Grain Size Dependence of Dynamic Strain Ageing During Low Cycle Fatigue in Type 304 Stainless Steel

Abstract

The grain size (GS) dependence of dynamic strain ageing (DSA) and its influence on low cycle fatigue (LCF) life, deformation and fracture behaviour of type 304 stainless steel has been investigated at 823K, employing the cyclic deformation rates from 1.6 × 10–4 to 1.6 × 10–2 s–1. A total strain range of 0.8% is used for all the tests. Grain sizes studied include 75, 310 and 700 μm. All the grain sizes show loss of fatigue life with reduction in strain rate (ε) due to DSA effects. Medium GS is more affected by DSA. DSA in LCF is manifested as a peak in the saturation stress, a minimum in plastic strain, increased work hardening rate and is at times associated with serrated stress-strain hysteresis loops. The fracture mechanisms and fatigue life are influenced to a great extent by prevailing deformation mechanisms. The inhomogenity of deformation results from DSA effects during LCF. Intense slip bands are observed in the interior of medium and coarse grained material. The impingement of these slip bands on grain boundaries caused brittle intergranular decohesion. TEM and SEM observations are presented to explain the variations in fatigue life as a function of GS and ε.