In sintering the base powder is made to glow by heating at temperatures that are below its melting point. Diffusion processes take place here between the touching particles of powder, which lead to increasing strength. One of the key factors in achieving optimum quality is the relationship of sintering temperature to sintering duration.
The furnaces used for sintering are usually electrically heated sintering furnaces with Cr-Ni heating elements. As conveyor belt furnaces are able to achieve temperatures of up to 1130°C, they are used primarily for sintering non-ferrous metals and iron.
The continuous (annealing) furnaces are charged with inert gas, which prevents the powder particles from oxidising. The gas used is predominantly cracked ammonia, although exothermic and endothermic gases, nitrogen and hydrogen are also used.
In the first stage of the manufacturing process the semi-finished and finished parts pass through the furnace’s stearate burn-off chamber. At temperatures of around 400°C to 600°C, this forces out the stearates, i.e. the compression moulding additives, to avoid them interfering with the sintering process. The actual sintering process, which is very precisely timed, takes place in the subsequent high-temperature zone at a heat of 1030°C to 1250°C.