Discussion on the segregation of carbon elements in relation to chromium and silicon suggests that chromium segregation typically aligns with carbon segregation, while silicon segregation tends to oppose it due to its exclusion effect, leading to lower carbon concentrations in high-silicon areas. However, the measured results show no clear pattern in the distribution of these elements, though the Si and Cr content in the core is slightly lower. It can be inferred 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 data points being 0.37%. These measurements generally reflect the carbon gradient from the edge to the core, indicating that carbon distribution does not significantly change. The observed microstructural differences are likely due to variations in the degree of supercooling during quenching rather than carbon segregation. Since the temperature difference between the surface and core of small samples is not substantial, the resulting microstructural variation is minimal. The hardness difference is attributed to this microstructural variation.
The impact of box segregation is difficult to assess without a comparative sample showing significant differences within the same furnace batch. However, based on available samples, although No. 1 and No. 2 exhibit two-stage segregation, the No. 1 sample failed during use, and when the system was adjusted, some samples showed about two levels of segregation but still met user requirements. This suggests that box segregation is not the primary cause of bolt failure. Comparing the average hardness values, the sample treated according to the user's heat treatment system (35.5 HRC) has higher hardness than the failed part (28.5 HRC). Based on this, the strength of the failed part should be significantly above 1200 kN/mm². Therefore, the failure is likely due to an inadequate heat treatment process that did not meet the required specifications, resulting in insufficient strength. It is generally believed that box segregation mainly affects lateral properties such as plasticity and toughness, with less impact on longitudinal performance. The slight variations in hardness across the bolt cross-section are primarily due to microstructural differences.
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 users have some machining allowance, but in most cases, especially for heat-treated components, machining allowances are not left before final treatment. This issue also applies to full-section specimens. The torque applied to the bolt is mainly concentrated in the outer layer, where the outermost 1 mm bears five times the load of the innermost 1 mm. This indicates 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 weakened its strength. The discussion on the heat treatment system highlights the importance of temperature control. For ML35CrMo, the user’s heat treatment system resulted in over 60% shrinkage, and necking was observed in the failed part. According to German DIN1654/4-1989, the minimum tensile strength (Rb) for quenched and tempered material is between 1250 and 1450 MPa, and the yield strength (Re) should be at least 400 MPa. This indicates that the current heat treatment system leads to excessive surface shrinkage. Considering the actual application of bolts, the plasticity requirements may need to be adjusted, and the surface shrinkage rate should be controlled below 50%. While the product meets national standard composition requirements (0.32%–0.40%), the wide tolerance range may affect performance consistency. To ensure reliable bolt performance, suppliers should narrow the tolerance range, and users should tailor heat treatment systems based on material composition and application needs. Additionally, reducing the quenching holding time and lowering the tempering temperature can help control delamination, reduce shrinkage, increase strength, and better utilize the steel’s properties to meet varying requirements.
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