Effect of W on tempering behaviour of a 3 %Co modified P92 steel
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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.
KeywordsAustenite Cementite Lave Phase Prior Austenite Precipitation Sequence
The authors acknowledge with gratitude the financial support received through the Russian Science Foundation, under Grant No. 14-29-00173 (N. D. and R. K., Belgorod State University) and A. F. greatly acknowledges the support by DAAD (Deutscher Akademischer Austausch Dienst) under Project No. A/12/86596 in the part of microstructural investigations by TEM. The authors are grateful to the staff of the Joint Research Center, Belgorod State University, for their assistance with instrumental analysis.
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The authors declare that they have no conflict of interest.
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