I have been using 4140 for hammers and it works great but there are somethings to keep in mind. It is an oil quenching steel. Yes large section will tolerate water but it is guess work. 1575f max on the Hardening temp and temper immediately . Both failures I have had happed after it was water quenched and left for 15 min before tempering. It hurts when you spend a lot of time on something only to have it crack on the final steps.