Alloys or metallic compositions – Ferrous – Nine percent or more chromium containing
Patent
1996-08-08
1998-05-19
Yee, Deborah
Alloys or metallic compositions
Ferrous
Nine percent or more chromium containing
420 54, C22C 3850
Patent
active
057531781
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to an austenitic stainless steel for use when hot.
2. Discussion of the Background
Many items of equipment such as, for example, aircraft engines, motor vehicle engines, steam turbines or steam generators include components which must withstand high temperatures. These components are, for example but not exclusively, bolts or couplings. They must be capable of working at temperatures that can reach 750.degree. C.
To manufacture these components, either alloys of the 286 type or martensitic stainless steels are employed.
Alloy 286 is an austenitic superalloy containing approximately 26% of nickel, 15% of chromium, 1.25% of molybdenum and 2% of titanium. The titanium is intended to form hardening precipitates of .gamma.' phase. These alloys can be employed up to 700.degree. C. but not beyond this because, above this temperature, the .gamma.' phase is unstable and tends to be transformed into the .eta. phase, which is less hardening. Moreover, since the nickel content is high, these alloys are expensive.
Martensitic stainless steels contain approximately 12% of chromium and little or no nickel, with the result that their price is substantially lower than that of the alloys of the 286 type but, on the other hand, they can be employed only up to 600.degree. C., and this is insufficient for some applications.
The aim of the present invention is to overcome these disadvantages by proposing a stainless steel for use when hot, which is more economical than the alloys of the 286 type and which has mechanical characteristics when hot that are comparable or even superior to those of these alloys.
SUMMARY OF THE INVENTION
To this end, the subject-matter of the invention is an austenitic stainless steel for use when hot, the chemical composition of which, by weight, includes: chemical composition additionally satisfying the relations:
The chemical composition, by weight, is preferably such that:
It is also preferable that the boron content should be between 0.005% and 0.020%.
The invention also relates to the use of a steel according to the invention for the manufacture of bolts for use when hot, which are intended especially to be fitted to motor vehicle engines.
The invention will now be described more precisely but without any limitation being implied.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The steel according to the invention is a stainless steel consisting of a stable .gamma. austenitic matrix hardened by .gamma.' phase precipitates Ni.sub.3 Ti or, better, Ni.sub.3 (Ti,Al) of cubic structure, containing enough aluminium to limit the transformation of the .gamma.' phase into .eta. phase of the same composition but of hexagonal structure, and not containing too much aluminium in order not to form the Ni.sub.2 AlTi phase.
In order to be capable of forming sufficient hardening precipitates, the steel must contain more than 1% of titanium, but the content of this element must remain lower than 3.5% and preferably lower than 3%, because beyond this it impairs the plastic deformability when hot, and this makes forming operations by rolling or by forging difficult. Moreover, when the titanium content is too high the steel must be remelted under vacuum in order to limit segregations, and this operation is very costly.
The aluminium content must not exceed 1.5% and preferably 1.2%, on the one hand in order to limit the segregations and the difficulties in forming by plastic deformation when hot and, on the other hand, in order to avoid the formation of an Ni.sub.2 AlTi phase. To ensure the stability of the .gamma.' phase, the aluminium content must preferably be between 0.45% and 1.2%.
For the hardening effect of the precipitates to be optimal, it is preferable that the titanium and aluminium contents should be such that:
The nickel content must be between 16% and 25% and must preferably be lower than 23%, and the chromium content must be between 16% and 18.5% and must preferably be lower than 18% in order that, after formation of t
REFERENCES:
patent: 2744821 (1956-05-01), Osman
patent: 3065068 (1962-11-01), Dyrkacz et al.
Davidson James Henry
Mihoub Williams
Imphy S.A.
Yee Deborah
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