ULTRASONIC CHARACTERISATION OF INTERGRANULAR STRESS CORROSION CRACKING IN AUSTENITIC STAINLESS STEEL WELDS
M. Bandyopadhyay, P. P. Nanekar, B. K. Shah, R. Ramanathan, M. D. Mangsulikar and P. G. Kulkarni
Atomic Fuels Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400 085, India
Keywords: Austenitic stainless steel; sensitization; ultrasonic testing; IGSCC; tip diffraction
ABSTRACT Welding of austenitic stainless steels (grades ANSI 304 and 316) results in sensitized microstructure and residual tensile stress in the heat affected zone (HAZ) making the material susceptible to Intergranular Stress Corrosion Cracking (IGSCC) in presence of a specific environment. IGSCC incidents are not safety related as 'leak before break' criteria is always met. However, they do affect plant availability. Periodic monitoring by ultrasonic testing for IGSCC initiation and growth is carried out as per the requirements of ASME Boiler & Pressure Vessel Code, Section XI. The code calls for depth of IGSCC to be evaluated by comparison of signal amplitude from the flaw to that of a reference reflection (10% wall thickness deep machined notch). This approach always results in undersizing of the flow as signal amplitude is a function of other factors not related to reflector size. Such factors include roughness, transparency, orientation, branching, attenuation etc. To overcome this limitation, two approaches are being standardised. One approach is to use IGSCC defect standard in place of machined notch and the other is to use ultrasonic sizing techniques which are not dependent on signal amplitude. The present study included the following: :
- One austenitic stainless steel heat exchanger tube (25 mm OD x 3.5 mm WT) which had shown leakage due to IGSCC was subjected to flaw sizing by conventional ultrasonic examination using 10% wall thickness deep notch as reference defect. Signal amplitude from the through- wall IGSCC was found to be less than that from the reference defect indicating thereby the limitations of IGSCC depth sizing based on signal amplitude comparison method.
- Experiments were carried out and feasibility established to create IGSCC of varying depths in sensitized and stressed austenitic stainless steel samples.
- Pulse arrival time technique (PATT), based on flaw tip diffraction, was used to size IGSCC in weld HAZ of an austenitic stainless steel pipe. This technique uniquely locates the flaw tip by maximising the tip signal. Distance of the flaw tip from the examination surface is estimated by noting the arrival time of tip signal.
The results obtained during this study indicate:
- machined notch reference defect standard is not suitable for sizing of IGSCC,
- amplitude based sizing method undersizes IGSCC, and
- tip diffraction method is observed to be more accurate for sizing of IGSCC.
Publication Source: Trends in NDE Science & Technology; Proceedings of the 14th World Conference on Non-Destructive Testing, New Delhi, 8-13 December 1996.Vol. 4, pages 2197 - 2200
Publisher: Ashgate Publishing Company
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