On the origin of the superior long-term creep resistance of a 10% Cr steel

A low-nitrogen 10% Cr martensitic steel containing 3% Co and 0.008% B was shown to exhibit an extremely long creep rupture time of ∼4·104 h under an applied stress of 120 MPa at 650 °C. The creep behavior and evolution of lath martensite structure and precipitates during creep at these conditions were studied. The main feature of the microstructure under long-term creep is retention of the lath structure until rupture. The following microstructural factors affecting the superior creep resistance were analyzed: (1) alloying by (W+Mo) elements; (2) particles of M23C6 and Laves phases; (3) homogeneously distributed M(C,N) carbonitrides. It was revealed that nanoscale M23C6 carbides and M(C,N) carbonitrides compensated the negative effects of W depletion from the solid solution and extensive coarsening of the Laves phase particles. M23C6 carbides demonstrate a high coarsening resistance under creep conditions and exert a high Zener drag pressure before rupture because of the coherency of their interfaces. The strain-induced transformation of a portion of the precipitated V-rich M(C,N) carbonitrides to the Z-phase does not affect the creep strength because the Z-phase particles are nanoscale and negligible in quantity.