Heat treatment is the process of heating and cooling metals, using specific predetermined methods to obtain desired properties. Both ferrous as well as non-ferrous metals undergo heat treatment before putting them to use. For that they develop new schedules or cycles to produce a variety of grades. Each schedule refers to a different rate of heating, holding and cooling the metal.
These methods, when followed meticulously, can produce metals of different standards with remarkably specific physical and chemical properties.
There are various reasons for carrying out heat treating. Some procedures make the metal soft, while others increase hardness. They may also affect the electrical and heat conductivity of these materials.
Some heat treatment methods relieve stresses induced in earlier cold working processes. Others develop desirable chemical properties to metals. Choosing the perfect method really comes down to the type of metal and the required properties.
In some cases, a metal part may go through several heat treatment procedures. For instance, some super alloys used in the aircraft manufacturing industry may undergo up to six different heat treating steps to optimise it for the application.
Heat Treatment Process Steps
In simple terms, heat treatment is the process of heating the metal, holding it at that temperature, and then cooling it back. During the process, the metal part will undergo changes in its mechanical properties. This is because the high temperature alters the microstructure of the metal. And microstructure plays an important role in the mechanical properties of a material.
The final outcome depends on many different factors. These include the time of heating, time of keeping the metal part at a certain temperature, rate of cooling, surrounding conditions, etc. The parameters depend on the heat treatment method, type of metal and part size.
Over the course of this process, the metal’s properties will change. Among those properties are electrical resistance, magnetism, hardness, toughness, ductility, brittleness and corrosion resistance.
Common Heat Treatment Methods
There are quite a few heat treatment techniques to choose from. Every one of them brings along certain qualities. The most common heat treatment methods include:
- Stress relieving
In annealing, the metal is heated beyond the upper critical temperature and then cooled at a slow rate. Annealing is carried out to soften the metal. It makes the metal more suitable for cold working and forming. It also enhances the metal’s machinability, ductility and toughness.
Annealing is also useful in relieving stresses in the part caused due to prior cold working processes.
Normalising is a heat treatment process used for relieving internal stresses caused by processes such as welding, casting, or quenching.
In this process, the metal is heated to a temperature that is 40° C above its upper critical temperature.
This temperature is higher than the one used for hardening or annealing. After holding it at this temperature for a designated period of time, it is cooled in air. Normalising creates a uniform grain size and composition throughout the part.
Normalised steels are harder and stronger than annealed steel. In fact, in its normalised form, steel is tougher than in any other condition. This is why parts that require impact strength or need to support massive external loads will almost always be normalised.
The most common heat treatment process of all, hardening is used to increase the hardness of a metal. In some cases, only the surface may be hardened.
A work piece is hardened by heating it to the specified temperature, then cooling it rapidly by submerging it into a cooling medium. Oil, brine or water may be used. The resulting part will have increased hardness and strength, but the brittleness increases too simultaneously.
Case hardening is a type of hardening process in which only the outer layer of the work piece is hardened. The process used is the same but as a thin outer layer is subjected to the process, the resultant work piece has a hard outer layer but a softer core.
This is common for shafts. A hard outer layer protects it from material wear. When mounting a bearing to a shaft, it may otherwise damage the surface and dislocate some particles that then accelerate the wearing process. A hardened surface provides protection from that and the core still has the necessary properties to handle fatigue stresses.
Ageing or precipitation hardening is a heat treatment method mostly used to increase the yield strength of malleable metals. The process produces uniformly dispersed particles within a metal’s grain structure which bring about changes in properties.
Precipitation hardening usually comes after another heat treatment process that reaches higher temperatures. Ageing, however, only elevates the temperature to medium levels and brings it down quickly again.
Some materials may age naturally (at room temperature) while others only age artificially, i.e. at elevated temperatures. For naturally ageing materials, it may be convenient to store them at lower temperatures.
Stress relieving is especially common for boiler parts, air bottles, accumulators, etc. This method takes the metal to a temperature just below its lower critical border. The cooling process is slow and therefore uniform.
This is done to relieve stresses that have built in up in the parts due to earlier processes such as forming, machining, rolling or straightening.
Tempering is the process of reducing excess hardness, and therefore brittleness, induced during the hardening process. Internal stresses are also relieved. Undergoing this process can make a metal suitable for many applications that need such properties.
The temperatures are usually much lower than hardening temperatures. The higher the temperature used, the softer the final work piece becomes. The rate of cooling does not affect the metal structure during tempering and usually, the metal cools in still air.
The heat treatment furnace play a very important role in imparting reproducible useful properties to the steel components. The design of the furnace is determined by the stock which is to be treated and the particular treatment which has to be carried out at the special temperature.
When parts are required to be heat treated at different temperatures, several furnaces may be required as a furnace which may be suitable for use at 1300°C, may be unsuitable for use at 300°C, although the latter temperature is within its maximum temperature range.