Discussion on the segregation of carbon elements in relation to chromium and silicon indicates that chromium segregation typically aligns with carbon segregation, whereas silicon segregation tends to oppose it. This is due to silicon's tendency to exclude carbon atoms, leading to lower carbon concentrations in high-silicon regions. However, the measured results show no clear pattern in the distribution of these two elements. The silicon and chromium contents in the core are slightly lower, suggesting that the carbon content in the core remains relatively stable. The average carbon content in the outer layer is 0.387%, with the lowest value among six measurements being 0.37%. These data generally reflect the carbon variation from the edge to the core, indicating that carbon levels do not significantly change. The observed differences in microstructure are likely not caused by carbon segregation but rather by variations in the degree of subcooling between the edges and core during quenching and cooling. Since the subcooling difference in small samples is not very large, the resulting microstructural differences are minimal. The hardness variation is attributed to these microstructural differences.
The impact of box segregation is difficult to assess, as there are no comparison samples within the same furnace batch showing significant differences. Therefore, no definitive conclusion can be drawn. However, examining the existing samples, although No. 1 and No. 2 exhibit two-stage segregation, the No. 1 sample failed during use. When the system was adjusted, some samples showed about a two-level segregation, yet they still met the user's requirements. Thus, box segregation does not appear to be the cause of the bolt failure. Comparing the average hardness values, the sample treated according to the user’s specified heat treatment system (35.5 HRC) has higher hardness than the failed workpiece (28.5 HRC). Based on this, the strength of the failed workpiece should be significantly greater than 1200 kN/mm². Therefore, the bolt failure is likely due to an inadequate heat treatment process and insufficient strength. It is generally believed that box segregation mainly affects lateral properties, particularly plasticity and toughness, with less impact on longitudinal performance. The slight differences in hardness across different points of the bolt section are primarily due to microstructural variations.
Regarding the decarburization layer, national standards require inspection for steels with carbon content above 0.3%. The total decarburization depth on each side should not exceed 1.5% of the nominal diameter. This is the maximum allowable value when the user has some machining allowance, but since heat treatment is the final step, machining allowances are usually not left before treatment. This issue also applies to full-section specimen processing. The torque applied to the bolt is mainly concentrated in the outer layer, where the outermost 1 mm of steel bears five times the stress of the innermost 1 mm. This implies that the presence of a decarburized layer negatively impacts the mechanical strength of the material. During inspection, a decarburization layer of approximately 0.2 mm was found on the surface of the failed bolt, which further compromises its strength. The discussion on the heat treatment system includes opinions on the temperature settings for the sample. Here, we focus on the heat treatment of ML35CrMo. According to the user’s provided heat treatment system, the sample shrinkage exceeds 60%, and necking was observed in the failed workpiece. According to German DIN1654/4-1989, the quenched and tempered material should have a minimum Rb of 1250 to 1450 and a minimum tensile strength of 7≥400 MPa. This suggests that the user’s on-site treatment system leads to excessive surface shrinkage. Considering the actual use of bolts, the plasticity requirements should be appropriately reduced, and the surface shrinkage rate should be controlled below 50%. While the products meet the national standard requirements, the carbon content tolerance range (0.32% to 0.40%) is relatively wide. To ensure consistent bolt performance, suppliers should narrow the tolerance zone to improve component accuracy. At the same time, users should develop tailored heat treatment systems based on varying compositions and usage conditions. Additionally, the quenching holding time should be shortened, and the tempering temperature reduced to control delamination, minimize shrinkage, enhance strength, and fully leverage the steel’s properties to better meet different application needs.
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