Effect of W on tempering behaviour of a 3 %Co modified P92 steel

The tempering behaviour of two 9Cr–3Co–0.5Mo–0.2 V–0.05Nb–0.005B steels containing 2 and 3 wt% W was examined. The decomposition of retained austenite induces the formation of semi-continuous films of W-rich M23C6 carbides along boundaries at 525 °C. The increase in W content from 2 to 3 wt% modifies the normal carbide precipitation sequence in the vicinity of these boundaries. The formation of W segregation at prior austenite grain/interlath boundaries leads to formation of the thermodynamically metastable W-rich M6C carbides. Finally, these M6C carbides transform to a stable Laves phase. Two-phase separation of MX carbonitrides into Nb-rich and V-rich dispersoids appears in both steels after tempering at 750 °C. This separation is thermodynamically metastable and attributed to the precipitation of Nb-rich MX carbonitrides with a round shape on dislocations at T ≤ 450 °C and V-rich MX carbonitrides within ferritic matrix at 750 °C. Moreover, in the steel with 2 %W, the additional separation of V-rich MX carbonitrides into W-rich particles located in the vicinity of the prior austenite grain/lath boundaries and W-free ones located within the interiors of laths was observed.