Spring with corrosion fatigue strength

Details Spring with corrosion fatigue strength Spring with corrosion fatigue strength

1998-11-16

2001-02-27

Yee, Deborah (Department: 1742)

Metal treatment

Stock

Ferrous

C148S580000, C148S909000

Reexamination Certificate

active

06193816

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of Invention
The present invention relates to springs with an improved corrosion-fatigue strength.
2. Description of Related Art
From the viewpoint of environmental protection and resource protection, it is strongly demanded to reduce the amount of toxic substances in exhaust gas and improve the fuel efficiency of automobiles. Since these are achieved by reducing the weight of an automobile to a considerable extent, efforts for reducing the weight of every automobile part are constantly underway. In the case of springs, including suspension springs, an increase in the employing stress (or the design stress) of a spring directly contributes to its weight reduction. But the increase in the employing stress usually causes fatigue and sagging of the spring. This problem is not limited to the springs used in automobiles, but also occurs to springs used in general machines.
Many kinds of spring material have been proposed so that that include various alloying elements to improve the fatigue resistance and sagging resistance. In the Publication of Unexamined Japanese Patent Application No. H3-2354, for example, a spring steel is disclosed including a lesser amount of Carbon, but including Nickel and Chromium and including a greater amount of Nitrogen than conventional ones. It is further proposed to include, in addition to those prescribed, one or more among the elements of Niobium, Vanadium and Molybdenum.
SUMMARY OF THE INVENTION
Sagging can be effectively reduced, in general, by increasing the hardness of the material. Under an ideal condition, increase in the hardness also improves the fatigue resistance to a certain extent. In the case of the automobile, though, the suspension springs are used in such places where they are constantly exposed to water and mud splashes. It is thus critical in actual use to consider the corrosion of the springs first, because corrosion forms minute pits on the surface of the springs and the pits initiate fatigue fracture of the springs.
The present invention is achieved to solve the problem. One of the objects of the present invention is to improve the fatigue corrosion resistance of the springs and provide springs having a high fatigue resistance in actual use, as well as having sagging resistance equal to or higher than conventional ones.
The spring of the present invention achieved to solve the above problem is characterized by:
a) using as a material a steel comprising C: 0.35-0.55%, Si: 1.60-3.00%, Mn: 0.20-1.50%, S: 0.010% or less, Ni: 0.40-3.00%, Cr: 0.10-1.50%, N: 0.010-0.025%, V: 0.05-0.50% and Fe balance;
b) heat treating to have a hardness of 50.5-55.0 HRC; and
c) shot-peening at a moderate temperature to render a residual stress of −600 MPa or more at a depth of 0.2 mm below surface.
A more preferable effect can be obtained by limiting the P (phosphur) content of the prescribed material to 0.010% or less.
The shot-peening “at a moderate temperature” (hereinafter it is referred to as a “warm shot peening”) means shot-peening a spring at a temperature higher than the room temperature. It is a matter of course that the temperature should be lower than the temperature at which the spring is heat-treated (or, more particularly, the tempering temperature) to prevent the hardness of the heat-treated spring from lowering. According to experiments by the present inventors, the most preferable corrosion-fatigue strength is obtained by setting the shot-peening temperature at 100-300° C. The temperature range is more preferably 200-250° C.
It is recommended to use shot particles having hardness of 450-600 Hv in the warm shot peening in order to obtain such a value of residual stress for the spring of such hardness. The hardness of the shot particle is further preferred to be 500-550 Hv.


REFERENCES:
patent: 5009843 (1991-04-01), Sugimoto et al.
patent: 5258082 (1993-11-01), Koyama et al.
patent: 03002354 (1991-01-01), None

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