INVESTIGATION OF THE MICROSTRUCTURE AND MECHANICAL PROPERTIES OF 40KH18N2M (EP378)

Abstract

This study presents an experimental investigation of 40Kh18N2M (EP378), aimed at examining its microstructure and mechanical properties after various heat treatment regimes. The steel was produced using modern industrial technology with the addition of deoxidizers and modifiers, followed by forging on industrial hammers. The influence of austenitizing temperature and cooling medium on the hardness of the steel was evaluated. It was found that hardness increases with the austenitizing temperature up to 1050 °C, due to the dissolution of carbide phases and enrichment of austenite with alloying elements. The microstructure of the steel was analyzed using optical and scanning electron microscopy. It was shown that the steel has a ferrite-martensite matrix with numerous secondary phases, including Me₂₃C₆ carbides, MnS sulfides, and Me₂P phosphides. A significant amount of carbide phase is distributed along grain boundaries, forming an almost continuous network, and also precipitates on non-metallic inclusions, particularly manganese sulfides. Specimens for mechanical testing underwent heat treatment, including quenching and tempering. Cold treatment was additionally applied to some specimens. The first series of tests was conducted on specimens of size No. 7, type III (GOST 1497–2023) with a sharp transition from the working section to the fillet, which resulted in a high scatter of the results. Fractographic analysis of the impact specimens revealed up to 100 % brittle fracture. To assess the influence of specimen geometry, a second series of tests was carried out on modified specimens of size No. 7, type IV (GOST 1497–2023) with a smooth transition from the working section to the fillet, which allowed the measurement of non-zero plasticity and more consistent results.