Fatigue Initiation Study of TMT Eutectoid Steel

Abstract

Eutectoid steel and 1060 steel samples were given a variety of thermomechanical treatments (TMT), described in Table I, which varied the pearlite interlamellar spacing, the cementite and inclusion orientation, the degree of cold-work in the ferrite matrix, and (for 1060 steel only) the proeutectoid grain boundary ferrite network. These samples were then evaluated as to resistance to fatigue crack initiation. The TMT designated, G, involves a subcritical anneal and results in a partially recrystallized condition which shows not only excellent resistance to fatigue crack initiation in the near threshold region but is second only to a fine pearlite microstructure, B, in fatigue crack propagation threshold value. It is believed that the excellent properties of the TMT, G, are related to the formation of subgrains in the interlamellar ferrite. On a scale normalized to tensile strength,(ΔK/√ρ)/σu, fine oriented pearlite in a soft ferrite matrix (rapid up-quench TMT such as E, F) shows the best resistance to fatigue crack initiation. A proeutectoid ferrite grain boundary network is poor at resisting fatigue crack initiation but good at resisting fatigue crack propagation. It should be emphasized that the combined high resistance of the subcritical TMT (G) toboth fatigue crack initiation and propagation, coupled with its much easier implementation relative to similar microstructures produced by difficult rapid up-quench TMT's (E, F) make it a very promising approach to improving overall fatigue resistance.