Fatigue Crack Initiation and Propagation at High Temperature in a Softening Martensitic Steel

Martensitic steels of the 9Cr1Mo family have been selected for advanced power generation systems, in particular because of their high thermal conductivity and low thermal expansion coefficient [1]. Typical in-service conditions with repeated start- and stop-operations lead to loadings of creep-fatigue type at high temperature (between 673 and 873 K), with very long hold times (typically one month). Submitted to cyclic loading these martensitic steels are known to soften both under pure fatigue and under creep-fatigue loading [2,3]. Pure fatigue and creep-fatigue mechanical tests were performed for different cyclic and creep applied strains. Fig. 1 shows that the number of cycles to failure decreases with increasing applied creep strain but the time to failure strongly increases. The interaction between fatigue and creep damage is illustrated by the map drawn in Fig. 2.