. 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

No abstract

0•15 wt-%. Owing to the lower nitrogen content the, optimum sintering temperature (OST) is higher than The sintering behaviour of high carbon-high 1150°C but lower than that of direct vacuum sintering. vanadium water atomised M35MHV HSS (1•8 wt-%C, Recently, the effect of nitrogen on the sinterability of 4•2 wt-%V) is analysed as a function of the nitro-M35MHV HSS has been shown through the calculation of gen pressure in the sintering atmosphere. Uniaxphase diagrams by means of ThermoCalc software and ially pressed compacts were sintered to full density their correlation with dilatometric and sintering data.11 It (Á98%TD) under different N 2 atmospheres in a range was demonstrated that densification takes place above the of pressures from vacuum to 8 bar. It is observed that solidus temperature within the austenite+carbide+liquid the optimum sintering temperature (OST) depends on phase field, and that nitrogen widens this region, being the absorbed nitrogen and is as low as 1050°C when responsible for increased sinterability with respect to the nitrogen content in the steel is 1•2 wt-%. The vacuum sintered HSS. This work combines the contribution absorbed nitrogen affects not only the OST but also of Bradford's team on the supersolidus sintering of the matrix and carbides composition and the phases HSSs12-14 with the more empirical research of CEIT on present after sintering. Compared with other powders the sintering of HSS under nitrogen atmosphere.1,2,4,5,9 It processed under the same conditions, it is shown that was also demonstrated that calculations of phase diagrams the amount of absorbed nitrogen depends not only on where the atmosphere effect can be taken into account the nitrogen partial pressure in the sintering atmoswould be a powerful tool for designing powder compositions phere but also on the amount of vanadium and and processes in order to obtain more competitive HSS carbon and even on the heating rate. Hardness, fraccomponents.11 ture toughness, and fracture strength values areThe aim of the present paper is to analyse the effect of reported after heat treatment.PM/0922 the nitrogen pressure on the sintering behaviour and microstructure of water atomised high vanadium M35MHV Dr Aguirre is in the Escuela Superior de Ingenieros HSS and to understand the factors affecting the absorption Industriales. University of Navarra. PO Box 1674, of nitrogen by the steel.

Resumen Se estudia la fabricación de matrices de corte de aceros rápidos para hierramientas mediante procesos de metalurgia de polvos. Los polvos de los aceros T42 con contenidos de carbono y vanadio modificados, M3/2 y M2, obtenidos por atomización en agua, se compactaron de tal modo que quedaron dotados de formas geométricas complejas. Después de sinterizados en una atmósfera industrial 90 N2-9 H2-I CH4, se trataron térmicamente en medios de distintas severidades. Tras el temple desde temperaturas de austenización cercanas a la temperatura óptima de sinterizado, se lograron durezas de hasta 1.100 HV. Se determinaron, también, las contracciones sufridas por las piezas sinterizadas.Palabras clave: Metalurgia de polvos. Acero rápido para herramientas. Matriz de corte. Sinterización. Tratamientos térmicos. High speed tool steel cut off dies made using powder metallurgy techniques AbstractThe present work summarises the results obtained by compacting water atomised powders of T42 modified carbón and vanadium, M3/2 and M2 high speed steels. The powders were compacted under complex forms (cut off dies) and sintered under a 90 N2-9 H2-I CH4 industrial atmosphere. Heat treatments in different severity media were performed. Hardness of 1,100 HV can be obtained in the case of T42 modified steel. Also, the shrinking of the sintered specimens was study.