Heat treat question

[wmhammond]

Morning guys. I use a propane forge with a steel baffle and a thermocouple to heat treat. I have been positioning my knife in the baffle and then lighting the forge and bring it up to critical temp. My question is this: Should I bring my forge up to critical temp. and then put my knife in or is my current way OK?

That makes me think of another question. The thermocouple measures the temp of the atmosphere inside the baffle not the temp of the knife. If I heat the forge up to critical temp first and then place the knife in it how long does it take the knife itself to heat up to that critical temp?

Thanks for the help

Wallace

 

[samuraistuart]

Whether you are using a forge to HT, or a programmable kiln, the best way is to ALWAYS have the temp where you want it prior to inserting the knife. Kilns and forges will overshoot the target temp as it settles in on your programmed temp. You don't want your knife exposed to hotter temps than your target temp (whether you are normalizing, cycling, or hardening, or tempering for that matter). In addition, if the blade is in your forge/kiln as it warms up, then your blade will be at an austenitic state longer than you want, compared to inserting it at the target temp. This will increase decarb, and depending on alloy may cause grain growth. For low alloy steels and simple cross sections like knives, the faster you can heat it the better. The main concern is the overshoot of the temperature, and having a blade exposed to it. I even wait 20-30 minutes after my kiln has settled onto my target temp before I insert the blade, just to ensure the atmosphere inside has settled down.

As far as how long it takes, that is going to depend on your knife (mass) and your equipment. You can always do the magnet test to tell you when the blade has passed ~1414f, understand that doesn't tell you the blade is AT your target temp. Also table salt melts ~1475f, which is the target temp for hyper eutectoid steels. A general guideline, tho, your blade should be at your desired temp in 2-3 minutes.