Steel Tempering: A Key Stage of Heat Treatment
Steel tempering is a critical process in metalworking that involves heating steel to a specific temperature, followed by controlled cooling. The process relieves internal stresses that can adversely affect the technical properties of metal products.
Main Stages of Steel Tempering
Steel tempering involves three key stages:
- Heating the alloy to the temperatures at which phase transformations begin.
- Holding at the required temperature.
- Cooling at a specified rate.
These stages make it possible to achieve the required technical properties of products and to minimize internal stresses. How effectively residual stresses are relieved depends directly on the heat treatment temperature and the cooling rate: the higher the temperature and the slower the cooling, the better the result.
The cooling rate varies depending on the chemical composition of the alloy and the desired result:
- Rapid cooling from +550…+650 °C increases the fatigue limit of steel by retaining residual compressive stresses in the surface layer.
- Slow cooling of complex-shaped metal products after high-temperature tempering helps prevent warping.
- Alloy steels susceptible to temper embrittlement are cooled rapidly after tempering at +550…+650 °C.
Types of Steel Tempering
Depending on the heating temperature, there are three types of steel tempering: low, medium, and high.
Low-Temperature Tempering
Low-temperature tempering is carried out by heating workpieces to +250 °C. Its main results are reduced quenching stresses and improved toughness without a significant loss of hardness. After low-temperature tempering, medium- and high-carbon steels reach a hardness of 58–63 HRC and high wear resistance. However, without a tough core, they are vulnerable to dynamic loads. Low-temperature tempering is commonly used for cutting and measuring tools, as well as for metal products after carburizing and carbonitriding.
Medium-Temperature Tempering
Medium-temperature tempering is performed at +350…+500 °C and is used for products such as springs, leaf springs, and dies. The process provides high fatigue and elastic limits, along with good relaxation resistance. The resulting structures are troostite or troostite-martensite, with a hardness of 45–50 HRC. For springs, water cooling after heating to +400…+450 °C creates residual compressive stresses on the surface, which improves the strength properties of the steel.
High-Temperature Tempering
High-temperature tempering is carried out at +500…+650 °C, producing a tempered sorbite structure. This treatment achieves the optimal balance between strength and toughness. The combined heat treatment of quenching followed by high-temperature tempering is known as quenching and tempering (Q&T). It outperforms various types of annealing and normalizing, increasing ultimate tensile strength, yield strength, impact toughness, and reduction of area.
For medium-carbon steels with 0.3–0.5% carbon that must meet high requirements for impact toughness and fatigue limit, quenching and tempering are key processes. They increase the material’s strength and reduce its sensitivity to stress concentrators and its tendency to crack.
Types of Temper Embrittlement
Although raising the tempering temperature usually improves a product’s properties, in some cases it degrades the alloy. Metallurgists distinguish two types of temper embrittlement: low-temperature (Type I) and high-temperature (Type II).
Low-Temperature Embrittlement
Low-temperature embrittlement occurs when the material is held for a long time at +250…+350 °C. The cooling rate does not affect whether it develops. It is caused by the uneven distribution of carbon along the crystal lattice, which distorts the alloy’s structure and sharply increases brittleness. This process is irreversible and severely reduces the alloy’s performance properties. To prevent it, low- or medium-temperature tempering is used.
High-Temperature Embrittlement
High-temperature embrittlement occurs when three factors combine: heating the alloy above +500 °C, a high content of Cr, Mn, and Ni in the steel, and slow cooling. This leads to an uneven distribution of carbon, chromium, manganese, and nickel atoms, which disrupts the steel’s crystal lattice. The problem worsens if products are held within the critical temperature range for 8–10 hours. It can be addressed by reheating to the specified temperature followed by rapid cooling, or by alloying the steel with tungsten and molybdenum.
Thus, steel tempering is a critical process for achieving optimal performance properties in metal products. Choosing the right tempering mode and controlling the process can significantly improve the strength, toughness, and other properties of steel, ensuring the durability and reliability of finished products.
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