The reason for the low stress brittle fracture of the bolt caused by the analysis of the fracture is as follows: the improper material selection design gives the bolt diameter M56, the performance grade is 1019, and the selected material is 42CrMo.42CrMo as the alloy quenched and tempered steel, with its excellent performance. Widely used in mechanical engineering. However, its application is limited by the cross-sectional dimensions of the components. Mechanical properties of 42CrMo. The heat treatment process is not suitable for 42CrMo as a hypoeutectoid steel. For larger cross-sectional dimensions (such as U50mm), it is more difficult to use a conventional quenching and tempering process to achieve a yield ratio of 019. In order to achieve the above objectives, it is inevitable to reduce the tempering temperature, that is, to use medium temperature tempering or lower temperature tempering. Thus, although the strength is increased, it will also cause a decrease in toughness and leave a defect in the metallographic structure (the second type of temper brittleness). The actual test results of these bolts: high strength (Rb>1200MPa), high hardness (HBS>400), metallographic structure is tempered troostite, which fully illustrates this point. Studies at home and abroad have shown that the higher the strength of steel, the greater its sensitivity to cracks. When Rb>1200MPa, if the toughness is insufficient, the low stress brittle fracture is most likely to occur. The anchor bolts are directly buried and no corresponding measures are taken. For steel, increasing its plasticity will result in a decrease in toughness. Straight-buried anchor bolts are used and no bushings are installed. On the one hand, the toughness of high-strength bolts is reduced under the constraints of concrete; on the other hand, scratches may be left on the surface of high-strength bolts during installation, causing stress concentrated. In summary, improper material selection is the main reason for the low stress brittle fracture of these anchor bolts. The wrong heat treatment process and installation form make the occurrence of low stress brittle fracture possible. Improvement measures Materials For engineering materials, it is important to have comprehensive mechanical properties, that is, the materials are required to have toughness that matches their strength. In engineering design, one-sided pursuit of high strength and neglect of matching toughness often produces unintended consequences. A large number of engineering practices have proved that the failures of use occurring below the creep temperature are mostly not caused by the general plastic deformation, but due to the fracture occurring under the condition that the nominal stress is lower than the yield stress, so-called low stress brittleness. fracture. In engineering design, in order to achieve a better combination of strength and toughness, in the selection of materials, one should pay attention to the organization of the material, because the organization determines the performance, for high-strength steel, the quenched structure should be low-carbon martensite or chin Second, we must pay attention to the size effect of the material, because the large structure and thick section have lower toughness than small parts and thin sections. When selecting high-strength steel parts, the carbon content should be reduced to a low level to ensure the toughness, and the high strength is guaranteed by alloying. National standard 5 steel structure with high-strength large hex head bolts, large hex nuts, washers technical conditions 6 (GB/T1231-1991) recommended for 1019 bolts, they are: 20MnTiB (D The installation form ladle turret anchor bolts are subjected to periodic alternating impact loads, and high-strength anchor bolts are used. The design should use replaceable anchor bolts (ie live bolts) for easy installation and replacement during use. Also pay attention to the structure of the bolt, reduce the groove, hole and sharp angle to eliminate the influence of stress concentration as much as possible. When direct-buried anchor bolts (ie dead bolts) are used, a certain length of casing (PVC pipe) should be installed. In this way, on the one hand, the constraint can be reduced, leaving a space for plastic elongation for the high-strength anchor bolts; on the other hand, the surface of the bolt can be protected from being scratched. Conclusion (1) The fracture of the anchor bolts is a low stress brittle fracture caused by insufficient toughness. Improper material selection is fundamental, and the wrong heat treatment and installation prompts it to happen. (2) The selection of high-strength steel materials should pay more attention to the matching of strength and toughness. The quenched structure should be low carbon martensite or lower bainite. Attention should also be paid to the size effect of the material. Aluminum Cnc Milling Parts,Custom Metal Parts,Cnc Metal Maachining,Custom Cnc Machining JIANGSU TONGDE INTERNATIONAL TRADE CO.LTD. , https://www.tongdetrade.com