Aging of metal, types, artificial, natural, how it happens and what it depends on

Aging of metals a fairly slow process that results in mechanical changes and alterations in physical and chemical properties.

On aging of metals is influenced by a number of factors, including:

• thermal motion of atoms and molecules;
• mechanical impact (various loads on bending/compression/tearing, etc.);
• light radiation (especially radiation invisible to humans);
• magnetic field (magnetization/demagnetization), etc.

The essence of metal aging is that it reaches an equilibrium state, during which the metal's properties deviate from the norm. Specifically, the material can become softer, brittle, less elastic, etc.

Types of metal aging

A distinction is made between natural and artificial aging.

Artificial aging of metal is the rapid acquisition of the desired composition and properties. Artificial aging is achieved through heat treatment and plastic deformation. For example, when producing duralumin, it is artificially aged for several hours.

Natural aging occurs naturally and does not require any additional conditions. However, the process is more intense with longer periods of time and temperatures approaching 20°C.

Application of aging processes in metallurgy and metalworking

Aging, as an additional treatment, is used as a final step. It is applied to certain metals and alloys in which a supersaturated solid solution can precipitate excess components and spontaneously decompose over time. This method is particularly useful for preparing materials for the production of individual components and parts for which the process described above is critical.

After aging, the metal's hardness and strength increase, but its viscosity and ductility decrease. However, it is important to note that these values are maintained throughout the entire service life of the material.

Aging of steel is performed to change the internal structure and is applied after quenching. The resulting solid ferrite solution, saturated with nitrogen and carbon, decomposes upon heating. Depending on the volume of carbon inclusions in the "aging" material, the internal structure takes on the following forms:

• cubic;
• spherical;
• disc-shaped (in the form of thin plates);
• needle-like.

Heat treatment (artificial aging of metal) is applied to alloys in which the solubility of one element in the solid state is significantly reduced. This property becomes more pronounced as the temperature decreases.

In steels with low carbon content, no higher than 0.05%, artificial aging causes the supersaturated solid alpha solution to decompose. As a result, excess phases precipitate. After such treatment, ductility decreases, but hardness and strength significantly increase. These are precisely the qualities often required in the final metallurgical product.

Orowan's model

The figure shows the Orowan model, which clearly illustrates dislocation movement. The maximum effect can be achieved through natural aging. However, this requires a significant amount of time, which is neither cost-effective nor practical for continuous, high-volume production (it's not like settling wine or cognac in barrels). Therefore, there are artificial methods to accelerate these natural processes (it's a shame you can't do the same with whiskey). However, it's worth noting that artificial aging will significantly reduce the strength properties of the material.

Hardness depending on aging time

The graph shown clearly demonstrates the problem described above: reducing the aging time of the metal does not increase its strength characteristics.

The aging process depends largely on carbon and nitrogen. This is especially noticeable in low-carbon steels. Nitrogen dissolves less readily in alpha iron as the temperature decreases. For example, at 590°C, the dissolved nitrogen content is 0.1%, but at 20°C, its content drops to 0.004%. During aging, the alpha solution releases nitrides. Therefore, the effect of nitrogen is less pronounced than that of carbon under thermal influences.

As the carbon content of steel increases, the structural changes produced by thermal treatment increase. The maximum amount of carbon that can dissolve in alpha iron is 0.02-0.04%. With this content, a quenched product subjected to natural aging has a hardness one and a half times greater than that after annealing.

Aging is the primary method for increasing the strength of heat-resistant alloys (high-nickel alloys). This group also includes aluminum-, copper-, and magnesium-based alloys. Furthermore, the altered structure of these metals and alloys imparts coercivity.

Aluminum and aluminum-copper alloys undergo degradation at different temperatures (above 100°C) due to differences in the structural decomposition temperatures of different metals. Thus, a distinction is made between low-temperature and high-temperature structural decomposition.

The decomposition of a solid solution occurs in two ways. In the first case, the formation and growth of phase particles occurs throughout the entire volume. In the second case, the decomposition is discontinuous (cellular). During this process, the cells grow in colonies. Colonies have a cellular structure, and growth occurs from the grain boundary and moves inward, decreasing in size.

Mechanical and thermal aging

There are two types of metal aging: thermal and mechanical. Let's look at each in more detail.

Thermal aging

The phase that strengthens the metal during thermal treatment occurs at its maximum point. This is where the metastable solution phase occurs in the Guinier-Preston zone. This type of strengthening of metals and alloys is commonly referred to as dispersion strengthening.

Dependence of strength on time and temperature of aging

With longer exposure, overaging begins, meaning a reduction in strength characteristics. This is influenced by:

• coagulation;
• partial replacement of particles by incoherent ones.

Types of thermal aging of metal:

• Two-stage – quenching, then holding at the substitution temperature, and then holding at an elevated temperature to achieve homogeneity of the solid solution.
• Quenching – quenching and one stage of holding with natural cooling.
• Natural - for aluminum alloys.
• Artificial – for alloys of non-ferrous metals by heating to a temperature higher than that used for natural destruction.
• Stabilization – high aging temperature and long holding period help to maintain the dimensions and properties of the part.

Mechanical aging of metal

Steel destruction by deforming forces occurs in the temperature range below the recrystallization process. This is caused by the formation and movement of dislocations. Cold plastic deformation increases the dislocation density, which further increases with increasing loads.

The changing mechanical properties of the metal cause carbon and nitrogen atoms to move toward dislocations located in the alpha solution. Upon reaching the dislocations, the atoms form clouds (Cottrell atmospheres). These clusters impede the movement of dislocations, resulting in a change in properties. The properties inherent to heat-aged parts appear.

While nitrogen, nickel, and copper significantly affect the aging effect of deformation, the addition of vanadium, titanium, and niobium completely eliminates this effect. Therefore, it is recommended to use steel with an aluminum content of 0.02-0.07%.

Recommended modes for aging

Heat treatment:

• for steels with high carbon content: temperature of about 130°C-150°C, holding time of about 25-30 hours;
• for non-ferrous metal alloys: temperature of about 250°C, holding time of about 1 hour.

Plastic processing:

• for a natural process: temperature of about 20°C;
• for artificial process flow: temperature of about 250°C, holding time of about 1 hour.

The heating temperature and holding time are selected individually for each grade of metal and alloy depending on their composition.