Why do low hardness, soft bands or soft spots sometimes appear when shafts are subjected to induction hardening?

When high-frequency quenching of shaft parts, there will be reasons for low hardness, soft bands or soft spots:

① The power of the equipment is not properly selected, the heating power is small, and the heating time is short;

② The design of the inductor and the cooler is unreasonable, and the inner diameter of the inductor is inconsistent with the workpiece, resulting in uneven heating and cooling;

③ The heating and cooling process is unreasonable, or there is water in the sensor, which is attached to the workpiece to form a soft spot after quenching, or the pressure of the cooling medium is small, the flow rate of the medium is small, and the water spray hole is blocked, resulting in insufficient cooling;

④ The heating power is too large and the heating time is long, overheating or coarse grains occur;

⑤ The original structure has coarse block ferrite, the carbon content of the material is too high or the hardenability of the material is too high or too poor;

⑥ uneven tempering temperature or insufficient tempering;

⑦ The quenching temperature is low or the moving speed is too fast;

The measures to be taken are:

① Select the power of the equipment reasonably, determine the reasonable heating process parameters, and carry out process verification;

② Improve the design of the inductor and cooler, so that the size of the induction coil is consistent with the requirements of the shaft to ensure that the surface of the part is fully and evenly heated and cooled;

③ Check the set heating and cooling parameters, re-quench after normalizing, adjust the heating temperature, time, and the pressure and flow of the cooling medium as required, and clean the water spray holes;

④ Adjust the heating process parameters and carry out the necessary metallographic inspection;

⑤ Replace the material (select the material that meets the requirements) or select a suitable cooling medium, and use pre-heat treatment such as normalizing or quenching and tempering to obtain a uniform and fine structure;

⑥ Strengthen the control of tempering temperature and random inspection of hardness;

⑦ Strictly control and adjust the moving speed of the shafts for the gold fork with heating temperature and hardness to ensure the quality is qualified;

⑧ Strictly control the composition and structure of the raw materials, remove the decarburized layer on the surface or replace the steel, improve the control of the atmosphere in the furnace for the preliminary heat treatment, carry out carbon supplementation treatment, and normalize the structure defects.