Mesure et prediction des contraintes residuelles dans une turbine hydraulique de type Francis

Welding is a common process in the assembly of Francis stainless steel (CA-6NM) water turbines. High temperatures cause deformations and residual stresses which can have a serious consequence over lifespan of turbines. Very little experimental data regarding temperature distribution and levels of stress during multi-pass welding in Francis turbines is available in the literature.;The mild steel experimental results measured a maximum stress of 220 MPa in tension at the center of the weld The maximum stress on a sample of stainless steel martensitic was measured at 341 MPa in compression at the center of the welded joint.;Using the numerical methods it was possible to predict the maximum stress intensity with a error of 12 and 9% for the mild steel and 415 martensitic stainless steel samples respectively.;The experimental approach consists of following measuring the variation of the stress during a complete cycle of manufacture of a turbine. Measurements were taken during the various manufacturing stages which are the machining, welding and heat treatment with strain gages. Moreover a continuous data recording measuring system was also developed and used to follow the temperatures and stress variations during the welding process.;The present report predicts the intensity and the distribution of residual stresses and its variation during the welding process or with time, in the welded joint of a turbine. We proceeded by stages, firstly experimental data were collected to estimate the intensity of the residual stress in martensitic stainless steel and in mild steel samples at the welded joint. Secondly a data-processing program was developed to predict the maximum intensity in a welded joint based on experimental measurements made in a mild steel sample. A model was developed to predict the maximum stress in 415 martensitic stainless steel sample. Finally the developed method was used to predict the maximum residual stress in the welded joint.;The measurements taken on the turbine blades, after machining, allowed a gross estimation of an average stress level at 155 MPa in tension. The continuous measurements during welding gave a variation of 47 MPa in tension. It was noted that the level of stress increased proportionally with the temperature during welding. When the temperature of the turbine cools, the stress level did not follow the same evolution as the temperature. The assumption of the effect of the constraint effects can be brought. The high temperatures gauges allowed measuring a maximum relieving of 169 MPa and the final stress level was established between 33 and 51 MPa in compression at the extrados exit of the blade. It is noted that the final level of stress is not null. It is noticed that the final level of constraint is not null, on the other hand compressive stresses can be beneficial in places highly solicited in tension.;Finally the numerical methods to predict the maximum stress on a CA-6NM turbine welded joint could not be checked due to a lack of experimental data.