On the α′ to γ transformation in maraging (grade 350), PH 13-8 Mo and 17-4 PH steels

The martensite to austenite transformation during continuous heating was studied over a range of heating rates for 350 grade maraging steel (M350), PH 13-8 Mo, and 17-4 PH stainless steel using a programmable dilatometer. The α′→γ transformation splits into two steps at lower heating rates for the M350 and PH 13-8 Mo, whereas occurs in one step for 17-4 PH. The activation energies for the precipitation and α′→γ transformation in each of the steels were determined. It was concluded that in M350, the first step occurs through a diffusion process, while the second step occurs though a shear process. In PH 13-8 Mo the two steps, seen at the lower heating rates, appear to be due to the partitioning of the material, and the transformation in both the steps appear to be controlled by long-range diffusion. In 17-4 PH, the α′ to γ transformation occurs via diffusion at low rates and changes over to a shear process at high rates. Precipitation in the three steels occurs through a diffusive mechanism. The continuous heating transformation diagrams, depicting the precipitation reaction and the α′→γ transformation, for the three steels were constructed.