Ageing of metals and alloys

Ageing is generally the final stage of strengthening heat treatment applied to alloys that have previously been solution heat-treated and quenched to retain a supersaturated solid solution. During holding, finely dispersed particles of strengthening phases precipitate within the material’s structure. These particles hinder dislocation movement, thereby increasing the strength, yield strength and hardness of the alloy.

Unlike quenching involving a polymorphic transformation, which retains a non-equilibrium structure through rapid cooling, ageing develops in a controlled manner the changes initiated during solution heating and quenching. It is therefore classified as a strengthening rather than a softening heat-treatment process.

The key condition for ageing is a decrease in the solubility of alloying or impurity elements in the alloy matrix as the temperature falls. During solution heating prior to quenching, these elements enter the solid solution. Rapid cooling prevents them from fully precipitating, creating a supersaturated and thermodynamically unstable state. During subsequent holding, the excess atoms gradually form strengthening phases. By selecting the appropriate temperature and process duration, the technologist controls the degree of solid-solution decomposition and the final properties of the component.

Why ageing strengthens metal

The strengthening effect is based on the mechanism of precipitation hardening. During the early stages of ageing, atomic clusters and fine precipitates form within the alloy. In certain aluminium alloys, Guinier–Preston zones are formed, followed by intermediate metastable and equilibrium phases. The exact transformation sequence depends on the chemical composition of the material.

Fine particles of the strengthening phase create local distortions in the crystal lattice and obstruct dislocation movement. The greatest increase in strength is generally achieved when the precipitates have an optimal size, density and uniform distribution. If the holding time is extended or the temperature is increased beyond the specified conditions, the particles coarsen, the distance between them increases, and the strength and hardness decrease compared with the peak-aged condition. This change is known as overageing.

Overageing is not always considered a defect. For certain components, it is intentionally used when improved ductility, toughness, dimensional stability or corrosion resistance is required. The treatment conditions are therefore selected not only to achieve maximum hardness but also to satisfy the full range of operating requirements.

Types of ageing

Depending on the processing temperature, ageing is classified as natural or artificial. Natural ageing of steel takes place at room temperature without additional heating: after quenching, the properties of the alloy gradually change over a specified period. Artificial ageing is performed at an elevated temperature in a furnace. It accelerates the decomposition of the supersaturated solid solution and makes it possible to achieve the required combination of strength, hardness, ductility and property stability within a controlled period.

According to the mechanism of structural change, thermal ageing and strain ageing are distinguished. Thermal ageing develops in a previously quenched alloy under the influence of temperature and time. Strain ageing occurs after cold plastic deformation, such as rolling, stamping or drawing, at temperatures below the recrystallisation temperature.

In low-carbon steels, strain ageing is mainly associated with the interaction of carbon and nitrogen atoms with dislocations. As a result, the yield strength may increase, while ductility and impact toughness decrease. Under certain conditions, this increases the risk of brittle material behaviour. Strain ageing must therefore be considered when designing the manufacturing process and determining the subsequent operating conditions of a component.

Materials subjected to ageing

Ageing is the primary strengthening method for many alloys that cannot be effectively hardened by quenching involving a polymorphic transformation. A classic example is aluminium alloys, including duralumin alloys: most of their strength increase after quenching is achieved through subsequent ageing.

The process is also used for beryllium bronzes and various copper, magnesium, titanium, nickel and other heat-resistant alloys. Maraging steels form a separate group. Their high strength is achieved mainly through the precipitation of intermetallic phases from low-carbon martensite, making it possible to combine high strength with good toughness and manufacturability.

Benefits of component ageing

When the correct treatment conditions are selected, ageing increases the ultimate tensile strength, yield strength and hardness of the material. In certain alloys, the treatment also helps stabilise the structure, reduce some residual stresses and improve the dimensional stability of the component. The extent of this effect depends on the alloy grade, component geometry, preceding quenching conditions and ageing parameters.

Dimensional stability is particularly important for precision components, tools, moulds and critical assemblies. However, ageing does not completely eliminate the possibility of component distortion if the process is not properly controlled or if the component has a complex geometry. The treatment conditions must therefore be selected individually and, where necessary, supplemented with intermediate checks of geometry and mechanical properties.

An increase in strength is usually accompanied by a certain reduction in ductility. The technologist’s task is to select a temperature-and-time cycle that provides the required balance of hardness, strength, toughness, ductility and dimensional stability.

Ageing of metals and alloys at KARBAZ

KARBAZ provides ageing treatment for metals and alloys, selecting the temperature-and-time cycle according to the material grade, component geometry and operating requirements. The treatment may be carried out as an independent operation after quenching performed by the customer or as part of a complete technological cycle that includes preliminary solution heat treatment and quenching.

The company’s own production and laboratory equipment makes it possible to control the processing temperature, holding time and cooling conditions, as well as to verify hardness and other parameters specified in the technical requirements. Material structure analysis can also be carried out when necessary. This approach helps achieve the required properties and reduces the risk of underageing or overageing the alloy.

KARBAZ specialists provide advice on selecting the appropriate treatment conditions based on the alloy grade, the initial condition of the workpiece and the intended application of the finished component. After treatment, the characteristics specified in the order are inspected. The service cost and available discount terms are calculated based on the material, component dimensions, complexity of the treatment cycle and batch size.