Improvement of Creep Rupture Strength of 9Cr-1Mo-V-Nb-N Steel by Thermo-Mechanical Control Process.

Abstract

A 9Cr-1Mo-V-Nb-N steel was subjected to the thermo-mechanical control process (TMCP), more specifically the direct quenching and tempering process, and the influences of heating temperature (Ts) and finish-rolling temperature (Tf) on the mechanical properties, including the creep rupture strength (CRS), were examined. The results were analyzed by observation of the substructure and precipitates, then the reasons for the improvement in CRS were discussed.Raising Ts and lowering Tf improve CRS for separate reasons. Raising Ts augments the coherency strain around VN through lattice expansion owing to the increased solution of Nb to VN. This increase in coherency strain is the reason for the improvement of CRS. On the other hand, lowering Tf disperses VN more finely by serving dislocations as nucleation sites for VN, resulting in improved CRS through decreased interprecipitate distance.When TMCP is applied, thin disc-like (V, Nb)N which nucleates on dislocations becomes a dominant type of VN precipitation, instead of NbN/VN complex precipitate which prevails when the steel is normalized and tempered. Furthermore, successive rolling from sufficiently high temperature is important to avoid coarse precipitation of VN and to exert the precipitation hardening during tempering to its full extent.