Formation and elimination of cracks during induction heating and quenching treatment of high-speed steel rolls

Using high-carbon high-speed steel instead of high-chromium cast iron to manufacture rolls has become one of the main development directions of rolls. The structure and performance of high-speed steel rolls are directly related to heat treatment. Since there are big differences between high-speed steel rolls and traditional high-speed steel in terms of composition and process conditions, the heat treatment of high-speed steel rolls cannot copy the traditional high-speed tool steel process.

The internal factors that affect the quenching cracks of high-speed steel rolls are mainly that the steel contains more alloy elements, has poor thermal conductivity, and has a high tendency to crack. Carbide segregation also has a significant impact on cracks in high-speed steel rolls. Under quenching heating conditions, coarse network carbides are difficult to fully dissolve, resulting in uneven roll performance. The greater the unevenness of the carbides, the lower its strength, toughness and plasticity. Under the same quenching conditions, the carbon alloy elements in the carbide accumulation areas High content, low melting point, prone to over-burning, high austenite stability, low Ms point, and high Ms point in areas with less carbide distribution, which leads to uneven martensitic transformation and anisochrony. When the rich area of carbide elements transforms to martensite, the low-concentration area has completed the martensite transformation and is in a hardened state, resulting in greater structural stress, thus increasing the tendency of quenching cracks and reducing and eliminating carbide segregation. , and at the same time, the modification treatment measures are adopted to make the carbides in the roll structure fine and evenly distributed, which has a good effect on improving the cracks of high-speed steel rolls during heating annealing.

When the high-speed steel roll is quenched and cooled, below the Ms temperature, due to the transformation of austenite to martensite, the volume expands, resulting in second-type distortion, second-type stress and macroscopic heat treatment stress, which can easily lead to quenching cracks. If the quenching cooling rate is too low, it will affect the degree of hardening and fail to meet the requirements of the roll quenching layer. The cooling medium has the strongest cooling ability at the temperature where supercooled austenite decomposes quickly. It is heated to 1025-1050 degrees Celsius in the heating furnace, placed in a controlled cooling device after insulation, and allowed to cool for 25 seconds during the roll cooling process. Rotate at a speed of -35r/min. At the same time, the surface of the roll neck is coated with heat insulation material, and the roll neck is placed outside the cooling device. In the controlled cooling device, the rolls are first spray-cooled with mist and gas coolers. When the roll surface temperature is lower than 300 degrees Celsius, the martensite transformation has been basically completed and can be put into the furnace for tempering treatment. By adjusting the pressure of water and air in the nozzle, the cooling rate is increased and the residual compressive stress is increased, which is beneficial to preventing quenching cracks. Below the Ms point, by reducing the water pressure and flow rate in the nozzle, increasing the air pressure, reducing the cooling rate, and reducing the phase change stress, it helps to prevent quenching cracks in high-speed steel rolls.