Heat Treatment of Steel: Softening, Stress Relief, and Annealing

Heat treatment is a process that involves a series of heating and cooling operations to ensure that the hardness, grain structure, and mechanical properties of a material meet the desired specifications.

Based on the characteristics of the heat treatment and the resulting properties, it can be classified into two main categories: Annealing and Hardening.

Annealing

Annealing is a heat treatment process where materials are heated to a specific temperature and then cooled slowly. This process aims to improve the machinability of materials, increase their plastic forming capacity, and correct internal structural properties.

Annealing processes are described by their processing temperatures and cooling methods. The primary annealing processes include:

Softening Annealing

Materials that have not undergone heat treatment exhibit different hardness levels depending on their carbon content. Some materials might be too hard to process. For plastic forming operations, materials are preferred to be at a minimum hardness. Therefore, softening annealing is performed to reduce the hardness of materials, making them easier to shape.

The microstructure of steel at room temperature consists of particles and carbon content that form carbides arranged in a sequence of fine, long plates, known as pearlite. The density of these carbide plates increases with the material’s carbon content, which increases hardness. Softening annealing transforms the fine, elongated carbide plates into a shorter, spherical shape, making the steel softer and more formable compared to its original state. This process is also referred to as spheroidizing annealing.

Stress Relief Annealing

Internal stresses occur in materials due to processes like welding, plastic forming, or excessive heating and rapid cooling. To relieve these internal stresses, the material is heated to a temperature above its maximum use temperature but below the phase transformation temperature and is held for up to two hours. This process helps eliminate unwanted internal stresses.

Recrystallization Annealing

Parts shaped through plastic forming methods may experience permanent deformation, especially in wall regions. This can lead to increased hardness and strength but reduced ductility and electrical conductivity. Recrystallization annealing is performed by heating the material below the phase transformation temperature and holding it for up to one hour, followed by slow cooling. This process helps restore the material's structure to a smooth and regular form, recovering the properties of the material prior to deformation. This is also known as recrystallization.

Normalization Annealing

While all annealing processes improve material properties, they can also lead to grain growth. To avoid large grain structures in specific applications, materials are heated to hardening temperatures and then allowed to cool in a controlled environment. What differentiates normalization from other annealing processes is that the material is cooled rapidly in still air rather than slowly. This results in a finer grain structure. This process is also called normalization.

Hardening of Steel

Depending on the operating conditions of the produced parts, it may be necessary for the entire part or just a portion of it to gain hardness, either all the way to the core or only along the surface. In such cases, different heat treatments must be applied depending on the desired property.

The hardening process is assessed under different headings based on its application characteristics and the final structural properties it achieves.

Tempering

Tempering is a heat treatment process involving quenching and tempering to achieve the desired hardness and mechanical properties. It is especially used when the entire cross-section of a part needs to be hardened.

Quenching

Quenching is the simplest way to describe hardening, where the material is heated to the hardening temperature and then cooled rapidly. The selection of hardening temperature, heating rate, quenching medium, and cooling rate are all interrelated and require expert knowledge to determine the correct values.

Hardening temperature ranges are determined through a series of experiments to ensure maximum hardness and the smallest possible grain structure. Heating below or above the optimal hardening temperature will result in lower hardness and a final structure that does not meet the desired specifications. The duration of holding at the hardening temperature is also important and is related to the material’s alloy content, the size of the grains, and their appropriateness for the process.

The choice of quenching medium is influenced by the material’s alloy content. For low-alloy steels, water and salt baths are commonly used, whereas for high-alloy steels, softer mediums such as oil are preferred due to the risk of distortion. The most commonly used cooling mediums include water, oil, salt bath, and air.

Water:

One of the most important aspects of water quenching is the temperature of the water used to cool the hot part. The optimal cooling water temperature for maximum efficiency is between 20–40 °C. Cooling at temperatures above 60 °C significantly reduces the cooling rate.

Oil:

Oil quenching provides a slower cooling rate than water. The most efficient cooling oil temperature ranges between 50–80 °C. Continuous, rapid stirring of the oil greatly enhances its cooling efficiency.

Saltwater Solution:

To enhance the efficiency of water quenching, sodium hydroxide or table salt can be added to the water. However, due to the corrosion caused by table salt on the parts, it is rarely used. Adding NaOH in a 10% ratio significantly increases the cooling rate, allowing for deeper hardening and reducing internal stresses.

Air:

Air quenching is the least efficient of the methods, mainly due to the very low cooling rate of air. Even calm air has a cooling rate that is less than 1% of that of water. As a result, this method is generally only suitable for high-speed steels.

Tempering After Quenching

After the quenching process, the final structure of the material becomes very hard and brittle, often accompanied by internal stresses formed during rapid cooling. Therefore, tempering is applied to improve the toughness of the material by reheating it, holding it at the same temperature for a period, and then cooling it slowly.

The tempering process can be performed at different temperatures depending on the desired toughness, hardness, and final structure. If tempering is done after the part has cooled completely, it may cause cracking. Therefore, tempering should be done as soon as the part cools to 60–80 °C.

Carburizing

Low-carbon steels, which are easy to machine, are subjected to carburizing after machining to increase their wear resistance and maintain a soft core. This process involves introducing carbon into the surface of the part, which enhances wear resistance while keeping the core soft, ensuring that the entire part exhibits toughness and high impact resistance.

Carburizing can occur in solid, liquid, or gas-phase environments. The most controlled and economical method is gas carburizing, where carbon sources such as CO or methane gas are used. In liquid carburizing, salts such as sodium cyanide and potassium cyanide are commonly used as carbon sources. Liquid carburizing is typically suitable for small parts. Solid carburizing, usually performed with charcoal, is harder to control and requires more expertise, making it less commonly preferred.

In carburizing, the goal is to increase the surface carbon content to about 0.7–0.8%. If the carbon content exceeds this range, it may lead to the formation of carbides, resulting in a brittle surface. The most important criterion for carburizing is achieving the desired depth of carburizing.

After carburizing, quenching is performed to harden the surface. This can be done by directly quenching from the carburizing temperature with water (direct quenching), cooling to room temperature and then tempering (single quenching), or quenching from the carburizing temperature and then tempering at a lower temperature. After the quenching process, tempering should always be done. The highest wear resistance after carburizing is achieved after tempering at approximately 300 °C, not at the highest hardness.

Induction Surface Hardening

Induction hardening is a surface hardening process in which the surface of the part is rapidly heated using induction currents and then cooled quickly. It is similar to flame hardening but is more efficient in terms of processing time and thermal accumulation on the surface. After the rapid heating by induction, the part is typically cooled with water, which can increase the risk of cracking, especially in high-carbon steels. To reduce cracking and internal stresses, the cooling water should be around 60 °C or salt may be used.

After hardening, tempering is done at 150–200 °C to relieve internal stresses.