Test analysis of eddy current detection with tangential and orthogonal coils of small-diameter carbon steel tube welds

To solve the problem of the detection of the surface-breaking cracks in the welds of the small-diameter carbon steel tube with the paint layer, the eddy current technique with tangential and orthogonal coils was introduced, and its signal character of the surface-breaking cracks of small-diameter carbon steel tube and its allowable liftoff were studied, as well as the detectability of the surface-breaking cracks in the welds of the small-diameter carbon steel tube by means of the specific samples. The experimental results showed that, eddy current signal of the carbon-steel surface-breaking crack was characterized by a constant signal phase and the signal amplitude increasing as the crack′s depth increases, decreasing as the liftoff increases, with an allowable liftoff up to 2.0mm, the eddy current impendence signals for the transverse and longitudinal surface-breaking cracks of the carbon steel weld were opposite, but weak to the longitudinal crack at the weld toe and insensitive to the 45° oblique crack, capable to detect the 5.00 mm×0.25 mm×1.00 mm (length×width×depth) transverse and longitudinal cracks in the welds of the small-diameter carbon steel tube with an insulating anticorrosive paint layer of 0.54 mm thickness, with the allowable outer diameter down to 20 mm, and the scan along the welds with the probe 45° orientation should be supplemented to detect the 45° oblique crack.