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#084 Fundamentals of Steel Materials for Dies (3) Properties of Steel Materials (Fatigue Resistance)

Category : Die Materials
July 1, 2011

Repetitive load acts on the parts such as punches, etc., in press forming operations. Repeated load becomes the cause of fatigue failure. Fatigue failure can be linked to fatigue strength.
The causes related to fatigue strength are the following:

1. 1.Hardness after thermal refining (after obtaining a hardness of about 45HRC)
2. Eliminating sharp corners (rounding or chamfering)
3. Making the surface smooth.

(1) Relationship with tensile strength

The fatigue strength is almost directly proportional to the tensile strength of the material. There is a difference between hardened material and raw material. The fatigue strength is about 50% of the tensile strength in the case of the raw (unhardened) material, and is about 45% in the case of the hardened material. This fact is not dependent on the material itself.

(2) Relationship with hardness

Even regarding the hardness of the material, the fatigue strength is higher for a harder material. The fatigue strength becomes the highest for a material that has been thermally refined to a hardness of 45HRC.

(3) Surface condition

Even if the tensile strength and the hardness are suitable, the fatigue strength decreases if the surface condition of the parts such as punches, etc., is bad. It is better to finish the surface to as good a state as possible.

(4) Shape

In the case of a shape with depressions and projections, the corners of depressions should not be made square but should be rounded. The effect is larger as the radius of curvature R is made larger.

(5) Residual compressive stress

Fatigue failure is generated from the surface of a part. Therefore, it is possible to increase the fatigue strength by making the surface (surface layer) hard and, in addition, if a residual compressive stress is present. It is very effective to carry out shot peening treatment of the surface in which the material surface is hit with steel ball or glass beads at a high speed.

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