furnaces with controlled atmospheres
These are essential for almost all sintering processes, to prevent oxidation and to promote the reduction of surface oxides. In practice dry hydrogen, cracked ammonia, and partially combusted hydrocarbons are mainly used, although the first named is often precluded because of cost. It is however, used for sintering carbides and magnetic materials of the Alnico type.
Dissociated ammonia containing 75% hydrogen and 25% nitrogen can readily be produced free from oxygen or water vapour, and having a dewpoint of the order of -50C. It can replace pure hydrogen for many applications at approximately one-third the cost, with the obvious exceptions where reaction with nitrogen cannot be tolerated. It is particularly useful for sintering iron, steel, stainless steel, and copper-base components.
The most widely used atmospheres primarily because of their lower cost, are produced by partial combustion of hydro-carbons. By variation of the air-to-gas ratio, a wide range of compositions is obtained. For practical applications, since the combusted gas contains water vapour it must be dried to a dewpoint of less than 0oC for satisfactory operation with iron components. Hydrocarbon gas, such as methane, butane or propane, reacted with a limited amount of air may contain up to 45% of hydrogen, some carbon monoxide and dioxide with nitrogen as the remainder. Because of the endo-thermic nature of this reaction, external heat has to be supplied, and for that reason the resulting atmosphere is called endogas.
If the hydrocarbon is burnt with just insufficient air for complete combustion, an atmosphere which may contain 5% or less of hydrogen and a very large percentage of nitrogen is produced, and as this reaction is exo-thermic, the atmosphere is called exogas. It is the cheapest atmosphere available, but its reducing potential is low and thus the removal of oxides from the powder compacts is less efficient and lower sintered strengths may result. For sintering steels, i.e. ferrous alloys containing carbon as an alloying element, the carbon potential of the atmosphere is very important. It should be in equilibrium with the steel; see the later section on 'Structural Parts'.
Finding increasing application are so-called 'synthetic' atmospheres, also called nitrogen-based atmospheres, since they are produced by careful mixing of predominantly nitrogen with hydrogen, and, for the sintering of steels, a hydrocarbon gas in predetermined proportions. These, though possibly more expensive than exo- or endogas, have the advantage of cleanliness, more reliable adherence to the specified composition, and inherently low water vapour content.