Elastic properties of heat resistant steels after long-term creep exposure

In order to clarify the effect of microstructural change during ageing and creep on elastic modulus, the elastic moduli of aged, crept and serviced (service-exposed) materials were investigated. In ferritic steels, the elastic modulus obtained by ultrasonic pulse does not change after long-term ageing and creep although the formation of creep voids slightly decreases the elastic modulus. In SUS304 and SUS316, the elastic modulus evaluated by ultrasonic pulse increases during ageing due to precipitation of M₂₃C₆ carbides, Laves phase and σ phase though the elastic modulus decreases during creep because of the formation of creep voids on grain boundaries. The increased elastic modulus of aged material with a large amount of σ phase was regarded as that of composite consists of matrix and σ phase. For STBA24, the elastic modulus obtained by a high temperature tensile test decreases after long-term service. The tensile test at a higher strain rate was proposed to avoid underestimation of elastic modulus of serviced materials at high temperatures.