Tempering: Heat Treatment Process and Performance Optimization of Steel

Tempering: Heat Treatment Process and Performance Optimization of Steel

Tempering (quenching + tempering) is a commonly used heat treatment process for steel, often employed to improve its performance. The main objective of tempering is to achieve a balanced microstructure in steel through appropriate heating and cooling processes, thereby enhancing its strength, hardness, and toughness. The process typically involves the following steps:

Heating (Austenitizing): The steel is first heated above its critical temperature (i.e., the austenitizing temperature) to transform it completely into austenite, a high-temperature microstructure of steel.

Soaking: The steel is maintained at high temperature for a period to ensure uniform microstructure. This step helps improve the quenchability of the material.

Quenching: The steel is rapidly cooled to facilitate the transformation of austenite into martensite, thereby increasing its hardness. The choice of cooling medium and the control of cooling rate are crucial for the success of the tempering process.

Tempering: The quenched steel is often too brittle and hard, thus tempering is necessary to reduce hardness and improve toughness. This step involves reheating the steel to a lower temperature, holding it for a certain period, and then cooling it.

The tempered steel exhibits high strength and hardness while maintaining a certain level of toughness. This makes it suitable for various engineering applications, especially those requiring a balance of strength and toughness. It is noteworthy that different types of steel and different tempering parameters (temperature, time, cooling rate, etc.) can result in different properties. Therefore, when performing tempering, it is necessary to develop an appropriate process plan based on the specific alloy composition and application requirements. Carbon steel can often be used without final heat treatment, but it can undergo annealing, normalizing, surface hardening, or tempering to enhance its manufacturing and mechanical properties.

Q235

Q235 is a common low-carbon structural steel with a carbon content ranging from 0.12% to 0.2%, equivalent to grades 10 and 20 steel. In theory, it can be quenched to obtain martensite, but due to the low supersaturation of carbon in martensite, the hardness after quenching is relatively low, around 170HBS. The hardness of this steel in its supplied state is approximately 144HBS (it has been normalized during production). Therefore, quenching Q235 does not significantly increase its strength and hardness, and it is also susceptible to deformation, cracking, oxidation, and decarburization during heat treatment, making it an uneconomical choice. Q235 is typically used without heat treatment, particularly in large-scale engineering applications where high mechanical requirements are not necessary. The steel is often used in its hot-rolled state, which includes a normalizing heat treatment. The reasons for not performing heat treatment include:

• These applications do not require high mechanical properties.

• The size of steel components is too large for heat treatment to be practical.

• The material is inexpensive and has low quality requirements, and being a low-carbon steel, heat treatment does not yield satisfactory results.

• If it is necessary to quench Q235 to achieve hardness, carburizing is required, but this is generally not economically viable.

45 Steel

45 steel is a commonly used medium-carbon tempered structural steel. It exhibits moderate cold plasticity, with better performance during annealing and normalizing compared to tempering. It possesses high strength, good machinability, and can achieve certain toughness, plasticity, and wear resistance after appropriate heat treatment. The material is easily available and suitable for hydrogen welding and argon arc welding, but less suitable for gas welding. Preheating is required before welding, and stress-relief annealing should be performed after welding. Normalizing can improve the machinability of blanks with a hardness less than 160HBS. After tempering, 45 steel exhibits superior comprehensive mechanical properties compared to other medium-carbon structural steels. However, it has low hardenability, with a critical quenching diameter of 12-17mm in water, and there is a tendency for cracking during water quenching. When the diameter exceeds 80mm, the mechanical properties of the steel after tempering or normalizing are similar. Medium and small mold parts can achieve high strength and toughness after tempering.

Quenching + Tempering Data for 45 Steel

Applications of 45 steel:

• It can be used as one of the materials for manufacturing DIN 6883 - 1956 wedge keys. It can also be used to manufacture bolts of grade 8.8, 9.8 with specifications of M16 and below, as well as bolts of grade 10.9 with specifications of M22 and below, nuts of grades 8, 9, and 10, and gaskets of grade 300HV. See JC/T 5057.40-1995 for details.

• It can be used to manufacture high-strength large hexagon bolts for steel structures of grade 8.8S with specifications of M20 and below, large hexagon nuts of grades 10H or 8H, and high-strength gaskets with performance grades of 35~45HRC. See GB/T 1231-2006 for details.

25CrMo

25CrMo is a kind of low-carbon alloy steel containing alloy elements such as chromium and molybdenum. It has high hardenability and no temper brittleness. 25CrMo has sufficient high-temperature strength below 500℃, good weldability, a small tendency to form cold cracks, good machinability, and cold strain plasticity. 25CrMo is generally used in the quenched or carburized and quenched state. The heat treatment specification for this steel is quenching at 880℃ followed by water or oil cooling, and tempering at 500℃ followed by water or oil cooling. 25CrMo alloy steel is used to manufacture high-pressure pipes and various fasteners that work in non-corrosive media and at working temperatures below 250℃, as well as more advanced carburized parts such as gears and shafts.