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| gas fired sintering furnaces with internal ENDO generator, Type 600+/ENDO |
Sintering ( I )
Controlled Atmospheres
Sinter-Hardening
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. 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'.
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; - but see the section on 'dimensional changes'.
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. 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.
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 1150C. 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 1350C. 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.
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.
Sinter-Hardening
- New types of sintering furnaces allow low alloy steel parts to be sintered with neutral carbon potential (without decarburization or carburization) and then to be hardened in a rapid cooling zone. The heat treatment is achieved by high speed circulation of a water cooled protective gas in the rapid cooling zone of the furnace with cooling rates of up to 50C/sec achievable between 900C and 400C. This results in a homogeneous martensitic structure in the PM steels. Close dimensional tolerances are maintained in the sinter-hardened parts thus eliminating the need for sizing.
The combination of sintering and hardening in one step has reduced the production costs of low alloy steel parts which need post sintering heat treatment. The sinter-hardening furnace also provides other cost benefits through its ability to generate the endothermic sintering atmosphere in the furnace itself from a combustion gas and air, and also through the use of the endothermic gas flowing out of the sintering zone to heat the PM parts from ambient temperature to approx. 500-600C.
Controlled Atmospheres
Sinter-Hardening
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. 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'.
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; - but see the section on 'dimensional changes'.
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. 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.
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 1150C. 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 1350C. 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.
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.
Sinter-Hardening
- New types of sintering furnaces allow low alloy steel parts to be sintered with neutral carbon potential (without decarburization or carburization) and then to be hardened in a rapid cooling zone. The heat treatment is achieved by high speed circulation of a water cooled protective gas in the rapid cooling zone of the furnace with cooling rates of up to 50C/sec achievable between 900C and 400C. This results in a homogeneous martensitic structure in the PM steels. Close dimensional tolerances are maintained in the sinter-hardened parts thus eliminating the need for sizing.
The combination of sintering and hardening in one step has reduced the production costs of low alloy steel parts which need post sintering heat treatment. The sinter-hardening furnace also provides other cost benefits through its ability to generate the endothermic sintering atmosphere in the furnace itself from a combustion gas and air, and also through the use of the endothermic gas flowing out of the sintering zone to heat the PM parts from ambient temperature to approx. 500-600C.
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