Metal induction hardening
Induction hardening of metals utilizes the principle of electromagnetic induction to generate eddy currents on the surface of a metal workpiece, rapidly heating the surface to the austenitizing temperature. This process is then immediately cooled by spraying water to achieve surface hardening. The core of this technology is to generate an alternating magnetic field using an induction coil. When the workpiece is within this magnetic field, induced currents (eddy currents) are generated on the surface. This electrical energy is converted into thermal energy, causing the surface to heat up very quickly. The core, however, remains cooler due to the shorter heat conduction time, thus achieving the effect of hardening the surface while maintaining the original properties of the core. Induction hardening offers advantages such as fast heating (typically completed within a few to tens of seconds), a small heat-affected zone, minimal workpiece deformation, and high production efficiency. It is suitable for surface hardening of parts such as shafts, gears, and cams made of medium-carbon steel and medium-carbon alloy steel. It is widely used in automotive manufacturing, engineering machinery, and machine tool manufacturing.
The process of metal induction hardening mainly includes pretreatment, induction heating, cooling, tempering and other steps. Pretreatment includes cleaning, degreasing and preheating of the workpiece to ensure the surface is clean and avoid oxidation or uneven carburization during heating. Induction heating is a key link. It is necessary to design a suitable induction coil according to the shape and size of the workpiece, and maintain a certain gap between the coil and the workpiece to ensure uniform heating. Heating parameters such as current frequency, heating time, power density, etc. need to be precisely controlled according to the material composition and quenching requirements: high-frequency current ( above 10kHz ) mainly heats the surface layer of the workpiece 0.5-2mm, which is suitable for small-sized parts; medium-frequency current (500Hz-10kHz) has a heating depth of 2-10mm, which is suitable for medium-sized parts; industrial frequency current (50Hz) has a heating depth of up to 10-20mm, which is suitable for large parts.
The cooling process directly affects the quenching quality. Water or water-soluble quenching media are usually used for spray cooling. The cooling rate needs to be fast enough to ensure that the surface austenite is transformed into martensite. If the cooling rate is too slow, pearlite or bainite structure will be formed, resulting in insufficient surface hardness; if the cooling rate is too fast, cracks may occur. Therefore, the design of the cooling system must ensure uniform cooling to avoid local overcooling or insufficient cooling on the workpiece surface. For parts with complex shapes, segmented cooling or local enhanced cooling can be used to ensure the quenching quality of each part. Tempering is usually carried out after induction quenching to eliminate quenching stress and improve the toughness of the workpiece. The tempering temperature is generally 150-250℃, and the holding time is determined according to the size of the part.
The quality inspection of metal induction hardening includes hardness testing, hardened layer depth measurement, metallographic analysis and deformation detection. The hardness test uses a Rockwell hardness tester or a Vickers hardness tester to measure the hardness of the surface and the core to ensure that the surface hardness meets the design requirements and the core hardness meets the toughness requirements. The depth of the hardened layer is measured by metallographic method or hardness gradient method, that is, the hardness is measured layer by layer from the surface to the core. When the hardness drops to the specified value, the depth is the hardened layer depth, which must meet the design standards of the part. Metallographic analysis observes whether the surface layer is a uniform martensitic structure and whether there are defects such as overheating, overburning or undissolved carbides. The deformation detection uses a measuring tool to measure the dimensional change of the workpiece to ensure that it is within the allowable range to avoid affecting the assembly and use of the parts.
With the advancement of industrial automation and intelligentization, metal induction hardening technology is constantly evolving. New induction heating equipment utilizes IGBT (insulated gate bipolar transistor) frequency conversion technology, improving power output stability and control accuracy, enabling multi-stage heating and precise temperature control. The use of automated production lines integrates loading, heating, cooling, and tempering processes, enabling continuous production and improving production efficiency and product consistency. The application of computer simulation technology can simulate the temperature field distribution during the induction heating process before production, optimizing induction coil design and process parameters and reducing trial-and-error costs. Furthermore, the development and application of environmentally friendly cooling media, such as biodegradable quenching fluids, reduces environmental pollution. In the future, metal induction hardening technology will develop towards higher precision, higher efficiency, and more environmentally friendly approaches, providing superior solutions for the surface hardening of metal parts.
