Surface hardening is used to produce a hard and wear-resistant surface layer on steel workpieces, while the toughness in the core is largely retained. You will find more information on the various processes and their advantages and disadvantages in this article.

• What are the characteristics of surface-hardened workpieces?
• How is the depth of the hardening layer controlled during flame hardening?
• What are the advantages of induction hardening compared to flame hardening?
• Why does laser hardening not require quenching with water?
• For which steels is case hardening suitable and what are the mechanical properties of case hardened components?
• What are single-quench hardening, double-quench hardening and direct hardening?
• For which steels is single or double quench hardening used in comparison to direct hardening?
• How does nitriding differ from all other surface hardening methods?
• What is the primary objective of nitriding?

Introduction

A hard surface layer is essential to increase the wear resistance of contacting components. In these cases hardening can be used as a possible heat treatment. The disadvantage, however, is the simultaneously decreasing toughness or 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. The most important ones will be discussed in more detail in the following sections.

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!

Induction hardening

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!

Laser hardening

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!

Case hardening

The toughness (ductility) of steels increases with decreasing carbon content, as then less brittle cementite is found in the microstructure. If components are to be very tough, they must inevitably be relatively low in carbon. At the same time, however, the hardenability of the material decreases due to the low carbon content, since the forced-dissolved carbon in the lattice in particular leads to the necessary formation of martensite. As a guideline, the carbon content should be at least 0.3 % for hardening. However, components such as toothed wheels must combine both contradictory properties:

• low carbon content in the core for high toughness (absorption of dynamic loads) and
• high carbon content on he surface for a hardenability of the surface layer (increase of wear resistance).

For such applications case hardening is suitable, which is generally structured as follows:

• carburisation
• cooling (not required for direct hardening)
• hardening (quenching & tempering)

In case hardening, a low-carbon steel (case hardening steel) with a maximum of 0.2 % carbon is first exposed to a carbon-containing environment. In the early days, the steel was practically placed in a “case” of glowing coke. The carbon then diffuses into the surface layer, where it leads to an enrichment of the carbon content to a hardenable level of about 0.8 % carbon, while the core remains low in carbon. This carbon accumulation in the surface layer is also called carburisation.

Since only the austenite structure is able to absorb sufficient amounts of carbon, the temperature during carburizing is above 900 °C with a carburizing time of several hours. Carburizing depths of 0.1 to about 5 mm can be economically achieved with this method. Since carburizing is a diffusion-controlled process, the carburization times can be reduced with higher temperatures, but at the same time the risk of coarse grain formation increases.

Carburizing can be carried out in different ways. During gas carburizing, the component is exposed to a carbon-containing atmosphere. This is particularly economical in mass production. Carburisation in salt baths is also possible. In addition, it is possible to carburize workpieces in powdered carbon granulate.

After the surface layer has been carburized to the desired hardenable level, the actual hardening process takes place, whereby the relatively low-carbon core is slightly quenched and tempered. The quenching required for this can be either done

• after a slow cooling from the reheated state (single and double quench hardening) or
• directly from the still hot carburizing state (direct hardening).

After quenching, the hardened components are always tempered and thus obtain their final service properties. Above all, the increase in fatigue strength makes case hardening very interesting for dynamically stressed components such as gears or drive shafts.

With case hardening, low-carbon steels are first enriched with carbon in the surface layer (carburisation) and then quenched! Such components are characterised by their high surface hardness combined with a very tough core (since low-carbon content)!

Single-quench hardening

Single quench hardening is a special case hardening process. It is suitable for steels that tend to form coarse grains during carburizing or for components that still require intermediate machining before hardening. In this process, the steel is cooled slowly after carburizing. For the actual hardening process, the steel is then heated again in a separate process step. The $$\gamma$$-$$\alpha$$-transformations cause a recrystallisation effect, which leads to grain refinement of the coarsely grown grains during carburisation.

The hardening temperature can be selected so that the austenitisation mainly takes place in the marginal area (due to the carburised surface, the temperature required for complete austenitisation is lower there than in the lower-carbon core area!) This austenitized state at about 750 °C is then quenched to achieve the desired martensite formation in the surface layer.

Due to the relatively low surface hardening temperatures, however, the lower-carbon core is not completely austenitized, so that no completely martensitic core structure is formed after quenching. Residual ferrite is to be expected in the core. In this case, the hardening temperature is preferably adapted to the desired properties of the surface layer in order to achieve optimum surface properties. This is why the process is also referred to as surface hardening or single quench hardening from surface hardening temperature.

In principle, the hardening temperature can also be selected so that the core is specifically austenitized. Due to the lower carbon content, however, higher temperatures of approx. 900 °C are necessary. This so-called core hardening temperature is then used for quenching. However, due to the high temperatures, a coarse needle-like microstructure in the surface layer is to be expected during core hardening. In this case, the temperature control is preferably adapted to the desired core properties in order to achieve optimum core properties. This is why the process is also referred to as core hardening or single quench hardening from core hardening temperature.

Single quench hardening specifically influences the properties of the surface (surface hardening) or the core (core hardening)!

Double-quench hardening

In principle, a combination of core and surface hardening is also possible. After carburizing, the workpiece is first cooled slowly and then reheated to core hardening temperature or cooled to core hardening temperature immediately after carburizing. The material is then quenched to adjust the core properties. Subsequently, the material is reheated to surface hardening temperature and then quenched in order to obtain optimum surface properties. However, due to the permanent change in temperature, the hardness distortion in this double quench hardening is relatively high.

With double quench hardening, first the desired properties of the core are adapted (core hardening) and then those of the surface (surface hardening)!

The reheating during single and double quench hardening makes these processes relatively energy- and time-intensive and therefore expensive. The advantage, however, is the grain refinement that occurs through the $$\gamma$$-$$\alpha$$-transformations. However, for steels that do not tend to form coarse grains in the first place (e.g. chrome-molybdenum steels), it is therefore economically more sensible to quench the steel directly after carburizing from the already heated state. Fine grain steels are also suitable for this direct hardening process, which is explained in more detail in the following section.

Single and double quench hardening is usually carried out on steels with a tendency to coarse grain formation, as a recrystallisation effect occurs during the $$\gamma$$-$$\alpha$$-transformations!

Direct hardening

Direct hardening is a special case hardening process. In this process, the steel is quenched directly after carburizing from the already heated state. Compared to single and double quench hardening, direct hardening is less time-consuming and energy-intensive and therefore less expensive, since reheating is not necessary. Direct hardening is always suitable when steels do not tend to coarse grain formation and therefore single or double quench hardening is not necessary.

Depending on the temperature in the surface layer or in the core, either the surface layer properties or the core properties of the workpiece can be specifically influenced during quenching. For this purpose, the component is either cooled down to core hardening temperature after carburizing or brought to case hardening temperature. After quenching, tempering takes place again at low temperatures.

In direct hardening, the heated steel is quenched directly from the carburized state!

Nitriding

The surface hardening processes explained so far 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.