18%Niおよび20%Ni, 25%Niマルエージ鋼の析出硬化

Abstract

Precipitation hardening and reversion behaviours in 18%nickel maraging steels bearing molybdenum and cobalt and 20%, 25%nickel maraging steels bearing titanium have been investigated by measuring the electrical resistivity at liquid nitrogen temperature and tensile strength. Transmission electron microscopic studies have also been carried out. The results obtained are summarized as follows.(1) The 18%nickel maraging steels show a good combination of strength and toughness in aged conditions. The strength after aging is considerably increased by adding relatively small amounts of titanium, the upper limit of the content of titanium is considered to be nearly 1.5%to avoid the drop in toughness after aging.(2) A considerable drop in toughness is observed in the aged 20%nickel maraging steels. This fact may be interpreted in terms of the existence of relatively coarse precipitates accompanying precipitation-free zones formed mainly at grain boundaries. The aging brittleness can be reduced by heavy cold working before aging. Addition of relatively small amounts of molybdenum or tungsten is also effective to reduce the aging brittleness.(3) A marked reversion phenomenon is observed in 18%nickel maraging steel bearing 5%molybdenum and 9%cobalt, pre-aged at 375°. However, the reversion in 25%nickel maraging steel bearing 1.8%titanium, pre-aged at 375°, is inconsiderable. From these results it may be concluded that a meta-stable phase is precipitated at a lower aging temperature in nickel maraging steels bearing molybdenum and cobalt but not in those bearing titanium.