Materials Science and Engineering, A 159 ( 1992) 103-109 103 Characterization of rapidly solidified ultrahigh boron steels J. A. Jim6nez, P. Adeva, M. C. Cristina and O. A. Ruano Centro Nacional de Investigaciones Metalfirgicas, C.S.L C., A v. de Gregorio deI Amo 8, 28040 Madrid (Spain) (Received March 31, 1992; in revised form June 23, 1992) 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/~m grains were developed in all materials after consolidation at tempera- tures 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. I. Introduction The effect of sman amounts of boron additions (up to 100 p.p.m.) to steels has been widely studied for over 50 years. It has been found that the presence of boron improves the hardenability of low alloy steels [1-4] and the creep rupture strength of austenitic steels [5, 6]. The solubility of boron in iron is very low. The maxi- mum solid solubility ranges between 16 and 33 p.p.m. in y-Fe at 1000 °C [7-9] which is consistent with the findings that almost all boron additions form borides [10-13]. These borides are very hard and coarsen at high temperatures. Ultrahigh boron steels are attractive as tool steels because the boride-containing microstructures should remain stable at high temperatures, at least up to about 900 °C. However, a very limited number of investiga- tions has been carried out on these materials [14-16]. Ultrahigh boron steels may exhibit many similar features to ultrahigh carbon steels (UHCS) and cast irons since both iron carbides and borides are hard phases and may strengthen the a or Y matrix. The UHCS and cast irons have high tensile strength but very low ductility at room temperature and show superplasticity in the temperature range 650-750 °C with a fine microstructure present [17-19]. Ultrahigh boron steels contain large borides when processed by conventional ingot metallurgy, resulting in poor ductility and fracture toughness. Improvements in the mechanical properties are expected when these steels contain a microstructure consisting of fine grains and a uniform dispersion of fine second-phase par- ticles. Such microstructures may be obtained using rapid solidification techniques. This investigation is aimed at characterizing the as- quenched and annealed powders of four ultrahigh boron steels produced by argon atomization, and at determining the influence of microstructure on the compressive properties of consolidated materials tested at high temperature. 2. Materials and experimental procedures The powders of the four alloy compositions listed in Table 1 were rapidly solidified using the argon atomization technique. The powders were sieved to produce particles of less than 80/~m. The binary alloy powders were outgassed at 400 °C and then consolidated by hot isostatic pressing (HIP) under 180 MPa for 2 h at 900 °C for Fe-2.2wt.%B and 800 °C for Fe-4.9wt.%B. The tool steels were consoli- dated by extrusion at 1150 °C with a reduction of 5 : 1. Powder particles with and without thermal treat- ments and consolidated materials were characterized using various techniques including X-ray diffraction, scanning electron microscopy (SEM) and energy 0921-5093/92/$5.00 © 1992 - Elsevier Sequoia. All rights reserved