Metal treatment – Stock – Ferrous
Patent
1993-10-26
1995-07-04
Yee, Deborah
Metal treatment
Stock
Ferrous
148908, C22C 3844
Patent
active
054296889
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Technical Field
The current invention relates to an improved stainless steel obtained by cold deformation, such as wire drawing and rolling. As a result, the steel provides a structure, made up of martensite and austenite, with high resistance to corrosion. Such properties suit its main application in the field of spring manufacture.
2. Background of the Art
Springs are submitted to a load cycle, and therefore require good fatigue resistance. A number of factors affect this resistance, but it is the superficial quality, without any doubt, that most regulates the spring's performance when submitted to fatigue conditions. The presence of superficial irregularities favors the nucleation of fatigue cracks. Nevertheless, resistance to fatigue is not guaranteed just by avoiding these defects, because superficial defects can be formed during spring use. One of the most prejudicial superficial defects created during spring use is corrosion. So, when the design conditions demand and the costs permit, stainless steel should be used in the manufacture of springs.
Stainless steel for springs was developed in order to increase the mechanical strength of springs, which was very low in the solubilized condition. Compositions that allow for hardening mechanisms and strength levels that exceed 2000 MPa, in some alloys and gauge, were developed. In addition, stainless steel provides the capacity to be cold worked, which eases the manufacturing process such as rolling and drawing.
Stainless steels that form martensite during cold deformation are called metastable. They provide high strength after cold deformation, as occurs during wires drawing, so they are the main stainless steels Used in spring manufacture. Strength is the result of a microstructure consisting of hardened martensite and austenite, having carbon as the main hardening element.
However, metastable austenitic stainless steel, or the current technical state, most used in spring manufacture, UNS S30200 steel, with up to 0.15% of C, 17.0 to 19.0% Cr, 8.0 to 10.0% Ni, up to 0.75% Si, up to 2.0% Mn, up to 0.045% P and up to 0.030% S, does not provide enough resistance to intergranular and pitting corrosion. Besides, due to the high carbon content, normally over 0.08%, these steels most be heat treated in a cycle known as solubilization, at higher temperatures and longer periods than other stainless steels. So, working with UNS S30200 steel involves more care and higher cost.
Also, the standard stainless steel for springs provides problems in durability when used in applications that require high resistance to corrosion. In the spring manufacturing process, a tempering heat treatment is normally carried out in order to increase the spring strength and durability. Depending on the temperature used, chromium carbide precipitation can occur, which reduces the resistance to corrosion.
The current invention solves these problems.
DETAILED DESCRIPTION OF THE INVENTION
The object of this invention is to produce a cold deformed stainless steel composition for spring manufacture, with a microstructure composed of a mixture of martensite and austenite, which yields better resistance to intergranular and pitting corrosion and does not require special care for solution heat treatment.
Specifically, the current invention provides a metastable stainless steel for spring manufacture that, after cold deformation, has a microstructure composed of austenite and martensite. This steel has 17.0 to 19.0% Cr, 8.0 to 10.0% Ni, 0.06 to 0.16% N, up to 0.03% up to 1.0% Si, 1.0 to 2.0% Mn, up to 0.80% Mo, up to 0.075% P and up to 0.030% S; the rest is iron and inevitable impurity.
The stainless steel according to the current invention provides high strength after cold deformation and high resistance to intergranular and pitting corrosion. Besides, the solution heat treatment of this steel does not involve special care, and can be eventually eliminated.
The chemical composition range of the new steel must have hardening properties similar to UNS S30200, wher
REFERENCES:
patent: 5314549 (1994-05-01), Misao et al.
Barbosa Celso A.
Tessler Marcelo B.
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