SYNERGISTIC INTERACTIONS DURING HIGH-TEMPERATURE FATIGUE OF TYPE-304 STAINLESS STEEL-GRAIN SIZE DEPENDENCE

The grain size dependence of time dependent low cycle fatigue (LCF) behaviour of type 304 stainless steel has been studied at 823 and 923 K, employing strain ranges from 0.05 to 1.2% and cyclic frequencies in the range 0.001 to 1.0 Hz. Grain sizes studied were 75, 310 and 700-mu-m. Exploratory strain controlled LCF tests conducted in the temperature range 623-973 K established the peak dynamic strain ageing (DSA) temperature as 823 K. All the grain sizes showed loss of fatigue life with decreasing frequency at 823 K due to DSA effects. DSA caused brittle intergranular decohesion in all the grain sizes with greatest tendency in medium grain size. A mechanism that creates deformation inhomogeneity and brittle intergranular cracking during DSA in LCF was identified. DSA in LCF manifested as a peak in the saturation stress, a minimum in plastic strain recorded per cycle, increased cyclic hardening rate and was at times associated with serrated stress-strain hysteresis loops. At 923 K, fine grained material exhibited better fatigue resistance than medium and coarse grained material at identical frequencies and strain ranges investigated. Four damage mechanisms namely DSA, inelastic deformation, creep and oxidation have been found responsible for the life degradation, their relative effects depending on the frequency and grain size. In fine grained material, DSA and inelastic deformation played a dominant role in reducing life as the frequency was lowered, whereas inelastic deformation and oxidation damage exerted a major influence in the medium and coarse grained materials. At 923 K too, crack initiation and propagation modes were influenced both by grain size and frequency. Oxidation caused cavity nucleation on the grain boundaries at frequencies < 0.1 Hz. Oxygen reactivity with grain boundary carbides is suggested to be the mechanism responsible for cavity nucleation. The role of frequency on Coffin-Manson relationship at 923 K has also been emphasized.