Formation mechanism of inspection defects in hundred-ton class 55NiCrMoV7 die steel forging

Using an ultrasonic flaw detector, this study precisely located the dimensions and distribution characteristics of defects during the manufacturing process of 55NiCrMoV7 die steel forgings weighing hundreds of tons. The probing defects of the forging are predominantly concentrated in the central region, spanning approximately 4200 mm in length, 400 mm height and 1500 mm width. Detailed research on the characteristics of these probing defects was conducted through dissection sampling and scanning electron microscope analysis. The research results reveal that the maximum diameter of an individual defect can reach Φ10 mm, and localised cracking phenomena exist in specific areas within the forging. The flaws exhibit characteristics such as millimetre-sized, large, elongated MnS inclusions, with spacing between inclusions ranging from 200μm to 500μm. Furthermore, based on elemental analysis, the sulphur content from the surface to the core of the forging increased from 0.001% to 0.002% to a maximum of 0.0085%. Through analysis using thermodynamic and growth kinetics models, it was determined that in regions with higher sulphur content (S = 0.0085) at a certain distance from the steel ingot surface, the radius of MnS inclusions can reach up to 380μm under low cooling rates (1 K/min). After forging deformation, these can evolve into millimetre-sized MnS inclusions, leading to internal inspection defects. Therefore, we propose that reducing the sulphur content in the molten steel and increasing the degree of undercooling can effectively inhibit the precipitation and growth of MnS inclusions. Practical results indicate that this improvement strategy effectively enhances the inspection quality of the 55NiCrMoV7 die steel forgings weighing hundreds of tons.