Many mechanical parts work under alternating loads and shock loads such as torsion and bending, and their surface layers are subjected to higher stresses than the core; In the case of friction, the surface layer is constantly worn. Therefore, the surface layer of forgings is required to be strengthened, that is, the surface has high strength, hardness, wear resistance and fatigue limits.

Surface heat treatment of forgings is a process that changes the structure and properties of the surface of the workpiece and only heat treats its surface. The surface is usually given high hardness and wear resistance, while the core still retains sufficient plasticity and toughness. In production, a certain composition of steel is selected to ensure that the mechanical performance indicators of the core meet the requirements, and then the surface heat treatment method is applied to strengthen the surface layer to meet the performance requirements. Surface heat treatment is divided into two categories: surface quenching and surface chemical heat treatment.

1. The surface of the forgings is quenched

Surface quenching of forgings is a heat treatment method that quickly heats the surface of the workpiece to the quenching temperature, and then cools it rapidly, so that only the surface layer obtains the quenching structure, while the core still maintains the pre-quenching structure. Commonly used are induction heating surface quenching and flame heating surface quenching. Surface quenching is generally medium carbon steel and medium carbon alloy steel forgings.

Induction heating quenching is the use of electromagnetic induction principle, through the alternating current on the surface of the workpiece induced huge eddy current, so that the surface of the forging is quickly heated, and the core is almost not heated.

Characteristics of induction heating surface quenching: martensite grain refinement after quenching, surface hardness 2 to 3HRC higher than ordinary quenching. There is a large residual compressive stress on the surface, which helps to improve the fatigue strength; It is not easy to produce deformation and oxidative decarburization; Easy to realize mechanization and automation, suitable for mass production. After induction heating and quenching, in order to reduce the quenching stress and reduce brittleness, low-temperature tempering of 170 to 200 °C is required.

Flame-heated surface quenching is a process method that uses a flame burned by oxyacetylene gas (temperature up to 3100 to 3200 °C) to quickly heat the surface of the forging above the phase change temperature, and then quench and cool.

Tempering at low temperatures is carried out immediately after quenching, or self-tempering is carried out using the waste heat inside the forging. This method achieves a hardening depth of 2 to 6 mm, is simple and cost-effective, and is suitable for single piece or small batch production.