PHASE CHEMISTRY AND PRECIPITATION REACTIONS IN MARAGING STEELS .2. CO-FREE T-300 STEEL

This article describes studies of phase transformations during aging in a commercial Co-free T-300 maraging steel. Atom-probe field-ion microscopy (APFIM) was the main research technique employed. Thermochemical calculation was used to aid the interpretation of results. The composition and morphology of precipitates were compared in the same maraging system aged at different temperatures for different times to investigate the aging sequence. Ni3Ti and Fe7Mo6 have been found to contribute to age hardening. The formation of the Fe7Mo6 phase took place at a much later stage of aging in the Co-free steel than in the equivalent Co-containing material. The matrix concentration of Mo was correspondingly higher at earlier stages of aging. No reverted austenite has been found during APFIM and transmission electron microscopy (TEM) examinations of specimens heat-treated at 510-degrees-C, even after 360 hours of aging.