Fracture Failure Analysis of 10.9 Grade High Strength Bolt

BoltThe fracture morphology 1.2 the location and state of the crack shows that the fracture is located between tooth 1 and tooth 2, and is concentrated at the root of the loaded surface of tooth 1. Looking up the assembly drawing, it can be seen that the 3rd to 6th threads are just located at the counterbore of the mating part, and the 2nd thread is the first thread matched with the internal thread of the mating part, I .e. the fracture is just at the initial joint position between the bolt and the mating part.

Obvious crack marks can be seen at the tooth root (thread 1~5) within the 4 pitch range. (A shows the low-magnification scanning electron microscope image of the root crack of the first to second thread, and the crack at the tooth root is clearly visible at the part indicated by the arrow. The metallographic examination showed that there were cracks at the root arc of the 1st to 5th teeth, and no cracks were found at the root arc of the 5th to 6th teeth with a depth of 0. 15 to 0.30. (B) The crack enlarged view at the 3rd to 4th tooth impressant arc is shown. The tooth profile angle of the bolt is about 59.6. After detection, the radius of the tooth root transition arc is about 0.32mm 2 components. The microstructure and hardness detection shows the microstructure photo at the edge of the thread side. The matrix is fine tempered sorbite + 3% insoluble iron cable with normal microstructure, meet the requirements of GB/T3098.1 ― 2000 to obtain more than 90% tempered sorbite; The edge color is obviously darker than other parts. Spectral chemical analysis (SFECTRCMEIER spectral analyzer) is carried out on different depths of the bolt cross section. Its chemical composition is shown in Table 1, which shows that its surface layer has been severely carburized. At the same time, according to GB/T3098.1 -2000, the surface carburization test is carried out by hardness method. The test part is tooth test along the middle diameter line. The hardness gradient value is shown in Table 2. The test results show that the surface hardness is significantly higher than the core hardness. The hardness of the bolt matrix shall be 359 ~ 39HRC at the 1/2 radius of the cross section.

Table 1 Chemical Composition Analysis Results (Mass Fraction, Distance from Surface/mm Heart Table 2 Hardness Method Measurement of Surface Carbon Depth/mm 0.42 Heart Adjacent Teeth Hardness/. 3 Defect Analysis 3.1 Fracture Analysis The F area shown in the fracture analysis is the section of the fatigue propagation area. The fatigue source of the section and the bolt axial direction is about 90. The crack starts at the thread root. (a) It is shown as a high-power scanning electron microscope image of the fracture crack source in F area. Because the fracture crack source has been severely worn, its section is smooth, but there are still micro-extrusion deformation marks and micro-cracks; (B) It is shown as a low-power scanning electron microscope image of the fracture edge in F area. The fatigue crack propagation area has a unique shellfish line of fatigue fracture, and the shellfish line has obvious advance marks, in the early stage, the shellfish line advances inward in an arc shape, with fine lines, while in the middle and late stages, it advances inward in a straight line, with thicker lines. As the cross section is repeatedly squeezed and rubbed, the area becomes brighter.