Relation between the Phase Transformations and the Magnetic, Physical and Mechanical Properties in 18Ni 350 Grade Maraging Steels

Abstract

The behavior of the martensite→austenite and austenite→martensite transformations in maraging steels containing 17.5%Ni, 12.5%Co, 4%Mo and 0-2.5%Ti has been studied by magnetic, dilatometry, and differential thermal analyses.
The martensite→austenite transformation during heating occurs in two successive stages in 17.5Ni-12.5Co-4Mo-Ti maraging steels with 0-1.3%Ti, but in one stage in a 17.5Ni-12.5Co-4Mo-2.5Ti maraging steel.
The reversion temperature shifts to higher temperatures as the titanium content is increased. Austenite reversed in the first stage is stable down to the liquid nitrogen temperature. However austenite reversed in the second stage transforms partially to martensite during cooling to room temperature, depending on the chemical composition of steels.
While the ferromagnetic to paramagnetic transformation always coincides with phase transformation on heating, they differ slightly on cooling. The Curie temperature of austenite reversed on heating is around 120-270°C, depending on the titanium content and elevated temperature.
The tensile strength increases with the increase of titanium content in steels, but the ductility decreases. The density increases as the volume of reverted austenite increases.