Homebrewed hammer die heat treatment

[Stormcrow]

I've made three different sets of hammer dies so far for my Appalachian-style guided helve hammer. These have so far all been made from pieces of railroad track and have done pretty decently since being put into service.

I'm making a set of main flat dies for the hammer now. In addition to being used by themselves to forge with, they'll also serve as tooling platforms (more information when completed and I have pictures). These dies are made from 3" square 4140 5" long. For quick reference, my die-holding system is here:

So I have these flat dies welded to their base plates, pre-and-post heating to make sure everything is as it should be. I need to do a little grinding and then I'll be ready to heat treat. My question is this: After hardening the dies, I'm not sure how far back to temper them. My hammer hits pretty hard (88 lb ram driven by a 3 hp motor), so I don't want to see high-speed shrapnel come off of these. I also don't want to have these expensive (for me) and time-consuming dies pick up dings and dents too easily. So I'm not sure whether it would be better to draw them back to blue (560-600 degree range) or purple (520-550 degree range).

This is a hammer that so far has proven to be as hard-working as a manufactured mechanical hammer, and the dies need to be tempered accordingly. It's not a hobbyist hammer, just a home-brewed workhorse.

[Spears]

Hello Stormcrow,

Below I have pasted the portion of my Power hammer build describing how I heat treated my 4140 dies. (very similar to yours) Between 350 and 400 degrees temper 1 hour for every inch of cross section should yield an approximate Rockwell 50C for a good depth of the outer surface. You cannot really over temper a part, but you can under temper. With material 3" thick, you should use 3.5 hours minimum to be good and sure. I've tempered for as long as 2 hours per inch of cross section. My dies have been slammed together by accident and even used on cold steel and are showing no dents or wear. Good luck.

Hardening the 4140 dies.

The method described here is a “limited equipment” method but does work. Consult a book like “Heat treatment, Selection and Application of tool steels by Author Bill Bryson for more proper processing.

In the simplest form, I describe the hardening process like this: 1. Get it hot. 2. Quench it. 3. Temper it.

What I did was weld a 4”X 4”X 1/2” plate of hot rolled mild steel to a 2”X 2”X 3.5” block of 4140 in first stage of the hardening process.

First I welded a small shelf to my forge stand which would hold the tacked together Die assembly face first in my gas forge. This setup allowed me to gradually move the part completely into the forge over a period of time. Efforts were made not to heat this up to quickly.

Step 1. I brought the die up to ~200 deg for tack welding in my tempering oven for about 20 minutes. After tacking the block to the plate near the corners, I placed it on the shelf near the mouth of the forge. The die was moved closer into the forge mouth over 10-15 min until starting to take color. The corners of the base plate were the first to turn red. I then removed the part, clamped it to the table and welded down both long sides and quickly returned it to the shelf in front of the forge mouth. Over the next 10-15 minutes I brought the part up to red. Target temperature was 1600+ degrees. Perhaps a bit hot according perfect processing but I made sure I completed step number one.

Step 2. Seven gallons of used crankcase oil quickly welcomed the part stirring vigorously up and down round and round. Do not do this if you’re scared to breath in a little oil smoke. It’s a 6lb chunk of red hot steel and when it first hits the oil you will see a bit of fire even at immediate submersion followed by delicious bubbles of smoke!

Step 3. The parts were allowed to cool to a 4 second bare hand touch and then they were put in a small oven set at 375 deg for four hours. Step three complete. Spears.

[Madmike]

I read somewhere (I think it was from Frosty or Nakedanvil) that tempering one of the dies a bit farther than the other will prevent chipping if the dies accidentaly get slammed together...
Seams to make sense

Perhaps should you also ask John Larson (Iron Kiss Hammers) how he does his selective hardening (hard face / soft body) but if I remember right he uses 4340