Dual-phase heat treatment process for large-diameter high-strength bolts used in steel space frames

High-strength bolts for steel space frames are important parts used for space frame bolt ball joints. They directly transmit the alternating internal forces caused by alternating loads, so they have high performance requirements. At present, most bolts are quenched and tempered, that is, quenched and tempered at high temperature. For large-diameter bolts, the common problems are: large diameter (generally 39~64 mm in diameter), difficult to harden, and its physical performance indicators It is difficult to meet the design requirements, especially the gap between the actual tensile fatigue performance and the corresponding fatigue strength index of 2 million cycles, and this fatigue performance index is an important basis for ensuring the safety of the steel space frame structure. In view of the above situation, we start from improving the heat treatment process, and apply the dual-phase technology to the heat treatment of large-diameter high-strength bolts. The test results show that the dual-phase heat treatment process is effective in improving the performance of large-diameter high-strength bolts.

Dual-phase heat treatment is a strengthening and toughening method developed in recent years. It is to properly quench the hypoeutectoid steel in the critical area to obtain a dual-phase structure composed of martensite and ferrite. Since the ductile phase ferrite surrounds the strengthening phase martensite, it is beneficial to increase the strength. At the same time, as the ductile phase, the strip ferrite can become an obstacle to crack propagation, which makes the crack divert and passivate, and greatly improves its toughness. Two-phase The heat treatment process can refine the grains, and grain refinement is the only mechanism that can benefit both strength and toughness. According to his own and other test results, D.V. Wilson believes that grain refinement is conducive to the improvement of fatigue strength, which is just as S.S.Mason pointed out that, in a certain sense, grain boundaries can be regarded as crack arresters. If there has been some crack initiation, finer grains mean more crack arrestors and shorter crack lengths.

Experiments have proved that 40Cr has obtained a dual-phase structure composed of ferrite and martensite after dual-phase heat treatment. In the obtained dual-phase structure, the core and edge structures have relatively little change. This homogenization effect is very beneficial on large-diameter high-strength bolts.
Tests show that the dual-phase structure obtained after dual-phase heat treatment is fine and uniform. That is to say, dual-phase heat treatment can refine grains, and grain refinement is the only mechanism that can benefit both strength and toughness. Iron and steel materials are still non-ferrous metals. Generally speaking, the fatigue strength of the material will always be improved if the grain is refined at least at room temperature. In addition, the fine grain also means that the entire processing process of the material is good. Prepared conditions for the material to have high fatigue performance. Compared with ordinary quenching and tempering treatment, dual-phase heat treatment has significantly improved plasticity under the premise of maintaining high strength. This result corresponds to its ideal microstructure. This strong the phenomenon that toughness can be improved at the same time also provides the possibility for large-diameter bolts to obtain high fatigue strength-because the fatigue strength of materials has a great relationship with strength, plasticity and toughness. From the point of view of the cooperation between the fatigue limit and the material strength and plasticity, the change law of the fatigue limit with the tensile strength is illustrated, and it is shown in the linear stage. is a dominant factor, and when the strength limit exceeds a certain value, then the plasticity will show its effect, that is, under the same strength condition, the higher the plasticity, the higher the fatigue strength. Generally speaking, it is difficult for materials to simultaneously It has both high strength and plasticity, and the use of dual-phase heat treatment technology for large-diameter high-strength bolts improves the strength and toughness at the same time, which lays a good foundation for obtaining high fatigue performance.