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Metallographic microscopic observation (1) The crack shape was examined at the location of the fracture. Under high magnification, the crack appeared slightly zigzag with a network-like morphology at the tip. No non-metallic inclusions were observed around the crack. Iron oxide within the crack was removed by reagent treatment. Upon inspection, the crack orientation aligned closely with the ferrite network. (2) The metallographic structure of 45 steel bolts after quenching and tempering should normally exhibit uniform tempered sorbite. However, the microstructure of the cracked bolt showed significant differences between the core and the surface. The core displayed tempered sorbite, while the surface layer consisted of pearlite combined with network ferrite or a mixture of pearlite and ferrite, along with other anomalies such as tempered sorbite. A severe decarburization layer was found on the surface of the cracked bolt, with three samples showing decarburization depths above 0.15 mm, reaching up to 0.19 mm. Generally, the depth was around 0.165 mm. Scanning electron microscope observation of the cross-section revealed a black region indicating intergranular fracture, with oxidation on the surface. Dimples were visible at the center of the fracture, while the surface remained unoxidized. Cracks were observed from the top of the bolt leading into the fracture area. (2) Longitudinal fracture analysis of another broken bolt showed a longitudinal crack starting from the fracture surface and extending toward the smaller end of the bolt. Upon opening the crack, a brittle fracture surface was observed, characterized by a dark appearance. (3) Electron probe microanalysis was conducted on the black region of the fracture. The results showed a Zn element count of 666 at the fixed point, gradually decreasing towards the center, with no Zn detected at the center. This suggests that the crack formed prior to the pickling and galvanizing process.
Based on an investigation of the entire production process and test results from the broken bolts, it can be ruled out that the fracture was caused by metallurgical defects. No cracks were found in the solid samples, and the levels of impurity elements were not excessive. From the manufacturing process analysis of the fastening bolts, cold rolling and quenching are the most likely causes of cracking. The batch of 45 steel used for the bolts had a carbon content of 0.148%, which is at the upper limit of the standard composition for 45 steel and near the lower limit of 50 steel. This steel has poor cold plastic deformation properties, especially considering the sharp right angle transition between the screw and the step, which leads to significant stress concentration during cold rolling—up to ten times higher than the average calculated stress. In many cases, internal stress cracks are induced, leading to fractures, as seen in engine connecting rods and cold-formed molds. In addition to minor design improvements, high-strength cold heading bolts, such as ML20MnTiB, should be used instead of regular 45 steel. It is well known that the internal and external quality requirements for cold heading steel are very strict. Only when materials are carefully selected, with tight control over cleanliness and composition, and with nearly flawless surfaces, can the quality of parts after cold rolling be ensured. From the microstructural analysis, another major cause of bolt cracking is the quenching process. According to the manufacturing process, the quenching temperature was set at 860°C using brine as the coolant. If the carbon content of this batch of 45 steel were at the lower limit of the standard composition, then 860°C would be appropriate, with no risk of overheating. However, the actual carbon content of this batch was 0.148%, at the upper limit of the standard composition. This reduced the Ac3 phase transformation temperature to approximately 750–760°C. If the material was still quenched at 860°C, overheating would occur, significantly increasing the likelihood of cracking. Additionally, the bolt surface exhibited a severe decarburization layer. After quenching, the surface layer would retain pearlite and ferrite, greatly reducing the strength of the outer layer and its resistance to crack initiation and propagation. The quenching coolant used was brine, resulting in a martensitic structure in the central region (tempered sorbite after tempering). Meanwhile, the outer layer experienced high tensile stresses, causing cracks to form first at weak points and propagate further. Oxidation was observed on the crack surface, and microstructural analysis revealed iron oxide at the crack tip, confirming that the crack formed after quenching and was subsequently oxidized during high-temperature tempering. Finally, the placement of magnetic particle flaw detection after thread rolling in the manufacturing process was unreasonable, potentially affecting crack visibility, especially for oblique or lateral cracks, which could lead to missed inspections.
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