Preventive Measures for Delayed Fracture of High Strength Bolts
Release time: 2023-11-23
At present, the delayed fracture caused by bolt connection is still a serious product quality problem. Delayed fracture is a phenomenon of sudden brittle failure of materials after a certain period of time under the action of static stress. It is a kind of environmental embrittlement caused by the interaction of materials, environment and stress. It is a form of material deterioration caused by hydrogen. Delayed fracture phenomenon is one of the main factors that prevent high strength bolts from increasing their strength.
Delayed fracture is caused by material processing, environment and other factors. Based on the production practice of tight firmware, this paper puts forward the preventive measures of delayed fracture for the reference of peers.
1. Deviation management of tensile strength
On the basis of ensuring that the bolts meet the mechanical performance indicators (especially the tensile strength and stress are controlled within the qualified range), the hardness of the bolts should be controlled within a certain range, as shown in Table 1.
High strength bolts are tested for hardness as a result of the process. (Control item) The final tensile strength value is the comprehensive measurement result.
2. Prevent the carburization of the bolt surface layer
Decarbonization is usually conducive to the improvement of delay failure performance, but the reduction of fatigue strength must be fully considered, and the bolt will not slip and slip. The pre-annealing process of the raw material must pay strict attention to decarburization and carburization, and the carburization of the thread surface is not allowed during the final quenching and tempering.
Before cold drawing, the wire rod needs to be softened or spherodized, in order to prevent decarbonization, a protective atmosphere is often used or charcoal is placed in the furnace. If the atmosphere in the furnace is not properly controlled or special circumstances occur, it will cause carburization on the surface of the raw material. Due to the surface carburization at the thread, the strength will increase and the plasticity will decrease, resulting in delayed fracture.
3. Prevent phosphorus infiltration
Drawing is the first process of producing high-strength bolts, but before drawing, the rod must be treated with zinc phosphide film to increase the finish, cold heading, quenching and quenching treatment to make bolts, there will be phosphorus infiltration into the surface layer. For high-strength bolts, the phosphating film can be cleaned with metal cleaning agent and pickling before heat treatment to prevent phosphorus infiltration. It should be noted that after the steel is phosphorized, the critical point of the steel A. Rising, there are more ferrites on the surface, and it also affects the subsequent blackening and oxidation coloring surface treatment
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