The body bolt often breaks. What causes it? Now we take a specific example to analyze for you. A certain body bolt is produced by a Carriage Bolt Manufacturer. Its specifications are MI2X1.25X44 and the material is 4OCr. Structural steel, bolts are quenched and tempered, and the hardness value is HRC34-39.
On the outside surface of the two screw holes of the hem arm, each part of the black paint is wiped off (indicated by the white arrow), but the wiped parts are different. For the screw holes of broken bolts, the rubbed part is above the screw holes; for the screw holes of the broken bolts, the rubbed part is below the screw holes. The paint was wiped to indicate that a local relative sliding occurred near the two screw holes between the lower arm and the rod seat which was fastened by the bolt. This relative sliding should occur after the body bolt was broken.
The whole fracture can be divided into two parts: one part of the fracture is basically perpendicular to the longitudinal direction of the bolt, which is black and gray, slightly rough, and is marked with the number 1; the other part of the fracture is perpendicular to the bolt, which is obliquely silver-gray, and is smooth. Out. The obvious differences in the morphology of the two parts of the fracture indicate that the bolt fracture has undergone two different processes. It should be noted that the screw hole adjacent to the fracture part shown by the number 2 also partially fractured (shown by the white arrow), and its morphology is similar to the fracture indicated by the number 2.
Fracture observation shows that the fracture of the body bolt includes two processes, brittle crystalline fracture and transient fracture. The former may be hydrogen embrittlement fracture, and the fracture of this part is corroded. The corrosive medium is related to sulfur and chlorine. The latter is an instantaneous fracture. After the brittle cracks have spread to a certain extent along the crystal, the remaining effective area of the bolt is not enough to withstand the actual working load and is broken. Whether the cause of brittle fracture along the crystal is related to corrosion depends on further analysis of the production process, installation and operating conditions of the bolt.
The results of the final scanning electron microscope and metallographic analysis showed that the bolts submitted for inspection had brittle fracture along the crystal, which was the cause of the early fracture of the Square Bolt.