Intermetallic phases and element distribution in carbon-free PM high-speed steels of the Fe-Co-Mo type

Abstract

In contrast to classical carbidic high-speed steels, carbon-free grades of the type Fe-Co-Mo(-W) obtain their strength by precipitation hardening through nm-size intermetallic µ phases; for example, (Fe,Co)7Mo6; coa­rser µ phases, in the low µm range, are required to prevent excessive grain growth during solution treatment. Precipitation hardening by intermetallic phases results in hardness >65 HRC combined with considerably higher temper resistance compared to carbides, which in turn results in improved cutting performance. On the other hand, the material is relatively soft as-quenched, <35 HRC, enabling machining and to some extent also cold working, and is hardened by an isothermal heat treatment without martensite formation, which strongly benefits geometrical precision. In this study, the intermetallic phases in the systems Fe-Mo, Co-Mo, and Fe-Co-Mo were investigated by pre­­paring specimens with varying composition from elemental powders through pressing and sintering. Metal­lographic sections were characterized by microscopy and SEM-EDX as well as X-ray diffraction. Compo­sition and crystallographic structure of the intermetallic phases were determined. It showed that the materials consist entirely of bcc matrix and µ phase; the ratio Fe:Co is almost the same in both phases and reflects the nominal one.