Interaction of Creep Damage and Low Cycle Fatigue Damage in a 1Cr0.5Mo Steel

Abstract

Non-destructive replica studies of creep cavitation in power plant components of low alloy steels is a frequently used tool in order to estimate the reminant life. In addition to creep such components are exposed to thermal stresses and strains during start-ups and shut-downs, which can lead to fatigue damage. In order to study the role of creep-fatigue interactions, the influence of creep damage on low cycle fatigue (LCF) in a 1Cr0.5Mo steel was investigated. LCF-specimens were creep tested to 5% elongation at 560 and 600°C resulting in carbide precipitation, bainite coarsening and a significant amount of creep cavitation. The damage development during LCF was studied using replicas for test series of pre-crept and virgin material. In addition, the influence of a hold time was investigated. The development of creep cavities and micro-cracks, as well as the amount of intergranular crack propagation were evaluated. Creep exposure resulted in a reduction of the number of cycles to failure. The relationship between this reduction and the amount of creep damage as well as the consumed creep life fraction was analysed. The influence of softening and increased inelastic strain range due to creep exposure was also studied.
Continuous cycling LCF on creep exposed material resulted in a small increase of the cavity density. Compared to virgin material the number of cycles to failure N_f was smaller at low strain ranges. LCF with 5 min hold time resulted in a reduction of N_f by approximately a factor of 1.8. Creep exposure before hold time LCF resulted in an enhanced development of creep damage during LCF and smaller amounts of intergranular crack propagation whereas no significant further reduction of N_f could be observed.