Étude du comportement au fluage de l'alliage 13Cr-4Ni en vue de simuler la redistribution des contraintes résiduelles lors du traitement thermique post-soudage

Abstract

The welding process generates a large amount of residual stress during the manufacturing of the 13Cr-4Ni martensitic steel turbine runners. A post-weld relaxation heat treatment is necessary and must be performed on the assembled wheel. However, the complexity of the geometry and the size of the wheel make temperature control difficult in the different sections of the wheel. The level of residual stresses after the post-weld treatment is unknown from the manufacturer and is only taken arbitrary into consideration for this design. The subject of this thesis is the development of a creep viscoplastic constitutive law that will help simulate the relaxation of these residuals stresses in a turbine runner. Creep tests were performed at the critical temperature of 620 °C at various stress levels. At this temperature, the amount of reformed austenite at its maximum. This phase has a positive effect on the fatigue life of the material. It is ductile and can absorb impacts on the blade runner. The plastic deformation rates of the 13Cr-4Ni material as a function time at various stress levels was derived from creep tests. A residual stress field of ± 500 MPa was introduced into four points bending test samples to observe the redistribution of the residual stresses following various typical heat treatments using the contour method. Different heat treatment combinations (temperature/dwell time) were applied to the bending samples: 400 °C/ 0h, 620 °C/ 0h, 620 °C/ 2h and 620 °C/ 48h. This experimental approach has shown that the residual stresses are redistributed even at a temperature as low as 400 °C. The predominant factor affecting the stress redistribution is the collapse of the true yield strength of the material with high temperature. The heat treatment 620 °C/ 2h is the metallurgical condition that reduces the most the residual stress field to a maximum of ± 90 MPa. In addition, these tests demonstrate that a longer dwell stay at the critical temperature could lead to a partial martensitic transformation during cooling and induce residual stress gradients.

Date	5 Feb 2018
Original language	French
Awarding Institution	École de technologie supérieure
Supervisor	Henri Champliaud (Supervisor) & Jacques Lanteigne (Co-supervisor)