Steel is an extremely important material on a global scale. By combining iron and carbon, we obtain an exceptionally durable alloy that forms buildings, car tanks, and broadly understood infrastructure. One of the properties of this material is hardness, which is resistance to deformations caused by denting, impact, or wear. Let's remember that steel has its hardness, which is not always sufficient for certain actions. To solve this problem, methods have been developed to significantly increase the hardness and other properties of steel. The methods we are talking about concern steel hardening. Why is it worth using a cooling bath? Which method has the best cooling speed? Can permanent deformations of the hardened element be caused? We write about the technique itself, its advantages, and effects below!
What is steel hardening?
Surface hardening of steel is usually carried out on the finished product, not on the raw material. In CNC machining, steel hardening is a final processing step performed on the workpiece. This is due to the fact that hardening an entire block of steel is not economical, as a large part of it will be removed during machining. Also, hardened steel is more difficult to process, as the hardness of the workpiece hinders tool penetration.
Internal steel structures and their hardness
Let's remember that depending on the application, steel can have different compositions. What distinguishes it is its internal structure. With the increasing demand for higher strength metals, steel hardening becomes necessary. Steel in its most basic form has relatively low strength and hardness. However, thanks to the possibility of modifying its structure, we can create a stronger version of it. Steel hardening enhances its hardness and strength. As for its internal structure, the following concepts should be presented:
Martensite
This is the hardest form of the internal crystalline structure of steel, where martensitic transformation occurs. Rapid cooling of austenitic iron creates martensite. Due to the lightning-fast cooling rate, carbon is trapped in the solid solution, causing hardening of the given part. Martensite has a needle-like microstructure, which looks like lenticular plates or plates separating the grains of the parent phase, always touching, but never intersecting.
Austenite
Austenite is the second hardest steel structure after martensite in the internal range. It refers to an iron alloy called gamma iron. It typically occurs below 1500°C and above 723°C.
Pearlite
Pearlite differs from martensite in that the structure of pearlite is formed by slow cooling. It is a layered system of ferrite and cementite. At a temperature of 723ºC, gamma iron transforms from its FCC structure into alpha iron, pushing out iron carbide (cementite) from the solution.
What are the methods of steel hardening?
There are various methods of steel hardening. The following techniques are particularly distinguished:
- thermal (e.g. isothermal hardening),
- mechanical,
- chemical
- a combination of two or more of them.
The process of thermal hardening is the most common method of steel hardening. It usually involves three basic stages, including heating the steel, maintaining it at a specific temperature, and cooling. The first step involves heating the metal to a temperature high enough to cause internal structural changes. It also facilitates work on the metal, e.g. changing its shape. Different methods of steel hardening include:
Cold working
How to harden steel in cold conditions? Cold working often changes the properties of steel or metal. This method of hardening steel simply deforms the metal at a temperature below its melting point. In this case, properties such as yield strength, tensile strength, and hardness increase, while plasticity and deformability of the material decrease. While about 90% of energy is dissipated as heat during cold working, the rest is stored in the crystal lattice, increasing its internal energy.
Solution Heat Treatment of Alloy
This technique involves adding alloying elements to the base metal to create a solid solution. From this point on, the metal hardens due to the presence of alloy atoms in the base metal lattice. The size difference between the atoms of the solute and the solvent affects the effectiveness of solid solutions. When the atoms of the solute are larger than the atoms of the solvent, a field of compressive stress appears. On the other hand, if the atoms of the solvent are larger than the atoms of the solute, there is a field of tensile stress. Dissolved atoms, which twist the lattice into a tetrahedral structure, cause rapid hardening.
Hardening and tempering
Thanks to this method, steel is heated to a critical temperature above the austenite range, maintained at it, and then quickly cooled or more often quenched in water, oil, or molten salt. In the case of hypoeutectoid steel, the heating temperature is 30-50ºC above the austenite dissolution line. For hypereutectoid steel, the temperature is higher than the eutectoid temperature. Quenching causes a martensitic transformation, which significantly hardens the steel. Unfortunately, we must remember that quenched steel is very brittle and care must be taken with this element of the metal surface.
Inductive hardening and flame hardening
This is a variant of heat treatment of the surface, where the element is quickly heated to prevent damage to the core of the material. The material then undergoes faster hardening. In this way, a high level of martensite is created on the surface.
Diffusion hardening (nitriding)
This is associated with changes in the composition of the surface zone. Fine particles are dispersed, allowing the selected gas to react and diffuse into the steel. During this process, the steel is subjected to heat treatment towards obtaining a tempered martensitic structure. Then, the element is exposed to the atmosphere of ammonia at a temperature of about 550°C for 12-36 hours. Small amounts of alloying elements, such as Al or Cr, enhance the formation of finely dispersed nitrides, which significantly improve surface hardness and wear resistance. This nitride composition greatly exceeds martensite in terms of hardness.
Carburizing
This involves exposing steel to an atmosphere containing carbon at high temperatures. The carbon atmosphere can be produced from high-quality coal or decomposed natural gas. Carbon atoms diffuse under the surface of the metal, creating a high-carbon coating, which is then hardened to form a hard, wear-resistant martensitic surface.
How to test steel hardness?
Although ordinary hardening can affect the hardness of steel, it is still worth knowing the maximum hardening depth and methods of testing hardening. Hardness does not have a specific unit of measure. 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 typical hardness tests include:
- Brinell hardness test - In this test, a steel ball of known diameter is applied to the surface of the material 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 pyramid's imprint is calculated, which is then used to calculate the hardness of the metal.
- Knoop microhardness test - This hardness test is intended for thin sheets or very brittle materials. The tip of the rhombic pyramid creates 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 - The Rockwell hardness is used to measure the difference in steel hardness before and after heat treatment. The penetrator can be a steel ball or a diamond cone penetrator. Hardness is measured by the depth of material penetration. Usually, two loads are used.
Types of steel that can be hardened
Currently, the most popular types of steel for hardening are considered to be:
- 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 type. The carbon content in each steel determines its hardenability and maximum achievable hardness. This applies especially to hardening, as carbon promotes the formation of martensite.
Summary
Hardening steel is an essential element for strengthening the material that naturally requires improvement. However, let's remember to do it professionally and in accordance with the above guidelines.