Low cycle fatigue and thermo-mechanical fatigue behavior of modified 9Cr–1Mo ferritic steel at elevated temperatures

► Softening rate independent of external environment. ► Common microstructural features under both CH and OP-TMF and under TH and IP-TMF. ► Mean stresses developed during TMF cycling, did not deteriorate TMF life. ► Dwell sensitivity controlled by environment. ► In air, oxides reduce life and in vacuum creep responsible for life reduction. In-phase (IP) and out-of phase (OP) thermo-mechanical fatigue (TMF) experiments were performed in vacuum between 623 K and 823 K on modified 9Cr–1Mo ferritic steel. Few isothermal low cycle fatigue (IF) experiments including one at the peak temperature of TMF cycle (823 K) were also performed in vacuum. Isothermal LCF tests resulted in the lowest fatigue life, followed by IP-TMF and OP-TMF respectively. In modified 9Cr–1Mo steel, oxidation played a major role in limiting fatigue life at elevated temperatures. However in the absence of oxides, other damaging effects such as creep influenced the fatigue life. In this paper, the deleterious effect of oxidation on fatigue life has been evaluated by conducting tension hold and compression hold experiments in air at 873 K. Isothermal and TMF experiment conducted in vacuum eliminated the deleterious effects of oxidation and hence creep effects alone were evident. The lower TMF life under IP condition was attributed to the microstructural rearrangements and cavity formation around the coarsened carbides that resulted in a mixed mode of failure.