Precipitation of Carbides in 12% Cr Steel during Tempering

Precipitation characteristics of 12% Cr steel during isothermal tempering were studied mainly by transmission electron microscope. As the result, the shape, nucleation site and habit plane of each carbide (Fe_3C, (Cr, Fe)_7C_3, (Cr, Fe)_C_6), and the orientation relationships between carbides and ferrite matrix were clarified. It was confirmed that the transformation of Fe_3C into (Cr, Fe)_7C_3 occurred by both in situ and separate nucleation, and that of (Cr, Fe)_7C_3 into (Cr, Fe)_C_6 by separate nucleation on grain boundaries. Furthermore, it was concluded that the secondary hardening might be induced by the distribution of fine granular particles of (Cr, Fe)_7C_3 nucleated separately on the dislocations in the region of high dislocation density