The process and its advantages
Solution annealing requires an electric furnace in which exact temperature regulation is possible. Just like annealing, solution annealing is divided into the three steps of heating, dwelling, and cooling.
In the first step, the workpiece is heated enough for the alloying elements to dissolve in the metal crystal. In order to achieve a uniform degree of solution in the entire workpiece, this step is followed by holding for the dwell time. Aluminium requires a dwell time of up to 6 hours between 450 ºC and 550 ºC. Steel and titanium require up to 20 hours at temperatures between 780 °C and 1,100 °C.
The final step is a rapid cooling of the material in gas or a liquid. Thick-walled workpieces and some types of steel require quenching with water. If the wall thickness is low or the workpieces are particularly large, cooling by air flow is suitable, because it carries a lower risk of warping. The cooling time depends on the shape of the component, the material type, and the desired outcome.
Hardening by quenching is not possible for solution annealing. Instead, a precipitation hardening process can follow. During this, finely distributed precipitation leads to increased strength. If it takes place at room temperature, this process is called cold natural ageing. At moderate temperatures, the process is referred to as artificial ageing.
The advantages at a glance
Solution-annealed workpieces benefit from the following advantages:
- Homogeneous microstructure
- Less warping
- Improved resistance to oxidation and corrosion
- Greater mechanical strength
Applications and suitable materials
Solution annealing is often performed as an intermediate treatment to achieve a homogeneous microstructure in mixed-crystal solidified alloys.
Uncontrolled carbide precipitation occurs especially in austenitic steels during hot forming between 500 °C and 800 °C. This may lead to intercrystalline corrosion during the use or further processing of the component. This can be prevented by solution annealing.
In addition, solution annealing can improve the mechanical properties of heavily contaminated materials. The prerequisite is that the impurities are soluble in the base metal and can diffuse from the grain boundary to the inside. This reduces the tendency to hot cracking.
Solution annealing is commonly applied to the following materials:
- Various copper alloys
- Austenitic steels
- Different alloys and grades of aluminium
Size of furnace
Maximum workpiece size for solution annealing under protective gas: 1,200 x 1,500 x 800 mm
Maximum workpiece size for solution annealing without protective gas: 1,100 x 1,000 x 700 mm