Brief analysis of the heat treatment process of high frequency induction heating equipment for backup rolls

The back-up rolls are divided into hot-rolled back-up rolls and cold-rolled back-up rolls. The back-up rolls are in direct contact with hot-rolled or cold-rolled work rolls. shape quality, and protect the work rolls from deflection. The back-up rolls are designed according to their rigidity. They are characterized by their large size and weight. Bearing large loads, it is easy to cause sliding wear, deformation hardening and roll breakage, so the backup roll is required to have sufficient hardness, wear resistance, fracture resistance, high contact fatigue strength and peeling resistance, and sufficient hardening Layer and transition layer depth, good metallurgical quality, and its common failure forms are roll fracture, spalling (dropping blocks), etc. It can be seen from its technical requirements that the roller body can obtain the required surface hardness, uniformity of hardness and effective hardened layer depth through heat treatment of high-frequency induction heating equipment. The surface heat treatment of the roller body is the key to ensure the above performance indicators , is also the core of backup roll manufacturing technology.

With the continuous improvement of the rolling force of the rolling mill, the roll body of the back-up roll is developing towards the trend of high hardness, deep hardened layer, uniform hardness and high toughness at the core. In the early 1980s, the technology of the domestic back-up roll Compared with foreign countries, there is a certain gap in the index. It can be seen that the domestic quality index is lower than that of Japan and the United Kingdom. Back-up rolls with different hardness and performance can be obtained by heat treatment process. As the material of the backup roll, it is highly sensitive to white spots. Therefore, during the heat treatment process after forging, dehydrogenation treatment should be carried out, and the holding time should be as long as possible, which is conducive to the desolvation and diffusion of hydrogen. For the heating method of 70Cr3Mo large backup rolls, domestic high-frequency induction heating equipment is used for rapid heating and step heating, which can reduce the maximum temperature difference in the elastic state, usually 550-570°C, and the final heating temperature is controlled at 880-920°C °C, it can not only ensure that the grains are not coarsened, but also meet the requirements of hardenability. In order to prevent heat cracking, oil cooling can be used. During the heating process of the differential temperature heating of the backup roll, when the furnace temperature is 950-1000°C, the temperature rise rate on the surface of the roll body is 150-250°C/h. During the whole heating process, the surface temperature should always be controlled within the range of 880-930°C, and the total holding time can be calculated according to 25-30mm/h. In order to avoid excessive heat treatment stress of high-frequency induction heating equipment, and at the same time achieve the required surface hardness and effective hardened layer depth, it is more effective to use water quenching and oil cooling for differential temperature heating. When duplex frequency is used for surface heat treatment of backup rollers, in order to prevent edge chipping defects due to good hardenability, early power-off at the end and delayed quenching should be adopted to prevent edge chipping. The back-up roll adopting this process is quenched and tempered first, so that the roll diameter meets the size requirement, and then the roll body is processed, leaving a margin of 2.5 mm, and then the deep hardened layer is subjected to power frequency heating treatment.