Masao Kinefuchi

EFFECT OF MICROSTRUCTURE ON FATIGUE PROPERTIES IN LOW AND ULTRA-LOW CARBON STEELS

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 epsilon-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.

Microstructural Design for Steels by Fatigue-Crack-Growth and Arrest Simulation.

The fatigue limits of single-, dual-and tri-phase steels were estimated using the crack-growth and arrest simulation, which was based on a model for continuous distribution of dislocations ahead of a small fatigue crack tip. The effects of microstructural parameters such as grain size, hardness and volume fraction of the three phases on the fatigue limit were systematically analyzed using this simulation. The simplified estimation equation for fatigue limit was derived from the simulation results as follows : δw=HV1·(k1+k2/√(D1))+Σj=2, 3(HVj-HV1)·Vj·(k3j+k4j/√(Dj)), where HVj is the hardness, Dj is the grain size, and Vj is the volume fraction of the j-th phase. Microstructural design for high fatigue strength steels is discussed with reference to this equation. Also, a more accurate simulation method is outlined including the effects of cyclic hardening and softening behavior of materials.