Good day everybody.

So, there I was busy training and performing assessments on two guys for MT Level II.

One of the test plates we used was a piece of mild steel welded to stainless steel. With the yoke (both poles) on the mild steel there is an attraction – of course. With the usual vibration and noise that comes with it. With the yoke (both poles) on the stainless section, there is no attraction, no vibration, no noise.

But with one pole of yoke on each material there is attraction from the mild steel only. Both my students failed to spot the difference in the “feel” of the yoke. I suppose this comes only with experience.

Another thing, that was quite obvious. In the centre of the weld there was a major linear indication;

a broad black line the entire length of the weld. I asked the students if they thought it was a crack.

Both wanted to reject the weld.

 

What happens is this:

We know that flux (magnetic field) leakage from the test piece is what attracts the metal powder. The metal powder collects and causes a black line – we call this an indication.

Where the mild steel is welded to carbon steel – in the middle of the weld, the flux reacts as if there was a major air gap, like at the edge of a magnet, or the edge of a magnetized plate in this case. The flux expands and “leaks” out of the plate attracting the metal particles, causing the thick black indication.

After I explained the situation, they both recommended a Penetrant test on the weld.

This brings us to another problem – which I did cover before – PT after MPI. The cracks are covered with white paint and possibly filled with MPI carrier fluid, paraffin. This will prevent dye (which is also paraffin based) from entering any cracks, which means that you may acce

Thanks guys.

Keep up the good work.

Hennie.

Published with the kind permission of Hennie de Wet