Influence of the microstructure on the premature fatigue failure of decarburized SAE 9254 spring steel

Fatigue life is a key issue on the performance of alloys employed in mechanical components manufacturing such as Si-Mn spring steels used in the automotive industry. In these alloys, the fatigue life is strongly affected by surface defects and microstructure. In this paper, the effect of total decarburization depth and nanohardness profile on the premature fatigue failure of the SAE 9254 spring steel is reported. Decarburization layer was developed during heat treatment (austenitization at 850 °C and oil quenching). Heat treated specimens with different decarburized layer thickness and controlled surface roughness were investigated to evaluate the interplay between the decarburized layer depth and fatigue behavior. Stress-controlled fatigue tests up to 400,000 cycles and fractographic analysis allowed determining that fully decarburized layers (ferrite layers) with thickness up to 25 μm did not fracture. Above this critical thickness, premature fatigue fracture took place. Microstructural characterization and nanohardness profile measurements indicated that premature crack nucleation during the fatigue tests is associated with an abrupt hardness variation at the interface between total and partial decarburization regions.