Dr. Balasubramaniam from Indian Institute of Technology did a material sciences analysis of the iron, and here’s what he found (in easy words):
If you melt pig iron and add phosphorous in it, it will start breaking up. The iron would never come together and make bond with each other. So, what the manufacturers do is that they take out phosphorous out of the molten iron. (Just like you filter out tea leaves out from your tea, where tea leaves are your phosphorous molecules in an analogy.)
What ancient Indians did was they never took out the phosphorous out of the pillar. They welded the cylindrical pieces of wrought iron. To remove the phosphorous from it, they started hitting it from all sides with a hammer. Phosphorous got pushed towards the surface of the cylinder from inside. So inside had no more phosphorous while outer surface had a protective layer of phosphorous. It was a win-win.
Credit : Quora
Picture Credit : Google