Evenheat too hot in the back?

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Tenebr0s

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Jun 3, 2012
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Today I used my new kiln (Evenheat KH418 120 volt) for quenching.... and I'm not happy with the results. I'm noticing a huge difference in temperature over the length of the knives coming out of the oven (this is both visible to the naked eye in the dark, but was also apparent in the change in grain size over the length of the blades I quenched. The tips are way too hot). I tried making some baffles out of 5 mm steel plate; but I think it made the problem worse (haven't tried refractory yet though). In the dark, the difference in color along the blade is clearly visible. I would say at least a 100 F difference in temp between the tip and tang,

I only put my blades in the oven after it had come up to temp and had been sitting for a while (10 minutes or so). I've tried playing with the ramp speed so that it ramps more slowly when the knives are in and it's recovering the lost heat from the door opening... didn't help.

The steel I'm working with is Hitachi White #3 (Simple carbon steel with .8-.9% carbon). I've been doing 1550 F normalization, then 1500 and 1450 thermal cycles. Then hardened at 1475.

Breaking the blade in sections, I find that the grain towards the tip is fairly coarse (visible individual grains), but gets progressively better closer to the heel, and is beautiful at the ricasso area. So I'm figuring the coils in the back of the oven are making the whole oven way hotter towards the back. Really wondering how to fix this.
 
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I don't have one of these ovens but I've been contemplating a purchase for a while now. I wonder what Evenheat will say if you contact them.
 
Not too sure why it is happening, possibly due to there being more coil area at the back due to the back wall. If you are doing something really long you could try putting it in handle first so that is the area that gets the higher heat exposure.
 
What Lo/Rez said handle first edge up. I have a evenheat but never checked my grain size in more than one place. Ill try it. They may need more time at temp to equalize. When I first got my oven I would soak for ten min and the blade wouldn't fully harden. Now I do longer soaks and haven't had issues. Are you normalizing?
 
I put a 1 inch thick half fire brick in the back end of my 24 inch Paragon. Some folks put them on all sides. that might be an option for your oven as the Evenheats are 10 inches wide, right?
 
I've been considering putting some firebrick in the back. Just seems bizarre to me that the oven would come with a design flaw* that would prevent it from being used normally as the manufacturer intended. Putting the blades in handle first doesn't seem like a great solution to me... I don't want to grab the blade portion with tongs when I'm quenching, and at any rate, it's not acceptable to me to have uneven grain along the length of the blade, even if the worst part is in the tangs.

*For the time being, I'm still assuming this is user error on my part.
 
Ive got a paragon so it might not be applicable, but when I set a slow ramp speed to a final temperature, and then open the door, and close it, it would ramp up to recover heat at full power and I assume would overheat the blade since the thermocouple is in the front. Are you able to change the ramp speed if its recovering heat during a hold? Have you tried putting in the knife while oven is hot, and delaying the program for 5~ mins to wait for the oven to equalise then slowly ramp up?
At the moment due to my oven ramping at full speed to recover heat during a hold i only harden once blade at a time.
 
Good point on tip first.I'm doing some blades today. Ill try to see if mine has a hot spot too.
 
jdm61 said:
I put a 1 inch thick half fire brick in the back end of my 24 inch Paragon. Some folks put them on all sides. that might be an option for your oven as the Evenheats are 10 inches wide, right?
Click to expand...

So you cover all the coils on the back wall?
 
maybe the heating coils are too thick toward the back and get hotter? have you asked the company about the problem ? the problem is in direct opposition to the company name :(
 
The first thing to do to stop the problem is to use slower ramp rates. 300°/hr is a good rate for most things. Never ramp a blade from cold in the oven. Start with a first step of a 5-10 minute pre-heat at 1200°F, and put the blade in after the oven reaches temp. The HT program steps should start from that point, using slower ramp rates.

I can pretty much guarantee that the above will end the problem. For some reason people want to ramp at 9999° per hour. That is a climb of almost 3° per second with the coils at full power with no cycling ... No wonder there is overheating of the thin tip. You can get away with that in stainless, but in carbon steel the tip may be 200-300° overheated before the oven gets near the set point.

There are all sorts of baffle systems people use. One is to set four fire bricks lengthwise down the center ( two each side) in a channel and set two more fire bricks on the top (kiln shelf is even better for the top). This makes a long shielded box that Keeps direct radiation off the blade. Moving the top bricks with tongs is simple when you need to remove the blade. Add a little soak time to assure full equalization of the bricks.
 
I've always done that as well.
Just lay a fire brick on edge at the end of the oven. It sort of helps even things out.


jdm61 said:
I put a 1 inch thick half fire brick in the back end of my 24 inch Paragon. Some folks put them on all sides. that might be an option for your oven as the Evenheats are 10 inches wide, right?
Click to expand...
 
Thanks for all the replies. Will definitely put in some thin insulating firebricks and use lower ramp speeds.

Stacy, I just want to make sure I understand your advice: did you mean use a 300 degree/hour ramp speed for all steps, including the 1200 degree preheat? And are you putting the knives in at the preheat temp or once the oven is at the target austenitizing temp?


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I always let the oven come up to temp before putting the blades in. Sometimes I put a big block of steel in the back of the oven to help even things out, but preheating should be good enough.
 
The per-heat ramp can be at 9999. Place the blade in the oven once the 1200F is stable. Ramps once the blade is in the oven should be at the 300/hr rate.
 
The spot where there is minimal distance from 3 active walls it's a focus hot spot in the oven and you need to shield some to get compensation.
I don't know why they put elements in the end wall.
 
Does anyone know if there's a difference in ramp speed between when the oven is coming up to temp (which would be the speed you programmed) vs if the oven is recovering from the door having been opened when it has already reached target temp and is trying to hold it?

I had assumed the oven would recover at whatever ramp speed you programmed for the initial heating phase, but now I'm not sure.


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You would be best to ask the tech guys at the makers site .. but:
From what I have seen in the flow charts for the controller on my oven, the ramp rate for each step is fixed at the programmed rate for that step. There are ways to program around that, but little need to do so.
 
Tenebr0s said:
Does anyone know if there's a difference in ramp speed between when the oven is coming up to temp (which would be the speed you programmed) vs if the oven is recovering from the door having been opened when it has already reached target temp and is trying to hold it?

I had assumed the oven would recover at whatever ramp speed you programmed for the initial heating phase, but now I'm not sure.


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Click to expand...

I know this response is coming late, but I had a similar issue with the TAP controller, and the manufacturer told me that the ramp rate does not affect the behavior while holding. So when, after reaching temperature, you open the door, the way it recovers from it won't change based on the ramp rate.
 
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