Proc. Of International Symposium on Materials Ageing And Life Management, ISOMALM-2000, Kalpakkam, October 3-6, 2000 AN AUTOMATED IMAGING SCHEME FOR QUANTITATIVE NON-DESTRUCTIVE EVALUATION OF STAINLESS STEEL PLATES AND WELDS B.P.C. Rao, Baldev Raj, T. Jayakumar, P. Kalyanasundaram, W. Arnold* and M. Kroening* Metallurgy and Materials Group Indira Gandhi Centre for Atomic Research Kalpakkam – 603 102, India * Fraunhofer Institute for NDT (IZFP) D 66123 Saarbrücken, Germany Abstract Eddy current test is a simple, versatile, fast and high sensitivity non-destructive evaluation technique widely used for detection and characterisation of surface and sub-surface defects in austenitic stainless steels. Fracture mechanics based approaches for structural integrity assessment demand NDE data such as length, width, shape, orientation, and distribution of defects. In EC test situations, this data can only be obtained by raster-scan imaging of eddy current probe on the material surface. Three major problems noticed during EC imaging of stainless steel plates and welds are the influence of disturbing variables, the blurring of images due to convolution of point spread function of probe with defects, and the larger imaging times. In this contribution, a new imaging scheme is presented for automated eddy current non-destructive evaluation of austenitic stainless steel plates and welds. This scheme uses artificial neural networks and imaging methods to identify all the defective regions quickly and then to automatically examine the defective regions in more detail to ultimately produce 3-dimensional picture of defects. Experimental studies on stainless steel plates and welds confirm that the automated scheme is capable of detecting defects quickly and characterising them accurately besides reducing the memory requirements for storage of inspection data. 1. INTRODUCTION Austenitic stainless steels are widely used in various industries as structural materials because of high level of fabricability and excellent corrosion resistance. Eddy current (EC) test is a simple, versatile, fast and high sensitivity non-destructive evaluation (NDE) technique widely used for detection and characterisation of surface and sub-surface defects in electrically conducting materials, e.g. austenitic stainless steels. EC test involves measurement of coil impedance change, which arises due to the interaction of induced eddy currents with defects [1]. The conventional single frequency EC test procedures, in general, use amplitude and phase angle of impedance change as parameters and apply threshold based criterion for defect detection and depth evaluation [2]. Austenitic stainless steels are not heat treatable like ferritic steels and cold work is usually given to improve strength. Cold work increases grain size, dislocation density and volume fraction of deformation-induced martensite and affects the coil impedance. Similarly, in case of austenitc stainless steel welds, due to non-linear thermal gradients that set in during welding, inhomogeneous microstructures are formed in the weld region with a distribution of grain size and volume fraction of magnetic delta-ferrite. Usually 3-8 % delta ferrite is intentionally formed in the welds to minimise the problems of hot cracking [3]. The variations in microstructure and delta-ferrite affect electrical conductivity and magnetic permeability, in turn produce continuous interfering signals during EC testing of welds. These interfering signals are of large amplitude and often mask the small amplitude signals of defects and as a result single frequency procedures become insufficient. For such situations, multi-frequency multi-parametric methods are necessary for detection of defects [3].. Fracture mechanics based approaches for