US2892702A

US2892702A - Hardenable stainless steel

Info

Publication number: US2892702A
Application number: US479855A
Authority: US
Inventors: Carl S Walton; William F White
Original assignee: United States Steel Corp
Current assignee: United States Steel Corp
Priority date: 1955-01-04
Filing date: 1955-01-04
Publication date: 1959-06-30
Anticipated expiration: 1976-06-30

Description

nited States Patent STAINLESSSTEEL (Jar-1 S; Walton, Sewickley, andWilliamF. White, Whitehall Borough, Pa., assignors to: United States Steel Gorporatiom; a corporation of- New Jersey No Drawing. Applicationlanuary 4, 1955 Serial-No. 479,855

3 Claims. (Cl. 75-424) This invention relates. to improvements intheart of.

annealing. We have discovered. thattlie: addition ofnitrogen within certain limits to steels of such type will prevent hardening following solution annealing and render the same uniformly responsive to transforming and precipitation hardening treatments.

The teachings of our invention are applicable to chromium-nickel stainless steels containing nickel in quantities insuflicient to render the same stably austenitic and which can be transformed and hardened by thermal treatment The preferred steel of our invention which has a desirable combination of corrosion resistance and mechanical properties and which at the same time will not transform following solution annealing is uniformly responsive to transformation and aging treatments has the following preferred composition ranges:

with the remainder iron and residual amounts of other elements.

In the absence of nitrogen, such an alloy will at times respond to transformation and aging treatments. However, the response varies widely for reasons unknown and frequently the steel hardens to above 95 R following solution annealing and cooling. This has materially affected the usefulness of these steels since many heats had to be diverted to other uses. We have discovered that the addition of nitrogen in suitable amounts will prevent such hardening after solution annealing and will provide a steel which will have hardness value of less than 95 R after solution annealing and air cooling. The nitrogen may be added by any conventional method, such as by the addition of ferro-chromium containing high nitrogen to the liquid steel melt. With our preferred analysis containing between .12 and .18%-nitrogen, after solution annealing, the steel can be treated either by heating and air cooling to transform it' or'by cooling;

to a low temperature. Heretofore, it has been necessary to vary the composition dependent on Whether itwas desired to transform the steel by heat treatment or'by' cooling to sub-zero temperatures. Following transformation our: steel can be aged to a hardness in excess of 38 R Steels having a composition within the foregoing ranges canbe suitably solution annealed by heating to a temperature between 1800 and 2000 F; for sufficient time to permitthe carbidesto go into solution. A preferred treatment is holding for 30 minutes at 1850 F. Following this they can-be air cooled to room temperature with substantially no hardening or transformation taking, place and will have a hardness value less than R Since the steel'insuch conditionis quite soft and ductile, it can be readily formed and Worked. After forming or fabrication, the steel can be transformed by either holding at a temperature below about 50 F. for sufficient time to cause the austenite to transform to' martensite or by holding for sufficient time ata temperaturebetween 1300 and 1500 F. to raise the M point sufficiently to permit the steel to transform on cooling to' room temperature. A preferred low temperature treatment is toholdforone hour at l10 F; and a preferred heat treatment is to hold it for about two" hours at 1400 F;

The steel can then. be further hardened to avalue above 38 R by an aging treatment which may vary considerably over a temperature range of 700 to 1100 F. At the lower temperatures considerably longer times are required but a suitable treatment is about 30 minutes at 900 F.

Hardness values following solution annealing, transforming and aging of the heats set forth in Table I will amply demonstrate the teachings of our invention:

Table I Heat 0 Mn .P S s1 N1 01' Al N The hardness values following solution annealing, transforming and aging of the foregoing heats are given in the following Table II, wherein all hardness values are Rockwell C unless otherwise specified.

Table II Transformation Transformation-l-Agiug Treatment Treatment 30 Mln. at Heat 1,850 F.

No. Air 0001 1 Hr. at 2 Hrs. at

to RJI. 1 Hr. at 2 Hrs at F.+ 1,400 F.+ -1l0 F. 1,40 F 30 Min. at 30 Min. at

1 Rockwell B.

Heat No. 1 containing .12% nitrogen was soft as an nealed and could be hardened to a minimum hardness Patented June 30,, 1 959- value of 38 R by both transformation treatments, where- I as Heat No. 2 containing only 0.034% nitrogen exhibited a high hardness after annealing.

Heat No. 3 having an aluminum content below the preferred range did not harden upon aging to the specified minimum.

Heat No. 4 containing .15% nitrogen reacted much like Heat No. l.

Heat No. 5 with three elements slightly outside the prefelred range did not transform and thus could not be hardened the desired amount.

Heat No. 6 with two elements outside the range on the side of high M temperature showed considerable transformation on annealing and thus was not satisfactory.

The steels of our invention are further characterized by an elongation of about 8% and a Charpy V notch impact value of at least 5 ft. lbs. when heat treated to a hardness value of R,, 41 and a corresponding tensile strength of 165,000 p.s.i.

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. A stainless steel characterized by a hardness value of less than 95 R after solution annealing and air cooling and susceptible to phase transformation and agehardening by double thermal treatment to a. hardness value of at least R, 38 containing between .05 and .08% carbon, 16.0 to 17.5% chromium, 5.50 to 6.75% nickel, .90 to 1.80% aluminum, .12 to .18% nitrogen and the balance iron and other elements in amounts which do not adversely affect the properties. 7

2. A stainless steel characterized by a hardness value of less than R after solution annealing and air cooling and susceptible to phase transformation and agehardening by double thermal treatment to a hardness value of at least R 38, said steel containing between .05 and .08% carbon, 1.5% maximum manganese, 1.00% maximum silicon, 16.0 to 17.5% chromium, 5.50 to 6.75% nickel, .90 to 1.80% aluminum, .12 to .18% nitrogen with the balance iron and residual amounts of other elements.

3. Age-hardened substantially martensitic stainless steel having a hardness in excess of 38 R characterized by initially having a substantially austenitic structure following solution annealing and air cooling and a hardness in said condition of less than 98 R said steel having been transformed to a martensitic structure by thermal treatment and thereafter age hardened, said steel containing between .05 and .08% carbon, 16.0 to 17.5% chromium, 5.50 to 6.75% nickel, .90 to 1.80% aluminum, .12 to .18% nitrogen and the balance iron and other elements in amounts which do not adversely affect the properties.

References Cited in the file of this patent UNITED STATES PATENTS 2,489,520 Camras Nov. 29, 1949 2,505,763 Goller May 2, 1950 2,703,298 Branson et al. Mar. 1, 1955 OTHER REFERENCES Uhlig: Trans. A.S.M., vol. 30," pages 949-980, especially pages 947, 954, 958, 978. 1942.

Claims

1. A STAINLESS STEEL CHARACTERIZED BY A HARDNESS VALUE OF LESS THAN 95 RB AFTER SOLUTION AMEALING AND AIR COOLING AND SUSCEPTIBLE TO PHASE TRANSFORMATION AND AGEHARDENING BY DOUBLE THERMAL TREATMENT TO A HARDNESS VALUE OF AT LEAST RC 38 CONTAINING BETWEEN 0.5 AND 0.8% CARBON, 16.0 TO 17.5% CHROMIUM, 5.50 TO 6.75% NICKEL, .90 TO 1.80% ALUMINUM, .12 TO .18E NITROGEN AND THE BALANCE IRON AND OTHER ELEMENTS IN AMOUNTS WHICH DO NOT ADVERSELY AFFECTED FROM THE PROPERTIES.