Effect of silicon and vanadium on corrosion-mechanical properties of high-nitrogen Cr – Mn steels

Abstract

The authors studied the phase composition, crystal lattice parameters, mechanical properties and stress corrosion resistance of high-nitrogen austenitic and austenitic-ferritic Cr – Mn steels after homogenizing treatment, aging and cold plastic deformation. It was established that alloying of Cr – Mn steels with silicon and vanadium can lead to the formation of different amounts of ferromagnetic ẟ-ferrite and, from its low content, to significant hardening due to the grain-boundary effect. The presence of ẟ-ferrite has a hardening effect both after homogenizing treatment and during cold plastic deformation. In vanadium-alloyed Cr – Mn steels, even after austenitization treatment at 1250 °C, a finer grain of austenite of 8 – 9 numbers is retained than those of steels alloyed with silicon, having after quenching from a lower temperature (1150 – 1170 °C) larger grain of 6 – 7 numbers. Formation of even small amounts of ẟ-ferrite leads to a decrease in corrosion cracking resistance of high-nitrogen chromium-manganese steels. At the same time, corrosion resistance of high-nitrogen steels with ẟ-ferrite is significantly lower than that of austenitic steels containing 0.4 % nitrogen and more single-phase Cr – Mn. Aging causes significant hardening of high-nitrogen, alloyed with both silicon and vanadium, Cr – Mn steels with ẟ-ferrite and is accompanied by a loss of ferromagnetism with a significant decrease in toughness and ductility. Disappearance of ferromagnetism seems to be due to the fact that ẟ-ferrite disintegrates into a σ-phase and a paramagnetic nitrogen-containing austenite. Microstructural and X-ray diffraction studies indicate that the aging of steel with ẟ-ferrite proceeds by a continuous mechanism, accompanied by a monotonous decrease in the lattice parameter of austenite due to the release of nitrides from it. Aging of two-phase steels, leading to the disappearance of ẟ-ferrite and ferromagnetism, caused a catastrophic decrease in corrosion cracking resistance.