Metal contact resistance heating quenching
Metal contact resistance hardening is a heat treatment process that utilizes the resistance heat generated when an electric current passes through the contact point between a metal workpiece and an electrode to locally heat the workpiece surface, followed by rapid cooling to achieve hardening. Its operating principle is based on Joule’s law. When current passes through the contact area between the workpiece and the electrode, the high contact resistance generates a large amount of heat, causing the area to rapidly heat to the austenitizing temperature. The current is then immediately cut off and a coolant is applied, causing the heated area to rapidly cool to form a martensitic structure, thereby achieving surface hardening. This process is suitable for hardening localized areas of parts, such as gear tooth surfaces, shaft journals, and bolt threads. It allows for precise control of the hardened area to avoid affecting the performance of non-hardened areas.
The metal contact resistance heating and quenching process primarily involves electrode design and selection, contact pressure adjustment, current parameter setting, and heating and cooling control. Electrode design and selection are crucial. Electrode materials are typically copper or copper alloys with excellent conductivity, high temperature resistance, and strong wear resistance. Their shape must match the hardened area of the workpiece to ensure good contact and uniform heating. The magnitude of contact pressure directly affects contact resistance and heating effectiveness. Too little pressure results in excessive contact resistance, generating excessive heat and even burning the workpiece. Excessive pressure increases electrode wear and may cause plastic deformation of the workpiece. Therefore, the appropriate contact pressure must be adjusted based on the workpiece material and thickness, generally between 0.5 and 5 MPa.
The current parameter settings include current intensity and heating time. The current intensity is determined according to the size of the hardened area and the resistivity of the workpiece material, and is usually between several hundred and several thousand amperes. The heating time is controlled according to the required heating temperature and the depth of the hardened layer, and is generally between 0.1 and 5 seconds. During the heating process, the current and time must be precisely controlled by a dedicated control system to ensure that the heating temperature reaches the austenitizing temperature but does not overheat. The cooling method usually adopts water spray cooling or water cooling inside the electrode. The cooling rate must be fast enough to ensure martensitic transformation. The cooling time is generally equivalent to or slightly longer than the heating time. For parts with complex shapes, a step-by-step heating and cooling method can be used to ensure uniform quenching quality in all parts.
Metal contact resistance heating quenching offers advantages such as fast heating, a small heat-affected zone (HAZ), low energy consumption, and simple operation. Due to the short heating time (typically within a few seconds), thermal deformation of the workpiece is minimal, eliminating the need for subsequent corrective treatment. The HAZ is limited to the surface of the contact area, minimally impacting the core performance and effectively preserving the toughness of the core. Compared with other heating methods, this process directly utilizes contact resistance to generate heat, resulting in minimal energy loss and low energy consumption. The equipment structure is relatively simple, making automated production easy and suitable for batch processing of parts. However, this process also has some limitations, such as rapid electrode wear, requiring regular replacement; poor contact performance for workpieces with high surface roughness, which can easily lead to uneven heating; and a shallow hardened layer depth, generally between 0.1 and 1 mm, making it unsuitable for parts requiring a deeper hardened layer.
With the development of automation and intelligent technology, metal contact resistance heating and quenching technology has been continuously upgraded. The new CNC contact resistance heating and quenching equipment can achieve precise control of electrode position, real-time adjustment of current and pressure, and automatic linkage of heating and cooling processes, thereby improving processing accuracy and production efficiency. Improvements in electrode materials, such as the use of dispersion-strengthened copper alloys, have improved the wear resistance and service life of electrodes and reduced production costs. In addition, the application of computer simulation technology can predict the temperature field distribution and hardening layer formation process during the heating process, optimize process parameters, and reduce trial and error costs. In the future, this technology will further develop in the direction of high precision, high efficiency, and long life, expanding its application scope in precision machinery manufacturing, automotive parts production and other fields.