The way it was explained to me is that modren steel (not wrought iron) has a crystalline structure. The easist way to visulise it is like a cell in the human body you heat it it expands. You cool it, it contracts. If you heat it evenly it expands evenly. The same with cooling. In welding to high of a temp will expand some cells and not others leaving uneven growth. leading to cracking. hope this helps.