Why the heck did the edge turn blue? (Heavy on pics)

M

MSCantrell

Knifemaker / Craftsman / Service Provider
Joined
Mar 12, 2005
Messages
1,213
Hey fellas,
Here's one I'm finishing up. It's an old Nicholson file, and I kinda wailed on it for a while in the forge and out came a KSO. I decided it's not leaving the house, so no need to finish it past 220 grit. Shop/bench knife, you know?
Anyhow, after I finished it up and sharpened it, I ran the brass rod test. A little bit of chipping on the edge. Obviously just a bit too hard. So back in the toaster oven at 450 she goes for an hour.

Comes out a perfect golden color, as you can see below. EXCEPT... (:mad: ) ... the edge. It's blue! Now I know good and well what blue means. Blue means hot and hot means soft.
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What I don't understand is why in the world the edge would get HOTTER than the rest of the knife. It's all in the same 450 (or so) atmosphere for an hour. Why would the edge be hotter than the rest of the knife? Doesn't make sense.

Anybody clue me in?
Thanks,
Mike
 
Pretty simple actually. Because the edge is thinner.Thinner cross sections will always heat faster and the blade should not be laid flat as you have it.Prop it edge up and watch your tem pering closer.Dave:)
 
But what I'm getting at is this: why would the edge end up hotter than the oven atmosphere? I know it will get hot sooner than the rest of the knife. No problem there. But why hotter?

The blue color should come between 560 and 600 (at least according to this chart: http://www.anvilfire.com/FAQs/temper_colors.htm).

Even if my toaster over is inaccurate 25 degrees plus or minus, it still should have been a long way from blue.

That's what's got me puzzled: how the edge ended up no less than 200 deg hotter the temp setting on the oven.

(Thanks for the quick reply though!:) )
 
Maybe sitting too close to the heating element? Heat in the toaster oven might not be uniform, as there's not much air circulation?
 
MSCantrell said:
But what I'm getting at is this: why would the edge end up hotter than the oven atmosphere? I know it will get hot sooner than the rest of the knife. No problem there. But why hotter?

The blue color should come between 560 and 600 (at least according to this chart: http://www.anvilfire.com/FAQs/temper_colors.htm).

Even if my toaster over is inaccurate 25 degrees plus or minus, it still should have been a long way from blue.

That's what's got me puzzled: how the edge ended up no less than 200 deg hotter the temp setting on the oven.

(Thanks for the quick reply though!:) )
Click to expand...

I think you need to put a sheet pan (or something) between the heater elements and the knife. The radiant heat is much higher than the ambient or atmospheric heat in the oven. Protect the knife from the direct heat of the elements.
 
My old toaster oven would turn the edge blue if the blade went in when the oven was turned on and elements were blazing. But if I waited and put the blade in after the oven had reached temp (400-450) no problems, just a nice straw color. So if the blade is in the oven when it coming up to heat, it can over heat the thinest part of the blade, those elements are putting out a lot of heat until your tempering temp is reached. I have also noticed this on my large HT oven but not as bad.

Check the edge on that blade, it may be OK.
 
Hi,
Nice to see a thread that follows my experience.
First drill a small hole in the side of your oven and insert a Brownell's lead thermometer - looks like a cooking dial thermometer only goes higher. I have found that yes, the oven temp can be off by that much. On top of that, it is not consistent. I tried about 10 or 12 ovens and found none of them to be consistently accurate. The ones that came closest were hardly ever touched in use. Next, have the oven up to temp before putting in the blade. This cuts way down on the direct infrared heat exposure to the element that was mentioned earlier. I have tried having a piece of square thinwall tube in the oven, but I didn't see much difference. Heavy wall pipe in a forge works wonders for heat treat. Even cooks do not trust built in temp dials on ovens.
The thermometer trick allowed me to use the family toaster oven as a precision tempering oven. Did I forget to mention the OWMBO never saw the hole? She worked evenings also.
Buzz the edge and brass rod it again. It may be ok. I had several blades discolor on reheat because of the contamination from testing and also I suspect from "loving" the edge too much with oils from my thumb. Several passed with flying colors despire a bit of Blue. I consider the colors mid way up to be a better sign. The fact that the colors exactly follow the grind makes me think oxidation of contaminants.
The fact that you are testing shows you really care and are going to be making some mean blades...Ken
 
I think the edge has been contaminated with oil or something else.... Always check your toaster oven with a thermometer. A brick or similar will help keep the temperature more uniform.
 
If this was done in a toaster, I would put my money on the thin edge gaining greater temp through radiated heat off the element which can accumulate on a part through direct exposure and result in an object being hotter than the ambient tempreature picked up by the thermostat. (or some similar phenomenon).
For example, I would refer to those radar-dish radiant-heat space heaters. If you stand in front of one of those, they'll burn you right where they're pointed, but the rest of you will be cold. They're good at heating objects you point them at, but not to good at heating the ambient air (compared with a regular fan/element heater).
It seems possible to me that if there's direct line-of-sight with an element, you can exceed (average) chamber temperature on a metal part with diverse cross section.
 
The temperture control in those toaster ovens is actually pretty crude and are not are not likely to be accurate nor consistent. It regulates the average temperature ty turning the heating elements fully on then fully off. The interior is probably smaller than your heat treating oven, making the temperature swings worse. R Coon is probably right about providing some shielding to reduce the exposure to the high heat.

Taking SimonSK's suggestion about using a separate thermometer one step further, it should be possible to use an inexpensive temperature controller (from e-bay) to control the toaster oven. It should be a big improvement.

Phil
 
So what I'm taking away from this thread, considered as a whole, is this:

1) Preheat the oven and

2) Shield the steel.


Now, followup question... does the shielding need to be something fancy, or can I just wrap the blade in, say, aluminum foil?

Thanks a ton guys,

Mike
 
R.Coon-Knives said:
I think you need to put a sheet pan (or something) between the heater elements and the knife. The radiant heat is much higher than the ambient or atmospheric heat in the oven. Protect the knife from the direct heat of the elements.
Click to expand...

I think this is the answer to that quesiton.

If I had read R.Coon's post first, I wouldn't have bothered to make my post since it says pretty much the same thing.
 
this happens with me as well, and what i have found is that the ellemets dont have any set setting there on or off. so the stove says hay the temp is 350 and it needs to be 400 so i need to turn on the ellements. that inturn has a radenat heat of what i think is 1200 at leastes on my stove, then when the air temp reaches 400 the ellements turn off. so you can see the if something is real thin it will heat way up way to hot befor the elements turn off. i have been thinking about geting a steel pipe with caps with a small hole in one end. then put the knife in the pipe and then in the stove, that should even out the heating cycle its like puting a cap inline with a dc motor it smothes out the cycles. my 2 cents
 
Test it, personaly since the blue is following the edge grind I suspect contamination.

What I do with my toaster oven is I've got a pan within a pan. Basicly I've got a drip pan and then in it a about 1/2" up is anouther pan with a bunch of holes in it. If I clean the blade and don't touch it I get a nice even color. Also insulating the top and bottom helps mine hold the heat and even out some of the temp. extreams. Insulation is on the outside, I've got a wallyworld cheapo. Also let it cycle at least two or three times and you'll get more acurate temps.

Worst case just grind the edge off past the blue and regrind the edge and test again, it should make a good shop knife at least.
 
I really don't think the problem is uneven heating, but rather more along the lines of what mete said. If the finish is not the same all over the blade, the colors will not be the same all over. Remember the colors your seeing are caused by oxidation. The deeper the oxidation, the darker the colors. Notice where the blade is darker, the grinding scratches are deeper. There's someplaces where the very edge is lighter in color, because you ground it finer, then it gets darker. Regrind it, this time making sure you get out all the deeper scratches, and it shouldn't do that.
If you test the blue part with a file, does it feel softer than the yellow parts?
 
Phillip Patton said:
I really don't think the problem is uneven heating, but rather more along the lines of what mete said. If the finish is not the same all over the blade, the colors will not be the same all over. Remember the colors your seeing are caused by oxidation. The deeper the oxidation, the darker the colors. Notice where the blade is darker, the grinding scratches are deeper. There's someplaces where the very edge is lighter in color, because you ground it finer, then it gets darker. Regrind it, this time making sure you get out all the deeper scratches, and it shouldn't do that.
If you test the blue part with a file, does it feel softer than the yellow parts?
Click to expand...

This is not bad advice.
I've done a lot of nitre blueing (which uses heat and salts) and some open-kiln 'Fire Bluing.' It is true that finish will affect the color, but (in my experience anyway) is not as big a factor as the actual application of the heat.
Contamination is also a very good culprit to mention, but my experience with that never seemed to involve an edge.
Of course, the experiences of other will doubtless vary.
 
Very simple. If the edge and a tip of a blade was dirty or just "more black" they will absorb more heat energy from the heater/ In the toster-roaster oven the main heat transfer is by means of IR radiation/ So the bright, more reflective part will get less energy. More dark surface will get more energy. In your case it looks like it eas a match of dark surface and thin metal.
My advice is to wrap the blade in fiberglass rug or some foil. Or put the blade in 2x2" metal tube.
 
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