Structure and mechanical properties of HSS HS6-5-2- and HS 12-1-5-5-type steel produced by modified powder injection moulding process

It was demonstrated in the project that the experimental steels of the HS6-5-2 and HS12-1-5-5 types manufactured with the PIM method demonstrate a lower sintering temperature compared to the same steel grades made by pressing and sintering. It was found out that the higher density, more homogeneous structure with fine precipitations of carbide phases are obtained during sintering, the steels formed from the slurry in the atmosphere of N-2-5%H-2 gases. The wider sintering temperatures' range of about 40degreesC features the unquestionable advantage of the high-speed steels made using this method compared with the pressed and sintered steel, for which this temperature range is only 5 degreesC. Moreover, the nitrogen concentration increase hampers the pillar precipitations of the M2C-type carbides, as this element stabilizes phases with the regular lattice and increases the solidus temperature. A higher carbon concentration in these steels, resulting from the binder degradation process, causes lowering of the solidus temperature, and consequently, of the sintering and austenitizing temperatures of these steels. Moreover, a higher carbon concentration causes increase of the retained austenite portion in these steels and lower hardness after quenching compared to the same grades manufactured with other methods. A high retained austenite portion after tempering at temperatures of 540 and 570degreesC is the reason for which the secondary hardness effect is not so evident as in steels manufactured using other methods, in spite of the precipitation of the dispersive carbides of the M4C3 and M2C types. Only tempering at the temperature of 600degreesC causes lowering of the retained austenite portion due to the martensitic transformation of this phase during cooling from the tempering temperature to 10% for the HS6-5-2 steel and to 8% for the HS12-1-5-5 steel, and their hardness increase after tempering at this temperature to the maximum values of 63.2 HRC for the HS6-5-2 steel quenched from the temperature of 1180degreesC and tempered at 600degreesC and hardness of 67.6 HRC for the HS12-1-5-5 steel quenched from the temperature of 1210degreesC and tempered at 600degreesC, respectively. (C) 2004 Elsevier B.V. All rights reserved.