| Microstructural evolution of martensitic Fe-16Cr-5Ni-0.06C-(0.1Nb) stainless steels during tempering were investigated utilizing scanning electron microscopy and transmission electron microscopy. The microstructure consists of tempered martensite laths together with reverted austenite, delta ferrite and some carbides. On tempering above 500℃, the lath-like reverted austenite formed preferentially at martensite lath boundaries and grew along lath boundaries, showing mainly K-S orientation relationship with matrix. Another type of reverted austenite, namely granular austenite, containing very low density of defects and twin was also found in higher tempering temperatures and the orientation relationship with matrix was found to be N-W relation. In both specimens wish and without Nb, M_(23)C_6 carbide precipitates along the martensite / reverted austenite boundaries or in the lath interior. On the other hand, in Nb containing specimen tempered above 550℃, very fine, spherical NbC carbides are also observed in the martensite lath interior, and this type of carbide presumably contribute to the small decrease of strength in tempering temperature between 500 and 620℃, as shown in previous paper. |
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