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Steel is an essential material globally. It is combining iron and carbon results in a solid alloy that forms buildings, car tanks, and infrastructure more widely. One of the properties of this material is hardness, or resistance to deformation caused by indentation, impact, or wear. Remember that steel has its hardness, which is only sometimes sufficient for certain activities. To solve this problem, methods have been created to increase the hardness and other properties of steel significantly. The ways we are talking about involve hardening steel. Why use a cooling bath? Which method has the best cooling rate? Is it possible to achieve permanent deformation of the hardened component? We write about the technique, its advantages, and its effects below!

What is the hardening of steel? 

Steel surface hardening is usually carried out on the finished product, not the raw material. In CNC machining, steel hardening is a finishing process performed on the workpiece. This is because it is not economical to harden an entire block of steel, as much of it will be removed during machining. Also, hardened steel is more difficult to machine because the hardness of the workpiece makes tool penetration more difficult.

Internal structures made of steel and their hardness 

Keep in mind that steel can have different compositions depending on its application. What sets it apart is its internal structure. As the demand for higher-strength metals increases, it becomes necessary to harden steel. In its most basic form, steel has relatively low strength and hardness. However, modifying its structure can create a more robust version. Hardening steel enhances its hardness and strength. As for its internal structure, these concepts should be presented: 


This is the hardest form of the internal crystal structure of steel, where martensitic transformation occurs. Rapid cooling of austenitic iron forms martensite. Due to the rapid cooling rate, carbon is trapped in the solid solution, causing the part to harden. Martensite has a needle-like microstructure that looks like lenticular plates or plates separating grains of the parent phase, always in contact but never intersecting. 


Austenite is the second hardest steel structure in the internal range after martensite. It refers to an iron alloy called gamma iron. It usually occurs below 1500°C and above 723°C.


Perlite differs from martensite in that perlite structure is formed by slow cooling. It is a layered system of ferrite and cementite. At 723ºC, gamma iron transforms from its FCC structure into alpha iron, pushing the iron carbide (cementite) out of the solution.

What are the methods of hardening steel?

There are various methods of hardening steel. In particular, there is a distinction between techniques:

  • thermal (such as isothermal hardening),
  • mechanical, 
  • chemical 
  • a combination of two or more of these. 

The thermal hardening process is the most common method of hardening steel. It usually involves three basic steps: heating the steel, holding it at a certain temperature and cooling it. The first step involves heating the metal to a temperature high enough to cause internal structural changes. It also makes it easier to work on the metal, such as changing its shape. 

The different ways to harden steel are:

Cold working – How to cold harden steel? Cold working often changes the properties of steel or metal. This method of hardening steel deforms the metal at a temperature below its melting point. In that case, properties such as yield strength, tensile strength and hardness increase, while the elasticity and deformability of the material decrease. While about 90% of the energy is dissipated as heat during cold processing, the rest is stored in the crystal lattice, increasing its internal energy.

Alloy quenching in solution – This technique involves adding alloying elements to the base metal to form a solid solution. After this point, the metal hardens due to the presence of alloy atoms in the network of the base metal. The size difference between the solute and solvent atoms affects the effectiveness of solid solutions. When the solute atoms are larger than the solvent’s, a compressive stress field appears. On the other hand, if the solute atoms are larger than the solute atoms, there is a tensile stress field. The solute atoms, which twist the lattice into a tetragonal structure, cause rapid hardening.

Hardening and tempering – With this method, steel is heated to a critical temperature above the austenite range, held there, and then rapidly cooled or, more commonly, quenched in water, oil or molten salt. For sub-eutectoid steels, the heating temperature is 30-50ºC above the austenite dissolution line. For supereutectoid steels, the temperature is higher than the eutectoid temperature. Hardening causes a martensitic transformation, which hardens the steel considerably. Unfortunately, we must remember that hardened steel is very brittle, and we must be careful with this part of the metal surface.

Induction hardening and flame hardening

This is a variation of surface heat treatment, where the component is heated quickly to prevent damage to the center of the material. The material then hardens faster. This creates a high level of martensite on the surface.

Diffusion hardening (nitriding)

This involves changes in the composition of the near-surface zone. Fine particles are dispersed, allowing the selected gas to react and diffuse into the steel. The steel is heat treated toward a tempered martensitic structure during this process. The component is then exposed to an ammonia atmosphere at about 550°C for 12-36 hours. Small amounts of alloying elements, such as Al or Cr, increase the formation of finely dispersed nitrides, which greatly improves surface hardness and wear resistance. This nitride composition significantly outperforms martensite in terms of hardness.


This involves exposing the steel to a carbon-containing atmosphere at high temperatures. The carbon atmosphere can be made from high-grade carbon or decomposed natural gas. The carbon atoms diffuse beneath the surface of the metal, forming a high-carbon coating, which is then hardened to form a hard, wear-resistant martensitic surface.

How do you test the hardness of steel? 

Although ordinary hardening can affect the hardness of steel, it is still helpful to know the maximum hardening depth and hardening test methods. Hardness does not have a specific unit of measurement. Instead, it is described by an index number. There are various hardness tests, and the metric used to describe the hardness of a material depends on the test used. Some common hardness tests are:

  • Brinell hardness test – In this test, a steel ball of known diameter is applied to the material’s surface as a load. 
  • Vickers hardness test – In Vickers hardness tests, the load is based on a square diamond pyramid. The load is applied to the material’s surface for about 30 seconds. The trace of the pyramid is calculated, which is then used to calculate the metal’s hardness.
  • Knoop micro-hardness test – This hardness test is designed for thin sheet metal or very brittle materials. The tip of the diamond pyramid forms a very small indentation in the material. The grooves made are then examined under a microscope and used to calculate the hardness of the material.
  • Rockwell hardness test – Rockwell hardness is used to measure the difference in hardness of steel before and after heat treatment. The penetrator can be a steel ball or a diamond conical penetrator. Hardness is measured by the depth of penetration of the material. Usually, two loads are used. 

Steel grades that can be hardened

Currently, it is considered that the most popular grades of steel for hardening are: 

  • Carbon steel
  • Alloy steel,
  • Stainless steel
  • Tool steel

The basic elements of steel are iron and carbon. However, different amounts of carbon and other alloying elements determine the properties of each grade. The carbon content of each steel determines its hardenability and maximum achievable hardness. This is especially true for hardenability since carbon promotes martensite formation.


High-carbon steels, alloy steels, and others have certain mechanical properties. Hardening and a critical transformation temperature produce improved mechanical properties and mechanical properties. Hardening steel is essential for strengthening a material that needs improvement in nature. However, we should remember to do it professionally and in accordance with the guidelines mentioned above.