Study of rail steel alloying by chrome and vanadium effect on dissociation of overcooled austenite

Abstract

To determine weldability and quality of a rail welded joint, information on kinetics of the rail steel overcooled austenite transformation is high importance. Thermo-kinetic diagrams of overcooled austenite dissociation of steels Э76ХФ, Э76ХАФ and Э76Ф, built based on results of dilatometric, metal science and durometric tests of rail steel samples. It was shown, that increase of chrome content in steels Э76Ф and Э76ХФ composition from 0.09 to 0.39% results in expanding of dissociation area of overcooled austenite at temperature scale for ferrite-cementite mixture and increasing of resistivity of the overcooled austenite against dissociation in the area of ferrite-cementite mixture formation. This can be characterized as decrease of critical quenching velocity from 100 to 30 °С/sec. It enables to obtain structural states of higher hardness at cooling with velocities in the area of 0.1−30 °С/sec. It was established, that increase of vanadium content from 0.04 to 0.07% does not cause quality changes at the thermo-kinetic diagram of overcooled austenite dissociation. However, it known, that vanadium is a strong carbide-formation element, which combines with carbon at low cooling velocities and removes it out of the solid solution. Due to this effect, sample of steel Э76ХФ with lower vanadium content at cooling with velocities from 0.1 to 10 °С/sec, had somewhat higher hardness level comparing with steel Э76ХАФ sample. Increase of chrome content in alloy content results in an increase of temperature of austenite formation completion at heating from 760 to 774 °С, while the temperature of martensitic transformation commencement at that remains practically unchanged at the level of 230 °С. In steels Э76ХФ and Э76ХАФ in the chemistry of which chrome was added in the amount of 0.37−0.39%, after cooling with velocities of 1 °С/sec and lower, apart from ferrite-carbide mixture of perlite type, formation of redundant ferrite with a volume share of 4−5% was observed as a result of overcooled austenite dissociation. However, in a steel Э76Ф sample, the content of which has chrome at the level of 0.09% at close content of carbon, after cooling in an analogue range of velocities, a ferritecarbide mixture of perlite type is formed with a slight trace of redundant ferrite in the structure as a result of overcooled austenite dissociation.