A novel oxide-dispersed strengthened T250 maraging steel (ODS-T250 steel) was prepared by mechanical ball milling followed by hot isostatic pressing. Martensitic transformation behavior, ultrafine-grained martensite microstructure and tensile properties of the ODS-T250 steel were compared to its oxide-free counterpart (T250 steel). The results indicated that ODS-T250 steel exhibited excellent microstructural thermal stability, maintaining an effective grain size of 0.348 μm even after quenching from 1200 °C, whereas T250 steel exhibited a much larger grain size of 5.473 μm. After quenching, the ODS-T250 steel showed significantly distinct variant selectivity, and its martensite morphology was unprecedented equiaxed structure rather than typical lath-type. Statistical analysis revealed that there was only a single variant formed when the prior austenite grain size (PAGS) was below 0.43 μm. Moreover, the martensite start (Ms) temperature of ODS-T250 steel was lower than the T250 steel due to its ultrafine PAGS and the dispersed oxide particles. The refinement of PAGS and the presence of oxide particles led to a simultaneous improvement in both strength and ductility of ODS-T250 steel.

The thermal stability of the microstructure, in particular that of the precipitates, has been known to be a critical factor for the creep strength of 9–12Cr martensitic heat-resistant steels. In the present study, four martensitic heat-resistant steels were designed to have different precipitate types and morphologies. Steel NS1 (nitride strengthened, NS) has a low density of fine nitrides and Steel NS2 is strengthened by a high density of fine nitrides. Steel CNS (carbide and nitride strengthened, CNS) contains large size carbides and fine carbonitrides and Steel CS (carbide strengthened, CS) mainly possesses large size carbides. Both aging and creep tests at 650 °C were employed to evaluate the stability of the microstructure and properties of the steels during high-temperature exposure as a function of the precipitate morphology. It was found that Steel NS1 and Steel NS2 displayed a good creep performance, but they already recrystallized after a relatively short-term aging treatment. On the other hand, Steel CNS and Steel CS gave poor creep strength, but they could still maintain the tempered martensitic microstructure during long-term aging. The relation between microstructural stability and precipitate morphology/density was linked to the dislocation-precipitate interaction.