RESOLVING THE DIFFICULTIES OF INSPECTING CASTINGS IN A NEW CORROSION RESISTANT ALLOY
D. S. Dulay, and D. J. Lovejoy
N. D. T. Consultants Ltd., United Kingdom
Keywords: New alloy, Corrosion resistant, Cast components, Defect detection, Eddy current
ABSTRACT The introduction of any new material, either in form or content, always raises interesting problems for non-destructive testing. Despite a backlog of data for similar materials it is impossible to predict the exact behaviour of a new material and it is necessary to be wary and accept that surprises are standard experiences in such circumstances. Some years ago in the marine industry it was decided that it should be possible to use an alloy other than nickel aluminum bronze (NAB) which would offer greater resistance to corrosion for castings. After considerable research a 70/30 copper/nickel alloy with 1.6% chromium was selected as it showed the required increase in resistance to corrosion and showed a number of other advantages over nickel/aluminum bronze and no obvious disadvantages. A considerable number of castings were produced at two foundries and tested non-destructively by the established methods of Visual inspection, penetrant testing using a colour contrast technique, and Radiography. While these techniques did find defects it was found that on machining defects which had been missed by the established techniques were revealed. Some were seen visually and others were revealed by the use of penetrant inspection. Of those defects seen visually only some were revealed by penetrant testing; this was because some were void and others were filled with material other than the alloy. The major contaminants in the defects being oxides of zirconium, titanium, and silicon. On destructive testing and sectioning many of the defects which had become apparent on machining were shown to be matrices or networks of discontinuities in the material, the majority were found to be interdendritic while a minority were intergranular. A non-destructive test method for revealing both the presence and the nature of these discontinuities was needed which revealed their presence before any machining was undertaken. This paper reports on a successful eddy current inspection method which was entirely successful in meeting the criteria set and the sensitivity of which could be adjusted to pre-determined limits of what was a defect and what was an acceptable discontinuity.
Publication Source: Trends in NDE Science & Technology; Proceedings of the 14th World Conference on Non-Destructive Testing, New Delhi, 8-13 December 1996.full paper not received
Publisher: Ashgate Publishing Company
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