Testing W2 Hardness Quenched with Parks 50 and 9% Brine Solution.

[Randomus]

My dad was a potter and we had a 5 foot high arch kiln in the backyard. He had peep holes for every shelf level that were plugged with cones made of soft firebrick. I think you can get them at pottery supply places. That way you don't have to worry about heat escape and can take a look any time.

 

[Jeff the Millwright]

I drilled a quarter inch hole in the front door of my oven to accommodate my probe to test the full-length of my oven for temperature variation today. And Don Hanson was absolutely correct the brick was extremely soft and it was like drilling through chalk.

I set up a ramp so that I could pull the probe in and out easily and keep it on the bottom of the oven and in the middle. I started 1 inch from the very back of the oven and pulled it out in two inches intervals waiting 30 seconds between each moved to let the probe acclimate. The last temperature reading was 1 ½ inches from the front of the oven. I did tests as soon as my target temperature of 1455°F was reached and then after 15 minutes, 30 minutes, 45 minutes, 60 minutes and 90 minutes.

By the way the probe I was using cannot be calibrated so I didn’t worry about the actual temperature instead was only concerned with change in temperature over the distance of the oven because later on I can figure out what temperatures I actually have to shoot for with W-2 with actual test pieces.

Here are my results and below that I made a graph for more clarity.

I really expected the temperature to even out much more than it did over time. The most even heat was after 90 minutes in the back of the oven where the temperature only varied 7°F over 6 inches. Perhaps if I were to let it heat up for three hours I might get an even longer and more even section back there but it seems like such a waste of energy.

I was really hoping the middle section of the oven would even out over time but it never really did significantly. The blue line with dots on it is the one that I did immediately upon reaching my set temperature and the green line with the dashes on it is my last reading at 90 minutes and if you look at the center section they are almost parallel which is a real disappointment and shows soaking doesn’t really even things out. Even with 90 minutes soaking the middle 8 inches of the furnace still had 25°F variation. From previous tests it seems like that kind of variation would have around two point hardness difference.

Looking at my chart I believe I wrote down the wrong number for my 45 minute line for the fourth to last number because it takes an odd rise.

Probably in practical applications one wouldn’t even notice that but it is sad that the most popular brand of oven is so poorly designed. If this oven just had heating elements on the two sides instead of also on the backside the middle would be the sweet spot and the heat would fall off evenly in both directions so you could position your blade accordingly and with a long soak it would probably be very even quite a ways out from the center.

I will definitely be doing the same experiment using the fire brick in the back to block the rear wall element and hopefully that will give me a more even heat even though I suspect eventually it will burn out my element. I have to work next week so I don’t know if I will do that tomorrow or not because I might be too busy.

And no samuraistuart I didn’t get the fact that Warren was using scrap steel on the floor of his oven to act as a heat sink; I was thinking he just used that to raise up his knives in the oven. I may try that once I get the oven as even as I can without it. Thanks for revisiting that.

 

[Jeff the Millwright]

My dad was a potter and we had a 5 foot high arch kiln in the backyard. He had peep holes for every shelf level that were plugged with cones made of soft firebrick. I think you can get them at pottery supply places. That way you don't have to worry about heat escape and can take a look any time.

Thanks Randomus but actually I didn't need a peep hole just a hole to allow my temp probe into my oven for my test; I will probably plug it with high temp wool in case I ever want to use it again i can just remove the wool.

 

[noseoil]

What about a length of pipe in the oven, just to even out the heating problem? Simple & effective, should help with the temperature control if the probe is inside the pipe & it reads the inside temp. That way you would have a "simulated controlled environment" in the oven.

 

[Jeff the Millwright]

What about a length of pipe in the oven, just to even out the heating problem? Simple & effective, should help with the temperature control if the probe is inside the pipe & it reads the inside temp. That way you would have a "simulated controlled environment" in the oven.

I appreciate the recommendation noseoil and I agree that your pipe solution is simple but effective unfortunately I must disagree.

Actually when I started out heat treating with my homemade natural gas oven I did put a pipe which had been cut lengthways in half in my furnace to disperse the flame and even out the heat and the only spot that would get red-hot was the part that was directly under the flame. Granted it helped but it was no panacea as a solution.

Besides being a millwright I used to also be a CWB certified all position stick welder and still weld and use torches such as oxygen acetylene, rosebuds, Tiger torches etc. on a regular basis at work. And I know from long experience that metal is not the great heat conductive equalizer you think it is. When you heat something with a torch the only part that gets red is the part directly under the torch and you have to move your torch the entire length of the piece you are heating if you want the whole thing red.

I think if you put a pipe in an oven that is being heated substantially more from one end you are going to get a pipe that is hot substantially more on one end than the other and the inside of that pipe pretty much is going to reflect the atmosphere outside it. And yes I concede that the conductive properties of the metal pipe may even out the heat marginally but I don’t think enough to be a practical solution.

I fear this is going to turn into a quench oil volume like fiasco all over again but I really don’t see this idea worth pursuing.

Today I am going to redo the same test I did yesterday but with my brick in the back blocking the rear element and see what kind of improvement I get.

I think the real solution to this problem is to eliminate the element in the back and I am toying with the idea of cutting my heating element into two pieces and eliminating the back section and rewiring it so I have two equal sides heating and no element in the back.

This is a big job because I would have to disassemble the whole furnace but I really think this is the best solution.

 

[noseoil]

OK Jeff, understood & good answer to my question. It sounds like the solution is even heat which is consistent from run to run, without any "hot spots" or cold spots. Appreciate this rabbit hole you are diving down & the willingness to share information you've shown throughout on these threads. Once it's running properly, you have the data you need for some good blades!

 

[Jeff the Millwright]

OK Jeff, understood & good answer to my question. It sounds like the solution is even heat which is consistent from run to run, without any "hot spots" or cold spots. Appreciate this rabbit hole you are diving down & the willingness to share information you've shown throughout on these threads. Once it's running properly, you have the data you need for some good blades!

Thank you noseoil and actually I am glad to share my information because I am so grateful to all those who know so much more than I do who have already shared and have helped me so much.

I actually figured out a pretty easy way to convert my oven so that I will just have two equal elements on each side and none on the back without totally dismantling it so I don’t think I will even try the brick test I was contemplating and instead just convert it over so there is no element in the back.

I have to work next week so it may be a couple of weeks before I can get around to it but I was thinking of starting a new thread with this information because I think people contemplating buying an oven could benefit from knowing that the Evenheat ovens are anything but evenly heated.

 

[Jeff the Millwright]

I decided today to throw caution to the wind and modify my Evenheat oven by rerouting the heating elements so that there are no elements in the back to try to even out the heat throughout the length of the oven.

I figured it would be easier in the long run to actually disassemble the entire thing and here it is once I removed the cover. At this point I was thinking this may be a bit of a nightmare fitting together again and I wasn’t completely wrong.:grumpy:

But I did manage to get it reassembled and up and running. Here is a picture when I first got it fired up with elements on the side and none in the back. It was a very gratifying moment considering there wasn’t a giant spark and the lights didn’t go out.

I was very curious what the heating line would look like on my graph after I modified it compared to the other ones I did previously before modification so I did the same heat variation test that I did before it was modified. See if you can figure out which is the latest line?

There are the numbers in case anyone cares.

Obviously it’s much better and after the first 4 inches the oven varies less than 10°F throughout its whole length. And I think I can improve the first 4 inches because the door latch didn’t line up properly after reassembly and the door needs to be fitted to close properly so I think it will be even better once I get it properly assembled. I’m really happy this worked out because now I can make a nice consistently hard blade.

Actually I just had a closer look at my numbers at 60 minutes and the whole oven is within less than ten degrees variance. I put up the 30 minute results because I didn't want to waste energy and heat longer than I have to so that is what I planned on using so that is what I plotted on the graph.

I really find it remarkable that Evenheat sells an oven with such an easily fixable but serious design flaw. I have dealt with them and their customer service is very helpful so I think I may recommend they change their design but I doubt they will listen.

I think I may make a new thread with these results because if I were buying an oven I would want to know how these Evenheat perform and most people won’t know it’s here because the thread title doesn’t indicate it.

 

[me2]

Any change you noticed in heat up time?

 

[Jeff the Millwright]

Any change you noticed in heat up time?

I didn’t notice any change in heat up time but there probably was a small amount of additional time needed. I was hoping to be able to compress the elements so that I could put the full lengths back in each side so that I wouldn’t lose any heating capacity but unfortunately the elements were extremely rigid and I was hoping they would have been somewhat springy but since they weren't I had to cut some off to fit in the reduced distance.

I figure I had to remove about 20% so my heat up time was probably about that much longer but I really didn’t notice the difference even though I was looking for it but I didn’t time it. I gladly trade off a 20% increase in warm-up time for a nice evenly heated oven. Besides this is a 220v oven so the warmup time is pretty good anyway and I just do other things while it is warming up; it isn't like I sit there and watch it and it has an external alarm to let me know when it is up to temp.

 

[Willie71]

Thanks for sharing this. I have the same oven, and it looks like I'll be modifying mine. :thumbup:

 

[Jeff the Millwright]

Thanks for sharing this. I have the same oven, and it looks like I'll be modifying mine. :thumbup:

I am definitely glad I did mine and ultimately it seems successful but before you do I think you should be warned that the element in that oven is extremely brittle and there is a chance you may break it and have to buy a new one.

When I was straightening out the bends that the element had from being routed the original way one of them snapped in half on me. Luckily it was the very last bend and just happened to be pretty much exactly where I needed to trim it anyways to fit the shorter route that I needed to eliminate the element in the back.The worst bends to straighten out where the 90° angles at the back were transitions from the back to the sides but the radius on the sides of the front did not present a problem and I didn’t heat that area when I was re-bending them to reinstall the element but you may want to.

After I snapped that one I got out a small propane soldering torch and heated the area I wanted to bend until it was slightly red and then slowly straightened them. Besides straightening bends I had to do this to about a three inch section where the element came out the back of the oven where I had to take the curl out. I tried not to heat the element very much and it was definitely a lot less hot that it would be when it was firing but still there is no way to know what the direct hot propane gas might have had so it may burn out early. This is a risk I’m willing to take because I really want an oven that heats evenly.

The other issue to look out for is how extremely brittle the bricks are and how easily they break. They seem to have the consistency of chalk. A few of mine broke during the process and luckily I had some furnace cement from attempting my hamons so I repaired them as I reassembled.

But on the plus side it wasn’t the daunting job I thought it would be. The main difficulty lies in managing to get all the bricks back in the right place while you try to rewrap it with the stainless cover. All I had to buy was 8 feet of high temperature insulated wire because I exited my leads out the back instead of right beside the controller as it was originally and I managed to get the same fittings for the wire ends where I got the wires but you could remove them from the shorter wires you replace. In all I think it took about three hours.

So it’s your decision if you want to take the risk but you might want to wait a while to see how long my element lasts. But I did fire it for over an hour and there didn’t seem to be any problem with it so hopefully I am okay?

I just looked online and a heating element for an Evenheat 22 ½ inch oven is $34 American so it isn’t a great tragedy if one is broken but it would be annoying not having an oven while you wait for it to be shipped. And that oven would have two of those and that is the price for one.

 

[Willie71]

I've got some canthal wire here anyway, as I was going to make an oven until I got a good tax return a few years ago, and just bought one. I have a box of fire bricks too. I was going to make a 36" or 42" kiln for swords and machetes with the parts. I might just do a long pid forge for those projects though.

I'll have to redo my tests with W2 if I change my oven though. I usually put the blades in the center of the kiln, with the middle of the edge right under the thermocouple. If the heat configuration changes, I'll need to know.

 

[Jeff the Millwright]

If you have got some canthal wire Willie you are all set if you decide to do it. I know I am being a little anal but I figured if I did a decent sized knife the way my oven was I would’ve gotten about a two point variation in hardness at best because of the temperature differential which probably no one would even notice but it just bothered me that my oven didn’t have even heat so even though I probably won’t even notice the difference in performance for some reason I had to do it.

 

[stezann]

This thread has an huge value!! Congratulations, and thanks for sharing your valuable experience, i'm sure it will be of benefit for everybody who run an electric kiln!! :thumbup:

Before i read the last post where you showed your brilliant solution, i was to suggest modifying the relative height of your oven legs, just to see if raising a bit the door side would have helped evening the inside gradient through convective air motion...nothing radical of course, not a vertical kiln

 

[duurza]

I appreciate the recommendation noseoil and I agree that your pipe solution is simple but effective unfortunately I must disagree.

Actually when I started out heat treating with my homemade natural gas oven I did put a pipe which had been cut lengthways in half in my furnace to disperse the flame and even out the heat and the only spot that would get red-hot was the part that was directly under the flame. Granted it helped but it was no panacea as a solution.

Besides being a millwright I used to also be a CWB certified all position stick welder and still weld and use torches such as oxygen acetylene, rosebuds, Tiger torches etc. on a regular basis at work. And I know from long experience that metal is not the great heat conductive equalizer you think it is. When you heat something with a torch the only part that gets red is the part directly under the torch and you have to move your torch the entire length of the piece you are heating if you want the whole thing red.

I think if you put a pipe in an oven that is being heated substantially more from one end you are going to get a pipe that is hot substantially more on one end than the other and the inside of that pipe pretty much is going to reflect the atmosphere outside it. And yes I concede that the conductive properties of the metal pipe may even out the heat marginally but I don’t think enough to be a practical solution.

I fear this is going to turn into a quench oil volume like fiasco all over again but I really don’t see this idea worth pursuing.

Today I am going to redo the same test I did yesterday but with my brick in the back blocking the rear element and see what kind of improvement I get.

I think the real solution to this problem is to eliminate the element in the back and I am toying with the idea of cutting my heating element into two pieces and eliminating the back section and rewiring it so I have two equal sides heating and no element in the back.

This is a big job because I would have to disassemble the whole furnace but I really think this is the best solution.

You sure about that? The heating elements radiate heat from their surfaces to a steel tube at a rate of no more than 200 [W/m^2 °K] (this is grossly overestimating it since it's really around 150). Even for half an inch of steel, it has a heat transfer coefficient for CONDUCTION at around 2000 [W/m^2 °K] which means that it is still 10x better at spreading the energy than the radiating heating elements.

The pipe should be good enough and this won't be the quench oil volume fiasco since you only need to put a pipe near the end of the oven. It only takes one test.

What you mentioned about steel getting red hot under the point of a flame occurs for two reasons:

1) The torch should have a much higher heat transfer coefficient than conduction through steel meaning that the energy going into the steel cannot dissipate as quickly throughout the steel. An increase in energy is an increase in temperature.

2) I don't suppose you are using your torch in an atmosphere at 1500 F. Since heat moves much slower through the steel than directly from the flame into the steel, what little heat that is moved away from the direct flame is either radiated away or lost via convection.


[EDIT] Check out the Biot Number (https://en.wikipedia.org/wiki/Biot_number). In short, it is a ratio of "What happens at the surface" vs "What happens within the body" and this goes for any system. If "What happens within the body" is 10x stronger than "What happens at the surface", you can assume (up to 5% error) that the entire body is at the same temperature.

 

[Willie71]

I looked in my kiln as I was thermal cycling some steel. The scrap steel on the floor was bright red throughout its length, except the last couple inches by the door. I use the steel for its thermal mass, as solids hold and radiate heat more consistently than gasses. How much it evens out hasn't been tested by me. I use radiant floor heat in my house and shop and it is much more even and efficient, similar principle to salt and sand pots. I've meant to make some strips of steel that are finned like an air cooled engine for the kiln. I thought of aluminum, but it melts at too low of a temperature.

Interesting discussion, anyway. I'll test the heat sink idea before I rip my kiln apart.

 

[Jeff the Millwright]

This thread has an huge value!! Congratulations, and thanks for sharing your valuable experience, i'm sure it will be of benefit for everybody who run an electric kiln!! :thumbup:

Before i read the last post where you showed your brilliant solution, i was to suggest modifying the relative height of your oven legs, just to see if raising a bit the door side would have helped evening the inside gradient through convective air motion...nothing radical of course, not a vertical kiln

Thank you stezann and I am happy you found my posts useful. Glad to give back because all those here who have been so helpful to me.

 

[Jeff the Millwright]

You sure about that? The heating elements radiate heat from their surfaces to a steel tube at a rate of no more than 200 [W/m^2 °K] (this is grossly overestimating it since it's really around 150). Even for half an inch of steel, it has a heat transfer coefficient for CONDUCTION at around 2000 [W/m^2 °K] which means that it is still 10x better at spreading the energy than the radiating heating elements.

The pipe should be good enough and this won't be the quench oil volume fiasco since you only need to put a pipe near the end of the oven. It only takes one test.

What you mentioned about steel getting red hot under the point of a flame occurs for two reasons:

1) The torch should have a much higher heat transfer coefficient than conduction through steel meaning that the energy going into the steel cannot dissipate as quickly throughout the steel. An increase in energy is an increase in temperature.

2) I don't suppose you are using your torch in an atmosphere at 1500 F. Since heat moves much slower through the steel than directly from the flame into the steel, what little heat that is moved away from the direct flame is either radiated away or lost via convection.


[EDIT] Check out the Biot Number (https://en.wikipedia.org/wiki/Biot_number). In short, it is a ratio of "What happens at the surface" vs "What happens within the body" and this goes for any system. If "What happens within the body" is 10x stronger than "What happens at the surface", you can assume (up to 5% error) that the entire body is at the same temperature.

You do have an interesting argument duuraz I have to concede that logically it does make sense that since the conductivity of the metal is much greater than the amount of heat that can be radiated from the elements it does make sense that the pipe would even out the heat but I just don’t see that happening in practice.

Because think about this. If this were true then you would not need a pipe at all because your knife being metal would conduct the heat inside itself faster than oven heat could be applied to it through radiation from the element. So if your argument were true the knife would be doing the exact same thing for itself that the pipe would do.

And that isn’t the case because one thing that made me think that my oven wasn’t evenly heated was when I made a knife it was obviously hotter the farther in it was because it was brighter and if your logic was true it would have been red consistently along its length because it would have been able to conduct the heat internally faster than the element could radiate heat to it unevenly.

I fixed my problem by moving my element so there is no point in me doing a test with a pipe because now I have even heat in my furnace so the results would be meaningless. But I would like to see someone try that just out of curiosity because I don’t think it would do much to even out the heat.

As a disclaimer to not ruffle any feathers this is just my opinion but all my experience tells me that putting a pipe in a in a furnace will not even out the heat inside the pipe but if someone does that experiment I would love to see it because I am curious.

 

[Jeff the Millwright]

I looked in my kiln as I was thermal cycling some steel. The scrap steel on the floor was bright red throughout its length, except the last couple inches by the door. I use the steel for its thermal mass, as solids hold and radiate heat more consistently than gasses. How much it evens out hasn't been tested by me. I use radiant floor heat in my house and shop and it is much more even and efficient, similar principle to salt and sand pots. I've meant to make some strips of steel that are finned like an air cooled engine for the kiln. I thought of aluminum, but it melts at too low of a temperature.

Interesting discussion, anyway. I'll test the heat sink idea before I rip my kiln apart.

Yes Willie if you do somehow test your heatsink to see how even the temperature really is please do post the results because I am very curious how much of an effect that would have.