If you don't know the steel, you're guessing at tomcats.
I've used old 18-wheeler truck axles, many of which were 0.45% carbon content, a medium carbon steel. Ideally, one doesn't harden the eye. If the entire head is heated to a cherry red and hardened in liquid, there is a good chance that the thin eye cheek may crack. In doing so, the eye quenches from the inside out and outside in, hardening before the hammer faces do, thus causing a difference in the contraction rate. Not good.
After normalizing the entire head, I take a localized cherry red heat on one of the faces at the edge of a coke fire. I quench vertically in water while agitating, withdrawing it when it quits screaming (making the cush sound). It should be around 150 - 200F when taken from the water; this helps prevent cracks. I abrade that face so that I may watch for tempering colors. The head goes into the vise, bare metal face upward.
I made a simple tempering tool of 7/8" square MS. It is a turned eye which makes a snug fit over the hammer head. I left enough extending from the circle to form a haft. The tool is used for conducting heat. Heating the circle to a full welding heat, it is placed over the hammer face until the face draws to a dark straw color, 465ºF. I hold that temper by pouring water on the face.
For the other face or peen, I use the "wet rag method." I wrap a wet rag around the tempered end and using large bolt tongs, I take the localized heat on the other face. The rag protects the finished face from undue heat. Again, I quench vertically keeping the rag in place. Agitate. This newly abraded face or peen faces upward and the head is clamped in the vise. The rag stays on while tempering. If it's a hammer face, I treat it with the tempering tool, as above. If it is a peen, I temper with an oxy tip flame "washing" the area near the eye, thereby chasing color toward the peen end, and this normally goes to a purple, 525ºF. Another water pour.
This is not a "my way or the highway." I'm just sharing my experiences.
http://www.turleyforge.com Granddaddy of Blacksmith Schools
