Conventional Heat Treatments
Introduction
Usually, after casting or forging processes, the materials become more hard.
Therefore, there are many conventional heat treatments that are available to alter the properties of the as-cast or as-wrought components. These processes increase the strength of the items, and change some of the mechanical properties.
These include improving machining capability, improving the formability, and acting to restore ductility after a cold working operation.
Hence, it is a very enabling manufacturing process. This can not only help other manufacturing methods, but can also enhance the capability of the product. This is obtained by increasing the strength, or other desirable characteristics.
Heat treatment types
There are a number of basic types of heat treatment processes used today:
- Solution heat treatment
- Annealing
- Process annealing
- Normalising
- Normalising and tempering
- Hardening
- Tempering
- Spheroidising
- Homogenising
These processes are briefly described here.
Davis Scientific Treatments can carry out any of these treatments, under high vacuum, which also preserves quality appearance of component parts.
Solution heat treatment
Solution heat treatment involves the heating of an alloy to a suitable temperature, for sufficient time to induce one or more of the constituents to enter into solid solution. This process is suitable for ferrous or non-ferrous materials.
Often this treatment is a precursor to precipitation or age hardening, for improved machinability, formability, and/or corrosion resistance. Constituents precipitate from solid solution to induce an appreciable increase in hardness.
Suitable materials include:
- aluminium alloys
- titanium alloys
- beryllium copper alloys
- inconel and similar heat resisting alloys
- nickel based alloys
- maraging steels
Annealing
Full annealing of a metal piece will restore its softness. Forgings & castings are heated to a particular temperature, holding it there for a definite time, then cooling to room temperature.
The metal is annealed to relieve internal stresses, to soften them, to make them more ductile, and to refine their grain structures.
Process annealing
This process is intended for low carbon steel forged parts. The piece is heated to lower temperatures than full annealing or normalising.
The piece is then allowed to air cool. This method changes the grain size and flow found in the forging.
Normalising
This type of heat treatment is only applicable to ferrous castings and forgings.
The metal is heated to a higher temperature.
The objective is to remove internal stresses caused by machining, forming, casting, forging, or other forming, and indeed by heat treatment.
If not controlled, stress can cause metal failure. Before hardening, first the normalising process should be carried out.
Usually castings are annealed, instead of normalising; however some castings need normalising.
Normalised steels are stronger and harder than annealed steels.
Normalising and tempering
This process combines the normalising treatment with tempering.
First the normalising process is carried out. Then the forging or casting is heated to a temperature between 400 and 600 deg C, to temper it.
This procedure improves ductility and strength.
Hardening
Metal hardening treatment consists of heating the parts to definite temperature, then cooling rapidly.
The majority of steels require rapid cooling [quenching] for hardening. However, a few can be air-cooled, for the same results.
Although hardening enhances the hardness and strength of the pieces, it makes them less ductile.
Usually the harder the steel, the more brittle it becomes. To remove some of the brittleness, the steel should be tempered after hardening.
Tempering
After hardening, steel is generally harder than required, and is often too brittle for many practical applications.
The tempering process is always performed after hardening, and so reduces internal stresses. It comprises of heating the material to a particular temperature [below hardening], and holding at that temperature, before cooling.
Final strength, hardness and ductility depend on the particular treatments, as well as giving a general softening of the pieces.
Spheroidising
Spheroidising is used in particular to improve the qualities oh high carbon steels, tool steel, and alloy steel forgings. This method improves machinability, by forming spheroids throughout the forging.
Homogenising
Homogenising is a separate heat treatment method, where the casting or forging is brought to an elevated temperature, often 80-90 % of the melting temperature.
This process allows the reduction or elimination of chemical segregation of composites. These include dendritic structures, or carbide and other deleterious phases.
In this method, the material is heated for a long enough period of time, to permit diffusion of alloying elements. the usage promotes uniformity of chemical
composition and microstructure.
Davis Scientific Treatments (DST)
At Davis Scientific Treatments, considerable R & D effort is being carried out, on a regular and continuing basis, to improve quality and speed with which castings and forgings are heat treated.
As a result, we have gained extensive experience in the development of some appropriate process treatments.
We are always ready and willing to discuss any new projects or applications.