Structural changes of tempered martensitic 9%Cr–2%W–3%Co steel during creep at 650 °C

The structural changes in tempered martensitic 9Cr–2W–3Co (wt%) steel during creep tests at 650 °C were studied. The starting material was a solution treated at 1050 °C and subsequently tempered at 750 °C. The tempered martensite substructure consisted of a mixture of crystallites with a lath-type morphology and equiaxed subgrains. The tempering resulted in the precipitation of numerous second phase particles. MX-type precipitates were homogeneously distributed within the ferritic matrix, whereas M23C6-type carbides were located on boundaries. Under creep conditions, Laves phase particles also precipitated at (sub)grain/lath boundaries. Laths and subgrains tended to coarsen during creep. It was shown that M23C6-type carbides played a major role in the stabilization of the tempered martensite lath structure by exerting a large pinning pressure. The transition to tertiary creep correlated with a coarsening of the carbides and a detachment of lath and subgrain boundaries from the chains of these carbides that led to a decreased pinning force from M23C6 carbides.