High temperature low cycle fatigue properties of a thick-section 9wt.%Cr-1wt.%Mo ferritic steel forging

Abstract

Total-axial-strain-controlled fatigue tests have been conducted in air to ascertain the influence of temperature (723, 773 and 793 K) on the low cycle fatigue behaviour of thick-section (300 mm) 9Cr-1Mo tube plate forging (where the chromium and molybdenum contents are in approximate weight per cent). The alloy was tested in a simulated post-weld heat treatment codition. A symmetrical triangular waveform and a constant strain rate of 1 × 10^-3 s^-1 were employed for all the tests performed over strain amplitudes in the range from ±0.25% to ±1.00%. The crack initiation and propagation modes were studied. Deformation and damage mechanisms which influence the stress response and endurance have been identified. A reduction in fatigue life was observed at all the strain amplitudes with increasing temperatures. The temperature effect on life was more pronounced at low strain amplitudes. The reduction in fatigue life at elevated temperatures was attributed to the combined effects of increased inelastic strain and fatigue-oxidation interactions. Thick-section forged 9Cr-1Mo steel exhibited inferior fatigue resistance compared with the hot-rolled material either in normalized-plus-tempered or in simulated thick-section heat treatment conditions. The poor fatigue resistance of tube plate forging was ascribed to its coarse grain size. The cyclic stress response of tube plate forging varied as a complex function of temperature and strain amplitude. The cyclic stress-strain behaviour could be described by a power law relationship at 773 and 793 K. At 723 K, the alloy exhibited a two-slope cyclic stress-strain curve. The crack initiation and propagation modes remained transgranular at all the conditions investigated.