Fatigue Properties of a New Martensitic Stainless Steel for Hot Stamped Chassis Parts

A new Ni-free martensitic stainless steel (MSS) was developed for hot stamped automotive parts, especially in order to design lightweight chassis part. After hot stamping simulation, the material exhibited a 1.2 GPa ultimate tensile strength with a minimum of 10% total elongation, in the as-quenched condition (Q) without any tempering treatment (Q+T). Moreover the material's chemical composition was optimized to improve the ductility at low temperature and during high strain rate mechanical testing. As a result, no brittle fracture in impact testing at −40°C was observed, and a good behavior in crash was recorded. To further assess the material's performances, high cycle fatigue properties of the grade have been characterized including the effects of machining and surface treatments. Results show that the fatigue limits at 2 million cycles for a stress ratio of −1, for both bare and shot peened surface are quite high and in the range of 580 MPa to 640 MPa. Then, the fatigue resistance on MaX martensitic stainless steel is significantly higher than that of boron steels. This result is confirmed at a 0.1 stress ratio, for which the endurance limit is found at approximately 860 MPa. These good fatigue properties after heat treatment arise from the fact that this new grade does not suffer from any surface decarburization. Therefore the surface keeps a martensitic structure and exhibits the same mechanical properties than the bulk of the material. Finally these good fatigue properties can be exploited for mass reduction of chassis parts. For instance, from a cold formed baseline made of 600MPa carbon steel, a 50% mass reduction can be expected with a hot stamped suspension arm made of MaX and included a new clamshell design.