CYCLIC SOFTENING IN HOT-WORKING DIE STEELS DURING LOW-CYCLE FATIGUE

The characteristics and microstructural changes of cyclic softening in hot-working die steels5CrNiMo and 5Cr2NiMoVSi were studied under strain controlled low-cycle fatigue.The re-sults show that the cyclic softening is featured in both steels hardened in different conditionsunder the strain controlled amplitude range of Δε_t/2=0.6-1.8×10~(-2).The softening effectmainly occurs in some initial cycles and the stress amplitude varies slightly in the sequentialcycles,i.e.the softening effect is minified.No obvious stress saturation phenomenon was ob-served during the whole cyclic deformation.The TEM analysis shows that the cyclic softeningis related to heterogenity of plastic deformation.The softening of the tested steels is caused bythe formation of the dislocation cell structure with low density and low internal stress,and bythe fragmentation and redissolution of fine carbides into matrix.