Numerical Simulation of Screw Manufacturing Process for the Assessment of Surface Integrity

Stress corrosion cracks have been observed on screws made of stainless steels grade 316 after some years of service in Pressurized Water Reactor (PWR) water environment. Grade 316 of stainless steel is not sensitive to corrosion unless it has been sensitized and/or subjected to a complex combination of factors including an important level cold work at the surface and in the bulk of the material. The tightening of the screw induces tensile stresses. This preload cannot explain the Stress Corrosion Cracking (SCC) defect appearing in the transition radius between the screw shank and its head. Thus, the question has been raised of the initial state of the screws after manufacturing. The simulation of the manufacturing processes has been carried out to have a better understanding of manufacturing process consequences on material degradation: solution annealing, cold drawing and machining. The dedicated “hybrid method”, specifically set up to simulate finish turning has been applied to obtain stress and strain states close to the surface. This method is detailed in the paper. The manufacturing process of these bolts is likely to induce high strain hardening since they have been cold drawn and then machined. It is suspected that tensile residual stress and cold work play a major role in the initiation of stress corrosion cracking of austenitic stainless steel grade of 316 type in PWR water environment. Simulation chaining method and results are highlighted in the paper with comparison with experiments. The main achievements are: the smaller the screw the less the cold work, the residual stress on the surface is mainly due to machining and the location of crack in the transition radius is well explained.