Ultrasonic Stress Measurement in Welded Component by Using Lcr waves: Analysis of the Microstructure Effect H. QOZAM 1,2,a , J. HOBLOS 1,2 , G. BOURSE 1,b , C. ROBIN 1 , H. WALASZEK 2,c , P. BOUTEILLE 2 , M. CHERFAOUI 2 1 Département Technologie des Polymères et Composites & Ingénierie Mécanique, Ecole des Mines de DOUAI, 941, rue Charles Bourseul, 59508 DOUAI-France 2 Centre Technique des Industries Mécaniques 52, av. Félix Louat 60304 SENLIS-France a qozam@ensm-douai.fr, b bourse@ensm-douai.fr, c Henri.Walaszek@cetim.fr Keywords: Ultrasonic measurement - Subsurface Lcr wave - Acoustoelastic constants Residual stresses - Welded components - Microstructure effect. Abstract. Welding, which is a largely used process in the mechanical manufacturing, well known to induce high-level residual stresses. The level of residual stresses is of great importance for the lifetime of welded components used in mechanical engineering industry. The use of the ultrasonic method for the evaluation of the residual stresses is based on the acoustoelastic effect, which refers to the change in velocity of the acoustic waves propagating in a strained solid. In the case of welding, the microstructure modifications observed in the heat affected zone (HAZ) and the melted zone (MZ) also induce variations of the velocity of the acoustic waves. The superposition of the two effects, stresses and microstructure, results in over-estimating the levels of stresses. This work which was completed in collaboration with CETIM is a contribution to this problem. The experimental study was carried out on P460HLE and P265 steels welded sheets. The results obtained by the ultrasonic Lcr wave technique were compared with those obtained by the hole drilling technique. This work confirms the possibility of evaluating the residual stresses induced by welding using the ultrasonic method. Introduction For the advanced safety design of technical components, both magnitudes and signs of the operating and residual stress states have to be known. The residual stress states in welded parts result of variety of parameters like material properties, geometry of the joints and the welding process. Many experimental techniques have therefore been developed to evaluate the stress states in order to assure the quality of welded structures. They range from destructive methods like the hole-drilling method to a large number of non-destructive methods based on X-ray or neutron diffraction, photo-elasticity, or the determination of magnetic properties and ultrasonic measurements. The most widely non-destructive method used to evaluate stress states is X-ray diffraction; it presents the advantage to be absolute because it allows the determination of stresses whatever the microstructure of material may be. However, the depth of investigation is limited to some tens of microns. Complementary to this method, ultrasonic techniques allow to evaluate not only the surface stresses but also the bulk stress. Their ability to perform fast measurements on site is the main reason of the industrial interest for these techniques. The present work concerns an ultrasonic technique based on the use of the critical refracted longitudinal wave (Lcr) to characterize the stresses in materials. The use of this technique for the surface stress measurements is described by many authors [1,2,3,4]. Previous works [2] described a method developed to take into account the microstructure effect, by the use of correction coefficients applied to ultrasonic measurements in the melted zone (MZ) and the heat affected zone (HAZ). These coefficients were "roughly" estimated in Materials Science Forum Vols. 524-525 (2006) pp. 453-458 online at http://www.scientific.net © (2006) Trans Tech Publications, Switzerland All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of the publisher: Trans Tech Publications Ltd, Switzerland, www.ttp.net . (ID: 80.124.139.217-14/09/06,18:30:24)