Dilatometry Study of the Sintering Behavior of Boron-Alloyed Fe-1.5 Pct Mo Powder A. MOLINARI, T. PIECZONKA, J. KAZIOR, S. GIALANELLA, and G. STRAFFELINI The effect of different boron concentrations on the sintering behavior of an Fe-1.5 pct Mo (wt pct) prealloyed powder was investigated. Sintering was carried out in a dilatometer so that all dimensional changes involved with the densification process could be followed. Several transformations were found to occur by heating powder compacts to 1200 °C and then cooling them to room temperature. At high temperatures, boron promoted the formation of liquid phases that, through a more-efficient sintering kinetics, promoted a satisfactory densification. Faster heating rates also had beneficial effects on the density of the final products. From a microstructural point of view, boron tended to destabilize the ferritic phase and to form iron and molybdenum borides. These borides can be found both in the intergranular regions, with a typical eutectic morphology, and dispersed in the ferritic matrix, in the form of nanometer-sized precipitates. This feature, having a significant effect on the hardness of the bulk material, has been ascribed to a bainite-like precipitation of borides from an undercooled austenitic phase. I. INTRODUCTION A microstructural characterization of the final products was carried out in order to identify the transformations occurring THE effect of boron on the sintering of iron and iron during sintering. alloys has been studied by several authors. Boron presents many favorable characteristics for the activation of the sin- tering process, as demonstrated by a quantitative approach II. EXPERIMENTAL PROCEDURE based on the figures of merit for activated sintering. [1,2,3] The specimens were produced by mixing together a preal- Boron, which may also promote the stabilization of the loyed Fe-1.5 pct Mo powder (Ho ¨ganas-grade AstaloyMo,  phase (ferrite), [4] thus further enhancing sintering, has average particle size of 50 m) with three different percent- beneficial effects on the hardness of iron and on the harden- ages (0.2, 0.4, and 0.6 pct, in wt pct) of an elemental boron ability of steels, with further positive effects on the techno- powder (Goodfellows product, average particle size of 2 m logical properties of the sintered components. These aspects and purity of 99.99 pct). Boron-free specimens were also have been demonstrated by previous investigations on iron produced for a comparison. and steels, [5,6] iron-molybdenum, [7] and martensitic [8] and The specimens for dilatometry were 14  4  4 mm 3 austenitic stainless steels. [9,10] prisms, uniaxially compacted at 600 MPa with die-wall lubri- Boron can be added under different forms, mostly as a cation. The compaction pressure was applied normally to compound. [11,12,13] In a few investigations only, elemental the dilatometry direction, i.e., the direction along which boron was used. [9,10,14] German et al., [14] as an example, used dimensional changes were measured. Sintering was carried elemental boron and molybdenum powders to investigate out at 1200 °C in a horizontal NETZSCH 402E dilatometer the microstructural and mechanical characteristics of Fe- under flowing hydrogen. The length-measuring system was Mo-B alloys with compositions in the Fe-rich corner of the connected to the specimen by a spring loaded to the 48cN, ternary system. Care must be used, however, in the selection the minimum load allowed by our dilatometer. The heating of the sintering atmosphere. Owing to the high affinity of rates to reach the sintering temperature were 5 °C/min, 10 boron to nitrogen and oxygen, in fact, very pure and dry H 2 °C/min, or 30 °C/min. The sintering time was 60 minutes, is recommended. [14,15] On the other hand, the high affinity and the cooling rate from the sintering temperature was 20 of boron to oxygen may be exploited during sintering, since °C/min in all the experiments. it may effectively contribute to the reduction of oxides pres- Table I summarizes the experimental parameters (boron ent on the metallic grains of powder. [15] concentration and heating rate) adopted for the studied mate- In the present investigation, elemental boron was added, rials and indicates the codes used for their identification in to different extents, to a prealloyed Fe-1.5 pct Mo powder. the following text. Then, sintering experiments were carried out in a dilatometer Density was measured by the water displacement method, in order to follow the relevant dimensional changes. The according to the ISO-2738 standard. The microstructural effect of three different heating rates was also investigated. characterization was carried out with an optical microscope and a scanning electron microscope (SEM). All observed samples were preliminarily etched, using a solution of 2 A. MOLINARI, Associate Professor, and S. GIALANELLA and G. STRAFFELINI, Senior Researchers, are with the Department of Materials pct HNO 3 in ethanol. Qualitative chemical analyses of the Engineering, University of Trento, 38050, Mesiano-Trento, Italy. T. microstructural constituents were carried out by energy-dis- PIECZONKA, Senior Researcher, is with the Academy of Mining and persive X-ray spectroscopy (EDXS). The morphology and Metallurgy, Cracow, Poland. J. KAZIOR, Associate Professor, is with the size of the pores were characterized and measured with an Cracow University of Technology, Cracow, Poland. Manuscript submitted July 7, 1999. image analyzer system using metallographically polished METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 31A, JUNE 2000—1497