Fatigue Behavior and Mechanism of FV520B-I in Ultrahigh Cycle Regime

Abstract

FV520B-I which is a kind of martensitic precipitated hardening stainless steels is usually applied to manufacture centrifugal compressor impellers. The centrifugal compressor impellers may fail from ultrahigh cycle fatigue induced by wake flow. Aim to assess ultrahigh fatigue life of impellers, fatigue tests up to 109 cycles were carried out by ultrasonic fatigue testing machine to research ultrahigh cycle fatigue (UHCF) behavior and mechanism of FV520B-I. The results show that the S-N curve of FV520B- I can be divided into two segments at 107 cycles apparently. The fatigue cracks almost all initiated from the surface of an impeller when the fatigue life less than 107 cycles and then from the subsurface inclusion of an impeller when the fatigue life more than 107 cycles. In other words, the fatigue behavior of FV520B-I is in accordance with UHCF except one specimen's crack occurred at surface surprisingly when fatigue cycles greater than 107. Through the fracture morphology to be observed and measured, the reasons of “fish eye” area and GBF area formation were discussed, and the relationship of fatigue strength, fatigue life, and characteristic area size were researched by relative models.