What is Hardening?
Hardening is a metallurgical metalworking process used to increase the hardness of a metal. The process starts with the heating of metal at a critical transformation temperature and quenches in a molten salt bath/water. Hypoeutectoid steels are heated from 30 to 50-degree c above Upper Critical Temperature (UCT), while hypereutectoid steel heats above Lower Critical Temperature (LCT).
If the cooling rate is higher than its critical value, austenite gets super cool to the martensite point. Due to rapid cooling, carbon freezes at its location, and microstructure appears like the colloidal solution of cementite. The microstructure is called martensite, which is very hard and brittle.
Hardened steel is very brittle and cannot use for practical purposes. After the hardening process on the steel, it must go through a tempering process to reduce brittleness and internal stresses. It gives desired mechanical properties to the material. In tool steel, tempering improves toughness, hardness, and wear resistance. While in structural steel, tempering increases toughness, strength, and ductility.
Factors need to consider during Hardening
1. Adequate Carbon Content
To produce hard structures like martensite, at least 0.5% of carbon must be present. Carbon increases hardness and up to 1% wear resistance of the surface.
2. Heating rate and heating time
The heating rate and heating time decide how much hardness the material has. The heating rate can hamper the hardness of the material. So, it requires to have proper heating rate and heating time. The heating time is different for every metal.
3. Quenching medium
In the martensite transformation, slower cooling reduces the formation of internal stresses, cracking, and distortion. A few quenching mediums are water, oil, air, aqueous solution, and molten salt. Water and aqueous solution have a higher cooling rate. Both solutions are using in the cooling of some low alloy steels and carbon. Quenching medium is important to improve the hardness of the material.
4. Quenching rate
The quenching rate is different for different materials and, it depends on the quenching medium.
5. Size and rate
Small and medium objects are easy to quench with their central axis perpendicular to both surfaces. On the edge, thin and flat objects can immerse. Heavy objects are not possible to immerse in the cooling medium. So the liquid is agitated on heavy objects.
6. Surface condition
When the object quenches in the water, only its surface gets in contact and, the heat dissipates from the surface to the water. The cooling rate is directly proportional to the surface area of the part.
Applications of Metal Hardening
- Machine Cutting Tools
As a cutting tool, these need to be stronger than the workpiece or the material to cut. If these tools are tough, then they can cut workpieces accurately without wear and tear.
Bearings are subjected to very high forces as it is the only rotating member that supports the whole machine. These forces cause stresses on the surfaces of the bearing. So, it is necessary to have hard surfaces to withstand the force.
- Armor plating
In bulletproof jackets, plates are there to avoid penetration of bullets through them. So, the hardness of the plate can prevent the penetration of bullets.
- Mechanical components
High hardness materials can prevent machine components from wear, tear, and breaking.