Thanks for all the replies. jlpservicesinc - I don't have any pics and have to dig out the chisel after work to get some (this happened at a hammer in and I haven't unpacked yet this week). littleblacksmith - Originally I was worried about slitting the 4140 as it was relatively thick so I was going to drill some 5/16" holes then slit the webs (thus the annealing). Well I got impatient with the lineup on the mill drill at our hammer in and decided I could just use the slitting chisel. I'll be sure to invest in a better coolant/lube, where did you buy your powdered graphite? Also its somewhat ironic that the piece I was slitting was for a Brian Brazeal Hammer Eye Punch. I'm just starting out and am in the process of forging most of my tools. ThomasPowers - Not sure what the alloy is, it wasn't supplied by CentaurForge when I bought it from them. It's clearly marked Peddinghaus made in Germany. Given the sound it made when it broke (a sharp "Ting" when I quenched it) I'm pretty sure you're right about it being a high carbon tool steel.
Was in the process of drifting a slit for my first hammer eye punch in a rectangular piece of 4140 about an 1 1/4 deep and 3/4 of the way one of the corners chipped off (about 1/4" of a 1"). I was cooling in my water quench bucket after every second/third hit, I didn't work the material cold (good orange heat), used a 2/5lb hammer. What did I do wrong? I should be able to salvage the chisel by regrinding and re-heat treating, I just don't want to repeat my mistake. Any advice (pertinent to my slitting chisel) is welcomed. Edit: Should also indicate that the 4140 billet was annealed in vermiculite for a couple hours after I pressed it from round into rectangle.