 0. 80 6. 28 0. 20 0. 65 0. 008 O. 012 0. 90 5. 0. 19 0. 63 0. 012 0. 021 0. 81 5. 53 O. 23 0. 63 0. 008 0. 012 0. 88 5. 45 0. 092 0. 67 0. 009 0. 010 0. 49 6. 82 0. 23 0. 67 0. 007 0. 024 0. 46 7. 03 0. 22 0. 63 0. 008 0. 023 0. 78 6. 00 0. 26 0. 64 0. 011 0. 026 0. 87 6. 45 0. 24 0. 67 0.008 0. 030 0. 91 5. 99 0. 23 0. 62 0. 007 0. 010 0. 86 6. 25 0. 26 0. 61 0. 007 9. 007 0. 92 6. 30 0. 25 0. 59 0. 005 0. 009 0187 6. 81 0. 25 0. 59 0. 005 0. 010 0. 86 6. 52 0. 25 0. 63 0. 006 0. 006 0. 90 6. 58 0. 26 0. 0. 006 0. 008 0. 99 6. 31 0. 26 0. 64 0. 008 0. 97 6.00

Steels of this invention.

b About 10 percent of the indicated eolumbium addition was tantalum.

The room temperature properties of hardened (aged) sheet specimens of certain of the steels of Table I are given in Tables 11 and HI:

TABLE III Room Temperature Tensile Properties in the 1000 F.

Aged Condition Noru.-The results are the average of two tests on specimens which were austenitized for minutes at 1950 F., conditioned for 15 minutes at 180N113 refrigerated for 16 hours at 100 F., and aged for one hour at 1000 The room temperature properties of the steels in the annealed condition are listed in Table IV:

TABLE IV Room Temperature Properties in the Annealed Condition Yield Tensile Elongation Steel Strength strength, in 2 inches,

(0.2% ofiset) p.s.i. percent p.s.i.

a Steels of this invention.

NorE.-'1he results are the average of two tests on longitudinal specimens which were austenitized for 5 minutes at 1950 to 1975 F.

The longitudinal tensile properties of the steels at 800 F. are given in Table V:

TAnLEv Tensile Properties at 800 F.

Yield Tensile Elongation Steel strength strength, in 2 inches,

(0.2% otiset), p.s.i. percent 1 Steels of this invention. b Broke outside gage marks. 0 Data not available.

Norm-The results are the average of two tests on specimens which were austenitized for 5 minutes at 1950" F.. conditioned for 15 minutes at 1800 F., refrigeratedfor 16 hours at 100 F., and aged for 2 hours at 950 F.

As indicated, the hardening treatment involves heating in the range of 1950 to 2050 F. and preferably Within the range of 1950 to 2000 F, i.e. to about 1975' F. On air cooling, the steel is substantially austenitic, being soft and ductile and readily workable. Rehea'ting within the range of 1750 to'1850 F. and preferably 1750 to 1800 F., i.e. to about 1775 -F., conditions the steel so that on cooling to below -50 F. and preferably to about -100 F. the steel will substantially transform to a martensitic structure. Thereafter the steel may be aged at temperatures above 800 F., preferably in the range of 900 to 1000 F.

The preferred heat treatment for the steel is asv follows:

(1) Anneal for 3 to 5 minutes at 1950 to 2000 and air cool to room temperature;

(2) Condition for about 15 minutes at 1750 to 1 800" F.

and air cool to room temperature;

(3) Refrigerate for 2 to 24 hours at -1 00 F.; and

(4) Age at 900 to 1000 F. for 30 minutes to 4 hours (the time is inversely proportioned to the aging temperature).

-is illustrated by steels 4, 6, 14, 16, and 24. These steels have a poorly balanced structure, having a relatively low nickel content (5.23 to 5.53%) with a relatively high chromium content (15.4 to 16.6%) with carbon contents in the range 0.19 to 0.25% and with aluminum contents in the range 0.55. to 1.49%, and these steels fail to meet one or more of the desired properties (about 20% elongation in the annealed condition, a minimum tensile strength of 225,000 psi. at room temperature in the hardened condition, and a tensile strength of at least 185,000 p.s.i. at 800 F. in the hardened condition). For instance, steels 6, 16, and 24 fail to meet the annealed elongation requirements; steels 4 and 14 fail to meet the room temperature tensile strength requirement in the hardened condition; and steels l4 and 24 fail to develop the desired properties at 800 F. In contrast, more balanced heats, such as steels 9 and 23 which also have a relatively low nickel content (5.43 to 5.50%) but a relatively low chromium content (14.3'and 14.7%), and steels 19 and 21 which have a relatively chromium content (16.5%) but also have a relatively high nickel content (7.10 and 7.11%), meet the desired properties. The importance of carbon in maintaining the proper balance between the austeuite-forming elements and the ferrite- -fonming elements is illustrated by these latter steels, 19

and 21. Except for carbon, both have about the same composition. Steel 19, which has a carbon content of 0.19%, develops a tensile strength of only 178,600 psi. at 800 F. whereas steel 21, which has a carbon content of 0.24%, develops a tensile strength of 199,200 p.s.i. at 800 F. These data indicate that although both steels are within the broad chemical composition. limits of the new steel, steel 19 is not. properly balanced and, therefore, does not develop the desired superior tensile properties.

The importance of remaining within the chromium content range is illustrated by steel 13, which has a chromium content (16.7%) that is above the specified range and has relatively poor elevated temperature properties (167,400 p.s.i. tensile strength at 800 F). The low side of the chromium content range is controlled by the necessity of having good resistance to atmospheric corrosion and elevated temperature oxidation.

The importance of remaining within the aluminum content range may be illustrated by steel 12, which has an aluminum content of 0.62% and has tensile properties at 800 F. (178,500 p.s.i.) slightly below those desired, and steel 10, which has an aluminum content only slightly higher (0.69%), and meets all the desired properties except for relatively poor elevated temperature ductility. The steels that have a low aluminum content generally have low elongation at elevated temperatures; therefore,

it is necessary to keep the aluminum content above the specified minimum. The high side of the aluminum composition range is controlled by the requirement of maintaining a relatively balanced composition.

"Steels 26, 27, and 28 illustrate the importance of a relatively high carbon content and the need for a carbide former. Steel 26 has low carbon but contains vanadium; and steel 28 is otherwise the steel of this invention except that it does not contain a carbide former. It should be noted that the elevated temperature strength of steel 26 falls considerably below the required elevated temperature strength. Thus a carbide former does not improve the elevated temperature strength of a realtively lowcarbon steel of this type. Also, steel 28, While showing relatively good room and elevated temperature strength, has poor ductility at elevated temperatures. This poor ductility at elevated temperatures is an indication that such steels have poor elevated temperature stability.

Thus it is seen that the steel of this invention combines the following qualities:

(1) Minimum room temperature tensile strength of 225,000 p.s.i. with a minimum 800 F. tensile strength of 185,000 p.s.i.; (2) Minimum ductility of about 20% elongation in 2 inches after annealing between 1950 and 2050 (3) Resistance to embrittlement during heat treatment and elevated temperature service; v

(4) Resistance to tempering (or aging) which enables the While we have shown and described several specific embodiments of our invention, it will be understood that these embodiments are merely for the purpose of illustration and description and that various other forms may be devised Within the scope of our invention, as defined in the appended claims.

We claim:

1. Age-hardenable stainless steel characterized by high strengths at room and elevated temperatures in the agehardened condition combined with good ductility in'the' annealed condition, said steel comprising Percent Carbon 0.18 to 0.30 Manganese 1.0max. Phosphorus 0.04 max. Sulphur 0.04 max. Silicon 0.25 to 1.25 Nickel 5.40 to 7.5 Chromium 14.25 to 16.50 Molybdenum 2.0 to 3.0 Aluminum 0.85 to 1.75 Stronger than molybdenum carbide 0.05 to 4.50

forming element of the class consisting of vanadium, tungsten, tantalum, columbium, titanium and zirconium or mixtures thereof in the following amounts:

Percent Vanadium Up to 1.25 Tungsten Up to 4.50 Tantalum Up to 4.40 Columbium Up to 2.25 Titanium Up to 1.25 Zirconium Up to 2.25

with the balance iron and other elements in amounts which do not adversely affect the properties, the austenite and ferrite formers being balanced within the foregoing ranges to provide a composition that transforms at temperatures below about -50 F. after austenitizing and conditioning. 2. Age-hardenable stainless steel characterized by high strengths at room and elevated temperatures in the agehardened condition combined with good ductility in the annealed condition, said steel comprising 7 Percent. Carbon 0.20 to 0.28 Manganese 0.50 to 0.80 Phosphorus 0.04 max. Sulphur 0.04 max. Silicon 0.60 to 1.00 Nickel 6.00 to 7.25 Chromium 14.50 to 16.00 Molybdenum 2.25 to 2.75 Aluminum 1.00 to 1.60 Stronger than molybdenum carbide 0.10 to 1.40

forming element of the class consisting of vanadium, tungsten, tantalum, columbium, titanium, and zirconium or mixtures thereof in the following amountsz Percent Vanadium Up to .30 Tungsten Up to 1.40 Tantalum Up to 1.30 Columbium Up to .5 5 Titanium Up to .30 Zirconium Up to .55

with the balance iron and other elements in amounts which do not adversely affect the properties.

3. Age-hardenable stainless steel characterized by high strengths at room and elevated temperatures in ageareasvo hardened condition combined With good ductility in the annealed condition, said steel containing with the balance iron and residual impurities.

4. Age-hardened stainless steel characterized by high strengths at room and elevated temperatures combined with adequate ductility, said steel comprising Percent Carbon 0.18 to 0.30 Manganese 1.0 max. Phosphorus 0.04 max. Sulphur 0.04 max. Silicon 0.25 to 1.25 Nickel 5.40 to 7.5 Chromium 14.25 to 16.50 Molybdenum 2.0 to 3.0 Aluminum 0.85 to 1.75 Stronger than molybdenum carbide 0.05 to 4.50

forming element of the clas consisting of vanadium, tungsten, tantalum, columbium, titanium, and zirconium or mixtures thereof in the following amounts:

Percent Vanadium Up to 1.25 Tungsten Up to 4.50 Tantalum Up to 4.40 Columbium Up to 2.25 Titanium Up to 1.25 Zirconium Up to 2.25

With the balance iron and other elements in amounts which do not adversely affect the properties, the ferrite formers and the austenite formers being relatively balanced, said steel having been hardened by austenitizing, conditioning, low temperature transformation and aging.

5. Age-hardened stainless steel characterized by high strengths at room and elevated temperatures combined with adequate ductility, said steel comprising forming element of the class consisting of vanadium, tungsten, tantalum, colurnbium, titanium and zirconium or mixtures thereof in the following amounts:

Percent Vanadium Up to .30 Tungsten Up to 1.40 Tantalum Up to 1.30 Columbium Up to .55 Titanium Up to .30 Zirconium Up to .55

with the balance iron and other elements in amounts which do not adversely affect the properties, said steel having been hardened by austenitizing in the range of 1950 to 2050 F., conditioned by heating within the range of 1750 to 1850 F., transformed by cooling to below about -50 F. and thereafter aged at temperatures above about 850F.

6. Age-hardened stainless steel characterized by high strengths at room and elevated temperatures combined with adequate ductility, said steel consisting of Percent Carbon 0.20 to 0.28 Manganese 0.50 to 0.80 Phosphorus 0.04 max. Sulphur 0.04 max. Silicon 0.60 to 1.00 Nickel 6.00 to 7.25 Chromium 14.50 to 16.00 Molybdenum 2.25 to 2.75 Aluminum 1.00 to 1.60 Vanadium .10 -to..30

with the balance iron and residual impurities, said steel having been hardened by austenitizing in the range of 1950 to 2000 F. for about 3 to 5 minutes, cooling and reheating within the range of 17 50 to 1800 F. for about 15 minutes, cooled to about F. for 2 to 24 hours and then age hardened by heating within the range of 900 to 1000 F. for 30 minutes to 4 hours, the time and temperature being inversely proportional.

7. Age-hardened stainless steel characterized by high strengths at room and elevated temperatures combined with adequate ductility, said steel comprising Percent Carbon 0.18 to 0.30 Manganese 1.0 max. Phosphorus 0.04 max. Sulphur 0.04 max. Silicon 0.25 to 1.25 Nickel 5.40 to 7.5 Chromium 14.25 to 16.50 Molybdenum 2.0 to 3.0 Aluminum 0.85 to 1.75 Stronger than molybdenum carbide 0.05 to 4.50

forming element of the class consisting of vanadium, tungsten, tantalum, colurnbiurn, titanium, and zirconium or mixtures thereof in the following amounts:

with the balance iron and other elements in amounts which do not adversely aflect the properties, the ferrite formers and the austenite formers being relatively balanced, said steel having been hardened by transforming and aging.

References Cited in the file of this patent UNITED STATES. PATENTS 2,894,867 Smith July 14, 1959 2,958,618 Allen Nov. 1, 1960 FOREIGN PATENTS 1,188,593 France Mar. 16, 1959 'UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,108,870 October 29, 1963 Richard R. Brady et al.

It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 7, line 11, after "relatively" insert high Signed and sealed this 21st day of April 1964.

(SEAL) Attest: EDWARD J. BRENNER ERNEST W. SWIDER Attesting Officer Commissioner of Patents

Claims (1)

1. AGE-HARDENABLE STAINLESS STEEL CHARACTERIZED BY HIGH STRENGTHS AT ROOM AND ELEVATED TEMPERATURES IN THE AGEHARDENED CONDITION COMBINED WITH GOOD DUCTILITY IN THE ANNEALED CONDITION, SAID STEEL COMPRISING