Effect of nitrogen on supersolidus sintering of modified M35M high speed steel I. Aguirre, S. Gimenez, S. Talacchia, T. Go ´ mez-Acebo, and I. Iturriza atmosphere, 90N 2 –9H 2 –1CH 4 . The diminution of the optimum sintering temperature and the widening of the Water atomised high vanadium M35M high speed sintering gate were proved. It was also demonstrated steel powders with different carbon contents were cold that sintering to full density of modified high carbon–high compacted at 700 MPa and sintered under an N 2 rich vanadium T42, T15, and M35M HSSs under 1150°C is atmosphere. Full density specimens were obtained at possible.10–12 This marks a historical milestone because temperatures between 1140 and 1105°C for carbon continuous mesh belt furnaces could now be used to contents between 1·8 and 2·5 wt-% respectively. The sinter HSS. sintering behaviour of these powder mixtures was cor- However, the interpretation of how nitrogen affects related with dilatometric curves and phase diagram sinterability was only made in a descriptive way. The aim calculations made by Thermo-Calc software. It is of this paper is to show the correlation between supersolidus demonstrated that densification takes place above the sintering behaviour and the nitrogen effect. The effect of solidus temperature, within the austenite +carbide + nitrogen has been studied through the sintering behaviour liquid phase field, and that nitrogen widens this of modified M35 HSS. Experimental results have been region, which is responsible for increased sinterability explained under the basis of the supersolidus theory and with regards to vacuum sintered high speed steel. the phase diagrams obtained by Thermo-Calc software PM/0836 and the SSOL databank. Calculations of phase diagrams where the effect of sintering atmosphere can be taken into The authors are at CEIT and Faculty of Engineering, account would be a powerful tool for designing powder University of Navarra, Po Manuel de Lardizabal compositions and processes in order to obtain more 15, Bo de Ibaeta, 20018 San Sebastia ´n, Guipuzcoa, competitive HSS components. Basque Country, Spain. Manuscript received 2 March 1999; accepted 11 June 1999. EXPERIMENTAL PROCEDURE © 1999 IoM Communications Ltd. Vacuum annealed water atomised M35MHV HSS powders were produced at Powdrex Ltd (UK). The chemical analysis of the powders provided by the manufacturer is Fe–1·80C– INTRODUCTION 4·0Cr–5·4Mo–5·5Co–0·035N–4·2V–6·0W–0·06O. Additions After three decades of study, two commercial PM routes of 0·2, 0·5, and 0·7 wt-% of elemental carbon (graphite) are now employed in the production of fully dense were made and dry mixing in a Turbula mixer was carried high speed steel (HSS) components; one is hipping of gas out for 3 h. Powders were cold compacted uniaxially at atomised powders and the other is vacuum sintering of 700 MPa and then sintered in a flowing atmosphere of water atomised powders.1 The latter allows HSS powders 90N 2 –9H 2 –1CH 4 . Specimens were heated at the sintering to be cold pressed and sintered to near net shape. It is well temperature, soaked for 60 min, and oil quenched. Densities established that direct sintering is one of the few commercial were measured using the Archimedes’ method. Transverse applications of the supersolidus processes.2,3 The presence sections were mechanically polished, etched with 3% nital, of a liquid phase associated with a narrow sintering window and observed by optical microscopy. As polished specimens (region of temperature where full density specimens with were observed by a Philips XL30 SEM fitted with an acceptable microstructures are obtained) makes an accurate energy dispersive X-ray system. Dilatometric curves were determination of the sintering gate necessary. This avoids obtained with a Netzsch Dil 402 E/7 dilatometer. Specimens distortion owing to an excessive volume fraction of liquid were heated at 20 K min-1 under a N 2 flowing atmosphere. or, on the contrary, a lack of densification because of an Specimens without carbon addition and those mixed with insufficient liquid phase. Both situations would produce 0·2 and 0·5 wt-%C were heated to 1250°C, while the undesirable mechanical properties. Commercial grade HSSs specimens with additions of 0·7 wt-%C were heated to exhibit a narrow sintering window (less than 3 K for M2 1230°C. and 10 K for T1) and thus vacuum sintering needs to After taking into account the carbon and nitrogen control accurately the temperature inside the furnace, contents after sintering, several phase diagrams were making the sintering of cold compacted near net shape calculated using the Thermo-Calc software.13 The thermo- components expensive. dynamic parameters in the calculations were taken from As a consequence, it is clear that a good understanding the SSOL database produced by the Scientific Group of how the sintering gate is affected by compositional and Thermodata Europe.14 In this software, each phase is processing variables will enhance the production routes modelled considering all the possible contributions to the for HSS parts, in order to make them cheaper and more Gibbs energy, including the magnetic contributions. The flexible. The effect of carbon, cobalt, and molybdenum effect of composition is taken into account by means of additions on the sinterability of different T and M grades different binary interaction parameters and, in some cases, (BT42, BT1, BM2) has been studied by several authors.4–6 ternary or even higher order interaction parameters. The They found that controlled additions of these elements not correlation between calculated phase diagrams (isopleths) only lowers the solidus temperature but also extends the and dilatometric curves was analysed in order to establish sintering gate. solidus lines and sintering windows for different compos- Since 1980, work has been carried out by Urcola and co-workers7–9 on sintering of HSS under a nitrogen rich itions. Powder Metallurgy 1999 Vol. 42 No. 4 353 ISSN 0032–5899