Steel Hardening Processes - An Overview

Steel is a versatile material in industry and is particularly popular for its robustness. However, this robustness is not a fundamental property of steel, but in many cases the result of purposeful thermal processes. In this article, we introduce the processes of steel hardening, provide an overview of hardening methods, and list common errors and problems.

How is steel hardened?

Steel is defined in the standard DIN EN 10020 as an alloy of iron with a carbon content of max. 2.06%. In addition to other alloying elements, the carbon content has a direct influence on whether or not a steel can be hardened. Structural steels with a carbon content below 0.2% can generally not be hardened. With a carbon content of approx. 0.25% to 0.4%, hardening by additional carbon input is possible (case-hardened steels). Starting at 0.4% carbon, steels can be hardened by common hardening methods. If the carbon content increases to more than 2.06%, the transition from steel to so-called cast iron begins.

The iron-carbon diagram is useful for making the effect of the carbon content on the material properties of steel a little clearer. The iron-carbon diagram shows the transformation of the individual phases (microstructure composition), provided that the steel always has sufficient time for the respective phase transformation. The microstructural change at different cooling rates cannot be shown. For this purpose, a separate time-temperature transformation diagram is created for each alloy.

The Iron-Carbon Diagram

In the iron-carbon diagram shown here, the range of the liquidus and solidus lines is only shown in simplified form. Above the liquidus line, the mixture is liquid; between the liquidus and solidus lines, the alloy has a mushy consistency. Below the solidus line, the alloy is solidified, but is present as a different phase depending on the carbon content with different intercalation mixed crystals depending on the respective phase.

The difference between hardness and strength

While hardness and strength seem to be the same at first glance, they describe two different characteristics of the steel. Hardness refers to the resistance of a body or material to mechanical surface deformation or intrusion of another body. The strength describes the force per surface area that a material can absorb by elastic and plastic deformation up to shear failure or destruction.

To obtain a harder or stronger steel, there are two basic methods: hardening and tempering. The selected method depends on the desired property: If high hardness is the main focus, hardening is recommended. If toughness is to be maintained in addition to high strength, tempering is the method of choice.

Both methods have three-stages.

• In the first step, the steel is heated to a temperature above the G-S-K line.

The iron-carbon diagram in the G-S-K line refers to the temperature at which the respective carbon content allows a transition to the gamma phase (austenite range). This temperature is at 911°C at point G and drops to point S with increasing carbon content. Point S on the iron-carbon graph indicates the point at which the lowest temperature is reached for the transition to the austenite range. This corresponds to a temperature of 723°C and a carbon content of 0.86% at point S. Between point S and point K, the temperature remains constant at 723°C for the transition to the austenite range.

• The steel is quenched (quickly cooled) in the second step.

Quenching occurs at a rate that is at least equal to the critical cooling rate. The critical cooling rate corresponds to the cooling rate at which the microstructure no longer has time to completely convert to the phase corresponding to the respective carbon content and temperature. Cooling too slowly leads to partial to complete regression of the phases without the desired microstructural change.

• In the third step, the steel is reheated to allow it to cool down slowly.

This process is called tempering. This is where the methods of hardening and tempering differ. Since hardening prioritizes the preservation of hardness, the tempering temperature must not be too high. The tempering process is used here for stress relief and allows the final hardness to be influenced in a purposeful manner. For tempering, the tempering temperature is deliberately selected to be higher since the hardness decreases significantly with increasing tempering temperature and a higher toughness is achieved in return.  

Hardening and tempering methods must be understood separately from other heat treatments, such as annealing processes. The difference is that the goal for hardening and tempering is to produce a thermodynamic imbalance of the microstructure, while annealing processes aim for a thermodynamic equilibrium. The rapid quenching during hardening and tempering prevents this equilibrium from being achieved.

But what is meant by hardening in the first place?

Hardening is the quenching of a steel from the homogeneous gamma phase (austenite) to the actual alpha phase (ferrite) at a rate that is at least equal to the critical cooling rate. In the γ phase (gamma phase), the lattice is cubic face-centered. The carbon is incorporated (dissolved) in the austenite lattice. The alpha lattice (ferrite) is intrinsically stable and therefore gives the carbon virtually no possibility of being dissolved. With slow cooling, the lattice structure would transform from austenite to ferrite only slowly, leaving sufficient time for the carbon to diffuse out of the austenite lattice. However, hardening purposefully prevents this. The hardness itself is achieved, so to speak, by "freezing" the lattice structure of the microstructure reached during heating.

Because the heated material cools more rapidly when quenched in the outer layers than in the core, the dissolved carbon atoms in the core have more time to diffuse out. The hardness achieved during quenching decreases from the outside to the inside. To learn how to determine the hardness of a material and the type of hardness testing that can be performed on steels, see our blog about hardness levels and hardness testing.

The minimum temperature required for hardening (austenitizing steel)

As shown in the iron-carbon diagram, the lattice structure of steel up to the transition to cast iron below the P-S-K line consists of pearlite or a mixture of pearlite and α-ferrite.

Pearlite is a phase mixture of α-ferrite and cementite with a lamella-like arrangement. It occurs in steels and cast irons with carbon content between 0.02% and 6.67%. The α-ferrite phase is body-centered cubic iron and the cementite phase is a compound of iron and carbon of the composition Fe₃C (iron carbide). These two phases together form a lamellar structure consisting of a low-carbon layer (α-ferrite) and a high-carbon layer (iron carbide).

The goal of heating above the critical minimum temperature is to dissolve the pearlite and convert it into austenite. For this purpose, the steel is heated to a temperature above the so-called GSK line in the iron-carbon diagram. The GSK line indicates the austenitization temperature or hardening temperature of steel, which varies by material composition. The originally body-centered cubic ferrite lattice becomes a face-centered cubic austenite lattice in which the carbon contained in the steel can dissolve.

During austenitization, cementite breaks down into its constituents and releases the carbon. This carbon is now soluble and can be absorbed by the austenite lattice. To ensure that this happens completely throughout the workpiece, the temperature is maintained for a longer period of time. If the steel were to cool down slowly again, the original structural state would also be restored. A hardness change can therefore only be achieved by quenching.

Quenching steel

Steel quenching is relatively fast compared to annealing methods such as normal or soft annealing. If the steel in the austenitic state is cooled/quenched faster than the critical cooling rate (quenching rate), the carbon does not have enough time to diffuse out of the austenite lattice and return to its original state. The carbon is essentially trapped/frozen in the austenite lattice. This lattice distortion leads to internal stress in the crystal lattice, which then, in a shear transformation, results in a body-centered tetragonal lattice and is reflected in the material as hardness. This new, hard structure is called martensite.

The hardening process leads to an increase in volume due to the distorted lattice structure. For this reason, the dimensional finishing is only carried out after the hardening process. It is important to ensure that all of material actually drops below the 100% martensite temperature throughout; otherwise residual austenite can remain in the material, which is subsequently transformed, with a delay, by time, temperature, or pressure and can thus again change the dimensions of the lattice structures (0.3% in the fiber direction) and therefore the dimensions of the components.

Steel in the martensitic state loses a large part of its deformability while its hardness and strength increase. Pure martensitic steel cannot be plastically deformed and is very brittle, i.e., it breaks immediately under load or even when dropped. This condition is also called glass-hard. In addition, the martensitic structure is susceptible to erosion and corrosion. In order for the steel to regain the toughness needed for further processing, it must be post-treated. This happens during so-called tempering.

Tempering steel

During tempering, the steel is reheated, but the temperatures remain below the G-S-K line (see iron-carbon diagram). The carbon atoms previously forcibly incorporated into the lattice structure can partially diffuse out again due to the slow and moderate heating. The structure approaches thermodynamic equilibrium again.

By adjusting the temperature, properties such as toughness, strength, and hardness can be controlled in a purposeful manner. The required temperatures can be read, for example, from so-called tempering diagrams or tables of steel hardness.

The higher the tempering temperature and the longer the tempering time (see ZTU, Time-Temperature Transformation), the higher the toughness with a simultaneous reduction of hardness and strength. In principle, at this point a distinction is made between whether the steel is hardened or tempered. Tempering means that the steel is tempered at higher temperatures up to 700°C, depending on its alloy. This ensures that the steel has high strength and high toughness. They can absorb high deformation energy compared to normally annealed steel. This is relevant, for example, for components such as shafts.

In addition to the tempering temperature, slow cooling is particularly important here.

At tempering temperatures of up to 400°C, the hardened steel remains brittle but also has a high hardness. This can be advantageous, for example, if toughness is not the priority, but wear resistance is. Hardened steels are highly resistant to abrasion and deformation. Cutting tools make use of this property. In principle, however, it should be noted that hardened steels can really only be used if deformation is practically ruled out, since they sometimes break even with minor deformation.

The aim of case hardening is to achieve an enrichment of the carbon content in the surface layers of the steel. This is achieved by carburizing, i.e. diffusing carbon into the microstructure lattice. Carbon diffusion takes some time and the steel must be therefore kept at temperature for longer.

In order to further increase wear resistance, nitrogen can be introduced in addition to carbon during the case hardening process. This process is called carbonitration. Carbonitration is suited for low-alloy steels, for example:

Overview of hardening processes

Depending on the requirements for steel hardness, toughness and application, there are different steel hardening processes. Many are primarily aimed at hardening the surface layer of the steel. Surface hardening is particularly sufficient if components require a wear-resistant and hard surface, but the core should still remain comparatively tough.

Typical application examples are linear shafts or gears. The combination of hard surface and tough core increases vibration resistance.

The following hardening processes aim to achieve this:

• Induction Hardening: Induction hardening involves heating the surface of the steel in a purposeful manner by means of induction coils. This is followed by rapid cooling to create a hard surface layer.
• Flame hardening: Flame hardening allows large surfaces or specific areas of a component to be quickly heated using powerful flames and hardened by subsequent quenching.
• Case hardening: Case hardening involves carburizing (enriching with carbon in a carbon-delivery medium) the workpiece in a first step. This occurs for example in a gas atmosphere or under negative pressure. After that, the workpiece is also cooled quickly to create a hard surface layer.
• Nitriding: Nitriding enriches the steel surface with nitrogen, which also helps to create a hard surface layer. This method assumes that sufficient carbon is already bound in the steel.

The following graphics illustrate the principles and applications of electron beam and laser beam hardening in addition to the methods of flame and induction hardening already discussed.

Common errors and problems with steel hardening

Some undesirable side effects may occur during steel hardening.

Stress cracks can occur, for example, if the wrong quenching medium was selected and the steel therefore cools too quickly or only on one side. Common media for this can be air, oil, or water.

Care must always be taken to ensure that the cooling temperature is reduced as evenly as possible.

Overheating during the hardening process also leads to undesirable results. Heating the steel for too long or at an excessive temperature can lead to grain growth, which affects mechanical properties.

The high temperatures during the hardening process can lead to surface decarburization. The hardness uptake at the surface is thus reduced. The negative effects of this influence can be avoided by a protective gas atmosphere or by a sufficient hardening allowance.

Some requirements must also be met for achieving the martensite state. The steel must have sufficient carbon content, otherwise sufficient hardness cannot be achieved. In general, a carbon content of 0.3% is sufficient for hardening in steels.

Since other alloying elements in addition to carbon often play a significant role in steels, the latter likewise influence the properties of hardness and toughness as well as the parameters of the hardening process itself during hardening.

Some alloying elements sometimes affect martensite formation to such extent that carbon solubility is suppressed and the steel remains in the austenitic or ferritic state. Such steels can generally not be hardened with thermal processes.

Choosing the right steel is therefore critical to the hardening process.