Question concerning using a pyrometer

samuraistuart

samuraistuart

Joined
Dec 21, 2006
Messages
3,158
Guten Tag,

I have a newbie question for you more experienced smiths out there. First, some background so you know where I'm coming from. I recently came across a nice laminated steel billet (White #2 surrounded by wrought iron), and I want to be ON TOP of the heat treat. Well, sort of ON TOP, since I'm using a paint can forge/MAPP. The "instructions" recommend a coal fire, to reduce carbon diffusion, and I know how much coal fires are looked down upon in the modern knifemaking community for heat treating, so I'm using what I have. I know this steel requires a soak time of 5-10 minutes at a certain temp, and I'm fairly confident I can hold that temp with my paint can forge by using several ultra low tech methods. One thing I am going to do before attempting the heat treat on this cherished billet/knife is buy a pyrometer/probe setup that was mentioned on another recent thread, so I know the exact temp.

OK, the theory and then the question......I need to know the temperature of the steel itself, not the surrounding atmosphere of the forge. I would imagine that 1500 degrees will be reached inside of the forge much faster than the steel reaching that temperature....right? Do I need to take the blade out of the forge in order to get a proper measurement of the temperature of the steel, by placing the probe directly onto the steel? It would seem to me that the only accurate measurement of the steel temp could be made outside of the forge, because the forge could be hotter than the steel is. Mind you, this is a simple paint can setup with MAPP running in....no sort of thermo controlled devices or anything really cool like that.

I really hope I'm making sense here to you guys, and not sounding like a complete bafoon! Maybe an example? If a blade being heated up inside the forge shows the color of 1500, the atmosphere surrounding the blade MIGHT or could be much hotter.....because it can get hotter than 1500. Even if I place the probe directly onto the blade, the probe might be giving me the temp of the air, not the steel. But I'm new to all of this, and have never used a pyrometer/probe.

Any advice, without too many laughs and chuckles, would be much appreciated. Help me, Obi-won. You're my only hope.
 
I wouldn't waste that good steel with your setup. Grind and sand your blade, and send it to a person who will HT it in a programmable electric oven.
A regular forge and a ceramic sheathed TC will work fairly well. A PID controlled forge will work just fine. A HT oven will work best....a paint can forge and a MAPP torch should not be used on anything more complex than 1084 and 5160.

I don't know why things like that "use a coal forge" statement still get sent with the steel ( I get the same note with some of the steel I get from Europe). A coal forge can be reducing, neutral, or oxidizing....just like a gas forge. A HT oven is about as neutral as it gets. They can even be made to exclude all oxygen.

A trick I see posted often is to use a steel muffle pipe when doing HT with a forge. Cap off one end, put in the blade, and put in a piece of hardwood charcoal. The charcoal will consume oxygen as well as emit carbon. I really doubt this will improve the blade much, but it is a basic good procedure to prevent decarb.
If you have a TC, place it in the muffle pipe on the blade. It will read the temperature of the blade. You can't do this well in your setup, because there isn't enough chamber space for the muffle and a good flame swirl. In a perfect forge, the muffle is positioned in the center of the chamber, not on the floor. To do that, set some fire bricks just outside the front and back ports to support the muffle in the center. Make a "U" in the top brick to keep the muffle pipe stable.
 
You are going to have to get control of your forge temp. You can not soak at 1500 degrees for 5-10 minutes in a 1800 degree forge. You have to have the forge be able to maintain the heat you are going to soak at.
 
One way you could do it would be to leave the billet out and turn the forge on with just the probe in it. Then adjust the gas flow in your chamber until you reach your desired soak temp. That way you know your forge temp is stable at your soak temp.
 
The more I think about this, Stacy, you're right. I'm going to send this off to have it heat treated once I get it ready. I want the MAX performance out of this steel, and I realize my setup is not going to give me the MAX performance. I've only been using 1080 1084 so far, but by adjusting the flame position, the flame's power, moving the steel in and out of the chamber, I feel confident that I can keep a blade just below 1500 even though the forge is capable of higher temps, and use some better steel like 1095 W-2. Um.....well.....looks like I gotta hold off on that W2!!! I suppose a pyrometer/thermocouple setup in a paint can forge is just not ideal anyway. Thanks for the input.
 
Unless you have a very well controlled forge with known min and max temperatures, set the forge at the temp you need BEFORE putting the blade in. I broke two O1 blades overheating them bringing the forge up to temp.
 
Thanks Pyromaniac and Willie, that's exactly what I'll do (with other steels). I'm thinking this White Steel I want done as best as possible.
 
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