Quenching and Tempering: Steel Heat Treatment Process and Performance Optimization
Quenching and tempering (quenching + tempering) is a common heat treatment process for steel, usually used to improve the properties of steel. The main purpose of quenching and tempering is to achieve a balanced organizational structure of steel through appropriate heating and cooling processes, thereby improving its strength, hardness and toughness. This process usually includes the following steps:
- Heating (Austenitizing): Steel is first heated above its critical temperature (i.e., austenitizing temperature), causing it to completely transform into austenite (austenite), which is a high-temperature steel structure.
- Soaking: Steel is kept at high temperature for a period of time to ensure the homogenization of the structure. This step helps improve the hardenability of the material.
- Cooling (Quenching): The steel is cooled rapidly, causing austenite to quickly transform into martensite, thereby increasing the hardness. The selection of cooling medium and the control of cooling rate are very critical to the success of the quenching and tempering process.
- Tempering: Quenched and tempered steel is usually too brittle and hard, so it needs to be tempered to reduce the hardness and improve the toughness. This step involves reheating the steel to a lower temperature, holding it warm for a certain period of time, and then cooling it.
Quenched and tempered steel has higher strength and hardness while maintaining a certain degree of toughness. This makes it suitable for many engineering applications, especially where a balance of strength and toughness is required. It should be noted that different types of steel and different quenching and tempering parameters (temperature, time, cooling rate, etc.) will result in different properties. Therefore, when performing quenching and tempering, an appropriate process plan must be developed based on the specific alloy composition and application requirements. Carbon steel is often used without final heat treatment, but it can be annealed, normalized, case hardened or tempered to enhance its manufacturing and mechanical properties.
ss400 Mild Steel
SSS400/Q235 is an ordinary low-carbon structural steel with a carbon content between 0.12% and 0.2%, which is equivalent to No. 10 and No. 20 steel. In theory, it can be quenched to obtain martensite, but due to the martensitic carbon The supersaturation is very low, and the hardness after quenching is very low, only about 170HBS. The hardness of this steel in its supply state is about 144HBS (it has been normalized before leaving the factory). It can be seen that when quenched with Q235, the strength and hardness increase is not obvious, and it also has to face the heat treatment disadvantages of deformation, cracking, oxidation, decarburization, and a heat treatment cost, which is quite uneconomical. ss400 Mild Steel is generally purchased without heat treatment. Generally, it is used in engineering projects where a large amount of steel is needed. The quantity is huge. It is usually used after hot rolling. Hot rolling means normalizing heat treatment. There are several reasons for not heat treating:
- These occasions do not require high mechanical requirements.
- The volume of steel components is too large, and heat treatment is not practical.
- The material is cheap, the quality requirements are relatively low, and it is low carbon steel, so the heat treatment effect is not very good.
- If you have to use Q235 to harden it, you can only carburize it, but it is not cost-effective.
1045/S45C Carbon Steel
1045/S45C steel is a commonly used medium carbon quenched and tempered structural steel. The cold plasticity of this steel is average. Annealing and normalizing are slightly better than quenching and tempering. It has higher strength and better machinability. After appropriate heat treatment, it can obtain a certain degree of toughness, plasticity and wear resistance. Material source convenient. Suitable for hydrogen welding and argon arc welding, but not suitable for gas welding. Preheating is required before welding, and stress relief annealing should be performed after welding. Normalizing can improve the cutting performance of blanks with hardness less than 160HBS. After quenching and tempering treatment, the comprehensive mechanical properties of this steel should be optimized compared to other medium carbon structural steels. However, the hardenability of this steel is low. The critical hardenable diameter in water is 12~17mm, and it has a tendency to crack during water quenching. When the diameter is greater than 80mm, its mechanical properties are similar after quenching and tempering or normalizing. Higher strength and toughness can be obtained after quenching and tempering treatment for medium and small mold parts.
Applications of 1045/S45C carbon steel:
- Can be used as one of the materials for manufacturing DIN 6883-1956 wedge keys; can be used to manufacture bolts of grade 8.8 and 9.8 M16 and below, bolts of grade 10.9 and M22 and below, nuts of grades 8, 9 and 10, and washers of grade 300HV , see JC/T 5057.40-1995.
- Can be used to manufacture high-strength large hexagonal bolts for steel structures of grade 8.8S M20 and below, large hexagonal nuts of grade 10H or 8H, and high-strength washers of performance grade 35~45HRC. See GB/T 1231-2006.
25CrMo4/35CrMo4/42CrMo4 Steel Q+T
The above materials are all low-carbon alloy steels, containing alloy elements such as chromium and molybdenum, and have high hardenability and no temper brittleness. 25CrMo4 alloy steel has sufficient high-temperature strength below 500°C, very good weldability, little tendency to form cold cracks, and good machinability and cold strain plasticity. 35CrMo4 steel is generally used in the quenched and tempered or carburized and quenched state. Heat treatment specifications for this steel: quenching at 880°C, water cooling, oil cooling; tempering at 500°C, water cooling, oil cooling. 42CrMo4 alloy steel is used to manufacture high-pressure pipes and various fasteners, and higher-grade carburized parts such as gears and shafts that work in non-corrosive media and media with operating temperatures below 250°C and containing nitrogen and hydrogen mixtures.
