Heat treatment is a set of technological processes aimed at changing the structure and properties of metals and alloys through controlled heating, holding and cooling.
Heat treatment makes it possible to increase the strength, hardness, ductility, wear resistance and durability of metal products, as well as to relieve internal stresses that arise after welding, casting or machining. The specific treatment mode is selected taking into account the material grade, the size of the part and its operating conditions.
Main Stages of Heat Treatment
The heat treatment process includes three main stages:
- Heating. The metal is heated to a specified temperature at which the required structural changes begin.
- Holding. The product is kept at the set temperature for a certain time. This is necessary for uniform heating and the completion of structural transformations.
- Cooling. The metal is cooled at a specified rate. It is the cooling conditions that largely determine the final hardness, strength, ductility and structure of the material.
Cooling can take place in a furnace, in air, in water, in oil or in a special medium. The choice of method depends on the type of treatment and the required result.
Main Types of Heat Treatment
Annealing
Annealing involves heating the metal to a set temperature, holding it and then cooling it slowly.
The main objectives of annealing are:
- reducing hardness;
- increasing ductility;
- improving machinability;
- relieving internal stresses;
- homogenising the material structure.
Annealing is often carried out before machining, welding or subsequent heat treatment of the product.
Normalizing
Normalizing is similar in principle to annealing, but differs in that the metal is cooled in air. The process helps to eliminate structural defects that occur after casting, forging or welding, and to improve the strength and uniformity of the material.
Normalizing is used to prepare products for further processing and to obtain a more uniform fine-grained structure.
Hardening
Hardening consists of heating the metal to the required temperature followed by rapid cooling in water, oil or another quenching medium.
The main purpose of hardening is to increase the hardness, strength and wear resistance of the product. At the same time, the material may become more brittle, so hardening is usually followed by tempering.
The choice of cooling medium is of great importance. Water provides more intensive cooling, while oil reduces the likelihood of distortion and cracking.
Tempering
Tempering is carried out after hardening. The metal is reheated to a temperature below the critical point, held and cooled.
The process makes it possible to:
- reduce brittleness;
- relieve residual stresses;
- increase ductility;
- maintain the required level of hardness and strength.
Depending on the temperature, a distinction is made between low-, medium- and high-temperature tempering. The specific mode is selected taking into account the purpose of the part.
Austenitizing
Austenitizing is the heating of steel to the temperature at which an austenitic structure forms. This stage usually precedes hardening or normalizing and provides the necessary conditions for subsequent structural transformations.
Isothermal Hardening
In isothermal hardening, the metal is cooled to a specified temperature and held in an isothermal medium for a certain time.
This process makes it possible to obtain a more stable structure, increase impact toughness and reduce the risk of cracking and distortion.
Comparison of Heat Treatment Types
| Type of treatment | Main purpose | Approximate temperature | Holding time | Cooling method |
| Annealing | Reducing hardness, relieving stresses, improving the structure | 600–850 °C | 1–10 hours | Slow cooling in the furnace |
| Normalizing | Improving the structure and mechanical properties | 850–950 °C | 1–2 hours | Cooling in air |
| Hardening | Increasing hardness, strength and wear resistance | 750–900 °C | 30–60 minutes | Rapid cooling in water, oil or a special medium |
| Tempering | Reducing brittleness after hardening | 200–600 °C | From 30 minutes to 2 hours | Cooling in air |
| Austenitizing | Obtaining an austenitic structure before further treatment | 900–1100 °C | Depends on the steel grade | According to the subsequent process |
| Isothermal hardening | Increasing impact toughness and reducing the risk of cracking | 250–450 °C | 1–3 hours | Holding in an isothermal medium |
The temperatures and holding times given are approximate. The exact mode is determined by the metal grade, chemical composition, product dimensions and required characteristics.
Heat Treatment of Welded Joints
During welding, the metal is subjected to uneven heating and cooling. This can lead to residual stresses, distortion, a coarse-grained structure and reduced strength of the welded joint.
Several methods are used to eliminate these effects.
Stress Relieving
The welded joint is heated to approximately 600–700 °C, held and slowly cooled. The treatment reduces residual stresses and lowers the risk of distortion or failure of the structure during operation.
Normalizing of Welds
Normalizing improves the metal structure in the heat-affected zone, reduces non-uniformity and restores the required mechanical properties of the joint.
Hardening Followed by Tempering
This treatment is used for critical welded structures that require increased hardness, strength and resistance to mechanical loads.
Specifics of Treating Carbon Steel
The properties of carbon steel largely depend on its carbon content.
- Low-carbon steel contains up to 0.25% carbon. It has good ductility, relatively low hardness and good weldability.
- Medium-carbon steel contains 0.25 to 0.6% carbon. It has increased strength but requires a more precise choice of treatment mode.
- High-carbon steel contains more than 0.6% carbon. This material has high hardness and wear resistance but is more brittle.
Annealing, normalizing, hardening and tempering are used for carbon steels. The mode is selected depending on the carbon content and the purpose of the product.
For parts that require a hard, wear-resistant surface while maintaining a ductile core, carburizing can be used. The process involves saturating the surface layer with carbon and is used in the manufacture of gears, shafts and other loaded parts.
Specifics of Treating Low-Alloy Steel
Low-alloy steel contains small amounts of alloying elements such as chromium, nickel, molybdenum, manganese or vanadium. These additions increase the strength, wear resistance and resistance of the material to external influences.
The main objectives of heat treatment of low-alloy steel are:
- increasing strength and hardness;
- improving ductility and impact toughness;
- relieving stresses after welding and machining;
- obtaining a uniform structure;
- reducing the risk of distortion and cracking.
Annealing, normalizing, hardening and tempering are also used for these steels. Oil is often used for hardening, as it provides less intensive cooling than water and reduces the likelihood of damage to the product.
Where Heat Treatment Is Used
Heat treatment is used in almost all industries related to the manufacture and operation of metal products.
Bridge Building and Shipbuilding
In the manufacture of large welded structures, heat treatment helps to relieve internal stresses, improve the structure of welded joints and extend the service life of steel structures.
Automotive Industry
Hardening and tempering are used in the production of crankshafts, gears, bearings and other parts operating under increased loads and friction.
Cutting Tool Manufacturing
Drills, knives, saws and other cutting tools are hardened and tempered to increase their hardness, wear resistance and ability to retain their cutting properties.
Agricultural Machinery
Heat treatment is used in the manufacture of ploughshares, mower blades and other parts that are in constant contact with the soil and subject to abrasive wear.
Conclusion
Heat treatment of metals and alloys makes it possible to purposefully change the structure and performance characteristics of a material. A correctly selected heating, holding and cooling mode increases the strength, hardness, ductility and wear resistance of products, and also helps to reduce internal stresses and the risk of premature failure.
To obtain a predictable result, the heat treatment mode must be selected individually, taking into account the steel grade, part geometry, previous technological operations and subsequent operating conditions.