Study on white spots in 42CrMo steel forgings

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42CrMo steel belongs to the chromium-molybdenum steel category of alloy structural steels. Thanks to its excellent comprehensive mechanical properties, high hardenability, and good workability, it is primarily used for manufacturing forgings that require high strength or have large quenched sections, such as large gears for locomotive traction, gears in turbochargers, rear axles, springs, and engine cylinders. However, practice has shown that 42CrMo steel is prone to the formation of white spots. The presence of these white spots severely impairs the mechanical properties of the steel, reducing the plasticity of large forgings. During heat treatment and quenching, they can easily cause cracking in the forgings; in service, they make brittle fracture very likely to occur. The production process route for 42CrMo steel is: EAF furnace melting → LF refining → casting → slow cooling in a pit → demolding → heating → forging → normalizing + quenching and tempering treatment. The specific manufacturing process for 42CrMo steel is as follows: 1) Stress-relief annealing is not carried out after the ingot is demolded ； 2) Pre-forging heating ； 3) Forging process: Reduction of about 20%, followed by direct elongation to Φ580mm×6000mm ； 4) After forging, normalizing + quenching and tempering treatment (water quenching, air cooling) is carried out immediately. To determine the true cause of the brittle fracture of 42CrMo steel mandrels during use, cross-sections of the mandrels were cut, and samples from typical defect areas were taken for analysis. After low-magnification acid etching, visual inspection revealed that the serrated, slender cracks distributed at a certain distance from the specimen surface or near its central portion exhibited a radial concentric circular or irregular pattern. The results of the low-magnification acid etching test indicated typical white spot crack defects. The formation of white spots is an inherent defect in medium-carbon alloy structural steels; in particular, such defects often occur in steel billets or forgings with large cross-sections, indicating that 42CrMo steel is indeed a steel susceptible to white spots. Rod forging belongs to the elongation of shaft-type forgings; the surface layer lies in the zone of difficult deformation and the zone of minor deformation, while the area from a certain depth below the surface up to the center is the zone of significant deformation. During the forging process, as the workpiece is turned, the surface layer alternates between zones of difficult deformation and zones of low deformation; meanwhile, the area from a certain depth on the surface up to the core remains in a zone of high deformation. Due to this uneven deformation, the central part of the cross-section is subjected to biaxial compressive stress, while the edges are subjected to axial tensile stress and compressive stress in the thickness direction. It is evident that the region of large deformation causes crack concentration; hydrogen accumulates at the cracks. Meanwhile, due to diffusion induced by residual stress, the hydrogen pressure increases to a level equal to the bonding force between metal atoms. This results in the breaking of atomic bonds, leading to crack growth and the formation of white spots. Through the analysis of white spot defects in 42CrMo steel, the methods for their control and repair are summarized: (1) Measures to reduce H content during the steelmaking process: 1) Increase the C content at the end of tapping to prevent over-oxidation operations ； 2) Increase the stirring intensity and raise the argon flow rate under vacuum ； 3) Improve equipment capacity and enhance the pumping capacity of vacuum devices. (2) From a technological perspective, forging and compacting methods that reduce sensitivity to white spots are employed; for example, high-temperature forging is used as much as possible, and the distance from the core of the forging to its surface is shortened, thereby increasing the diffusion rate of H and the relative surface area of the forging, which facilitates the precipitation of H from within the forging and reduces the likelihood of white spot formation in the steel. (3) It is necessary to strictly control the quality of raw materials in order to significantly reduce the content of impurities. A suitable heat treatment process should be determined based on the H content and the size of the cross-section of the forged part, in order to eliminate residual stresses caused by thermal stresses during cooling and uneven deformation.