Abstract
Precipitation characteristics in 18%, 20% and 25% nickel type maraging steels were investigated by measuring the changes in electrical resistivity at liquid nitrogen temperature. Generally, the changes in resistivity are large in the remarkably precipitation-hardened steels, and the aspect of the changes in resistivity well coincides with that of the changes in tensile strength.
In the 18% nickel type maraging steel, the precipitation kinetics and its mode are assumed to be different in the temperature ranges above about 425°C and below. Consequently, the precipitates formed in the temperature ranges also seem to be different with each other. Activation energy for precipitation is about 35kcal/mol, and it can be deduced from this result that the precipitating atoms may diffuse through martensitic matrix with the help of some kinds of lattice defects. However, at the stage of over about 70% in total resistivity change, the activation energy seems to be increased to 44-49 kcal/mol.
In the case of 20% and 25% nickel type maraging steels, the resistivity increase, assumed to be due to the formation of cluster zone, was found at the early stage of precipitation. Activation energy for the formation of cluster zones was determined as 55-65 kcal/mol but, in the early stage of the formation, the activation energy was calculated as about 35kcal/mol. The precipitation rate in the 25% nickel type maraging steel was faster than that of the 20% nickel type maraging steel and the maximum precipitation-hardened state appeared in the stage of the growth of cluster zones. In comparatively higher aging temperature range, stable austenite, retained even in liquid nitrogen temperature, was formed and resulted in resistivity increase. The formation of austenite was more easily in the 18% and the 25% nickel maraging steels than in the 20% nickel maraging steel, and was accelerated by cold work.
Precipitation rates in nickel maraging steels were accelerated by cold work but its effect was not so significant as might be expected.
