Preventive measures for delayed fracture of high strength bolts

At present, delayed fracture caused by bolted joints is still a serious product quality problem. Delayed fracture is a material under the action of static stress, after a certain period of time after the sudden brittle destruction of a phenomenon, is the material of an environmental stress interactions and the occurrence of an environmental embrittlement, is a form of hydrogen leading to material deterioration. Delayed fracture phenomenon is a major factor preventing high strength bolts to improve strength.

   Delayed fracture is related to material processing, environmental factors. In this paper, from the fastener production practice, delayed fracture prevention measures for peer reference.

   1, tensile strength deviation management

   In order to ensure that the bolt to achieve the mechanical properties of the indicators on the basis of (especially the tensile strength and stress control in the qualified range) to control the hardness of the bolt within a certain range, see Table 1.

   High-strength bolts in the test hardness as a result of the process. (Control items) ultimately to the tensile value of the strength value for the integrated measurement results.

   2, to prevent the bolt surface layer of carbonization

   Decarburization is usually conducive to delayed damage performance, but must give full consideration to the decline in fatigue strength, and not to the bolt decoupling, slippery teeth. Pre-annealing process of raw materials must pay strict attention to decarburization and carbonization, and in the final tempering quenching for the thread surface carbonization is not allowed.

   Before cold drawing, the coils need to be soft annealed or ball annealed, in order to prevent decarburization, often using a protective atmosphere or charcoal placed in the furnace. If the atmosphere in the furnace is not properly controlled or if special conditions occur, this can cause carbonization on the surface of the raw material. Due to the thread at the surface carbonization, will make the strength increase and plasticity decline, resulting in delayed fracture.

   3, to prevent phosphorization

   Drawing is the production of high-strength bolts of the first process, but before drawing, coil shall be treated with zinc phosphate film, increase the finish, cold heading, tempering quenching treatment made of bolts, there will be phosphorus infiltration into the surface layer. For high-strength bolts, heat treatment can be used before the metal cleaner and pickling to wash the phosphating film to prevent the purpose of phosphorus seepage. It should be noted that the steel seepage of phosphorus, the critical point of steel A. Rise, the surface appears more ferrite, but also affects the back-order blackening and oxidation coloring surface treatment.