Characterization of rapidly solidified ultrahigh boron steels

Abstract

Rapidly solidified powders of two binary FeB alloys and two boron-containing tool steels exhibiting a fine lamellar eutectic microstructure were investigated in both rapidly solidified and consolidation conditions. Both binary alloys contained ferrite and the metastable boride Fe3B, the latter of which was transformed into the stable boride Fe2B upon annealing at 610°C. The hardness of the as-quenched powders was very high (up to 1050 HV) because of high volume fractions of both phases. A loss of hardness with increasing annealing temperature was observed and may be attributed to the transformation of the metastable boride. Both tool steel powders showed the presence of austenite and stable borides. Fine, uniform microstructures consisting of 2–3 μm grains were developed in all materials after consolidation at temperatures ranging from 800 to 1150°C. When compression tested at 900°C, all materials exhibited a low stress exponent of 2–3, which may be explained in terms of grain boundary sliding. It appears that the borides containing chromium, nickel and/or molybdenum in both tool steels are harder than iron-borides at high temperatures, making the tool steels superior to the binary alloys over the entire temperature range up to 1100°C.