Cr12MoV steel rolls are used for the preliminary rolling of the rear wheels of small four-wheel tractors. During production, cracks were found after the rolls were quenched by intermediate frequency quenching equipment, which caused the workpieces to break and be scrapped.
After the roll was quenched, radial cracks first appeared from the sharp corners of the keyway bottom of the inner hole during the stay, and then wavy cracks were found on the R80 round fox surface. After that, many new radial cracks appeared on the surface of the inner hole, which lasted for a week. The hardness of the rolled blank after quenching by intermediate frequency quenching equipment is 207-225HBW; the workpiece quenching process shows that the hardness of the roll after heat treatment is required to be 58-64HRC, and the surface hardened layer depth is >10mm. The hardness of the invalid roll after quenching is 54-57HRC, and it can be seen that the hardness of the roll is 4-7HRC lower after treatment
According to the analysis, when the Cr12MoV steel roll is austenitized by heating at 950 °C, due to the low saturation of carbon and alloying elements in the austenite, the martensite transformation point increases, resulting in the amount of martensite and residual carbonation of the workpiece after quenching. The amount of matter increases, while the amount of retained austenite decreases, the cross section of the workpiece is hardened, and a large stress is generated due to the volume expansion of martensite transformation, which is the main reason for crack initiation and propagation.
On the other hand, after the workpiece is cooled for 15 minutes, the surface temperature is about 160°C, while the core of the roll is higher than 160°C, and the Martensitic transformation continues, so that the workpiece forms a stress state in which the surface is subjected to compressive stress and the core is subjected to tensile stress; The martensitic transformation redistributes the stress state of the workpiece. Since the structural transformation does not occur at the same time, the surface of the workpiece generates a large stress. After quenching, the structural transformation of the workpiece is not completed, which leaves hidden dangers for the initiation and expansion of cracks and the fracture of the roll when it stays after quenching. .
The roll is quenched and heated at 950°C, Ms is about 250°C, and Mf is about 100°C. After the roll is quenched, it is air-cooled to room temperature, and the retained austenite continues to transform into martensite, so martensite is not transformed at the same time, and the surface layer is first transformed and hardened by the core. The post-transformed structural stress and thermal stress are superimposed on the bottom area of the keyway in the inner hole of the roll to generate stress concentration and tensile stress. When the tensile stress is greater than the tensile strength of the material, cracking occurs. The tangential and radial cracks that appear on the outer circle and both ends of the roll are due to this.
According to the above analysis, the large and insufficient stress of the larger Cr12MoV steel roll after quenching at 950 °C is the main reason for the fracture of the workpiece. Therefore, process improvement should increase supplementary quenching to eliminate stress, stabilize the structure, and avoid hidden dangers of quenching cracks and fractures caused by excessive stress.
The improved process is: after quenching, the roll is immediately quenched twice at 180-200°C to reduce the structural stress and remove the crack defects of the workpiece.
