PRESINTERING & SINTERING - POWDER METALLURGY

PRESINTERING :

Presintering is the process of heating the green compact to a temperature below the sintering temperature. This is done to remove the lubricants and binders added during blending and to increase the strength of the compact. All metals do not require presintering. But some metals like tungsten carbide are easily machined after presintering. After sintering they become so hard that they cannot be machined.

SINTERING :

After being compacted into a briquette having the shape of the finished workpiece, the cold-welded aggregate of metal particles is heated in a furnace to a temperature close to the melting point of the basic metal which goes into the mixture. This is carried out in controlled atmosphere furnaces. It may also be carried out under protective gas normally hydrogen or in a vacuum if material tends to react with the protective gas. The heating causes the metal particles to sinter, that is, a proportion of them partly melt and by so doing cement the remaining particles together in a cellular structure. From the economic point of view, the sintering time should be as short as possible , but the time must be long enough to obtain the required properties in the workpiece.

Sintering is performed to achieve all possible final strength and hardness needed in the finished product. The three most important variables governing the sintering process are temperature, time and sintering atmosphere. The workpiece dimensions change during sintering. Such changes may be either a shrinkage or growth. In general, bronze tends to expand and iron and brass to contact.

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