Precipitation of Intermetallic Compounds in Austenite and Its Effects on Toughness of As-Quenched Martensite in 13%Ni Maraging Steel

Abstract

The precipitation of intermetallic compounds in austenite and its effects on the toughness of as-quenched martensite in 13%Ni maraging steel have been studied by means of Charpy impact tests and structural observations. The heat-treatment process investigated comprises intermediate isothermal holding in the temperature range from 800°C to 1100°C after homogenization at 1200°C reheating of the martensite to the austenite region below 1100°C after quenching to room temperature from 1200°C.
Not only the intermediate holding but also the reheating introduced a drastic decrease in Charpy impact energy of the as-quenched martensite. Structural observations revealed the coarse precipitates on prior austenite grain boundaries and within the grains for both heat-treatments. Prolonged intermediate isothermal holding was necessary to cause the precipitation within the grains. However, when the martenisite was reheated to the range between Af and the austenite recrystallization temperature, the precipitation within the grains was promoted in short time, due to the presence of the nucleation sites formed in martensite on the heating up to the austenitic region, or probably to the internal strains of the reverted austenite.
These precipitates were considered to be intermetallic compounds of Fe₂Mo or (Fe, Co) ₇Mo₆. The major loss in the toughness of as-quenched martensite of 13%Ni maraging steel was associated with the precipitation of intermetallic compound containning Mo during holing in the austenite regions.