Influence of surface quenching on morphology and phase composition of ferritic-pearlitic steel

Abstract

The study was carried out by means of transmission electron microscopy on thin foils to investigate the changes in matrix morphology and phase composition occurring in ferritic-pearlitic steel of St2 grade (Russian) under plasma electrolytic surface quenching. In the original state St2 steel is a material which underwent quenching under the temperature of 890 °C (2 – 2.5 h) with cooling into warm water (30 – 60 °C) and further tempering under the temperature of 580 °С (2.5 – 3 h). Surface quenching was conducted in aqueous salt solution during 4 seconds under the temperature of 850 – 900 °C, voltage of 320 V, and current rate of 40 A. In the original state morphological components of the steel matrix were lamellar pearlite and non-fragmented and fragmented ferrite. Surface quenching resulted in the following transformations of morphology and phase composition: 1 – to martensitic transformation (morphological components are lath martensite, lamellar low-temperature and high temperature martensite), 2 – to steel self-tempering (inside all martensite crystals there are thin plate-like precipitations of cementite), 3 – to diffusion transformation γ → α and precipitation of retained austenite (γ-phase) given as thin layers along the boundaries of laths and plates of low-temperature martensite and inside all the crystals of lamellar martensite in the shape of “needles” like in twin type colonies. Surface quenching led to precipitation of special carbides of Мe23С6 phase. It was revealed that carbide precipitation is attributed primarily to decomposition of retained austenite and martensite and also to partial dissipation of cementite and, moreover, it is due to carbon removal from dislocations and the boundaries of α-phase crystals. That means that in all cases carbon from retained austenite, α-solid solution, cementite particles and defects of crystal lattice is used for the formation of special carbides.