Application of ultrasonic induction heating equipment in carburizing process of steel

Carburizing is a common heat treatment process that can be used to meet a variety of quality requirements. Some basic instructions and results about the carburizing process are as follows.

The temperature range of carburizing is usually 913~954℃. In this temperature range, especially for some steels, the austenite grains not only on the surface but also in the center will become coarse. If the grains are too coarse to be ignored, the coarse grains can be improved through two processes: one is to use ultrasonic heating equipment to heat it to 913°C to refine the core tissue, and then use ultrasonic induction heating equipment to heat it to Refine the surface structure at 788℃; or if it is just the refinement of surface grains, just reheat to 788℃.

In either process, heating at 788°C is followed by quenching and hardening. This two-time heating or one-time heating process to refine the grains is relatively expensive and can easily cause deformation. It is more appropriate to select appropriate steel, which requires that the grains of the steel will not become coarse at the carburizing temperature, and it can also be quenched directly using ultrasonic heating equipment. Many steels, especially some low-alloy steels, will not cause coarse grains at carburizing temperatures for a long time, and are suitable for direct quenching using ultrasonic heating equipment. Some carburized workpieces require high hardness on the surface and low hardness in the core. Such workpieces cannot be quenched directly from the carburizing temperature. Because this will cause the hardness of the cylinder core to be too high. The temperature before quenching of these workpieces should be high enough to harden the surface, but not high enough to harden the core, usually around 774°C.

If the mass fraction of carbon on the surface of the part reaches 0.9%, network carbides will be formed on the surface of the part during slow cooling. Carburized parts with high surface carbon content should avoid slow cooling from the carburizing temperature to prevent the formation of network carbides. Quenched workpieces with network carbides are difficult to grind and can easily cause spalling during service.