Abstract:
Inspection of non-ferrous cast material is routinely carried out looking for
casting defects inherent for the manufacturing process. The NDT methods
employed are Radiography and Ultrasonics, primarily for sub-surface or
internal defects and Penetrants fo r surface breaking defects.
Electromagnetic techniques have not been used on cast material except
maybe for conductivity determination. This limitation resulting fi7om the
surface roughness normally associated with cast surfaces and the fact the
other techniques mention above have been very successful in finding and
evaluating the discontinuities sought.
The possible application of Electromagnetic techniques on surfaces in the
as-cast condition of non-ferromagnetic material came about because of
specific problems experienced by industry.
Two major investigations were offered namely;
1) Investigation of CNC material - CuNiCr [1.6%Cr] castings exhibiting
oxide entrapment in the form of networks.
2) Investigation of NAB - Nickel Aluminium Bronze exhibiting selective
phase corrosion on immersion in seawater.
The detection and measurement of both oxide entrapment and phase
selective corrosion was difficult and in cases impossible with conventional
NDT methods employed for quality control of these material/component
types. Time of Flight DiMaction Ultrasonics did give some 50%
detectability of phase selective corrosion, but the method was found to be
expensive and very time consuming.
The metallurgical properties of the material and morphology of the defects
have been studied for both Cupro Nickel Chromium and Nickel Aluminium
Bronze cast alloys. An investigation was then conducted to study the
effects of eddy current signals and their potential in detecting, both linear
and cluster type defects which were predominantly interdendritic with some
reported as intergranular in nature.
For inspecting Cupro Nickel Chromium castings two successful eddy
current methods have been developed. Detection of surface flaws was
achieved by high frequency [2MHzj examination and subsurface flaws by
using low frequencies [1--'IKHz] but using specifically developed sensors that provided good penetration but maintained sensitivity to the fine
defects.
In the case of Nickel Aluminium Bronze material, the investigation was to
look at electromagnetic techniques, which best utilizes the inherent feature
of permeability/conductivity associated with this non- ferromagnetic
material and any changes that phase selective corrosion may produce.
Some meaningful results were obtained using a combination of edd'y
current excitation with detection via magneto -re s istive sensors.
Testing through 30-40mm of material to detect small magnetic variation
produced by only 1-2 mm of corrosion penetration was difficult to quantify.
Detection and assessment however appeared hopeful when examination
was carried out from the corroded surface. With quantifiable samples a
meaningful technique using eddy current excitation and magneto-resistive
sensor for detection can be developed.