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The creep strength of ultra-fine grained tempered martensite ferritic steels with 9 to 12 wt.% chromium is significantly increased when small amounts of boron (in the 100 wt-ppm regime) are added. In the present note, experimental evidence from the literature is reviewed and previous explanations for the role of boron are summarized. The explanation for the effect of boron on creep strength must leave space for the simultaneous effect of boron on ductility. A scenario is suggested where boron segregates to micrograin boundaries which often have subgrain-boundary character. Boron can decrease the subgrain-boundary energy, decrease the velocity of subgrain-boundary migration, hinder knitting reactions between free dislocations and subgrain-boundaries and thus contribute to creep strength. This decrease of intensity of interface plasticity contributes to a decrease in ductility. Boron may moreover decrease the strength of the interface and thus promote brittleness. Further work is required to fully account for the role of boron on the mechanical properties of tempered martensite ferritic steels.

Boron segregation and creep in ultra-fine grained tempered martensite ferritic steels