Inspection of non-ferrous cast material is routinely carried out looking forcasting defects inherent for the manufacturing process. The NDT methodsemployed are Radiography and Ultrasonics, primarily for sub-surface orinternal defects and Penetrants fo r surface breaking defects.Electromagnetic techniques have not been used on cast material exceptmaybe for conductivity determination. This limitation resulting fi7om thesurface roughness normally associated with cast surfaces and the fact theother techniques mention above have been very successful in finding andevaluating the discontinuities sought.The possible application of Electromagnetic techniques on surfaces in theas-cast condition of non-ferromagnetic material came about because ofspecific problems experienced by industry.Two major investigations were offered namely;1) Investigation of CNC material - CuNiCr [1.6%Cr] castings exhibitingoxide entrapment in the form of networks.2) Investigation of NAB - Nickel Aluminium Bronze exhibiting selectivephase corrosion on immersion in seawater.The detection and measurement of both oxide entrapment and phaseselective corrosion was difficult and in cases impossible with conventionalNDT methods employed for quality control of these material/componenttypes. Time of Flight DiMaction Ultrasonics did give some 50%detectability of phase selective corrosion, but the method was found to beexpensive and very time consuming.The metallurgical properties of the material and morphology of the defectshave been studied for both Cupro Nickel Chromium and Nickel AluminiumBronze cast alloys. An investigation was then conducted to study theeffects of eddy current signals and their potential in detecting, both linearand cluster type defects which were predominantly interdendritic with somereported as intergranular in nature.For inspecting Cupro Nickel Chromium castings two successful eddycurrent methods have been developed. Detection of surface flaws wasachieved by high frequency [2MHzj examination and subsurface flaws byusing low frequencies [1--'IKHz] but using specifically developed sensors that provided good penetration but maintained sensitivity to the finedefects.In the case of Nickel Aluminium Bronze material, the investigation was tolook at electromagnetic techniques, which best utilizes the inherent featureof permeability/conductivity associated with this non- ferromagneticmaterial and any changes that phase selective corrosion may produce.Some meaningful results were obtained using a combination of edd'ycurrent excitation with detection via magneto -re s istive sensors.Testing through 30-40mm of material to detect small magnetic variationproduced by only 1-2 mm of corrosion penetration was difficult to quantify.Detection and assessment however appeared hopeful when examinationwas carried out from the corroded surface. With quantifiable samples ameaningful technique using eddy current excitation and magneto-resistivesensor for detection can be developed.
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