Steel hardening is a heat treatment process in which steel is heated to the right temperature, held there and then cooled quickly to increase its hardness and strength. Thanks to this, steel hardening extends the life of tools, gears and other parts exposed to abrasion. Below we explain what steel hardening involves, how heating, cooling and tempering proceed, what types of hardening there are and when it is worth outsourcing it to a contractor.
Table of contents:
- What is steel hardening?
- How does the steel hardening process work?
- Types of steel hardening
- Which steels can be hardened?
- Properties of steel after hardening
- Hardening, annealing and tempering – what is the difference?
- The most common problems in steel hardening
- Steel hardening and CNC machining
- Applications of hardened steel
- When is it worth outsourcing steel hardening?
- Steel hardening – FAQ
What is steel hardening?
Hardening is a heat treatment process for steel. It consists of heating the steel to the austenitizing temperature, holding it at that temperature and cooling it quickly. With rapid cooling a hard microstructure called martensite forms in the steel, so the steel becomes harder and stronger. The cooling rate determines the microstructure: with slower cooling softer structures form, and with rapid quenching the steel reaches high hardness. Hardening increases hardness and strength, but at the same time it makes the metal more brittle, so its purpose is to improve mechanical properties in a controlled way. At higher temperatures steel transforms into austenite, and with rapid quenching austenite turns into martensite.
Steel hardening is usually done on a finished or nearly finished part, not on a raw block. Hardening a whole block is not economical, because a large part of the material will be removed during machining. Hardened steel is also harder to machine, because high hardness makes work difficult for cutting tools.
How does the steel hardening process work?
Hardening consists of three stages: heating and soaking, rapid cooling, and tempering. Each of them affects the mechanical properties of the finished part, so it requires control of temperature and time. Changing any parameter changes the hardness, strength and risk of distortion.
Heating and soaking
Steel hardening takes place at temperatures from 800°C to 1000°C, and high-alloy steel can be hardened even up to 1300°C. Steel hardening requires precise temperature control, because the austenitizing temperature affects the structure of martensite. Too low a temperature does not give full hardness, and overheating can lead to a worse structure and cracking. During soaking the part must reach a uniform temperature across the whole cross-section, but too long a time at high temperatures worsens the properties of the steel.
Cooling
The cooling rate affects whether martensite forms in the steel. The faster the cooling, the greater the hardness, but also the greater the stresses. Cooling takes place in a cooling medium, most often water or oil. Quenching in water is one of the oldest methods and gives very fast cooling, but it increases stresses and the risk of cracks. Quenching in oil minimizes the risk of cracking in steel, so it is chosen for parts with complex shapes. The choice of cooling medium depends on the type of steel and the shape of the part, and in alloy steels gentler cooling is often enough, because alloying elements make it easier for martensite to form. For each type of steel, the material manufacturer gives the recommended temperature and way of cooling.
Tempering
After hardening, steel is very hard but brittle and cracks more easily under impact. Tempering consists of reheating the hardened steel to a lower temperature and reduces brittleness and internal stresses. In practice tempering is an indispensable part of the process: hardening and tempering are always used together, because only after tempering does the steel have good strength and impact resistance. Tempering restores part of the ductility, at the cost of a slight drop in hardness.
Types of steel hardening
The main methods of steel hardening are through hardening and surface hardening. The choice of method depends on the steel grade and the property requirements, and the hardening process is applied to parts that are expected to have high hardness.
Conventional (through) hardening
Through hardening consists of heating the entire cross-section of the part and cooling it quickly. Conventional hardening gives high hardness throughout the volume of the part and works well for tools and small parts.
Surface hardening
Surface hardening improves the mechanical properties of steel at the surface, while the core of the part remains more ductile. As a result the part has high resistance to abrasion and at the same time withstands impact loads. Surface hardening includes, among others, induction hardening and flame hardening.
Induction hardening consists of rapidly heating the surface with induction currents. Flame hardening uses a gas torch to heat the steel. In both cases heating covers only the outer layer, followed by rapid cooling.
Stepped hardening
Stepped hardening reduces structural stresses in steel. After rapid cooling to a set temperature, the part is held at that temperature for some time and only then cooled to the end. As a result the safety of the process increases for complex shapes.
Isothermal hardening
Isothermal hardening consists of holding the steel at a constant temperature until the transformation is complete. This type of hardening reduces the risk of distortion and cracks, and the steel has good ductility.
Which steels can be hardened?
Whether steel can be hardened is decided above all by its carbon content. Steel with very low carbon content gains little from hardening, so ordinary steel can be hardened only if it has enough carbon. Tool steels and many alloy steels harden well. Structural steel can be hardened if it has sufficient carbon content. This applies, for example, to shafts and transmission parts. Gears are often surface hardened, because the teeth must have a hard surface and a core resistant to impact. Alloying elements change the behavior of steel during cooling, so separate parameters are selected for a given steel.
Stainless steel can be hardened only if it has a martensitic structure. Other grades do not gain hardness through hardening, so if you want to harden stainless steel, the grade must be checked. Corrosion resistance depends on the grade of steel, not on hardening itself, so after hardening it must be assessed separately for a given grade. Before ordering it is therefore worth stating the type and grade of steel, and in case of doubt checking the composition in the material certificates.
Properties of steel after hardening
Hardening increases the hardness of steel, making it resistant to wear and abrasion. Hardening increases the mechanical strength of steel under loads, and hardened parts have a longer service life thanks to greater wear resistance. The life of cutting tools is extended thanks to hardening.
Steel that is too hard can be brittle and cracks more easily under impact, which is why tempering is used after hardening. Hardening does not in itself protect against corrosion: corrosion resistance is provided by the composition of the steel, for example stainless steel, and not by hardness alone. During rapid cooling, distortion and deformation of the part can also occur, so grinding is often needed after hardening.
Hardening, annealing and tempering – what is the difference?
Hardening, annealing and tempering are different heat treatment processes for metal. Hardening increases hardness and strength, annealing softens the steel, reduces stresses and improves ductility, and tempering follows hardening and reduces brittleness. In steel, the general term "hardening" usually means precisely quench hardening.
Annealing may be needed before hardening, when the steel has to be softened, machining made easier or stresses reduced after earlier operations. Reducing stresses also reduces the risk of distortion during later hardening.
The most common problems in steel hardening
The most common problems are cracking, deformation and distortion of the part. Cracking results from stresses: the formation of stresses depends on the cross-section, because parts with a varying cross-section cool unevenly. To limit them, quenching in oil, stepped hardening and tempering immediately after hardening are used. Overheating the steel worsens its structure, and too long soaking at high temperature weakens strength. That is why proper hardening requires experience and control of parameters.
Steel hardening and CNC machining
In CNC machining, steel hardening is usually a finishing operation. A typical process is milling or turning, hardening and then finishing. After hardening, the hardness makes cutting difficult, so parts are finished by grinding or electrical discharge machining. More about these stages on the pages about heat treatment, CNC grinding and electrical discharge machining.
Applications of hardened steel
Hardened steel is used in the tool industry. In the machine industry hardened steel is used for transmissions and gears. Hardened steel is also key in the production of automotive parts. Steel hardening increases its wear resistance, which is why it is an indispensable part of many technologies.
When is it worth outsourcing steel hardening?
It is worth outsourcing hardening when a part is to work under heavy load, is exposed to abrasion or is to have a longer service life. Outsourcing to a contractor who combines heat treatment with milling, turning and grinding shortens the path from material to finished part. It is also worth agreeing with the contractor whether the part will be ground after hardening, because machining allowances depend on it. For a quote, a technical drawing with the steel grade, the required hardness and the quantity is enough.
Steel hardening – FAQ
What does steel hardening involve?
Steel hardening consists of heating to a set temperature, soaking and rapid cooling, which increases hardness and strength. Tempering is used after hardening.
How do you harden steel properly?
You need to heat the steel to the right temperature, soak it, cool it in a suitable medium and temper it. Proper hardening requires temperature control and selecting parameters for the steel grade.
Can ordinary steel be hardened?
Ordinary steel can be hardened if it has sufficient carbon content. Steel with very low carbon content does not reach high hardness.
How is steel hardened?
Steel is heated to the austenitizing temperature and then cooled quickly in water or oil. The methods include through, surface, stepped and isothermal hardening.