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.

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.

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.

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!

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!

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.

Surface hardening like flame hardening, induction hardening, laser hardening and case hardening all have in common that the hard surface layer is achieved by a martensitic microstructure. However, such a transformation could become a problem if a workpiece has to be dimensionally accurate, since the microstructure transformation generally leads to hardening distortion. The scale layers that form may also have to be reworked. In such cases nitride hardening (nitriding) can provide a remedy, which does not require any microstructural transformation. Nitriding is therefore not one of the classical surface hardening methods by means of microstructure transformation.

Nitriding is not based on the formation of martensite but on the formation of hard and wear-resistant nitrides on the surface of the component!

During nitriding, the alloyed steel is exposed to a nitrogenous environment at temperatures of about 500 °C. The nitrogen atoms diffuse into the surface of the steel and combine there with present alloying elements such as aluminium, chromium, molybdenum, vanadium and titanium to form hard and wear-resistant nitrides.

Nitriding requires special steels containing nitride-forming alloying elements, so-called nitriding steels (e.g. 34CrAlMo5). The nitrides formed on the surface also lead to stresses in the material. However, these do not represent any weaknesses but increase the fatigue strength of the component to a special degree due to the residual compressive stresses caused! The nitride layer also improves corrosion resistance.

Nitriding is used in particular to improve the fatigue strength of dynamically stressed components!

While the surface hardness increases strongly due to the nitrides formed, the properties of the component core remain unaffected, as the nitrides only form on the surface. The layer thicknesses range from 0.1 mm to 1 mm. Thicker nitride layers are only possible with very high effort. The long annealing times of sometimes several days can make nitriding very time-consuming and therefore expensive.

Laser-beam hardening (laser hardening) offers even shorter heating times of the surface than in induction hardening. This significantly reduces the already low hardness distortion and scaling. Under inert gas, oxidation of the surface can even be completely prevented.

In laser hardening, a laser beam with a very high specific power (about factor 10 compared to induction hardening) is guided over the workpiece surface to be austenitized. The enormous thermal output of the diode laser of several kilowatts results in a temperature just below the melting point in a very short time! Since the heat input is limited only to the local focal spot of the laser, unnecessary heating of unwanted areas is avoided. This means that the locally heated area is quickly quenched by the cooler surrounding areas. This so-called self-quenching eliminates the need for quenching with water.

With laser hardening, the surface is heated by a laser beam and quenched by heat dissipation in the workpiece (self quenching)!

The laser spot covers a track width of 1 to approx. 50 mm, depending on focusing and process control. Larger surface layers have to be scanned line by line with the laser. Typical hardening depths with laser hardening are in the range of 0.1 mm to 2 mm. As is the case with induction hardening, the smaller the surfaces to be hardened and the shallower the surface layer depths, the greater the cost-effectiveness of laser hardening. Laser hardening is particularly suitable for areas that are very difficult to access, such as tapped holes.

With laser hardening, only small surfaces can be hardened economically. The hardening depth can be kept very low!

The flames during flame hardening generally lead to a large heat-affected zone. With small geometries, this can lead to undesired full hardening over the entire cross-section. In order to harden even such thin-walled workpieces only on their surface in the range of a few tenths of a millimetre, so-called induction hardening can be used.

The principle of induction hardening is based on the induction effect, which is also used in induction cookers or transformers. A high-frequency alternating current is generated in a copper tool electrode (“primary coil”) which is adapted to the shape of the workpiece to be hardened. This in turn leads to a constantly changing magnetic field around the electrode, which penetrates into the adjacent workpiece and generates eddy currents due to the induction effect (“secondary coil”). These very large eddy currents of up to several thousand amperes per square millimeter lead to heating of the workpiece.

The fact that heat is mainly generated on the surface rather than inside the material is due to another physical phenomenon, the so-called skin effect. While the current density in a conductor cross-section is constant with direct current, with alternating current the current density increases with increasing frequency in the outer areas and decreases inside. The frequency of the eddy currents in the workpiece depends on the frequency of the alternating current in the electrode (also called inductor). This also results in relatively simple control of the hardening depth. The higher the frequency, the stronger the skin effect and the thinner the layers to be hardened.

The frequencies to be adjusted therefore depend on the thicknesses of the hardness layers to be achieved. At a utility frequency of 50 Hz, hardening depth in the range of 20 mm to 10 mm can be achieved. In the medium frequency range from 1 kHz to about 10 kHz, hardening depth of about 5 to 1 mm can be achieved. In the high-frequency range of up to several megahertz, even hardening depth of only a few tenths of a millimeter can be achieved.

With induction hardening, the workpiece is heated by induced eddy currents. The hardening depth is controlled by the frequency of the alternating current!

With induction hardening, the austenitized surface is usually quenched by means of downstream water showers, which are pulled evenly over the workpiece together with the inductor. In cases where only very low hardening depths are achieved, quenching can also take place without water by the relatively cool material core (self-quenching). Because a very high hardness can be achieved at the surface during induction hardening, high residual stresses can occur. This may require subsequent tempering at low temperatures.

The heating times are generally considerably shorter with induction hardening than with flame hardening, since a specific heating output of several kilowatts per square centimetre can be achieved, which is approx. 10 times greater. This has the advantage that the scaling is relatively low and the post-processing effort is reduced accordingly. This also significantly reduces the risk of hardening distortion.

In addition, no (toxic) exhaust gases are produced during induction hardening compared to flame hardening. Another advantage of induction hardening is the more even heating of the surface, provided the inductor is optimally adapted to the workpiece. This requires a high design tooling effort in advance so that induction hardening is economical, especially in automated production lines with high batch sizes. Due to the high electricity costs, economic efficiency increases when only small surface sizes have to be hardened on a workpiece.

Induction hardening can be easily automated and is suitable for complex geometries, especially in mass production! Scaling and hardening distortion are less than with flame hardening!

Introduction

A hard surface layer is essential to increase the wear resistance of contacting components. In these cases quenching an tempering can be used as a possible heat treatment. The disadvantage, however, is the decreasing toughness or increasing embrittlement of the steel, which can lead to unforeseeable material failure. For this reason, it may only make sense to harden the surface of a workpiece so that the component core still retains its toughness (partial hardening). This is known as surface hardening.

With surface hardening, only the surface layer is hardened to increase the wear resistance, so that the component core remains tough!

Toothed wheels are typical cases where surface hardening is used. However, crankshafts or camshafts are usually also surface-hardened after quenching and tempering. Depending on the application, different surface hardening methods have developed. These include:

• Flame hardening
• Induction hardening
• Laser hardening
• Case hardening
• Nitriding

Flame hardening

With flame hardening, a burner flame is passed over the workpiece surface to be hardened, which is then austenitized. Water nozzles are installed directly behind the burner flames, which then provide the necessary cooling to form martensite (quenching). Subsequent tempering is not usual for flame hardening! This generally also applies to the other surface hardening processes, as the unhardened core provides sufficient toughness.

The thickness of the hardened surface layer depends on the speed at which the burner flames are moved over the workpiece surface (called feed). The slower the speed, the deeper the heat can penetrate and austenitize the microstructure and the thicker the hardened surface layer will be after quenching. At the same time, of course, the necessary cooling rate for martensite formation in the deeper marginal layers must also be ensured! Since alloying elements generally reduce the critical cooling rate, deeper surface layers can be hardened with high-alloy steels.

With flame hardening, burner flames are moved over the workpiece and quenched with water nozzles! The hardening depth is controlled by the feed rate!

Due to the relatively bulky arrangement of the nozzles, flame hardening is limited, especially for small components with complex geometries. Flame hardening is also generally inferior to induction hardening and laser hardening in terms of accuracy (adjustment of the hardening depth).

In principle, heating should be carried out as quickly as possible in order to keep the heat-affected zone on undesired areas to a minimum. Otherwise there is a risk of thermal stresses or distortion of the component geometry (hardening distortion). In addition, long heating times lead to increased scaling, which usually requires special postprocessing. In the case of rapid heating, however, it must be noted that there is no longer a thermodynamic equilibrium state in the microstructure. As a result, the transformation temperature for austenitization shifts towards higher temperatures!