Tempering (quenching + tempering) is a common heat treatment process for steel, usually used to improve the properties of steel. The main purpose of tempering is to improve the strength, hardness and toughness of steel by achieving a balanced organizational structure through appropriate heating and cooling processes. The process usually includes the following steps:

　　Heating (Austenitizing): Steel is first heated to its critical temperature (i.e., austenitizing temperature) above the complete transformation into austenite (austenite), which is a high-temperature steel tissue form.

　　Holding: The steel is held at elevated temperatures for a period of time to ensure homogenization of the organization. This step helps to improve the hardenability of the material.

　　Cooling (Quenching): The steel is rapidly cooled to allow rapid transformation of austenite to martensite, thereby increasing hardness. The choice of cooling medium and control of the cooling rate are critical to the success of the tempering process.

　　Tempering: Tempered steel is usually too brittle and hard, so it needs to be tempered to reduce hardness and increase toughness. This step involves reheating the steel to a lower temperature, holding it for a certain period of time, and then cooling it.

　　Tempered steel has a high level of strength and hardness while maintaining a certain level of toughness. This makes it suitable for many engineering applications, especially where a balance of strength and toughness is required. It is important to note that different types of steel and different tempering parameters (temperature, time, cooling rate, etc.) will result in different properties. Therefore, it is important to develop an appropriate process program based on the specific alloy composition and application requirements when performing a tempering treatment. Carbon steel is often used without final heat treatment, but it can be annealed, normalized, case-hardened or tempered to enhance its fabrication and mechanical properties.

　　Q235

　　Q235 is an ordinary low carbon structural steel with a carbon content of between 0.12% and 0.2%, equivalent to 10 or 20 gauge steel, which can theoretically be quenched to obtain martensite, but due to the low degree of martensitic carbon supersaturation, the hardness of the quenched steel is very low, only about 170HBS. And the supply state hardness of this steel is about 144HBS or so (factory has been normalized). It can be seen, with Q235 quenching, its strength and hardness increase is not obvious, and also have to face deformation, cracking, oxidation, decarburization of the heat treatment disadvantages, and a heat treatment costs, is quite uneconomical.Q235 generally buy with no heat treatment, generally it is used in engineering a large number of places where the need for steel, a huge amount of, generally hot rolled on the use of hot rolling is that there is a normalizing of this heat treatment. There are several reasons for not heat treatment:

　　- These occasions do not need too high mechanical requirements.

　　- The volume of steel components is too large, heat treatment is not practical.

　　- Material is cheap, quality requirements are relatively low, and is a low carbon steel, heat treatment is not very good.

　　- If you have to use Q235 hardened hardness that can only carburize, but a very uneconomical thing.

　　No. 45 steel

　　45 steel is a commonly used medium carbon tempered structural steel. The cold plasticity of the steel in general, annealed, normalized than the tempering to be slightly better, with high strength and good machinability, after appropriate heat treatment to obtain a certain degree of toughness, plasticity and abrasion resistance, the material source is convenient. Suitable for hydrogen welding and argon arc welding, less suitable for gas welding. Preheating is required before welding, and stress relief annealing should be carried out after welding. Normalizing improves the cutting performance of billets with hardness less than 160HBS. The steel after tempering treatment, its comprehensive mechanical properties to be optimized over other medium-carbon structural steel, but the steel hardenability is low, the critical quenching diameter in water is 12~17mm, water quenching has a tendency to crack. When the diameter is greater than 80mm, after tempering or normalizing, its mechanical properties are similar to medium-sized and small mold parts can be obtained after tempering treatment of high strength and toughness.