Structural preparation of rolling from pearlite steels for uploading automobile bolt products

Stud bolt products manufactured by cold forging from pearlitic steels are subject to special requirements in terms of structural and mechanical characteristics. A very common and productive method for obtaining finished metal products with high mechanical properties is their heading on cold heading (cold heading) machines from rolled metal, which must have a certain quality along the entire length - sufficient plasticity, uniform structure and parameters of technological properties, absence of defects. Hardware enterprises receive the initial blank for the manufacture of various fasteners in the form of hot-rolled steel, which cannot be used for upsetting high-strength hardware due to poor quality structure and surface layer. Therefore, calibrated rolled products are subjected to intermediate heat treatment before drawing, which should guarantee the quality of hardware at cold heading transitions. Further, upset products of strength class 8.8 and higher from alloy steels must be subjected to quenching followed by tempering. However, such a thermal operation often leads to surface decarburization, cracking and warpage of finished products, which increases labor intensity, energy consumption and, consequently, the final cost. The paper proposes an alternative method for the structural preparation of rolled products, which includes isothermal treatment of 38X and 38XA pearlitic steels, excluding recrystallization annealing and hardening with tempering of finished bolted products from the technological process and leveling the appearance of scratches, warping and decarburization on the rolled products and products. The rolled structure induced in the dies and surface hardening by thread reduction and knurling provide the mechanical properties of M8 bolts and studs corresponding to strength class 9.8.