Most of the smaller tools and all the cutting edges are made of plain carbon steel. The carbon content makes it so I can heat-treat them to get the right hardness.
The bodies on most of the larger tools are made of mild steel for a few reasons. The first is that I can not get the kind of carbon steel in the size I need to make the whole tool. So I forge the body of mild steel and weld in a carbon steel edge. Mild steel is also easier to forge, especially in larger crosssections. An additional benefit is that having part of the bevel out of mild steel makes the tool easier to sharpen.
I purchase some new steel and get some at the scrap yard. All cutting edges are made with new carbon steel which, when heat treated in my tempurature controlled oven, yields consistent performance.
In the accompanying image of my first carving ax you can see these two steel layers clearly.