Tensile Properties and Fracture Toughness of Two-phase 0.2% C-Ni-Cr-Mo Steel

Abstract

Two-phase steels of ferrite-martensite were used to study the tensile properties and plane strain fracture toughness which was estimated by experimentally determined critical J-value. The ferrite-martensite two-phase structures of a 0.2% C-Ni-Cr-Mo steel were obtained either by heating at intercritical temperatures and quenching or by hot-rolling (rolling reduction of area≈50%) at that temperature range and direct quenching.
Main results obtained are as following.
(1) The 0.2% offset proof stress and tensile strength decrease with an incresae in the volume fraction of the ferrite phase; this shows a deviation from the so-called mixture rule.
(2) The tensile ductility is not improved by mixing of the ferrite phase in the range of small volume fraction.
(3) The fracture toughness remains nearly constant in the range of small volume fraction of ferrite phase. This may be attributed to the fracture at ferrite-martensite interfaces.
(4) The tensile properties and fracture toughness are improved by hot-rolling and direct quenching in comparison with those by heating at intercritical temperatures and quenching. One of the main reasons of the improvement in the fracture toughness seems to be the relaxation of plastic constraint due to the fracture along the mechanical fibers produced by the hot-rolling.