Effect of carbon content on microstructural evolution of 8Cr steel in austenitizing process

Abstract

Steel samples with a Cr content of 8 wt% and different C contents ranging from 0.56 to 1.14 wt% (termed as 8Cr steel) were prepared by vacuum melting, and the microstructural evolutions of their carbide and matrix in the austenitizing process and subsequent oil-quenching were investigated. There were three different forms of primary carbide generated in the final solidification region, namely Mo-rich M2C, (Cr, Fe)-rich M7C3 and mixture carbides, and the formation order of which depended on the mixing enthalpy. These coarse primary carbides would dissolve while the fine secondary carbides M23C6 would precipitate first and then dissolve (above 1323 K) during austenitizing. With the C content increase from 0.56 to 1.14 wt%, the primary carbide fraction increased from 0.67 to 9.94 vol%, and the corresponding temperature required for their complete dissolution increased from 1323 to 1473 K. The carbide decomposition caused a C-enrichment in the adjacent austenite, and when it contained less than 0.87 wt% C, complete martensite transformation would take place in subsequent quenching, but the higher C content would made austenite preferentially remain there. Due to sufficient martensite transformation, appropriate lattice distortion, and reasonable carbide fraction, the 8Cr steel with 0.74–0.86 wt% C could achieve a high and stable hardness (HRC 60–63) after austenitizing at 1323–1423 K.