3. The heat treatment of non-ferrous metals and alloys

None of the non-ferrous metals and only a very few non-ferrous alloys can be quench hardened like plain carbon steels. The majority of non-ferrous metals can only be hardened by cold-working processes. Alternatively they can be manufactured from cold-rolled (spring temper) sheet or strip, or they can be manufactured from cold-drawn wire. Work-hardened nonferrous metals can be annealed by a recrystallization process that is similar to the process of annealing for plain carbon steels. The main difference is that non-ferrous metals do not have to be cooled slowly. They can be quenched after heating and this has the advantage that the rapid cooling causes the metal to shrink suddenly and this removes the oxide film.

In the case of copper and its alloys this is even more effective if the metal is pickled in a weak solution of sulphuric acid whilst still warm. Heat treatable aluminium alloys (‘duralumin’ is such an alloy) require somewhat different treatment. They can be softened by solution treatment and hardened by natural ageing or they can be hardened artificially by precipitation treatment. The alloy ‘duralumin’ contains traces of copper, magnesium, manganese and zinc; aluminium makes up the remainder of the alloy.

3.1 Solution treatment

To soften duralumin type aluminium alloys, they are raised to a temperature of about 500◦C (depending upon the alloy). At this temperature alloying elements can form a solid solution in the aluminium. The alloy is quenched from this temperature to preserve the solution at room temperature.

Gradually, the solid solution will break down with age and the alloy will become harder and more brittle. Therefore solution treatment must be carried out immediately before the alloy is to be processed. The breakdown of the solution can be delayed by refrigeration at between −6◦C and −10◦C. Conversely it can be speeded up by raising the temperature.

3.2 Precipitation treatment

The natural hardening mentioned above is called age hardening. This is the result of hard particles of aluminium–copper compounds precipitating out of the solid solution. This hardens and strengthens the alloy but makes it less ductile and more brittle. Precipitation hardening can be accelerated by heating the alloy to about 150◦C to 170◦C for several hours. This process is referred to as artificial ageing or precipitation hardening. The times and temperatures vary for each alloy and the alloy manufacturer’s heat treatment specifications must be carefully observed, especially for critical components such as those used in the aircraft industry.

4. Heat treatment furnaces

The requirements of heat treatment furnaces are as follows:

• Uniform heating of the work. This is necessary in order to prevent distortion of the work due to unequal expansion, and also to ensure uniform hardness.

• Accurate temperature control. We have previously discussed the critical nature of heat treatment temperatures. Therefore, not only must the furnace be capable of operating over a wide range of temperatures, it must be easily adjustable to the required process temperature.

• Temperature stability. Not only is it essential that the temperature is accurately adjustable but, once set, the furnace must remain at the required temperature. This is achieved by ensuring that the mass of the heated furnace lining (refractory) is very much greater than the mass of the work (charge). It can also be achieved by automatic temperature control, or by both.

• Atmosphere control. If the work is heated in the presence of air, the oxygen in the air attacks the surface of the metal to form metal oxides (scale). This not only disfigures the surface of the metal, it can also change the composition of the metal at its surface. For example, in the case of steels, the oxygen can also combine with the carbon at the surface of the metal. Reducing the carbon content results in the metal surface becoming less hard and/or tough.

• To provide atmosphere control, the air in the furnace is replaced with some form of inert gas which will not react with the workpiece material. Alternatively the work may be totally immersed in hot, molten salts.

Economical use of fuels. This is essential if heat treatment costs are to be kept to a minimum. If the furnaces can be run continuously on a shift work basis considerable economies can be made. The fuel required to keep firing up furnaces from cold is much greater than that required for continuous running. Thus it is more economical for small workshops to contract their heat treatment out to specialist firms who have sufficient volume of work to keep their furnaces in continuous use.

• Low maintenance costs. The furnace is lined with a heat resistant material such as firebrick. Since the furnace must be taken out of commission each time this lining is renewed, it should be designed to last as long as possible.