After stamping, metal contact pieces often develop residual stress due to plastic deformation, leading to uneven hardness, decreased toughness, and even cracking or accelerated wear during service. Heat treatment, through precise control of heating, holding, and cooling processes, can effectively eliminate residual stress and optimize the microstructure, thus significantly improving the mechanical properties of the contact pieces. When selecting a heat treatment process, material properties, part service conditions, and process feasibility must be comprehensively considered. The following analysis focuses on key process types and their mechanisms of action.
Annealing, by heating the contact piece to an appropriate temperature and holding it for a certain time, followed by slow cooling, can eliminate residual stress generated during stamping, making the internal microstructure of the metal more homogeneous. For carbon steel or alloy steel contact pieces with high carbon content, annealing can reduce hardness, improve machinability, and prevent cracking or deformation during subsequent processing. For example, if high-carbon steel contact pieces are directly machined after stamping, excessive hardness may lead to accelerated tool wear; annealing can adjust the hardness to a suitable range, improving processing efficiency and surface quality.
Normalizing is similar to annealing, but with a faster cooling rate, typically achieved through air cooling. This process is suitable for carbon steel or alloy steel contact parts with low carbon content. By refining the grain size and improving microstructure uniformity, it enhances the strength and toughness of the parts. Normalized contact parts can better resist alternating loads during service, extending their service life. For example, in gear contact parts of transmission systems, normalizing refines the core microstructure, simultaneously improving surface hardness and wear resistance, meeting the demands of high-load conditions.
Quenching involves rapidly heating the contact part to above its critical temperature, forming a high-hardness martensitic structure on the surface while maintaining toughness in the core. This process significantly improves the wear resistance and anti-galling properties of the contact parts, but it also introduces significant internal stress, requiring tempering to eliminate brittleness. For example, mold contact parts that are not tempered after quenching may suddenly fracture during use due to excessive brittleness; low-temperature tempering retains high hardness while reducing brittleness, ensuring the contact part maintains stability under impact loads.
Tempering is a crucial follow-up process after quenching. By heating the contact parts to a temperature below the critical temperature, holding it at that temperature, and then cooling it, the balance between hardness and toughness can be adjusted. Depending on the service conditions, tempering can be categorized into three types: low-temperature tempering, medium-temperature tempering, and high-temperature tempering. Low-temperature tempering is used for high-hardness tool-type contact parts, such as blanking dies; medium-temperature tempering is suitable for spring-like parts requiring high elastic limits; and high-temperature tempering achieves comprehensive mechanical properties through quenching and tempering, and is widely used in structural parts such as shafts and gears. For example, after quenching and tempering, the surface hardness and core toughness of automotive driveshaft contact parts reach a balance, effectively resisting fatigue fracture.
Chemical heat treatment further enhances the wear resistance, corrosion resistance, or fatigue resistance of contact parts by altering the surface chemical composition. Carburizing is suitable for low-carbon steel or alloy steel contact parts. By infiltrating carbon atoms into the surface layer to form high-carbon martensite, it significantly improves surface hardness while maintaining core toughness. Nitriding, by introducing nitrogen atoms to form nitrides, improves surface hardness, wear resistance, and corrosion resistance. It involves relatively low processing temperatures and minimal deformation, making it suitable for precision contact components. For example, high-temperature alloy contact components in aero-engines often undergo nitriding to meet service requirements in extreme environments.
Surface hardening, through localized heating and rapid cooling, hardens the surface of the contact component while maintaining core toughness. It is suitable for parts requiring high surface wear resistance and core resistance to impact loads. Induction heating and flame heating are commonly used methods; the former offers fast heating speed and precise control, while the latter requires simple equipment and is low-cost. For example, induction surface hardening of machine tool guideways significantly increases surface hardness, effectively resisting wear, while core toughness ensures long-term stability without deformation.
The selection of heat treatment processes should be based on the material properties of the contact component, service conditions, and cost. Low-carbon steel contact components are best treated with normalizing or carburizing quenching to improve strength; high-carbon steel contact components require quenching followed by low-temperature tempering to achieve high hardness; alloy steel contact components can utilize quenching and tempering to optimize overall performance. In addition, process feasibility, equipment conditions, and production cycle are also important considerations. For example, for mass-produced contact parts, efficient processes such as induction heating surface hardening are preferable, while for single-piece or small-batch production, lower-cost flame hardening can be used. By scientifically selecting and combining heat treatment processes, the mechanical properties of stamped metal contact pieces can be significantly improved, meeting diverse engineering needs.
