How Even is an EvenHeat? How to Operate Furnaces Effectively

Larrin

Larrin

Knifemaker / Craftsman / Service Provider
Joined
Jan 17, 2004
Messages
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I'm impressed how accurate the EvenHeat oven temperature is. I sure wish they'd had the LB series without the coils across the back when I purchased my oven years ago. I put a longer TC in my oven so the tip is only a couple of inches above the floor so it's right at the blade. From your charts it looks like that wasn't really needed since the temperature is so even thru-out the oven.

Thank you for another GREAT article.
 
so glad you called it a furnace and not a kiln :)

The temp drop is my biggest problem with my Paragon. In the carbon steel heating range, the temp drop when I open the door is dramatically silly.
It's why I favor my Lindberg muffle furnace. With elements behind the refractory walls, there is almost no temp drop when opening the door.
 
Another great article..... thanks Larrin.

About six years ago "jeff the millwright" posted on Bladeforums.com about modifying his EvenHeat because of un-even heating. This was before the LB models were introduced. Jeff redid his EvenHeat eliminating the rear heating coils, greatly improving the furnaces performance.
https://www.bladeforums.com/threads...9-brine-solution.1434614/page-4#post-16578891
I did the same mod on my own EvenHeat. I did not dis-assemble the bricks, just pulled the old element and cut two sections out. I was able to reuse my existing element - Evenheat informed me that my model had a NiChrome element that maintained its cold ductility even after many firings.
Saludos
J
 
Nicely written and studied, Larrin! Pays off in understanding and peace of mind to drill a couple extra holes, get a quality shielded thermocouple-as-probe and meter, and profile ones furnace. It's comforting to know what the temp is at the target piece rather than at the wall, and that they are close. Thanks for the data and discussion!
 
Great article Larrin, thanks again!
I have one of the older Evenheat models with the rear coils. I got a second thermocouple and placed it low in my furnace and found that the lower one reads 60 degree cooler than the factory one in the ceiling. I did a bunch of coupons and found that going off the lower one lead to marked better results, especially for carbon steel.
 
Hey Larrin,

I love your articles and the new steel you designed. I don't disagree with you very often and I hardly ever disagree with you publicly. You're pretty smart and you have a PhD. I hope you'll entertain a little bit of a contrary POV.

I'm one of the people who prefer "better" ovens. I'm also one of the people who pre-heat the Evenheat oven (while empty) to a temperature above my target temperature, load my work and turn it back on at the target temperature. I started doing this to address issues that I observed with my Evenheat that I did not experience with commercial ovens I used in school or my own commercial oven. I'm not doing this for the fun of it, I'm doing it to solve a problem. This is a problem you might also experience if you were to heat treat a large number of knives and run them through their paces and start to debug the root cause of different failures. You might notice "Hey, some of these are behaving like it has a burned spot". "Oh neat I can make this sort of thing go away by modifying my process". "My goodness, this very inexpensive oven works but it isn't 100% superfantastic"

I think the small size of the oven and the close proximity of the exposed coils expose the blade to uneven infrared radiation that can over heat areas of the work and areas on the blade can exceed the air temperature being measured by the thermocouple. This is exacerbated by the relatively low thermal mass and relatively high heat loss through the walls of this grade of oven necessitating extended burns of the coils. I don't know about the physics relating to the thermocouple in the oven and the thermocouple you're using vs the blackbody absorption of a particular workpiece, but I'm not sure it's safe to assume that your thermocouple measurements exonerate the oven. If you're measuring the air temperature and there are localized areas on the blade that can exceed the air temperature I think that you could easily have an issue and not realize it. My experience tells me this is true.

A great example of this phenomenon was the time I fried an egg on the trunk on a car. The air temperature was much lower than the truck. (you should never do this BTW, it's terrible for the paint)

I'm fairly certain you will get small random spots on your work well above the air temp if it sits in there next to blazing coils for an extended amount of time. If you'll soak the oven (empty) at temp above your target temp and load your work, your oven will safely come down to temp well before your work overshoots. And it will prevent your work from sitting in there with the coils blazing. This is a real solution to a real problem. Also as a hedge your can set up a scrap piece of foil to cast some shade on your work, the same as a sun umbrella above your head.

Also, I think there is almost never a need to pre-heat a knife blade. In practice you're not going to get any significant distortion from skipping this. You're going to get distortion from how you handle the blade and in quench that dwarfs anything you'll see from the heat up. And there are ways of addressing distortion in your quenching and tempering practices that makes any minor movement during the heat up moot.

:thumbsup::)
 
Larrin said:
New article where I used a secondary thermocouple to check the temperature distribution of an EvenHeat LB 22.5 compared to the furnace thermocouple reading. I also measured what happens when the door of the furnace is open, and measured what happens when you preheat the furnace to a temperature above the final setpoint.

https://knifesteelnerds.com/2021/04/13/how-even-is-an-evenheat-how-to-operate-furnaces-effectively/
Click to expand...
Excellent article, I agree with what you wrote based on my experience.

I've heat treated a long list of different steels, some very exotic from Ferrotitanit WFN to Vancron SuperClean, Maxamet, CPM Rex 45 and CPM SPY27, etc

Even some more basic steels like 52100, 8670 etc

The recommended practices in your article worked best.


When I first got my Evenheat I tried different techniques but the results on the hardness curves were off leading to overaustenitizing on sensitive steels that had to be tossed.
Which hurts bad when you're trying to earn a living at making handmade knives or when you get a very rare steel that cannot be replaced.


I settled for practices similar to what worked best in the article.


Strong work on the article.


The results speak.

 
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Enjoyed the article. I have a very old Evenheat 22.5” model and have wondered what differences there might be regarding blade placement in the oven. My oven has the elements on three sides with the door obviously not having any elements. Back before I knew better, I used to ht with the blades near the door. I could never quite get my ht process to get as high Rockwell figures as I should. The thermocouple is placed about 1/3’rd of the way from the back. I rubbed both of my brain cells together and concluded I should be placing my blades as far back as I can. Same steel (CPM154), same 1950 degrees, etc....I gained two points on every blade by just keeping it as far from the door as possible.

Your article answered a good number of questions for me.Thanks for posting!
 
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