Thermoplastic processing and debinding behavior of NbC-M2 high speed steel cemented carbide

Abstract

The aim of this study is to evaluate the thermoplastic processing on a new generation of hard metals based on NbC bonded with high speed steel. The conventional technique in producing cemented carbide parts is dry pressing with paraffin binder and post production. To overcome shape restriction in pressing and post production, thermoplastic shaping processes can be used. For this purpose, a previously known binder system for ceramic feedstocks containing paraffin wax and ethylene vinyl acetate was selected and used for the first time with cemented carbides. The stated organic binders were mixed with NbC-12 wt.% M2 high speed steel powder and feedstocks with solid loading content from 50 to 60 vol.% were obtained. The rheological behavior of the feedstocks as well as the critical solid loading content were assessed using a capillary rheometer. A model fitting approach was used for the first time in hardmetal feedstocks to estimate the critical solid loading. Experimental values were fitted to a model from Chong (1971) which is based on bimodally disperse powder mixtures and a critical powder loading of 70 vol.% could be calculated. However, based on the abrasion problems during mixing at high filler content, feedstock with 57.5 vol.% powder loading had the optimum processability characteristics, and was therefore selected for further shaping processes. Thermo gravimetric analysis on green samples was performed after shaping in order to develop an optimum thermal debinding profile. The thermal debinding program was adjusted with special attention on decomposition onset, maximum mass loss rate and decomposition offset temperatures. Initial trials using decomposition offset temperature as holding steps, resulted in fragmented samples after debinding. The final designed heating profile resulted in a consistent mass loss rate according to thermo gravimetric analysis which limited crack formation during debinding. Debinding under a forming gas (95N2-5H2) atmosphere using optimized heating cycles resulted in sound sintered bodies only when green parts were embedded in a powder bed during debinding.