Influence of Atmosphere on Sintering of T15 and M2 Steel Powders S. JAUREGI, F. FERNANDEZ, R.H. PALMA, V. MARTINEZ, and J.J. URCOLA The sintering behavior of T15 and M2 water-atomized high-speed steel powders under vacuum and N2-H2-CH4 atmosphere is analyzed. Sintering under the gas mixture allows high densities [99 to 100 pct total density (TD)] to be obtained at temperatures 45 to 50 K lower than for vacuum sintering for steel T 15, but the high densities are reached at slightly higher temperatures (15K) than for vacuum sintering for steel M2. Chemical analysis of gas-sintered specimens has shown that a higher amount of nitrogen is absorbed during sintering of steel T15 (0.72 to 0.78 pct) than in the case of steel M2 (0.34 to 0.38 pct). Although MC and M6C primary particles are observed during vacuum sintering, in gas sintering, the MC carbides change to MX carbonitrides. These particles are very fine (1 /~m) and are very resistant to coarsening during oversintering. Sintering in the gas atmosphere allows important oversinterings (up to 50 K above the minimum sintering temperature for achieving a density higher than 99 pet of theoretical density) to be reached in the case of steel T15 without the formation of eutectic films. In the case of M2 steel, for gas sintering, the eutectic films appear for oversinterings smaller or equal to those for vacuum sintering. I. INTRODUCTION WORK carried out in the last two decades on the sintering of high-speed steel powders has resulted in ob- taining high-density components with near net shapes. One of these routes is direct sintering, consisting of cold compaction of annealed water-atomized powders and subsequent vacuum sintering to full density. This tech- nique has been applied successfully to TI,f~J M2, flj T6, r2] T42, ~3,41 T I 5 , ~51 and M42. t6t In many cases, to compensate for the loss of carbon due to its reaction with the oxygen present at the surface of the powders, some elemental carbon is added to the prealloyed powders before sinteringJ 7"8'91 Such addition also results in an acceleration of the sintering kinetics, explained by some authors as the effect of carbon in re- ducing the steel solidus temperature, t~~ More recently, Wright till has proposed that the sintering mechanism in this type of steel is "supersolidus." Previous work at CEIT has shown the possibility of an alternative route to vacuum sintering, reaching the- oretical densities by sintering in a flowing atmosphere of N2-H2-CH4. Work has been performed on M2, f~zj T15, It3] T42, H3,141T6, tlSI and M42 t161 steels. In a recent article on T42, t~71 a consistent decrease in the optimum sintering temperature (the minimum temperature at which a density >99 pct of the theoretical density is achieved) compared with vacuum has been reported. A finer micro- structure is normally produced due to the substitution of massive MC carbides by fine MX coarsening-resistant S. JAUREGI and F. FERNANDEZ, Ph.D. Students, are with Escuela Superior de Ingenieros Industriales (ESII) de San Sebasti~in. J.J. URCOLA, Senior Researcher, is with the Centro de Estudios e Investigaciones Tecnicas de Guipuzcoa (CEIT), E-20009 San Sebasti~in, Basque Country, Spain. R.H. PALMA, Lecturer, is with the Universidad de Atacama, Copiap6, Chile. V. MARTINEZ, Lecturer, is with the Universidad de Santiago de Chile, Santiago, Chile. Manuscript submitted March I 1, 1991. carbonitrides. It is also reported that oversintering by up to 40 K in the gas atmosphere does not significantly modify the microstructure and continuous films of eu- tectic are not observed. Fracture toughness tests per- formed on these steels gas-sintered, r~6'18-2~jquenched, and tempered have shown Klc values similar to vacuum- sintered high-speed steels. In the present work, the vacuum sintering behavior of steels T15 and M2 is compared with sintering in the gas mixture N2-H2-CH4. The effect of addition of 0.2 wt pct elemental carbon to these two steels is also studied. After sintering, the carbon, oxygen, and nitrogen contents are analyzed together with the microstructure characterized by the austenite content, initial grain size, and the com- position, size, and volume fraction of the primary car- bides. Sintering in the gas atmosphere will allow cost saving due to the low-temperature sintering and im- provement in microstructure, mechanical properties, and performance in service and offers the prospect of sig- nificant competitive advantage to manufacturers of tools and wear parts. II. EXPERIMENTAL PROCEDURE Vacuum-annealed, water-atomized T15 and M2 pow- ders were bought from Powdrex Limited, Tonbridge, United Kingdom. The composition and sieve analysis of the powders provided by the manufacturer are given in Tables I and II. Additions of 0.2 wt pct of elemental carbon in the form of graphite, 15/xm mean size, were made, and appropriate amounts of dry powders and graphite were mixed for a period of 4 hours. Compacts of 16-ram diameter, weighing 4 g, were cold compacted uniaxially at a pressure of 500 MPa, the walls being lubricated. This resulted in a green density of 5.6 • 103 and 6.0 • 103 kgm -3 for the T15 and M2 steels, respectively. These values are equivalent to 68 pet total density (TD) for T15 and 74 pct TD for M2. METALLURGICAL TRANSACTIONS A VOLUME 23A, FEBRUARY 1992--389