Effect of Aluminum on Microstructure of HP40 Steel

Microstructures of HP40 steels with 5-20 wt. % Al in displacement of Ni fabricated by vacuum-arc melting were investigated by optical microscope and EPMA-1600 electron probe. The results show that matrix of the steels transformed from γ-Fe to α-Fe phase and then from α-Fe to (Fe, Ni)Al phase with Al content increasing. (Ni, Fe)Al intermetallic phase appeared in the steels and its content increased with Al. Shape of the (Ni, Fe)Al phase changed from precipitate to dendrite and then from dendrite to equiaxed grains. σ phase appeared in the steels with 15 and 20 wt. % Al.