10Ni-18Co-14Mo系マルエージ鋼におけるオーステナイト中の析出とその強靱性におよぼす影響

Abstract

When a thermomechanical treatment (TMT) was applied to a 350 kgf/mm2 grade 10Ni-18Co-14Mo maraging steel containing a large amount of Mo, the austenite grain size was refined to below 10μm, but precipitation in the austenite occurred during TMT, leading to a detrimental effect on toughness before aging. This study was carried out to show the precipitation characteristics during TMT and accelerated precipitation behavior by hot-working, and then to investigate the relationship between the change in microstructure and mechanical properties. To evaluate the effect of hot-working on precipitation behavior, a continuous cooling treatment without working (CCT) was also carried out.The results are as follows:1) Precipitation in the austenite did not occur during CCT with a cooling rate of 4.2°C/s corresponding to that of TMT. 2) When the cooling rate during CCT was reduced to 0.42°C/s, precipitation occured along the grain boundaries in the 10Ni-18Co-14Mo steel. 3) When applying the TMT, precipitation occurred after sixth pass and with further increase in pass number the amount of precipitates increased greatly. 4) These precipitates were identified as the intermetallic compound of molybdenum and titanium sulfide. 5) Toughness before aging was drastically reduced by the presence of these precipitates. 6) It is difficult to detect the detrimental effect of the precipitates from the change of tensile properties and fracture toughness after aging.