US3873378A - Stainless steels - Google Patents

Abstract

Disclosed are high-strength stainless steels of the following composition: ElementWeight Percent Carbon 0.12 to 0.20Nitrogen 0 to 0.07Chromium12 to 16Molybdenum 0 to 7Vanadium 0 to 0.6Cobalt12 to 16Nickel 0.5 to 1.5Niobium 0.1 to 0.3Iron Balance These steels exhibit high strength, fracture toughness, ductility and uniform elongation.

Description

O United States Patent 11 1 1111 3,873,378 Webster Mar. 25, 1975 [5 1 STAINLESS STEELS 3,340,048 9/1967 Floreen 148/38 [751 Invent 3 Webster, Island, ijififili if 02824711111111:31113113132111: 132/33 [73] Assignee: The Boeing Comppany, Seattle, Primary Lovell w h. Attorney, Agent, or Firm-Christensen, OConnor,

G & H 1k 22 Filed: Dec. 13, 1972 ave a [21] Appl. No.: 314,686 [57] ABSTRACT Related Application Data Piselosed are highstrength stainless steels of the fol- [62] Division of S61. No. 171,181, Aug. 12, 1971, Pat. No. Owmg compo Element Weight Percent Carbon 0.12 to 0.20 [52] US. Cl 148/37, 148/125, 148/135 g 13 [51] Int. C22C 39/10, C22C 41/04, C2ld 1/18 Molybtlenum 0 to 7 [58] Field of Search 148/37, 38, 125, 134, 135, Xgggfl g 148/143 Nickel 0.5 1015 Niobium 0.1 to 0.3 [56] References Cited Baum UNITED STATES PATENTS These steels exhibit high stength, fracture toughness, 3,131,097 4/1964 Mantel 148/143 X ductility and uniform elongation 3,154,412 10/1964 Kasak 148/37 3,251,683 5/1966 Hammond 148/37 8 Claims, 4 Drawing Flgures resistance and decreased fatigue crack growth rate. It is another object of this invention to provide heat treatment procedures by which the mechanical properties of the steels of this invention can be optimized.

SUMMARY OF THE INVENTION STAINLESS STEELS This is a division, of application Ser. No. 171,181, Brow? Range 3 Preferred a ge filed Aug. 12, 1971 now U.S. Pat. No. 3,756,808. (Wlght%) wei ht 7%) Carbon 0.12 to 0.20 0.13 to 0.18 BACKGROUND OF THE INVENTION 5 Nitrogeln to 0.07 0.02 m 005 This invention relates to stainless steels and more 1 2:; i8 2:? particularly to hardenable stainless steels which exhibit Vanadium 0 ,to 0.6 0.1 to 0.4 Cobalt 12 to 16 13.2 to 14.2 combinations of high strength, toughness, ductility and Nickel to L5 085 to I L25 uniform elongation not heretofore obtainable. Niobium to ()3 on to 0,25

Two recently introduced stainless steels, AFC-77 l0 Balance Balance and AFC-260 (both developed by Crucible Steel Company of America See p NO 26,225) Preferably, the elements are individually maintained sess combinations of strength and elongation which are the Preferred ranges mdlcatedlmpurlties Such superior to previous stainless steels. The compositions as slllcofli manganese, f Phosphorus can be of these two Steels are Shown in Table 77 is present in the steels of this invention. Silicon and manmartensitic in character, while AFC-260 is semiaustegallese Contents Should generally be maintained below time 1 0.35 and 0.25 weight percent, respectively. Sulfur and The compositions of both are balanced such that heat phosphorus contents should each be maintained below treatment will substantially eliminate retained austenabo 0 5 weight p and p eferably are mainite. AFC-77 can be heat-treated to strengths of up to 20 tamed below about 0.010 weight percent. about 290 ksi by tempering at from 900 to 1,100 F. Typical of the new steels of this invention is a steel However, its fracture toughness and ductility are limerred to herein as Alloy B which has the following ited by the small amounts of austenite retained at such COH'IPOSIIIOIII 7 TABLE I AFC-77 (wt. 7 AFC-260 (wt. 76 Element Normal Range Aim Normal Range Aim Carbon 0.13 to 0.17 0.15 0.05 to 0.09 0.07 Silicon 0 to 0.30 0.20 to 0.35 0.25 Manganese 0 to 0.25 0.15 to 0.30 0.25 Sulfur 0 to 0.010 0 to 0.015 Phosphorus 0 to 0.010 0 to 0.015 Nickel 0 to 0.25 1.7 to 2.0 1.85 Chromium 14.0 to 14.7 14.5 15.2 to 15.8 15.5 Molybdc- 4.7 to 5.1 5.0 4.25 to 4.75 4.5 num Vanadium 0.3 to 0.4 0.5 None Cobalt 13.0 to 14.0 13.5 12.6 to 13.6 13.0 Niobium None 0.10 to 0.25 0.15 Nitrogen 0.03 to 0.06 0.05 0.01 to 0.05 0.03 Iron Balance Balance Balance Balance tempering temperatures. In its austenitic condition, ement Weight% AFC-260 is characterized by low yield strength and Carbon 0J6 reasonable ductility. With thermal treatment of AF- gliqn'oge n 51,332 C-260 to effect austenite-to-martensite transformammlum I M 1 bd 5.22 tion, tensile strengths of about 265 ksi can be obtained. g i jg 009 Up to its maximum tensile strength, AFC-260 does exgol leg; hibit good toughness. }f, It is an ob ect of this invention to provide high Silicon 0.06 strength stainless steels exhibiting combinations of 2133 32 83( strength, toughness, ductility and uniform elongation Phosphorus 0.018 which are superior to those of AFC-77, AFC-260 and Balance other prior art steels. It is a further object of this inven- I tion to provide high strength stainless steels of the type Thi invention i l di d to a novel h described which also exhibit improved stress corrosion treatment process b hi h th i r ha i al properties of the steels of this invention can be optimized.

BRIEF DESCRIPTION OF THE DRAWINGS Various mechanical properties of the steels of this invention and comparisons thereof with prior art steels are graphically depicted by the accompanying drawings, wherein:

FIG. 1 shows tensile stress-strain curves for AFC-77 and AFC-260 and tensile stress-strain curves for Alloy B showing the effect of various heat treatments on strain to fracture values:

FIG. 2 shows a comparison of the toughness and tensile strength of Alloy B with existing stainless steels; and

FIG. 3 shows a comparison of elongation values of Alloy B with existing stainless steels.

FIG. 4 shows a comparison of the uniform elongation values of Alloy B with those of existing stainless steels.

DETAILED DESCRIPTION OF THE INVENTION The compositions of the steels of this invention are critically balanced such that, even after complete heat treatment, they have a duplex structure consisting of a dispersion of on the order of from to of soft austenite in a matrix of hard martensite. The retained austenite, however, is sufficiently unstable that it transforms to martensite when stressed, the enhanced strength and ductility of these steels being attributed to the occurence of this transformation preferentially at regions of highest stress. During tensile tests of Alloy B, areas which attempt to neck raise the local stress at that portion of the gage length and cause the formation of stress-induced martensite. This hardens the local area sufficiently to obviate any further tendency to neck. This process occurring continuously along the entire gage length insures a high uniform elongation.

Composition-wise, the steels of this invention can be considered to be modifications of AFC-77, the modifications being'the inclusion of 0.5 to 1.5% by weight of nickel and 0.1 to 0.3% by weight of niobium. The niobium addition effects refinement of the austenite grain size which increases strength and stress corrosion resistance and decreases fatigue crack growth rate. The nickel addition stabilizes the austenite, causing more austenite to be retained at high tempering temperatures.

AFC-260 can also be considered a modification of AFC-77 (see Table I), the most significant differences between the two being that AFC-260 contains 1.7 to 2.0% by weight nickel and 0.10 to 0.25% by weight niobium and has a reduced carbon content (0.05 to 0.09%). The lower carbon content in AFC-260 results in lower strength and tends to negate the austenitestabilizing effect of the nickel.

The preferred procedure for heat treating the steels of this invention is as follows:

In Stage I, the steel is austenitized at from l,600 to l,800 F., and preferably at about I,700 F., and then cooled to ambient temperature. This stage is designed to refine the austenite grain size of the steel by optimiz ing the size and dispersion of the niobium carbides. Austenitizing for only a few seconds at the indicated temperatures will effect some austenite grain refinement, but times of at least one hour are preferred. The rate of cooling to ambient temperature is not critical and can be in air or by oil quenching.

Stage 2 involves austenitizing at from 1,950 to 2,300 F. for at least about one-quarter hour, cooling directly to a lower temperature of from 1,800 to 2,000 F and holding within the latter range for at least about one-half hour to remove delta ferrite, an undesirable brittle phase. The second temperature should be at least 50 F. lower than the first. The method and rate of cooling are not critical. Preferably this stage is carried out by austenitizing at from 2,000 to 2,200 F. for at least one hour, cooling to from 1,850 to 1,950

F. (preferably about l,900 F.) and holding for at least about I hour within the latter range. Holding for one hour at about 1,900 F. is sufficient in most cases to remove all delta ferrite but holding times of as much as hours can be beneficial if alloy segregation is severe enough to markedly slow the removal rate of delta ferrite. After the holding period, the steel is cooled in air or by oil quenching. The steel can be further cooled to -65 F. to -l50 F. (preferably to about l00 F.) and held there for from one-half hour to 20 hours to effect further hardening by transforming austenite to martensite. The subzero cooling step can be omitted to reduce yield strength and increase elongation and toughness.

Stage 3 involves tempering the steel at temperatures of from 500 to I,l00 F. for at least one-half hour and preferably for 2 2 hours (2 hours at temperature followed by air cooling to ambient temperature followed by an additional 2 hours at the same temperature). A variation on this tempering treatment is to temper at two different temperatures, the first being a low one designed to render the austenite more stable at the second higher tempering temperature. A promising range for the first tempering temperature has been found to be from 650 to 750 F. (preferably about 700 F.). Some increase in elongation can be achieved for a given final tempering temperature by first tempering at 700 F. for 2 hours. However, this increased ductility is obtained with some sacrifice in strength.

The heat-treatment procedure described above has general applicability to stainless steels comprising from 0.0] to 0.25% by weight carbon, II to 16% by weight chromium, l0 to 20% by weight cobalt and up to 10% molybdenum.

One of the most important properties of the steels of this invention is high uniform elongation, i.e., the elongation before local necking or reductions in area occur. In FIG. 1 are shown the tensile stress-strain diagrams for AFC-77 (FIG. 1A), AFC-260 (FIG. 1B) and Alloy B in five different heat-treated conditions (FIG. 1C 1G). The AFC-77 and AFC-260 specimens tested had been subjected to the following heat treatments (see U.S. Pat. No. 3,563,813): austenitizing at 2,IO0 F., cooling to and holding at l,900 F. for 1 hour, oil quenching to ambient temperature and tempering for 2 2 hours at 900 F. (AFC-77) and l,000 F. (AF- C-260). The Alloy B specimens used to produce the five heat-treated conditions had been austenized at l,700 F. for 1 hour, cooled to room temperature, austenized at 2,lO0 F cooled to and held for one hour at 1,900 F. and then cooled to room temperature by oil quenching. To produce Condition I (FIG. 1C), Alloy B was tempered at 800 F. for 2 2 hours without having been previously cooled to subzero temperatures. In this condition Alloy B contains substantial amounts of retained austenite which produces a low yield point (-40 ksi), but its work hardening capacity is sufficient to produce an ultimate tensile strength of 238 ksi. Alloy B in this condition is particularly applicable whenever extensive plastic deformation in a fully heat-treated condition is required. In condition II (FIG. 1D), Alloy B is tempered at 800 F. for 2 2 hours after a subzero treatment (1 hour at l00 F.) which removes a large amount of retained austenite. This results in a substantial increase in yield strength with only a slight decrease in ductility. The effect of cold working on Alloy B in Condition II has been examined by cold rolling 0.080 inch sheet. Cold reductions up to 68% were obtained without edge-cracking. There is a significant increase in both yield and tensile strengths as a result of cold working, and tensile strengths of up to 384 ksi have been obtained by this technique. Ageing at 800 F. after the working produces a further increment of strength. The elongation drops rapidly with cold working but ductility as measured by reduction of area remains at a high level for a material of this strength.

In Conditions III, IV and V (FIG. 1E IG), Alloy B was subjected to a subzero treatment as in Condition II and then was tempered at 900 F. for 2 2 hours (Condition Ill); tempered at 700 F. for 2 hours, cooled to room temperature and tempered at 800 F. for 2 2 hours (Condition IV); or tempered at 700 F. for 2 hours. cooled to room temperature and tempered at 900 F. for 2 2 hours (Condition V). Alloy B in Condition III has a higher strength than in Condition II, at

strength steels. Uniform elongation is a measure of a materials formability and is also a design parameter in some bending applications.

The corrosion resistance, strength, toughness and ductility of the steels of this invention render them useful in many diverse applications. One application for the steels of this invention is the production of highstrength rivets which are tough and ductilevenough to be gun driven without cracking, and which, after driving, have shear strength of over 170 ksi. The rivet must possess its high ductility in a fully hardened condition, since further heat treatment in situ is not usually practical. The properties of Alloy B in Conditions I and II are compared in Table II with a widely used stainless rivet material A 286.

* Measured by a double shear test on V. inch diameter bar. Tested according to the requirements of Aerospace Research and Testing Committee report number ARTCflflfl-aslener Ensron. Double Shear and Lap Joint Testing Procedures."

some sacrifice in toughness and elongation. As dis- From Table II it will be observed that Alloy B offers a cussed previously, the 700 F. treatment in Conditions IV and V, stabilizes the austenite and prevents its transformation to martensite at the higher temperatures (800 900 F). This results in some increase in both total and uniform elongation, with a slight decrease in strength.

One of the objects of this invention is to provide a stainless steel possessing a combination of strength and toughness not available in commercial steels. The extent to which this object has been achieved can be seen from FIG. 2 which compares Alloy B in Conditions II, III and VI with existing stainless steels. Condition VI was produced in the same manner as Conditions II V except that tempering was at l,000 F. for 2 2 hours. The fracture toughness of Alloy B in Condition I is so high that measurement thereof was impractical because of the large size of specimen that would have been required. The steepness of the fracture toughnessstrength relationship for AFC-77 is a reflection of the diminishing austenite content at the higher strength levels. In AFC-260 the austenite is maintained at a high level up to the maximum strength of 265 ksi so that the strength-toughness line is considerably less steep than for AFC-77. Alloy B retains austenite up to an ultimate strength of 290 ksi and exhibits a better combination of strength and toughness than either AFC77 or AF- considerably higher tensile and shear strengths while maintaining a high level of toughness. During installation of the rivets by either gun driving or squeezing, Alloy B in Condition 1 work hardens rapidly due to martensite formation so that there is a marked increase in shear strength from 126 ksi to I74 ksi. Condition I material is the most suitable for gun driving, since its low yield strength will allow formation of the bucktail with less force than would normally be required for a material of this shear strength.

Another application for the steels of this invention is the manufacture of razor blades. The following is a thermomechanical technique which has been successfully used to produce razor blade stock from Alloy B: Step 1.

Alloy B plate (4 in. X 12 in. X 0.6) was hot rolled at 2,l00 F. to produce a coiled band approximately 0.008 inch thick and 4 inches wide.

Step 2.

The coil was austenitized at from 2,000 to 2,200 F. for 1 hour, cooled to and held for one hour at l,900 F., cooled to ambient temperature and further cooled to and held for 1 hour at l00 F.

Step 3.

The coil was then tempered at 500 F. for 2 hours, cold rolled to a thickness of approximately 0.004 inches (50% reduction) and then retempered at 800 to 1,100 F., preferably l,000 F., for 2 2 hours.

The resulting razor blade stock was found to be harder than all commercial stainless steel razor blades tested and markedly more corrosion resistant.

A number of factors render the steels of this invention particularly attractive for use in automobile bumpers. First, unlike other stainless steels of similar strength, these new steels can be heat-treated (Condition I) to possess the high ductility required in the formation of bumpers. Second, the steels of this invention are inherently corrosion resistant and therefore do not require the platings necessary on conventional bumper steels. Third, because these steels harden locally on impact, the effect of a collision is spread over the entire bumper and local damage is reduced.

What is claimed is:

1. A heat-treated stainless steel consisting essentially of:

Element Weight Percent Carbon 0. l 2 to 0.20 Nitrogen to 0.07 Chromium 12 to lo Molybdenum 0 to 7 Vanadium 0 to 0.6 Cobalt 12 to 16 Nickel 0.5 to 1.5 Niobium 0.] to 0.3 Iron Balance said steel having been subjected to the following sequence of heat treatments:

21. austenitized at a temperature of from l,600 to l,800F. and cooled to ambient temperature; and

b. austenitized at a temperature of from 1,950 to 2,300F., cooled to and held at a lower temperature of from l,800 to 2,000F., and cooled to ambient temperature; and

c. tempered at a temperature of from 500 to 2. A steel according to claim 1 which, after having been subjected to treatment (b) and before having been subjected to treatment (c), has been cooled to and held at a temperature of from -65 F. to -l50 F.

3. A steel according to claim 1 which in step (b) was air cooled or oil quenched to ambient temperature.

4. A steel according to claim 2 which in step (b) was air cooled or oil quenched to ambient temperature.

5. A heat-treated stainless steel consisting essentially of:

said steel having been subjected to the following sequence of heat treatments:

a. austenitized at a temperature of from l,600 to 1,800F. and cooled to ambient temperature; and

b. austenitized at a temperature of from l,950 to 2,300F.. cooled to and held at a lower temperature of from l,800 to 2,000F., and cooled to ambient temperature; and

c. tempered at a temperature of from 500 to 6. A steel according to claim 5 which, after having been subjected to treatment (b) and before having been subjected to treatment (c), has been cooled to and held at a temperature of from 65 to l50 F.

7. A steel according to claim 5 which in step (b) was air cooled or oil quenched to ambient temperature.

8. A steel according to claim 6 which in step (b) was air cooled or oil quenched to ambient temperature.

Claims (8)

1. A HEAT-TREATED STAINLESS STEEL CONSISTING ESSENTIALLY OF:

2. A steel according to claim 1 which, after having been subjected to treatment (b) and before having been subjected to treatment (c), has been cooled to and held at a temperature of from -65* F. to -150* F.

3. A steel according to claim 1 which in step (b) was air cooled or oil quenched to ambient temperature.

4. A steel according to claim 2 which in step (b) was air cooled or oil quenched to ambient temperature.

5. A heat-treated stainless steel consisting essentially of:

6. A steel according to claim 5 which, after having been subjected to treatment (b) and before having been subjected to treatmenT (c), has been cooled to and held at a temperature of from -65* to -150* F.

7. A steel according to claim 5 which in step (b) was air cooled or oil quenched to ambient temperature.

8. A steel according to claim 6 which in step (b) was air cooled or oil quenched to ambient temperature.

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