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Metallographic microscopic observation was conducted to analyze the crack. The shape of the crack appeared slightly zigzag under high magnification, with a network-like structure at the crack tip. No non-metallic inclusions were observed around the crack. Iron oxide within the crack was dissolved by the reagent. Upon further inspection, it was found that the crack orientation aligned closely with the ferrite network. Regarding the metallographic structure, 45 steel bolts undergo quenching and tempering, typically resulting in a uniform tempered sorbite structure. However, the microstructure of the cracked bolt showed significant differences between the core and the surface. While the core exhibited tempered sorbite, the surface layer contained pearlite combined with network ferrite or a mix of pearlite and ferrite, indicating abnormal structures. A severe decarburization layer was observed on the surface of the cracked bolt, with a thickness exceeding 0.15 mm in three samples—reaching up to 0.19 mm in the most affected area. The fracture surface was analyzed using a scanning electron microscope (SEM). After ultrasonic cleaning, the black region was identified as intergranular, while the surface showed oxidation. Dimples were visible at the center of the fracture, but the surface itself was not oxidized. Cracks were observed from the top of the bolt extending toward the fracture point. In another case, a longitudinal crack was found starting from the fracture surface and extending toward the smaller end of the bolt. Upon opening the crack, a brittle fracture surface was observed. Electron probe microanalysis was performed on the black region of the fracture, revealing a zinc (Zn) content of 666 at the fixed point, which gradually decreased toward the center, with no Zn detected at the center. This suggests that the crack formed prior to the pickling and galvanizing process.
Based on the investigation of the entire production process and test results of the broken bolts, it can be ruled out that metallurgical defects caused the fracture, as no cracks were found in solid samples, and impurity levels were within acceptable limits. From the manufacturing process analysis, 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 near the upper limit of the standard composition and close to the lower limit of 50 steel. This resulted in poor cold plastic deformation properties. Additionally, the sharp right-angle transition between the screw and the step led to significant stress concentration, which could increase local stress by up to ten times compared to the average calculated value. Internal stress cracks often occur under such conditions, leading to fractures, as seen in engine connecting rods and cold-formed molds. To improve performance, the design and material selection should be revised. High-strength cold-heading bolts, such as ML20MnTiB, should be used instead of regular 45 steel. It is well known that materials for cold heading must meet strict internal and external quality standards, including high cleanliness, precise composition control, and a nearly flawless surface to ensure good quality after cold rolling. Microstructural analysis also indicated that quenching is a major contributor to bolt cracking. The quenching temperature for these bolts was set at 860°C with brine as the coolant. If the carbon content of this batch of 45 steel were at the lower end of the standard, 860°C would be appropriate without overheating. However, the actual carbon content of 0.148% placed it at the upper end of the standard, lowering the Ac3 phase transformation temperature to 750–760°C. Quenching at 860°C in this case would cause overheating, significantly increasing the risk of cracking. Furthermore, the bolt surface had a severe decarburization layer, which, after quenching, retained pearlite and ferrite in the outer layer, reducing its strength and resistance to crack initiation and propagation. The core, however, transformed into martensite during quenching (tempered sorbite after tempering). The outer layer experienced high tensile stress, making it prone to crack formation at weak points. Oxidation on the crack surface and the presence of iron oxide at the crack tip suggest that the crack occurred after quenching, followed by oxidation during high-temperature tempering. Lastly, the placement of magnetic particle inspection after thread rolling was inappropriate, potentially affecting crack detection, especially for oblique or lateral cracks, increasing the likelihood of missed inspections.
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