Intercerical Quenching of Low-Carbon Steel with the Formation of a Disperse Multiphase Structure
Introduction. The advanced steel 10H3G3MFS (C = 0.1, Mn = 2.51, Cr = 2.75, Mo = 0.40, V = 0.12, Si = 1.25), developed for oil-producing engineering, has good workability and strength characteristics, but the level of impact strength after the traditional modes of heat treatment is at a sufficiently low level. The paper is devoted to the investigation the possibility of metastable structural states formation during intercerical quenching of steel 10H3G3MFS for the purpose of increase the level of impact strength without significant loss of strength characteristics. The subjects of study are the steel structure formation processes when heated in an intercerical temperature range (ICTR) with subsequent quenching. The purpose of this work is to study the possibilities of controlling the structure and properties of steel 10H3G3MFS with the use of isothermal austenitization in ICTR to obtain a dispersed structure. Methods. Dilatometric analysis using the Linseis hardening dilatometer R.I.T.A. L78, metallographic analysis using a light inverted microscope OLYMPUS GX 51 and electron microscopy using a transmission electron microscope FEI Tecnai 20 G2 TWIN are being in use. Uniaxial tensile tests are carried out using the universal hydraulic system for static tests INSTRON-SATEC 300 LX and the toughness is determined by pendulum coprometer KM-30, followed by fractographic analysis on a light microscope Olympus SZX-16 and a scanning electron microscope Hitachi S-3400N. Results and Discussion. Based on the results of the study of the process of investigated steel continuous heating, a thermokinetic diagram of the formation of austenite with the designation of the critical points A<sub>C1</sub> and A<sub>C3</sub> is constructed. It has been established that as the heating rate increases, the critical temperature A<sub>C1</sub> decreases and A<sub>C3</sub> increases. A study of the isothermal austenitization process showed that 27% of the γ-phase is formed at 710 °C, 59% of the γ-phase is formed at 750 °C, 76% of the γ-phase is formed 800 °C, and during the soaking at 860 °C occurs complete austenitization and 100% of the γ-phase is obtained. It was also found that with an increase in the temperature of isothermal soaking, the proportion of athermic austenite increases, and the isothermal content decreases. An isothermal diagram of austenite formation the initially hardened steel 10H3G3MFS is constructed. The study of steel 10H3G3MFS structure formation process has revealed that during the austenitization process at 715 °С the first stage of austenite formation occurs: austenitic grains form along the boundaries of former austenite grains and martensitic packages. An increase in the austenitization temperature up to 750 °C leads to the development of the second stage of austenitization: austenitic grains form along the martensitic stripe boundary. At a temperature of 800 °C, the second stage is further developed, which, after quenching, leads to the formation of a martensitic framework along the interstitial boundaries of the initial α-phase. These interlayers of the initial α-phase are fragmented by dislocation boundaries and strengthened by a small amount of carbide particles. Thin layers of residual austenite are present in the martensite framework. There are freshly quenched areas of the polyhedral shape that are formed at the grain boundaries of the original austenite or the boundaries of the original packets. The raise of heating temperature to 860 °C causes the end of the α → γ transformation during the soaking process, and after following quenching the structure of packet martensite whith twins is formed. According to the strength and plasticity test, it is established that quenching from 800 °C leads to a slight decrease in the tensile strength brake (by 8%), but the yield strength does not practically decrease. The percent elongation remains at the level of the initially hardened steel, and the percent reduction significantly increases (from 54 to 60%). The KCT toughness level of the steel under study significantly increases up to 0.76 MJ/m<sup>2</sup> (by more than 70%). According to the data of fractographic analysis, samples after dynamic tests have viscous fracture mode. The received mode of heat treatment allows to increase the level of toughness of the steel under study without loss in strength of products of any overall dimensions for oil-producing machine building.