Thermo-mechanical fatigue behavior and microstructure evolution of 4Cr5Mo3V hot work die steel

Thermo-mechanical fatigue (TMF) testing was used to study the fatigue behavior of 4Cr5Mo3V hot work die steel to reflect the in-service conditions. The synergistic effect of cyclic temperature and mechanical load was investigated using a strain-controlled method. Our results confirmed that the cyclic softening degree of 4Cr5Mo3V steel increased as the mechanical strain amplitude get bigger, leading to a reduction of the fatigue life. The microstructure evolution was studied before and after the TMF testing, and it was found the maximum texture strength in as-received steel decreased from 7.22 to 2.34 when the strain amplitudes was set as +/- 1.1 %. Experimental evidence showed that martensite decomposition, oxidation, dislocation density reduction and carbide segregation all contributed to the final TMF failure. Severe deformation existed in the vicinity of the newly precipitated M2C carbide during the TMF process, acting as fatigue crack initiation sources due to stress concentration. Four different models were interrogated including Basquin-Coffin-Manson model, Smith-WatsonTopper (SWT) model, Ostergren model and 3SE energy model; and it was found that SWT model yielded the best fatigue life description. Nevertheless, all the four models showed reasonable fitting capability within the 2x scatter band, which proved the reliability of the experiments established in this study.