Formation and Elimination of Cracks in High Speed Steel Rolls During Heat Annealing

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. Due to the large 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.

 

High Speed Steel Rolls, Heat Annealing

 

The internal factors that affect the quenching cracks of high-speed steel rolls are mainly that the steel contains more alloy elements, poor thermal conductivity, and a large crack tendency.

Carbide segregation also has a significant effect on cracks in high-speed steel rolls.

Under quenching and heating conditions, the coarse network carbides are difficult to fully dissolve, resulting in uneven roll performance. The greater the degree of carbide unevenness, the lower its strength, toughness and plasticity. Under the same quenching conditions, the carbon alloy elements at the carbide accumulation High content, low melting point, prone to over-burning, high austenite stability, low Ms point, and high Ms point in parts with less carbide distribution, which leads to inhomogeneity and isochronism of martensite transformation, When the enriched area of carbon elements transforms to martensite, the low-concentration area has completed martensite transformation and is in a hardened state, resulting in greater structural stress, thus increasing the tendency of quenching cracks, reducing and eliminating carbide segregation At the same time, the modification treatment measures are adopted to make the carbides in the roll structure become fine and evenly distributed, which has a good effect on improving the cracks of the high-speed steel rolls during heating and annealing.

When the high-speed steel roll is quenched and cooled, below the Ms temperature, due to the transformation from austenite to martensite and volume expansion, the second type of distortion, the second type of stress and the macroscopic heat treatment stress are generated, which 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 hardened layer of the roll.

The cooling medium has the strongest cooling capacity at the temperature at which supercooled austenite decomposes quickly. It is heated to 1025-1050 degrees Celsius in the heating furnace, and placed in the cooling device after heat preservation, and the roll is cooled for 25 Rotate at a speed of -35r/min, while coating the heat insulating material on the surface of the roll neck, and the roll neck is placed outside the cooling device.

In the controlled cooling device, the roll is cooled by spraying with mist and air cooler first. When the temperature of the roll surface is lower than 300 degrees Celsius, the martensitic transformation has been basically completed, and it 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 to increase the residual compressive stress, which is beneficial to prevent quenching cracks.

Below the Ms point, by reducing the water pressure and flow in the nozzle, increasing the air pressure, reducing the cooling rate, and reducing the phase transformation stress, it is helpful to prevent quenching cracks in high-speed steel rolls.

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