Finishes, test standard modifications

The purpose of the nondestructive control consists in detecting local modifications of the material parameters which, by their presence can endanger the quality of the half-finished or finished products. The electromagnetic nondestructive control permits to render evident surface and subsurface discontinuities in the electroconductive material under test. The present tendency of this control is to pass from a qualitative evaluation (the presence or absence of the material discontinuities which give at the output of the control equipment a signal higher or at least equal to that coming from a standard discontinuity whose shape and severity has been prescribed by the product standards) to a quantitative one, which enables to locate as exactly as possible the discontinuity and to make predictions over its shape and severity. 

A test procedure which has proved very useful was first described by Hatfield. The samples are cylinders 32 x 12-5 mm in diameter with a standard abraded finish which are supported on open-ended refractory boats in a tubular furnace. In the original test the atmosphere, which was produced by burning towns gas with a 50% excess of air, was passed over the specimens at a standard velocity after first preheating to test temperature over refractory packing in a separate furnace chamber. More latterly, natural gas has been used with suitable modification of air gas ratio to give... 

Finally, modifications to standard test methods must sometimes be made. This is especially true if a product has finishes that prevent water absorption or soiling. 

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