Carbide-free-bainite in low carbon construction steels

Abstract

In this work the structure and mechanical properties of chromium-nickel-molybdenum steels with a carbon content of 0.1 to 0.2% after austenitization and slow continuous cooling in the bainitic region at a rate of about 5°C /min were investigated. It was shown that at a carbon content of about 0.10...0.15% after such heat treatment, a carbide-free bainite structure is formed in steel, which is a two-phase mixture of bainitic ferrite and residual austenite supersaturated with carbon. Using transmission electron microscopy, the features of the formed bainitic ferrite and the distribution of retained austenite, the amount of which ranged from 10 to 14%, were studied. Using X-ray phase analysis, the carbon content in the retained austenite was determined, which turned out to be equal to 0.8 ... 0.9% C. An increase in carbon content up to 0.2% in such steels leads to the precipitation of carbide particles during bainitic transformation with continuous cooling. After additional doping with silicon in an amount of about 1% in the steel of such an alloying system with 0.2% C, the formation of carbides is almost completely suppressed and carbide-free bainite is formed, while the amount and degree of enrichment of retained austenite in carbon slightly increases. Comparison of the mechanical properties of the investigated low-carbon steels showed that at approximately the same level of strength, steels with carbide-free bainite have a higher level of toughness compared to steel containing carbide precipitates. Retained austenite in carbide-free bainite is substantially carbon enriched and contains a significant portion of the total carbon content in steel.