Effect of Microstructure on Fatigue Properties in Low and Ultra-Low Carbon Steels

Abstract

Ultra-low carbon steel containing phosphorus and copper (P-Cu steel) has both a higher fatigue limit and better crack propagation resistance than conventional low carbon steels with the same tensile strength. In this paper, the mechanism for improving the fatigue properties of P-Cu steel is discussed on the basis of microscopic observations by electron microscope and measurements of crack closure behaviour for small and long fatigue cracks. The excellent fatigue limit and small-crack propagation resistance in P-Cu steel can be attributed to solution hardening caused by phosphorus and precipitation hardening caused by ε-Cu. On the other hand, the superior resistance to long-fatigue crack propagation was caused by grain coasening which occurs with reduction of carbon content, leading eventually to roughness-induced crack closure.