PROPERTIES OF A LOW CARBON AND HIGH NITROGEN MARTENSITIC STAINLESS STEEL MELTED IN A PRESSURIZED INDUCTION FURNACE

摘要

A low carbon and high nitrogen martensitic stainless steel, consisting of 0.004C-18Cr-1Mo-0.5N (mass percent), has been produced in a pressurized induction furnace under 0.9MPa, and the hardness and corrosion resistance of the steel have been investigated in various heat treatment conditions. The hardness of the steel after sub zero treatment is about 52HRC, which is smaller by about 6 points than type 420 in quenching conditions. On the other hand, the steel showed secondary hardening behaviour and exhibited a maximum tempering hardness of 56HRC at 723K. This is comparable to type 420 tempered at 473K, which is one of the most commonly used conditions. The corrosion resistance of as-quenched material and tempered materials under 723K was very high and comparable with type 316 evaluated by pitting potential in 3.5 percent NaCl aq. solution. However the corrosion resistance of the steels tempered over 723K deteriorated drastically. TEM observations revealed that the deterioration in corrosion resistance was due to the precipitation of hexagonal beta-Cr_2N at pre-austenite grain boundary.