Reversible temper embrittlement in a steel deformed under high pressure

Abstract

A steel containing 0.30% C, 0.95% Mn, 1.10% Si, 1.10% Cr, 0.013%P, 0.008% Ni and 0.009% Cu and known for its susceptibility to reversible temper embrittlement was studied. After quenching and tempering at 500, 600 and 700 °C, the steel was hydro-extruded with deformations of up to 50%. The deformed steel was subjected to embrittlement tempering at 650 °C followed by slow furnace cooling. The main characteristics of the mechanical properties were determined. An electron microscopy study of the structure at separate stages of the treatment was carried out. It is established that the best combination of fracture energy and strength is observed when the steel is tempered before deformation at 600 °C. The transition temperature decreases by 40 °C in this case. It is shown that the improvement in steel properties is connected with the formation of a misoriented cellular substructure during deformation under a high hydrostatic pressure. On heating, a thermally stable homogeneous structure with a subgrain dimension of 0.5 – 1 μm appears based on this substructure. The embrittlement decrease is assumed to be due to the decrease in phosphorus concentration and the degree of localized carbide precipitation at the prior austenite grain boundaries as a result of the segregation of phosphorus and carbon to newly formed boundaries.