Abstract
Refinement of prior austenitic grain size through the austenite ⇆ martensite cylcic heat treatment was investigated on Fe-15Ni-5–30Co-5–10Mo-0–2Ti alloys with initial coarse austenitic grain size. The minimum austenitic grain size attainable depends on the chemical composition of the alloys as well as the conditions of thermal treatment. The grain size of all alloys was reduced to less than 50 μm by cycling five times at a suitable austenitizing temperature. The increase of cobalt content and the addition of titanium promoted grain refinement, whereas the increase of molybdenum content suppressed grain refinement.This treatment was applied to two 400 ksi (2 750 MN/m2) grade maraging steels (13Ni-15Co-10Mo and 15Ni-25Co-7Mo) and a commercial 350 ksi (2 410 MN/m2) grade 18Ni maraging steel with a coarse austenitic grain size, and the relationship between structures and mechanical properties was investigated. A superior ductility was obtained by choosing appropriate conditions of cycling for each steel without degrading its strength and toughness. This treatment was thus found to be a remarkable grain refining method in high-strength maraging steels.
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