45 steel (slowly cooled after forging) low-magnification structure and non-metallic inclusions

45 steel (forging slow cooling) is a kind of middle and low carbon steel, and its main element is carbon. The microstructure of 45 steel after forging is mainly composed of ferrite and pearlite. The carbon content in the steel is 0.42%-0.50%, which is called low carbon steel. After slow cooling of 45 steel, its microstructure structure is ferrite and pearlite, low carbon content and low strength, but it has good toughness, plasticity and machinability, and its comprehensive mechanical properties are better.

After the hot processing of 45 steel, a wide range of non-metallic inclusions such as oxides, sulphides, phosphides and nitrides will be formed in the steel. These inclusions exist in the form of needle shape, ellipse, flow shape and defect bubbles. Generally, it is harmful to the mechanical properties, toughness and plasticity of the steel.

Low carbon steel has poor hardenability, so it is not suitable for quenching and tempering treatment, and usually does not require heat treatment and does not need to be normalized. However, the hardness of the steel can be improved by homogenization or carburization heat treatment after forging. After carburization heat treatment, the surface hardness of the 45 steel can be increased to 45-50 HRC.

Advantages and Disadvantages of 45 Steel

The advantage of 45 steel is its good plasticity, machining performance and welding performance. Its low carbon content makes it more suitable for punching and shearing. Low carbon steel can achieve a better weldability through proper welding process and reduce the welding stress, but it should pay attention to the standard welding current. Its plasticity and toughness are also suitable for forming.

The disadvantage of 45 steel is that its strength is relatively low after slow cooling. After slow cooling in the heat treatment of the steel, its strength and hardness are not high. Therefore, if the requirements for the strength and hardness of the parts are not high, the 45 steel can be chosen. But if higher strength and hardness are required, other grades of steel should be used. Furthermore, the non-metallic inclusions in the steel can also reduce its mechanical properties.

In conclusion, 45 steel has good plasticity, machinability, weldability and forming properties. It is widely used in many fields, such as agricultural machinery, electric appliances, chemical industry and construction industry. Its low carbon content and good machining performance make it more suitable for cutting. The disadvantage is that its strength and hardness are not high, and the non-metallic inclusions can affect its mechanical properties. But when it is used in parts with low requirements for strength and hardness, 45 steel still has a good application effect.

45# steel (cooled slowly after forging) is a medium-carbon alloy steel, which is used for making various types of parts requiring higher strength and toughness, such as military equipment, machine tools, and automotive self-driving components. The chemical composition of 45# steel is shown in Table 1.

Table 1. Chemical Composition of 45# Steel (%)

C: 0.42-0.50

Si: 0.17-0.37

Mn: 0.50-0.80

P: ≤ 0.035

S: ≤ 0.035

Cr: ≤ 0.25

Ni: ≤ 0.30

Mo: ≤ 0.25

45# steel has good mechanical properties, such as good hardenability, quenching hardness, brittle point, and small deformation after heat treatment. After quenching, it can gain a higher hardness of more than 25HRC, and the structure of the steel is homogeneous. In addition, the steel has excellent wear resistance, fatigue strength and toughness.

When using 45# steel, it is prone to cracking issues, which is mainly caused by its low ductility after heat treatment or non-metallic inclusions such as gas. Therefore, attention should be paid to the heat-treatment process and the cleanliness of the material. The non-metallic inclusions should not exceed 0.1%, and the total number of grain pulling should not exceed 60 grains.

In addition, the structure of 45# steel is not very stable, which may cause the quenching strength and toughness to degenerate. For this reason, after quenching and tempering, it is necessary to perform tempering in a timely manner at an appropriate temperature to ensure its good work performance.

Therefore, to ensure the best results, it is important to control the whole process,especially heat treatment methods, cleanliness of steel, and smaller grain pulling number.