On the contribution of carbides and micrograin boundaries to the creep strength of tempered martensite ferritic steels

The objective of the present study is to find out how much micrograin boundary carbides contribute to the creep strength of tempered martensite ferritic steels. For this purpose we compare the creep behaviour and the microstructural stability of a tempered martensite ferritic steel (German grade: X20) with that of a strongly deformed binary Fe10Cr alloy. The binary Fe10Cr alloy was subjected to equal channel angular pressing (ECAP). The ECAP parameters are adjusted such that the Fe10Cr alloy develops an ultrafine grain microstructure which is very similar to the microstructure of X20 with one important difference: there are no carbides on micrograin boundaries. We perform creep experiments for both materials and compare the creep curves as well as the evolution of microstructures during creep. The results show that boundary particles keep ultrafine grain structures from recrystallising. They moreover slow down micrograin boundary migration and impede reactions between dislocations and subgrain boundaries. It is concluded that micrograin boundary carbides reduce creep rates by several orders of magnitude.