Why will the film thickness gage read the coating thickness on the weld joints between two stainless steel pipes and on the flanges but not on the pipe itself?

Q. I am in charge of quality control and check dry paint film thickness on the steel tubes and pipes we produce before they leave the plant. In many cases, the pipes are butt welded together to form long runs. In other cases, flanges are welded to the pipe ends. We have just started producing a line of pipes made of stainless steel, and I have encountered a strange thing while checking the dry film thickness on them. The film thickness gage will read the coating thickness on the weld joints between two pipes and the flanges but not on the pipe itself. Can you explain this? O.F.

A. Obviously you are using a magnetic dry film thickness gage. Certain grades of stainless steel are non-magnetic and therefore would not give you a reading using a magnetic gage. However, when the stainless steel tubes and pipes are welded, the high temperature can cause a change in the chemical composition of the metal, making it magnetic and enabling you to get a reading in those areas.