Microstructural aspects of superior creep resistance of a 10%Cr martensitic steel

The microstructural evolution and the dispersion of secondary phases were studied in a low-nitrogen 10%Cr martensitic steel with 3% Co and 0.008% B additives at 650 °C under an applied stress of 140 MPa. It was demonstrated that the superior creep strength of this steel can be attributed to the high resistance of M23C6-type carbides and Nb-rich MX carbonitrides against coarsening, resulting in a stable of the tempered martensite lath structure (TMLS) under short-term creep conditions. The TMLS remains slightly changed under creep: lath coarsening occurs with a two-fold decrease in the lattice dislocation density. M23C6-type carbides were found to give the main contribution in hindering the transformation of interlath boundaries to subgrain boundaries, impeding the migration of low-angle boundaries by exerting a large pinning pressure. A high Zener drag pressure is maintained up to rupture. The precipitation of a Laves phase under creep conditions results in a minor contribution to the overall pinning pressure. V-rich MX carbonitrides tend to dissolve with increasing time. No formation of a Z-phase was detected. M23C6 carbides retain their orientation relationship with ferritic matrix up to rupture. No significant strain-induced coarsening of M23C6 carbides, Laves phase, or MX carbonitrides was observed.