Effect of Alloying Elements on the High-Temperature Tempering of Fe-0.3N Martensite

Abstract The precipitation behaviors of alloy nitrides during the high-temperature tempering of Fe-0.3mass%N-1mass%M (M: Cr, Mo, Mn, or Si) martensite obtained by gaseous nitriding and quenching was investigated. Resistance to temper softening was observed with the addition of Mn, Cr, and Mo and secondary hardening occurred in the Cr and Mo alloys at tempering temperature above 673K although Si addition did not result in clear change in temper softening in comparison to the Fe-N binary case. X-ray diffraction analysis and conventional TEM observation shows the precipitation of iron nitride (γ'-Fe4N) and alloy nitride during tempering at 773K. Cs-corrected STEM-HAADF observation revealed that metastable mono-layered nitrogen-alloying element clusters are formed along {001}α' plane and they are eventually thickened into B1-type MN precipitate in the Cr and Mo alloys. In the Mn alloy, B1-type Mn nitride was detected which presumably changes to η-Mn3N2 by thickening. 3DAP analysis confirmed the ratio of nitrogen and alloying element of the nitride corresponds to the structure deduced by STEM-HAADF. Clustering analysis of 'matrix' indicated that there are still finer clusters which could not be clearly visualized in STEM-HAADF and 3DAP. Hardness after tempering was examined by strengthening by precipitation and dislocations, suggesting that undetected Cr-N clusters should contribute to hardening in the Cr alloy.