Microstructural changes in cast martensitic steel after creep at 620°C

Microstructural changes in the cast steel GX12CrMoWVNbN10-1-1 (Fe-0.11 C-0.31 Si-0.89 Mn-9.57 Cr-0.66 Ni-1.01 Mo-1.00 W-0.21 V-0.06 Nb-0.05 Cu-0.05 N in wt %) have been investigated after tests for long-term strength at a temperature of 620 degrees C in the range of stresses of 120-160 MPa. Upon short-term creep (up to 5000 h), the tempered troostite structure and distribution of particles of proeutectoid constituents change insignificantly, except for the precipitation of particles of the Laves phase similar to 100 nm in size along boundaries of laths, blocks, packets, and initial austenite grains. Upon long-term creep (to 10000 h), the tempered troostite partially transforms into the subgrain structure, which is accompanied by a decrease in the dislocation density from 6.4 x 10(14) to 3.1 x 10(13) m(-2) and connected with growth of sizes of M23C6 carbides of 105-150 nm and particles of the Laves phase to 380 nm, due to the dissolution of these particles located along path boundaries. Upon long-term creep, the average size of V(C,N) particles increases from 45 to 64 nm (while Nb(C,N) particles increase from 48 to 87 nm), and the Nb content in V-enriched carbonitrides and the V content in Nb-enriched M(C,N) particles substantially decrease. No formation of the Z phase has been revealed. The combination of M(C,N) nanoparticles with the presence of W in the solid solution has been found to be responsible for the enhanced high-temperature strength of the steel.