Substructural recovery in a cold worked Ti-modified austenitic stainless steel during high temperature low cycle fatigue

Abstract

Total axial strain controlled fatigue tests have been conducted in air at 823 and 923 K to ascertain the influence of cold work on LCF behaviour of a 15Cr-15Ni, Ti modified austenitic stainless steel, designated as Alloy D9. The LCF behaviour of the 20% cold worked alloy was compared with that of solution annealed alloy. A symmetrical triangular waveform at a constant strain rate of 3 × 10^-3 s^-1 was employed for all the tests performed over the strain amplitude in the range of ±0.25 to ±1.00%. The cyclic stress response varied as a complex function of microstructure, temperature and strain amplitude. The cyclic stress response of solution-annealed alloy was generally characterized by initial hardening to the maximum stress followed by a regime of nearly stable peak stresses. The degree of initial hardening was higher at 823 K due to dynamic strain ageing effects. The cold worked alloy displayed a gradual softening prior to the attainment of saturation stress response stage. The plastic strain fatigue resistance of the cold worked alloy was inferior compared to that in solution-annealed condition at both 823 and 923 K. At low strain amplitudes, the cold worked alloy exhibited better total strain fatigue resistance at 923 K. The observed variations in fatigue life and cyclic stress response behaviour have been explained on the basis of crack initiation, development of substructure, precipitation behaviour and evolving changes in plastic strain during cycling.