Process design of high-strength bolt of fully pearlitic high-carbon steel

In this study, fully pearlitic high-carbon steel, known as a higher delayed fracture strength material, with an ultimate tensile strength of 1410 MPa was used for manufacturing a high-strength M8 bolt. Softening behavior was observed by a compression test of the material. The tensile strength of the material was measured after application of several drawings. On the basis of these test results, two multi-stage manufacturing processes were designed by a knowledge-based expert system in order to develop a high-strength bolt using either the upsetting mode or the extrusion mode for the second preform. The load requirement and possible defect formation during the process were predicted by employing a rigid-viscoplastic finite element analysis. When the second preform was made in a laboratory experiment according to the process design on the basis of the upsetting mode, the specimens cracked at the flange in the final stage of forming. However, the bolt was successfully manufactured using the second preform obtained using the extrusion mode and showed extended tool life in the laboratory test. Finally, the manufactured bolt was confirmed via experiments to have tensile strength of 1600 MPa and delayed fracture strength.