Fractographic evaluation of high cycle fatigue strength of a high V alloyed powder metallurgy tool steel

The high cycle fatigue strength and the fracture surface morphology of a high V alloyed powder metallurgy tool steel were investigated and connected with microstructure. The inclusion with size above 30 μm was demonstrated to be responsible for the reduction in fatigue strength. The shape and size of inclusion and the stress amplitude affected the position of crack origin in fish eye. Based on fractographic evaluation and statistical analysis, the most harmful inclusions with the largest size proportion for steel A are ∼30 μm, and that of B₁, B₂ and B₃ is roughly 50, 60 and 60 μm respectively. A distinctive granular-like zone with specific morphology was found to be confined in a very small area, suggesting that the fatigue crack propagation was mainly controlled by Paris Law regime. A linear fracture mechanics approach was implemented to evaluate the effect of grain size, yield stress and inclusion size on fatigue strength.