Heat treatment of rollers. There are four automatic lines for thermal processing of rollers using HDTV at 1 GPP. The rollers in the installation heat up to 970 ° C in an inductor for 25-30 seconds, cool under a water-cooled shower and are released in a resistance furnace with convective heat exchange (Img. 1).

Heat treatment of rollers, installation scheme for heat treatment of rollers

Image1. Installation scheme: 1 – tempering furnace; 2 – quenching rolls; 3 – rejection node; 4 – inductor; 5 – pusher mechanism.

Four types of rollers are subjected to heat treatment with induction heating, and the inductors for them are the same – 45 turns each. Each line is fully automated. Constant temperature is provided by the stabilization of the generator voltage.

When the unit is turned on, the temperature of 970 ° C is reached in 25-30 seconds. During this time, the rollers with a special device are sent to a separate cassette and after 25-30 seconds are automatically fed to the rolls.

In fig. 2 that the shutter speed of the rollers after passing through the critical point of the γ-region is ~ 2/3 of the total heating time; This provides a more complete saturation of austenite with carbon.

The temperature distribution of the rollers along the length of the inductor during induction heating

Image2. The temperature distribution of the rollers along the length of the inductor.

High-quality heat treatment and hardening of the rollers

To achieve high-quality quenching, the rollers rotate at a speed of 200-300 rpm on smooth cylindrical rollers under running water during cooling. The increase in cooling rate with this cooling method is caused by the fact that falling jets compress the vapor film, and the part rotation causes the vapor film to break, so in this case the thermodynamic equilibrium interval (horizontal section in Fig. 3) should be reduced and the cooling rates increase even with more high temperatures, i.e., the stage of bubble (nuclear) boiling comes earlier. The cooling water temperature is 20 ° C, the cooling time is 25-30 seconds.

Cooling curves

Image3. Cooling curves: solid line – immersion in water; dashed line – shower cooling.

The microstructure of the rollers that have been quenched by high-frequency heating is the same as that of the rollers that are heat-treated in resistance furnaces. After quenching, the rollers in an OKB-799 type drum furnace are subjected to tempering at 150-160 ° C for 2 hours. Since quenching from high-frequency heating produces heterogeneous martensite, optimal tempering conditions for such rollers (the best combination of hardness and viscosity) should be other than for the rollers after the furnace heating.

The microstructure of the rollers, heated by HDTV and in the furnace

Image4. The microstructure of the rollers heated by HDTV (a) and in the furnace (b).

The use of automatic lines for rollers with HDTV heating allows to obtain hardness within GOST (after tempering HRC 61-62), significantly reduce weight loss (0.1% instead of 0.77% when furnace heating), reduce the production area (by 30-40 %), increase productivity (1.5-2 times) and eliminate the washing operation before pre-grinding.

ISSN 0026-0819. “Metallurgy and heat treatment of metals”, No. 9. 1968
This article was taken from this resource.