Metal hardening is a heat treatment process aimed at increasing the strength and hardness of a material. This process plays a key role in the production of steel products, ensuring their durability and reliability. In this article, we will look at the main aspects of hardening technology and the methods of hardening steel.
Fundamentals of Metal Hardening
Metal hardening involves heating steel to a certain temperature, holding it at this temperature and then cooling it rapidly. The purpose of the process is to change the microstructure of the metal, which leads to an improvement in its mechanical properties. The main parameter determining the nature of hardening is the critical temperature, at which phase transformations take place in the steel.
The Hardening Process
- Heating. Steel is heated to a temperature above the critical point (usually in the range of 800–900 °C), during which austenitizing takes place — the transformation of the metal structure into austenite.
- Holding. Once the required temperature is reached, the metal is held at this temperature for a certain time so that the phase transformation is completed.
- Cooling. Rapid cooling is carried out in various media, such as water, oil, air or special quenching mixtures. Cooling leads to the transformation of austenite into martensite — a harder and stronger structural constituent.
Recommended Hardening Parameters for Various Steel Grades
The table shows approximate heating temperatures, cooling media, hardness values and tempering parameters for selected steel grades.
| Steel grade | Heating temp., °C | Medium | Hardness, HRC | Tempering, °C (final) |
|---|---|---|---|---|
| St3 | 880–920 | air | not specified | not performed |
| Steel 20 | 880–920 | air | not specified | not performed |
| Steel 45 | 820–850 | water | 45–56 | 180–220 |
| 40Kh | 840–860 | oil | 45–56 | 180–220 |
| 65G | 800–830 | oil | 50–62 | 200–250 |
| U8 | 780–800 | water / oil | 60–64 | 160–220 |
| KhVG | 820–850 | oil | 60–64 | 160–220 |
| ShKh15 | 830–860 | oil | 60–64 | 150–180 |
| 40Kh13 (stainless) | 1000–1050 | oil / air | 48–56 | 180–250 |
Exact modes depend on the cross-section of the part and the drawing requirements.
Steel Hardening Methods
There are several main methods of hardening steel, each of which is used depending on the required properties of the final product and the characteristics of the initial material.
- Conventional (simple) hardening. The most common method, involving heating to the austenitizing temperature and rapid cooling in water or oil. This method is suitable for most carbon and low-alloy steels.
- Hardening followed by tempering. Involves conventional hardening followed by heating to a lower temperature (150–650 °C) and holding at this temperature. Tempering reduces internal stresses and increases the toughness of the metal while maintaining high hardness.
- Differential hardening. This method involves cooling different parts of the product differently, which makes it possible to obtain different hardness in different areas. This approach is used for tools and weapons, where a combination of strength and elasticity is important.
- Hardening with induction heating. A method based on heating the metal surface by means of induction currents, which ensures very rapid and localised heating. After heating, the surface is rapidly cooled, creating a hard outer layer with high wear resistance.
- Salt bath hardening. Used for products of complex shape or large size, where uniform cooling in water or oil is difficult. Salt baths provide slow and uniform cooling, which reduces the risk of distortion.
Unique Characteristics of Steel after Hardening
The hardening process gives steel a number of properties that make it stronger and harder. Here are some of them:
- Increased hardness: Hardening significantly increases the hardness of steel. Instant cooling “freezes” the microstructure of the metal, forming martensite — a very hard and brittle phase. The more intensive the strengthening, the higher the hardness.
- Increased strength: Hardening also increases the strength of steel. The formation of martensite and the change in microstructure make the metal more resistant to deformation. This is especially important for products that require high strength, such as knives or springs.
- Improved wear resistance: Hardened steel has increased wear resistance. A hard material copes better with friction and wear, which makes it possible to keep cutting edges sharp and to use products for a long time under high loads.
- Limited ductility: However, it should be noted that hardening can lead to increased brittleness of steel. The martensitic structure can reduce the ductility of the material, making it more prone to cracking and fracture.
- Reduced impact toughness: Hardened steel may have low impact toughness. This is due to its increased hardness and brittleness, which may be undesirable in some applications. To improve impact toughness, additional heat treatment, such as tempering, may be required.
Metal hardening is a key process in metallurgy, providing the necessary mechanical properties of steel products. The variety of hardening methods makes it possible to choose the optimal method for each specific case, which is important for achieving high performance characteristics of products. Understanding the fundamentals of hardening technology and the competent application of various steel hardening techniques play an important role in modern industry and manufacturing.