Homogenizing annealing – reducing segregation and homogenizing the material composition

Homogenizing annealing, also called homogenization or diffusion annealing, is a process heat treatment Primarily used to reduce local differences in chemical composition that occur during the solidification of metals and alloys. It involves heating the material to a sufficiently high temperature, prolonged annealing to allow for the diffusion of alloying elements, and controlled cooling.
Unlike processes such as normalization, stress relief annealing hardening and thermal improvement, the primary purpose of homogenization is not to directly achieve final hardness, strength or stress removal. The process is primarily intended reduce the chemical inhomogeneity of the material inherited from solidification.
Homogenization is carried out in the solid state. The temperature must be high enough to allow diffusion of slower-moving alloying elements at a technologically useful rate, but at the same time it must remain below the temperature at which melting can begin in locally enriched regions of the material.
The most important goal of homogenization is to reduce segregation occurring during coagulationThis treatment is primarily used on ingots, castings, and large semi-finished products prior to further plastic processing or heat treatment. It applies to both steels and alloys. aluminum material, nickel and other metals.

What is chemical segregation?

During the solidification of an alloy, the composition of the solid phase that forms is usually not identical to the composition of the remaining liquid. Individual elements may be rejected to varying degrees by the growing crystals or incorporated into the emerging solid phase.
As solidification progresses, the composition of the remaining liquid changes. Consequently, the previously formed portions of the dendrites may have a different chemical composition than the areas formed in the final stage of solidification.
A characteristic pattern is created dendritic segregation.Dendritic cores and interdendritic spaces may differ in content. coal, manganese, chrome, nickel, molybdenum, silicon and other alloying components.
Such differences influence the local transformation temperature, the share of individual phases, hardenability, susceptibility to deformation and the material's reaction to subsequent changes. hardening and thermal improvementIn some alloys, segregation may also promote the formation of low-melting components, structural banding, or unfavorable precipitates.

Homogenization annealing does not completely equalize the composition of the entire ingot. Its primary purpose is to reduce inhomogeneity, primarily at the microstructural scale. Macrosegregation spanning distances of centimeters or greater is much more difficult to remove, as the required diffusion time would be technologically unacceptable.

Why is segregation bad?

A non-uniform distribution of elements means that different areas of the same ingot or casting may behave as materials with slightly different chemical compositions.
This can lead to:

• local differences in hardenability,
• uneven course of changes during hardening,
• differences hardness and mechanical properties,
• the formation of structural banding after rolling or forging,
• uneven distribution of carbides and other precipitates,
• local lowering of the initial melting temperature,
• deterioration of the material's susceptibility to plastic processing,
• uneven reaction of the material to subsequent heat treatment.

Homogenization is therefore primarily a procedure that prepares the material structure for further processing , and not a typical final processing of a machine part.

Diffusion – the mechanism of homogenizing annealing

The homogenization of the composition takes place by diffusion of atoms in the materialThe differences in the concentration of alloying elements are reduced by their movement between adjacent areas.
The driving force behind this process is the system's desire to reduce chemical potential differences. In a simple case, the process can be conceptualized as the movement of a component from an area of ​​higher concentration to one of lower concentration, but in real multi-component alloys, the mechanism is more complex.
The rate of diffusion increases significantly with temperature. Therefore, homogenization requires high temperatures and a sufficiently long heating time.
However, individual elements move at different speeds. Coal in steel it is an interstitial element and usually diffuses much faster than substitutional alloying elements such as chrome, nickel, manganese molybdenum.
It is the slow diffusion of substitution elements that is one of the main reasons why industrial homogenization annealing requires a long time.
The approximate relationship between the process time and the diffusion path can be written:

• t – characteristic diffusion time;
• L – characteristic distance at which the train is to be equalized;
• D – diffusion coefficient of a given component in a specific phase and temperature.

For the same diffusion coefficient, doubling the transport distance translates to approximately quadrupling the time. This relationship well explains why it is possible to limit the compositional differences between the dendrite core and the interdendritic region, but it is much more difficult to equalize the composition between the axis and the surface of a large ingot.
However, this is not a ready-made formula for determining industrial homogenization time. In a real material, the diffusion coefficient depends on temperature, phase, element type, and alloy composition.

The effect of temperature on the homogenization rate

The diffusion coefficient increases very strongly with increasing temperature. From a technological point of view, this means that increasing the temperature can significantly shorten the time needed to reduce segregation.
However, you cannot increase the temperature arbitrarily.
Interdendritic regions can be enriched with components that lower the local melting point. The material may therefore begin to melt locally at a temperature lower than that corresponding to its average chemical composition.
This phenomenon is one of the most important limitations of the homogenization process.

Homogenizing annealing process

1. Material evaluation. The chemical composition, state after casting, segregation scale, type of phases and presence of components that may be subject to local melting are determined.
2. Temperature selection. The temperature should ensure sufficiently rapid diffusion, but remain below the temperature at which uncontrolled local melting begins.
3. Controlled heating. The heating rate is selected according to the dimensions, geometry and susceptibility of the material to cracking due to temperature gradients.
4. Cross-section temperature equalization. In large ingots and forgings it is necessary to take into account the time required to heat the core.
5. Homogenizing annealing. The material is maintained at a high temperature for a time sufficient to limit segregation and achieve the desired phase transformations.
6. Controlled cooling. The cooling method depends on the type of alloy, the required structure and the next technological stage.
7. Further processing. Depending on the material, forging, rolling, extrusion are used, normalization, other types of annealing or further heat treatment.

Not every homogenization process ends with very slow furnace cooling. Especially in aluminum alloys , the cooling rate can affect phase separation and the material's subsequent processability.

Homogenization temperature and time

There is no single homogenization annealing temperature suitable for all materials. The parameters depend on the alloy composition, local melting onset temperature, distance between dendrite arms, type of precipitates, size of the workpiece, and required homogenization efficiency.

Single-stage and multi-stage homogenization

The simplest cycle involves heating the material to a set temperature and heating it for a specified time.
However, in alloys with a more complex composition it can be used multi-stage homogenizationPreheating at a lower temperature may result in partial transformation or dissolution of components, the presence of which limits the ability to safely use a higher temperature.
After such a stage, it is possible to increase the temperature and accelerate the diffusion of slower-moving elements.
A multi-stage cycle is not a universal solution. It must be tailored to the specific alloy, as an incorrect temperature sequence can lead to localized melting, undesirable precipitation, or excessive grain growth.

Homogenizing annealing of steel

In steels, homogenization serves primarily to reduce dendritic segregation in ingots and castings before further processing.
The differences in distribution are important, first of all manganese, chrome, nickel, molybdenum, silicon and other alloying elements.
In alloy steels, the uneven distribution of these components can cause local differences in hardenability. After subsequent hardening different areas of the material may then exhibit different microstructures and properties.
The process may be a stage of preparing the material for large open-die forgings and semi-finished products from alloy structural steels for thermal improvement.
Examples of species from this group are: 40HNMA / 40NiCrMo6 / 40NiCrMo7 and 30H2N2M / 30CrNiMo8.
This does not mean that each product made from these grades requires a separate homogenization annealing. The need for this process is determined by the ingot or casting production technology, the size of the semi-finished product, the level of segregation, and the subsequent processing technology.

Homogenization and forging and rolling

Homogenization often precedes hot plastic processing.
Reducing microsegregation can facilitate more uniform material behavior during deformation. Plastic processing itself also plays a significant role in shaping the structure: it breaks down and lengthens the casting structure, influences the phase distribution, and enables the desired geometry of the semi-finished product.
In the case of large open-die forgings the combination of properly designed homogenization and subsequent forging can be part of the entire process leading from the ingot structure to a material with more uniform properties.
However, homogenization does not replace plastic processing and vice versa – these processes solve various technological problems.

Homogenization and normalization

Long-term exposure of steel to high temperatures may cause grain growthThe material after homogenization does not have to have a microstructure appropriate for the finished element.
Therefore, for appropriate steel grades, further processing may include plastic working or normalization.
Normalizing serves a different purpose than homogenization. Its primary goal is to properly shape the steel's structure after austenitizing and air cooling, while homogenization primarily serves to reduce chemical segregation.

Homogenization of aluminum alloys

In the feet aluminum material Homogenization of ingots is an important step in preparing the material for further processing, including rolling or extrusion.
The process may:

• limit microsegregation occurring during coagulation,
• change the composition and morphology of interdendritic phases,
• cause the dissolution of certain components in the matrix,
• shape the distribution of fine precipitates,
• influence the ingot's susceptibility to subsequent plastic processing.

Silicon , copper , manganese and other components of a given alloy may play an important role .

Homogenization of nickel alloys

In selected nickel-based superalloys The segregation that occurs during solidification is of particular importance due to the large number of alloying components.
Next to nickel there may be significant amounts chrome, molybdenum, niobium, titanium, aluminum, cobalt and other elements.
Homogenization can reduce segregation after solidification and prepare the microstructure for further processing.
However, the choice of temperature is particularly important. Locally enriched areas may have a lower melting onset temperature than a material of average chemical composition, so overheating can cause localized melting.

The influence of selected elements on homogenization

Homogenization and other types of annealing

Benefits of homogenization annealing

A properly selected process can:

• reduce microsegregation of alloying components,
• limit compositional differences between dendrite cores and interdendritic areas,
• reduce the heterogeneity of the distribution of specific phases,
• improve the uniformity of the material's response to further heat treatment,
• facilitate subsequent plastic processing,
• limit some of the causes of structural banding,
• prepare large ingots and castings for further forging or rolling.

However, automatic increases in hardness , tensile strength , or impact strength should not be assumed . Ultimate properties also depend on grain size, microstructure, phases, and subsequent technological steps.

Limitations and risks of homogenization

• Homogenization does not effectively remove macrosegregation occurring over large distances.
• It is not a method of purifying the alloy z phosphorus, sulfur or permanent non-metallic inclusions.
• Does not remove shrinkage cavities, cracks or casting porosity.
• Too high a temperature may cause local melting., especially in areas enriched with low-melting components.
• Excessive heating may cause grain growth.
• Long process times increase energy consumption and processing costs.
• An inappropriate atmosphere can lead to surface oxidation.and in the case of steel also for decarburization.

How is the effectiveness of homogenization assessed?

The effectiveness of the process is assessed primarily by comparing the microstructure and distribution of components before and after annealing.
The following may be used:

• optical microscopy,
• scanning electron microscopy SEM,
• local composition analyses using the EDS method,
• electron microanalysis,
• mapping the distribution of elements,
• studies of the proportion and distribution of phases,
• local measurements hardness, if they can provide auxiliary information about the heterogeneity of the structure.

The average chemical composition of the sample is not sufficient to confirm homogenization. Two materials can have the same average composition and yet differ significantly in the distribution of elements between the dendrite cores and interdendritic regions.
Diagnostics mechanical properties complement the assessment, but the hardness result itself, tensile strength Rm and/ or impact strength does not prove equal chemical composition.

Homogenization annealing is a high-temperature diffusion treatment primarily intended to reduce microsegregation occurring during solidification. The process is mainly used in the technology of ingots, castings and large semi-finished products before further processing.
The effectiveness of homogenization depends on temperature, time, diffusion path length, type of segregating elements, and the phases present. High temperature accelerates diffusion, but also increases the risk of local melting and grain growth. Homogenization does not replace normalization, hardening and thermal improvement or other final processes shaping the microstructure and mechanical properties. Rather, it is an earlier technological step, the purpose of which is to prepare a more chemically homogeneous material for subsequent operations.

Methods of heat and thermochemical treatment of steel

Homogenization annealing is just one of a group of processes used to modify the microstructure and properties of steel. Depending on the desired effect, normalizing, stress relieving, hardening, tempering, and thermochemical processes such as carburizing and nitriding are also used.

Steel heat treatment methods

• Normalizing steel – grain refinement and microstructure unification.
• Stress relief annealing – reduction of residual stresses.
• Softening of steel – reducing hardness and improving susceptibility to further processing.
• Full annealing – austenitization and slow cooling.
• Isothermal annealing – transformation carried out while holding at a properly selected temperature.
• Ball annealing – spheroidizing – change in the morphology of cementite or other carbides.
• Recrystallization annealing – removal of the effects of crushing and restoration of plasticity.
• Homogenizing annealing – reducing segregation and homogenizing the material composition.
• Hardening and heat treatment – shaping the microstructure and properties by hardening and tempering.
• Tempering steel – controlled modification of steel properties after hardening.
• Induction hardening – surface induction heating and rapid cooling.
• Flame hardening – surface hardening after local heating with a flame.
• Oversaturation – obtaining a supersaturated solid solution by an appropriate thermal cycle.
• Aging – controlled phase separation leading to material strengthening.

Methods of thermochemical treatment of steel

• Nitriding of steel – diffusional enrichment of the surface layer with nitrogen.
• Carburizing steel – diffusional enrichment of the surface with carbon.
• Carbonitriding – simultaneous enrichment of the surface layer with carbon and nitrogen.
• Nitrocarburization / nitrocarburization – enrichment of the layer primarily with nitrogen and carbon.
• Drilling – diffusion saturation of the surface with boron.
• Diffusion chrome plating – diffusion enrichment of the layer with chromium.
• Diffusion aluminizing – diffusion enrichment of the layer in aluminum.