The 4140 steel 1.5"x1.5"x3" power hammer dies were welded to a 1"x4"x4" hot rolled steel plate. The dies were pre-heated and "Super Missileweld" electrodes were used.
After welding, the die and plate were stress relieved by heating to a bright cherry red temperature [non-magnetic] and allowed to cool slowly inside the propane furnace.
Then the bottom of the die plate was faced off flat on a milling machine to assure a good contact with the anvil or hammer. The die back plates were drilled for 4 each 3/8" holes for bolting to the hammer and anvil.
An angle grinder was used to rough the basic angles of the die surfaces and feather these angles to smoother curves. Hand filing removed the grinder marks and brought the dies to their finished shape. The top surface was sanded with 600 grit to remove all file marks. The die surfaces have about a 6" radius.
I will use the dies in the annealed condition for awhile. I am told, as long as I am hitting red hot steel, the dies don't have to be hardened. Apparently some smiths never harden their dies.
To harden the die it must be heated in the forge to a non-magnetic condition, that is, bright cherry red, and quenched in oil. Ideally, when the temperature of the die cools to around 140 degrees it should be reheated in a oven to maybe 375 or 400 degrees for 3 hours to reduce the brittleness. I use a toaster oven in the smithy to keep the wife happy.