Comparison of structure and properties of the HS12-1-5-5 type high-speed steel fabricated using the pressureless forming and PIM methods

Structure and mechanical properties of the HS12-1-5-5 type high-speed steels manufactured by powder injection moulding and with the pressureless forming were investigated and compared in the project. Basing on the structure examinations, occurrences of big spherical pores in the pressureless formed test pieces were revealed, that could not be eliminated in the debinding and sintering processes. The specimens fabricated with the powder injection forming method are characteristic of the higher density and uniform distribution of the high-speed steel powder in the binding agent matrix. In addition, employing the bicomponent binding agent in the form of paraffin and polyethylene made cutting the debinding and sintering time possible by about 10 h in comparison to manufacturing the steel with the pressureless forming method. Hardness tests in the as quenched state of the HS12-1-5-5 type steel specimens have demonstrated that the pressureless formed steels display lower hardness compared to steels manufactured with the PIM method. This is, undoubtedly, caused by a higher carbon concentration, which lowers the Ms and Mf temperatures of the martensitic transformation and increases the contents of the retained austenite. Yet, pressureless forming has its advantages. This method does not require practically any additional equipment apart from the furnace for thermal debinding of the binding agent and for sintering. Moreover, specimens formed with the pressureless method have lower sintering temperature and higher density.