| As the term ‘sintered part’ implies, sintering is a key part of the operation. It is here that the compact acquires the strength needed to fulfil the intended role as an engineering component. In general, sintering requires heat. Definition of Sintering The ISO definition of the term reads: “The thermal treatment of a powder or compact at a temperature below the melting point of the main constituent, for the purpose of increasing its strength by bonding together of the particles.” What Happens During Sintering? Theories about exactly what happens during sintering have provided the subject matter of innumerable conferences and learned scientific papers, but these need not concern us here. Suffice to say that atomic diffusion takes place and the welded areas formed during compaction grow until eventually they may be lost completely. Recrystallisation and grain growth may follow, and the pores tend to become rounded and the total porosity, as a percentage of the whole volume tends to decrease. Sintering Atmospheres The operation is almost invariably carried out under a protective atmosphere, because of the large surface areas involved, and at temperatures between 60 and 90% of the melting-point of the particular metal or alloys. Liquid Phase Sintering For powder mixtures, however, the sintering temperature may be above the melting-point of the lower-melting constituent, e.g. copper/tin alloys, iron/copper structural parts, tungsten carbide/cobalt cemented carbides, so that sintering in all these cases takes place in the presence of a liquid phase, hence the term liquid phase sintering. It is, of course, essential to restrict the amount of liquid phase in order to avoid impairing the shape of the part. Sintering Furnaces Control over heating rate, time, temperature and atmosphere is required for reproducible results. The type of furnace most generally favoured is an electrically heated one through which the compacts are passed on a woven wire mesh belt. The belt and the heating elements are of a modified 80/20 nickel/chromium alloy and give a useful life at temperatures up to 1150°C. For higher temperatures walking beam furnaces are preferred, and these are increasingly being used as the demand for higher strength in sintered parts increases. Silicon carbide heating elements are used and can be operated up to 1350°C. For special purposes at still higher temperature molybdenum heating elements can be used, but special problems are involved, notably the readiness with which molybdenum forms a volatile oxide. Molybdenum furnaces must operate in a pure hydrogen atmosphere. |