This article provides answers to the following questions, among others:
- What are the objectives of annealing processes, such as normalizing, soft annealing, coarse grain annealing, recrystallisation annealing, diffusion annealing, solution annealing and stress-relief annealing?
- In which temperature ranges are the annealing processes carried out?
- Why is an improvement in machinability also achieved with soft annealing?
- How can the generally poorer properties of a coarse-grained microstructure be subsequently removed?
- What is the significance of recrystallisation annealing for transformation-free steels?
- What is hot forming or cold forming?
- Why is diffusion annealing relatively cost-intensive?
- Why does the workpiece have to be cooled slowly after stress relief annealing?
As already explained in the chapter deformability of metals, fine roundish grains generally lead to better toughness and strength values compared to large grains. A uniform microstructure is desirable, which has similarly small grains over the entire microstructure. This is the only way to ensure that the material meets the strength requirements at every point to the same extent.
However, achieving a homogeneous microstructure already during solidification requires high demands, since the solidification conditions will not be identical across the entire melt. For example, cast steel may cool faster at the contact points to the mould wall than inside the melt. While finer grains will develop in the edge areas due to the stronger undercooling, larger grains will form in the middle of the melt.
A heterogeneous grain microstructure can also occur during forging, as the grain boundaries shift due to diffusion processes and the grains can unite. This results in a new grain formation with the consequence of a heterogeneous structure. The same effect of heterogeneity can also be seen in welded workpieces in the area of the joint.
For this reason it is necessary to standardize (homogenize) a non-uniform microstructure by means of a heat treatment. This gives the steel its “normal” properties, which are always reproducible. For this reason, the targeted homogenization of a steel microstructure is also called normalizing.
The aim of normalizing is to achieve a uniform homogeneous microstructure with reproducible properties!
During normalizing, the steel is heated to just above the GSK-line so that the pearlite is completely converted to austenite. The austenitized steel is then slowly cooled in air. Since the grains form anew during the \(\gamma\)-\(\alpha\)-transformation, grain refinement occurs and makes the microstructure homogeneous.
The temperature during normalizing should not exceed approx. 30 °C above the GSK-line, otherwise there is a risk of coarse grain formation. The reason for this is that, from an energy point of view, large roundish grains are more favourable than many small ones. Therefore, the microstructure always strives to form a single large grain. This requires, among other things, diffusion processes which are favoured by higher temperatures. Therefore, it is important to keep the temperature as low as possible during normalizing in order to avoid the formation of coarse grains. For this reason, hypereutectoid steels are not completely heated up to the austenite region (above the SE-line).
Normalizing is preferably used for hypoeutectoid steels whose microstructure has been negatively influenced by manufacturing processes such as forging, rolling, casting, welding, etc. During rolling, normalizing can already be carried out during the rolling process (normalizing rolling). A normalized microstructure is generally characterized by very good toughness and strength values due to its homogeneous, fine structure.
Normalizing is often carried out on hypoeutectoid steels after forging, rolling, casting or welding!
Not every material has to be designed to withstand high mechanical forces. With a curved sheet metal with milled out slots, for example, it is not necessary for the material to be able to absorb high forces. Rather, the focus in the selection of materials is on good formability and machinability of the steel. This plays an important role, especially in automated production with large batch sizes, in order to make production economical.
For this reason, it may be necessary to adapt the microstructure of a steel in such a way that it can be better formed and/or machined. Particularly with regard to formability, it is therefore necessary to produce a correspondingly soft microstructure. This can be achieved by so-called soft annealing.
The aim of soft annealing is to improve formability and machinability!
During soft annealing, hypoeutectoid steels are heated to just below the PS-line so that the cementite does not just yet decompose. The lamellar cementite now has enough time to transform through diffusion processes into the thermodynamically more favourable, roundish form. The strip cementite in pearlite transforms into spherical cementite (spheroidal cementite). After the cementite has disintegrated into the round shape, the steel is slowly cooled. In contrast to hypoeutectoid steels, hypereutectoid steels are heated during soft annealing just above or oscillating around the PSK-line.
A particularly homogeneous microstructure with finely divided spheroidal cementite can be achieved by hardening the steel before soft annealing. The spherical cementite is then formed from the relatively homogeneous martensite microstructure.
After soft annealing, the steel shows much better formability due to the spherical cementite shape. The cause lies in the facilitated dislocation movement. While the strip-shaped cementite lamellas partially extend completely from one end of the grain to the other, the cementite spheres are only occasionally present in the grain. The dislocation movement is thus less strongly hindered by the spheroidal cementite than with the completely penetrating strip cementite. The deformability increases accordingly, while the hardness decreases.
This facilitates subsequent rolling, bending, deep drawing, etc. due to reduced forming forces. In addition, the spherical cementite achieves better machinability, as the cementite spheres offer less resistance to the tool cutting edge compared to the lamellar cementite form. This increases the durability of the tool accordingly.
Spheroidal cementite improves the machinability of the microstructure compared to lamellar cementite!
The micrograph below shows a soft-annealed steel C45 with the cementite lamellae disintegrating into small roundish spheres.
Hypoeutectoid steels with a carbon content below approx. 0.3 % carbon are generally not soft annealed, as these are relatively soft anyway. Although these steels already have good formability, their machinability is unfavourable due to the tendency to form built-up cutting edges at the tool edges. In order to give these low-carbon steels good machinability, the coarse-grain annealing described below can be used as heat treatment.
In general a coarse-grained steel microstructure is undesirable due to the relatively low toughness and strength values. The only advantage of a coarser grain is the resulting better machinability, which is due to the increased brittleness of the coarse grain (note that brittleness and toughness always behave in reverse). Especially for low-carbon steels with a carbon content of less than 0.3 %, coarse grain annealing is an alternative to soft annealing in order to improve machinability.
The aim of coarse grain annealing is to improve machinability!
During coarse grain annealing, the steel is annealed in the range between 950 °C and 1100 °C. At these high temperatures, diffusion processes can take place to a sufficient extent so that the atoms can reattach to the grain boundaries and thus cause them to grow. The driving force is the reduction in surface energy, which is accompanied by a larger grain instead of many small ones. Since the diffusion processes take time, depending on the thickness of the workpiece, the annealing process takes several hours.
Due to the generally unfavourable mechanical strength properties, coarse grain annealing is limited to low-carbon steels and is only rarely used (e.g. in high-temperature applications). After the coarse grain microstructure has been machined, it can be removed by normalization to regain better strength properties.
The generally poorer strength values of a coarse-grained structure can be removed by normalizing!
The microstructure of rolled, bent or deep-drawn workpieces is strongly deformed by the high forming forces. This also changes the material properties. In the case of rolled sheets, this can lead to a strong anisotropy through the elongated crystals, also known as rolling texture. In addition, strain hardening takes place in the forming area, which increases the strength and reduces the deformability accordingly. If the workpiece is to be further formed in this condition, the risk of cracking increases. Multi-stage forming processes are therefore not possible without further ado. However, many workpieces and semi-finished products have to be formed several times in the course of their production in order to reach their final state. For example, a steel block that is several centimetres thick cannot be rolled in a single pass down to a few millimetres.
The aim must therefore be to restore the deformed crystals (grains) of a reshaped microstructure to their original shape before every multi-stage forming process. This can be achieved by recrystallisation annealing.
The aim of recrystallisation annealing is to restore a deformed microstructure to improve its deformability!
During recrystallisation annealing, the steel is annealed below the PSK-line in the range between 550 °C and 700 °C. Therefore, no lattice transformation takes place, as is the case with normalizing or partially also with soft annealing, although a recrystallisation effect also occurs with these two processes. During recrystallisation annealing, the grain boundaries can migrate through diffusion processes and the grains thus form anew. The deformed grains regain their original shape and the material regains its deformability.
In addition to the annealing time and temperature, the size of the recrystallized grains depends in particular on how strongly the individual grains were deformed before. A high degree of deformation with very fine, elongated crystals allows the microstructure to recrystallize rather fine-grained. A lower degree of deformation leads accordingly to a coarser grain after recrystallisation. Especially for a slightly deformed microstructure, however, there is also the danger of coarse grain formation. This risk can occur particularly for low-carbon steels with carbon concentrations below 0.2 %, so that normalizing may be more suitable for the formation of new crystals.
Recrystallization annealing is the only method for transformation-free steels (where \(\gamma\)-\(\alpha\)-transformation is completely suppressed by alloying elements) to achieve grain refinement.
In order to always maintain the ductility of the material in multi-stage forming processes, the microstructure must be recrystallized between each forming step. This process is then also called intermediate annealing.
The effect of recrystallisation can also be used during the forming process itself by forming above the recrystallisation temperature. This is known as hot forming. However, if the material is formed below the recrystallisation temperature (e.g. at room temperature) it is called cold forming. Hot forming places much higher demands on the machines involved, so that economic efficiency must always be checked.
In hot forming, the workpiece is formed above the recrystallisation temperature, in cold forming, however, below the recrystallisation temperature!
When steels with high alloy concentrations solidify, the alloying elements may not be distributed homogeneously in the microstructure or in the individual crystals. Such concentration differences within the individual crystals are also called microsegregations.
With different alloy concentrations, there are also different properties within a grain which may weaken the microstructure. That is why such microsegregations must always be prevented. However, the formation of such concentration differences cannot always be prevented from the outset due to the finite cooling rates. For this reason, differences in concentration within a microstructure must be eliminated by subsequent heat treatment. This can be done by diffusion annealing.
The aim of diffusion annealing is to compensate for concentration differences!
During diffusion annealing, the steel is annealed to relatively high temperatures in the range between 1050 °C and 1300 °C. This ensures that the diffusion processes can take place to a sufficient extent so that the atoms can cover the relatively long diffusion paths. However, this usually requires several hours of annealing time.
Disadvantage of diffusion annealing is the formation of coarse grains due to the high temperatures. Although this could be reduced by reducing the temperature, longer annealing times would then be necessary and diffusion annealing would no longer be economical. If coarse grain formation cannot be prevented during diffusion annealing, the coarse grain structure must be subsequently removed again. This can be achieved by normalizing.
Since microsegregations occur during the solidification of a steel, they are usually removed directly in the steel mill on the ingot. Especially as the process is very energy-intensive and places high demands due to the high temperatures and long annealing times.
During welding or hot forming, microstructural changes occur in the steel due to the influence of heat and uncontrolled cooling. This is particularly the case with austenitic steels, where the high temperatures in the range of 500 °C to 800 °C can cause carbide precipitation at the grain boundaries. This leads to intergranular corrosion (intercrystalline corrosion) due to the different electrochemical properties. To avoid this, the formed precipitations must be dissolved again. This can be achieved by solution annealing in the range of approximately 1000 °C to 1100 °C. In the case of deformed steel microstructures, the effect of recrystallisation also occurs.
The aim of solution annealing is to dissolve formed precipitates!
Solution annealing is also used as an intermediate step in the value chain to temporarily improve machinability. The precipitates responsible for the poor machinability are dissolved by annealing. If the workpiece is then rapidly cooled, a supersaturated (metastable) solid solution microstructure without precipitates is formed. In this condition, a better machinability of the material is temporarily achieved. The precipitates are then formed by subsequent cold ageing or warm ageing and the material regains its original properties. This process is used, for example, in the precipitation hardening of some special aluminium alloys.
Stress relief annealing
If a steel is heated or cooled unevenly, internal stresses can arise, also known as residual stresses. Such residual stresses are often induced during welding, for example, because the workpiece is heated not evenly but only locally at a certain point and then cooled down. However, residual stresses can also occur in the workpiece during milling or turning, as high temperatures can occur in the machining area of the workpiece. During hardening, the quench distortion is also based on the residual stresses caused by the uneven cooling.
Such residual stresses can, among other things, reduce the strength of the workpiece. Furthermore, residual stresses during a subsequent machining process can also lead to distortion of the workpiece if the residual stresses are suddenly released as a result. Therefore, in some cases it is necessary to eliminate residual stresses in the workpiece. This is achieved by stress relief annealing.
The aim of stress-relief annealing is to release residual stresses!
In stress relief annealing, the workpiece is annealed below the PSK-line in the range between 550 °C and 650 °C. The effect of the stress relief is based on the fact that the strength of the heated material decreases at a higher temperature. If the yield strength (hot yield strength) falls below the value of the residual stresses, these are reduced by plastic deformation. The dislocations begin to move accordingly.
The residual stresses can therefore only ever be reduced to a maximum of the corresponding hot yield point, never completely. After annealing, the workpiece must then be cooled slowly to avoid a renewed occurrence of stress. In most cases, the workpiece remains stationary in the switched-off annealing furnace.