When it comes to hammers. That depends on how they are to be used. For hammers that are to be hitting struck tools (top tools, chisels, punches, etc. You want a softer hammer than the tools struck, so not high carbon. For hammers used to hit hot steel on the anvil, I like 4130. It hardens reasonably well and tempers to a range of colors. Of course one can always use wrought iron and forge weld a steel face on.