Rob,
I use a variety of steels. Everything from standard simple high carbon bar stock to 0-1, or W-2 round stock. I use a lot of recycled steels. In particular forged hatchet and axe heads, high carbon crowbars, breaker bars, etc.....
I harden the blade and hammer on each hawk to 58 rockwell and temper them back to 50 to 51 rockwell. I have found that hawk blades need to be a bit softer than a knife to avoid cracks along the cutting edge.