Influence of prior thermal ageing on creep and low cycle fatigue behaviour of forged thick section 9Cr-1Mo ferritic steel at 793 K

Detailed investigations have been carried out to understand the influence of prior thermal ageing on creep and low cycle fatigue (LCF) behaviour of thick section 9Cr-1Mo ferritic steel forging. Creep tests were conducted in quenched and tempered (Q+T), simulated post weld heat treatment (SPWHT) and thermally aged (TA) conditions at 793 K. Total-axial-strain controlled continuous cycling LCF tests have been performed on SPWHT and TA specimens at 793 K. No significant variation in creep rupture properties was observed in the three material conditions investigated. The failure mode remained transgranular ductile characterized by void coalescence under all test conditions. LCF resistance of thermally aged material was found to be higher compared to SPWHT material. Both SPWHT and TA materials exhibited progressive cyclic softening and similar cyclic stress-strain behaviour obeying power-law relationship. The crack initiation and propagation modes remained transgranular in both material conditions. However, thermally aged material exhibited extensive particle decohesion and higher secondary cracking. The improved LCF resistance of thermally aged material has been ascribed mainly to the reduction in stress intensity at the tip of propagating main crack caused by enhanced particle decohesion and secondary cracks.