Low cost grain refined steels as alternative to conventional maraging grades

Abstract

It is proposed that Fe–12%Ni steels alloyed with 1 or 2%Al, and 1%Mo for pitting resistance, may offer a steel competitive with 18Ni (300) maraging steel in strength and toughness, but at one-third of the alloy cost. Previous alloy development research with a similar objective based on Fe–Cu alloys is reviewed. Although aging binary iron alloys containing up to 6%Cu at 485°C leads to considerable increase in hardness, impact energies are very low owing to intergranular failure. Alloys containing 4%Cu alloyed with 9%Ni to increase hardenability, hot workability and impact toughness showed no edge cracking on forging to 1 in (25 mm) plate at 1050°C. Grain refining the same alloy with 0·24%Nb and 0·04%C resulted in a ductile–brittle transition temperature below −100°C in the aged condition. The balancing of Nb and C contents for grain refinement in martensitic steels is discussed. To increase hardness, additions of 0·9%Al and 1·6%Al were made to the grain refined steel containing 4%Cu and 9%Ni. This resulted in considerable increments in hardness on aging at 450°C, but the alloys are no longer commercially viable owing to the high cost of Cu. Work on the proposed cost competitive Fe–12Ni–(1–2)Al–1Mo grain refined steel is continuing.