Influence of thermal ageing on the creep behaviour of a P92 martensitic steel

It is well established that thermal ageing reduces the life of engineering components operating at elevated temperature. However, there is still not an accurate approach to estimate the change in the service life as a result of thermal ageing of the material. In this study, the effect of accelerated thermal ageing, for a thousand hours prior to creep testing at a temperature of 650 degrees C for a martensitic P92 steel has been compared to un-aged steels. The effect of thermal ageing on a primary-secondary stress dependent creep model has been explored, by conducting a set of uniaxial creep experiments on both un-aged and aged P92 steel. It was observed that thermal ageing enhanced the creep deformation for a narrow range of stresses, and systematically reduced the creep rupture life. These applied for conditions by up to a quarter of the original life. Results from creep crack growth tests for similarly aged material reduced incubation time and accelerated the crack growth rate. However, creep stress relaxation tests revealed no evidence of the influence of thermal ageing on rate of relaxation.