The process: Treatment steps
Precipitation hardening takes advantage of the fact that the solubility of some alloy constituents falls as the temperature decreases. The desired outcome requires the completion of three steps: solution annealing, quenching, followed by ageing (during which the actual precipitation takes place).
Solution annealing (diffusion annealing, homogenising)
In order for the subsequent precipitation to succeed, all necessary elements must first be present as dissolved elements. Solution annealing is used for this purpose. During this step, the correct temperature is of key importance. It must be high enough to minimise the presence of coarse particles. However, it must also not be too high, because otherwise structural components will melt, rendering further processing impossible.
This process may only take a few minutes, but can also last several hours. This depends on the size of the component, the fineness or coarseness of the microstructure, the alloy type, and the processing of the semi-finished product (e.g. forged, pressed, etc.).
The so-called dispersoids are precipitated as early as solution annealing. These particles impede the movements of the grain boundary and thus control recrystallisation. Due to their size and low concentration in the material, they only cause a negligible increase in strength.
The next step in the process is quenching in a suitable medium. Suitable media are water or oil, and also gas or compressed air. The quenching medium depends on the material. Quenching prevents diffusion, and causes the mixed crystal to assume a metastable, oversaturated, and single-phase state.
The actual precipitation/ageing takes place in the final step of the process. The temperature determines the duration and the type of the precipitation. Factors such as nucleation and precipitate maturation can be adjusted.
The correct temperature for precipitation depends primarily on the material alloy. For aluminium alloys and maraging steel, the temperatures range between 150 °C and 190 °C and between 450 °C and 500 °C respectively.
While the diffusion or precipitation is accelerated by the temperature increase, the oversaturated, single-phase mixed crystal is transformed into a two-phase alloy.
The first phase is the matrix – it is coherent in terms of volume, and usually makes up the larger portion. The newly formed second phase is the precipitate– a homogeneous microstructure of many small precipitates that can be adjusted specifically.
The advantages at a glance
Compared to other hardening processes, precipitation hardening takes place at relatively low temperatures. This creates minimum warping.
Precipitation hardening can be applied only to alloys that meet the following requirements:
- At least one alloying element is capable of forming mixed crystals with the base metal.
- The alloying elements must be decreasingly soluble in the base metal as the temperature falls.
- The precipitation temperature must generate a driving force and a diffusion speed that create sufficient nucleation.
- Homogeneous distribution of the precipitates in the material is important for preventing coagulation at the operating temperature.
Age hardening of aluminium alloys
Precipitation hardening plays a major role especially for aluminium alloys, as they cannot be hardened by martensite formation. Instead of polymorphic conversion, diffusion must be used to harden them.
For example, rivets made of duraluminium are subjected to age hardening. Besides aluminium, duraluminium contains 4% copper and 1% magnesium. It undergoes solution annealing at 495 °C to 505 °C. Quenching is followed by the ageing step in order to attain the final strength. This can take place by natural ageing at room temperature or by artificial ageing. Aluminium alloys treated by age hardening are considered susceptible to corrosion as the treatment does not allow the formation of an oxide layer. Freezing at -18 °C can be used as a means to delay age hardening, in order to achieve a longer processing time.
Precipitation annealing must always be preceded by solution annealing. When in their solution-annealed state, the following materials are suitable for age hardening: special stainless steels (e.g. maraging steel 1.4542), aluminium (e.g. duraluminium), titanium, bronze alloys, and other non-ferrous metals.
Information necessary prior to age hardening
The following information helps us prepare a precipitation hardening proposal for you:
- Material designation including, if available, material data sheet
- Dimensions of the component
- Information about the microstructural state – the workpiece must have been solution-annealed, but the effects of this may lost, however, through mechanical or thermal effects
Size of furnace
Treatment in a convection furnace with cooling in the air:
Ø 1,010 x 1,000 mm
Treatment in a protective gas/vacuum furnace with cooling in a nitrogen atmosphere:
1,200 x 1,500 x 800 mm