Discontinuity defects of the bolt head bearing surface

Abstract

One of very common defects of bolt products are failures resulting from the plastic forming processes of materials. Surface discontinuity defects can take the form of folds, forging laps and cracks. These defects are known phenomenon associated with the production of bolt products, and they are described in the standard DIN EN 26157–3. The reasons for their formation may be different and the knowledge about improving the quality of bolt products is very important. Prevention of these defects is one of the key issues related to an improvement of product quality and implementation of a new bolt technology.The work presents and analyzes mechanisms of generating forging laps and cracks formed in a four- and five-stage cold forming processes of bolts. Three cases of defect generating were selected and analyzed using numerical simulations by the QForm3D software. The FEM results were compared with results of an experiment of bolt forging on a forging machine.In the first case, the influence of the volume of the material determined on the forging of the bolt head was examined. A variant with a constant volume and a variant with a volume that changed with successive stages were prepared. In the second case, dies with different geometry were used (constant or graded corner radii dies shaping the transition of the pin into the bearing surface of the bolt). In the third case, the shaping of the bearing surface of the bolt in one or two steps of forging was considered.Potential errors in the design of the process of shaping the bearing surface and threats in further use of bolts were indicated. Forging laps and cracks based on the microstructure of the material flow lines, photos and FEM simulations were presented.