High strength, high fracture toughness alloy

Details High strength, high fracture toughness alloy High strength, high fracture toughness alloy

1992-06-30

1993-12-07

Yee, Deborah

Metal treatment

Stock

Ferrous

148335, 420 95, 420 96, 420 97, 420107, 420108, C22C 3852

Patent

active

052680444

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND OF THE INVENTION

This invention relates to an age-hardenable, martensitic steel alloy, and in particular to such an alloy and an article made therefrom in which the elements are closely controlled to provide a unique combination of high tensile strength, high fracture toughness and good resistance to stress corrosion cracking in a marine environment.
Heretofore, an alloy designated as 300M has been used in structural components requiring high strength and light weight. The 300M alloy has the following composition in weight percent:


______________________________________ wt. % ______________________________________ C 0.40-0.46 Mn 0.65-0.90 Si 1.45-1.80 Cr 0.70-0.95 Ni 1.65-2.00 Mo 0.30-0.45 V 0.05 min. ______________________________________ tensile strength in the range of 280-300 ksi.
A need has arisen for a high strength alloy such as 300M but having high fracture toughness as represented by a stress intensity factor, K.sub.IC,.gtoreq.100 ksi .sqroot.in. The fracture toughness provided by the 300M alloy, represented by a K.sub.IC of about 55-60 ksi .sqroot.in, is not sufficient to meet that requirement. Higher fracture toughness is desirable for better reliability in components and because it permits non-destructive inspection of a structural component for flaws that can result in catastrophic failure.
An alloy designated as AF1410 is known to provide good fracture toughness as represented by K.sub.IC .gtoreq.100 ksi .sqroot.in. The AF1410 alloy is described in U.S. Pat. No. 4,076,525 ('525) issued to Little et al. on Feb. 28, 1978. The AF1410 alloy has the following composition in weight percent, as set forth in the '525 patent:


______________________________________ wt. % ______________________________________ C 0.12-0.17 Mn .05-.20 S 0.005 max. Cr 1.8-3.2 Ni 9.5-10.5 Mo 0.9-1.35 Co 11.5-14.5 REM 0.01 max. ______________________________________ REM = rare earth metals to be desired with regard to tensile strength. It is capable of providing ultimate tensile strength up to 270 ksi, a level of strength not suitable for highly stressed structural components in which the very high strength to weight ratio provided by 300M is required. It would be very desirable to have an alloy which provides the good fracture toughness of the AF1410 alloy in addition to the high tensile strength provided by the 300M alloy.


SUMMARY OF THE INVENTION

Accordingly, it is a principal object of this invention to provide an age-hardenable, martensitic steel alloy and an article made therefrom which are characterized by a unique combination of high tensile strength and high fracture toughness.
More specifically, it is an object of this invention to provide such an alloy which is characterized by significantly higher tensile strength than provided by the AF1410 alloy while still maintaining high fracture toughness.
A further object of this invention is to provide an alloy which, in addition to high strength and high fracture toughness, is designed to provide high resistance to stress corrosion cracking in marine environments.
Another object of this invention is to provide a high strength alloy having a low ductile-to-brittle transition temperature.
The foregoing, as well as additional objects and advantages of the present invention, are achieved in an age-hardenable, martensitic steel alloy as summarized in Table I below, containing in weight percent, about:


TABLE I ______________________________________ Broad Intermediate Preferred ______________________________________ C 0.2-0.33 0.20-0.31 0.21-0.27 Mn 0.20 max. 0.15 max. 0.05 max. S 0.0040 max. 0.0025 max. 0.0020 max. Cr 2-4 2.25-3.5 2.5-3.3 Ni 10.5-15 10.75-13.5 11.0-12.0 Mo 0.75-1.75 0.75-1.5 1.0-1.3 Co 8-17 10-15 11-14 Ce small but small but 0.01 max. effective effective amount up amount up to 0.030 to 0.030 La small but small but 0.005 max. effective effective amount up amount up to 0.01 0.01 Fe Bal. Bal. Bal. ______________________________________ from the desired

REFERENCES:
patent: 3502462 (1970-03-01), Dabkowski et al.
patent: 3585011 (1971-06-01), Matas et al.
patent: 4076525 (1978-02-01), Little et al.
patent: 4152148 (1979-05-01), Machmeier
L. Luyckx et al., "Sulfide Shape Control in High Strength Low Alloy Steels", Metallurgical Transactions, vol. 1 (Dec. 1970), No. 3341.
W. M. Garrison, Jr., "Ultrahigh-Strength Steels for Aerospace Applications", JOM (May 1990).
L. Luyckx et al., "Current Trends in the Use of Rare Earths in Steelmaking", Electric Furnace Proceedings, (1973).
P. E. Waudby, "Rare Earth Additions to Steel", International Metals Reviews, (1978) No. 2.

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