The metallurgy of iron and steel is enormously complex, involving chemical compounds with carbon and other metals, crystal structure and grain structure, and how these differ with temperature. There are a number of websites that cover it all, but I'm not going to recommend any -- if you're interested, you'll have to find one that explains at a level you can follow, depending on your knowledge of chemistry. It's mind-boggling to think of the time and patience needed to develop and master the knowledge and skills of iron working in ages before the mechansims were understood. It's no wonder smiths were thought to be magical!

Steve