I encountered this problem in a shipyard where I worked in the 1970s, the rolled steel (12mm thick) plates were of the right quality (purity) steel, but during the rolling proccess slag was rolled out into a thin layer midway in the plates, thus the quality was the problem, this was discovered using an ultra-sound device!
"Twenty years from now you will be more disappointed by the things that you didn't do than by the ones you did do. So throw off the bowlines. Sail away from the safe harbor. Catch the trade winds in your sails. Explore. Dream. Discover." -- Mark Twain