Phase composition study of 40Kh18N2M (EP378)

This study presents the results of an investigation into the ferritic-martensitic corrosion-resistant steel 40Kh18N2M (EP378), designed for operation under contact loads in aggressive environments. The primary objective in developing this steel was to improve its machinability by alloying with sulfur (up to 0.25%) and phosphorus (up to 0.15 %) while maintaining high mechanical strength and corrosion resistance. Two industrial heats with chemical compositions corresponding to the lower and upper limits specified in TU 6400-009-59532330–2018 were used as the material for this study. The study involved thermodynamic modeling of crystallization and phase transformations during cooling using the Thermo-Calc software package (TCFE6 database), as well as experimental methods including X-ray diffraction analysis and dilatometric measurements. Thermodynamic simulations of solidification and cooling process was carried out. It was shown that, under equilibrium cooling conditions, Me23C6 carbides, the intermetallic σ-phase, Me3P phosphides, and MnS sulfides are formed. X-ray analysis revealed that the steel has a ferritic-martensitic structure. In the studied samples, the martensite content was low (up to 10 vol.%), but this fraction can be increased to as much as 70 vol.% through heat treatment. It was found that cryogenic treatment for 90 minutes at minus 70 °C has little effect on the phase composition due to the low content of retained austenite. As a result of dilatometric studies, the critical temperatures of phase transformations were determined for steels with chemical compositions corresponding to the lower and upper limits of the main elements specified by the technical specifications. For the steel with the minimum alloying content, the critical temperatures were 762.5 °C and 807.8 °C, and the martensite start temperature (Mₛ) was 230 °C,for the steel with the maximum alloying content, the critical temperatures were 753.4 °C and 801.1 °C, with an Mₛ temperature of 202 °C, respectively.