Characterization of Precipitates in the 18-1-1 and 18-1-1-5 High-Speed Steels During Tempering After Previous Quenching

Abstract

In this paper the precipitates formed during the tempering after quenching (in salt bath) at temperature 1240 °C for 18-1-1 and 18-1-1-5 high-speed steels are investigated using an analytical transmission electron microscope (A-TEM). The study of this tempering is carried out in isothermal and anisothermal conditions, by comparing the results given by dilatometry and hardness. Tempering is performed in the range of 350-750 °C. Coarse primary carbides retained after heat treatment are of the V-rich MC and W-V-rich M6C types. In turn, it gives a significant influence on the precipitation of dispersive secondary carbides (M4C3 type), that is secondary hardening during tempering. The hardness peak is observed in the tempering range of 500-550 °C (66.8 HRC after a double tempering at 550 °C). It was found out that the cobalt in the concentration limited to about 5 % results in increasing the effect of the secondary hardening by 2 HRC. Between 700 and 750 °C, the impoverished phase (retained austenite) transforms on heating. In the end, in this temperature range of tempering, other more stable carbides of the M6C and M23C6 types are formed developing at the expense of the carbides M4C3 and M7C3 formed before, which then disappear.