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Heat Treating and Metallurgy Discussion of heat treatment and metallurgy in knife making.

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  #1  
Old 01-03-2014, 11:09 AM
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racinca racinca is offline
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Tempering mistake

I forged, normalized, rough ground, and hardened a 5160 blade. Then I put it in the oven to temper for one hour at 400 degrees. After cooling back to room temperature I put it back in the oven and set the timer for one hour, but I forgot about it and went to bed. It actually stayed in the oven for about 10 hours.

After the first tempering cycle, it came out a perfect dark straw color. After the second (really long) cycle, it came a purplish-brown color which should indicate that it was tempered to something around 500 degrees, but the oven was set to 400 (and it actually runs about 390 according to my previous checks with thermometer).

So could the long time oxidize the steel to a higher temp color due to the long time in the oven or did the oven actually get up to 500 degrees at some point? Do I need to re-harden and start over?
Tony


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Old 01-03-2014, 12:28 PM
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R. Yates R. Yates is offline
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Scrap it and start over! It is trashed ! Chalk this up to a loss and lesson learned (Never go to bed with a knife in the oven .)

Sam


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Old 01-03-2014, 02:59 PM
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Eli Jensen Eli Jensen is offline
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I'd bet its fine. Oxidation is a function of temperature and time, so if its in there forever, you'll get those colors usually indicitive of higher temps. Its actually pretty common.
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Old 01-03-2014, 05:24 PM
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GHEzell GHEzell is offline
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Oxide colors are notoriously deceptive. Time is not nearly as important as temperature once the bare minimum is reached. Odds are very good you have not adversely affected the hardness of the blade. Wrap some duct tape around the handle and test it by cutting various materials (whittling oak heartwood is a good test), if it preforms to your liking, finish it.


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Old 01-03-2014, 06:38 PM
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racinca racinca is offline
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Thanks guys. I think it's ok. I sharpened it and chopped into a 2x4 pretty good with no edge deformation. I also rolled the edge over a 3/32 rod - it flexed as it rolled over but left behind no deformation.


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Old 01-04-2014, 05:28 AM
Ed Tipton Ed Tipton is offline
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Racinca...I usually give my blades 2 hours at 400 degrees. I temper in our kitchen oven (electric), and I wrap my blade in aluminum foil to help shield it from the direct radient heat from the heating coil, and I place it in the upper part of the oven to get it as far as possible from the heating element.
Your blade is probably OK, but technically speaking it was in too much heat for too long. The amount of time spent in the correct heat really shouldn't matter provided it is in long enough. An overnighter should not hurt your blade. It sounds as though you may have placed the blade a little too close to the heating element which would explain how it got too hot. The heating element gets to red hot indicating that the element itself is considerably hotter than 400 degrees. Try wrapping your blade in some aluminum foil and then place it as far as possible from the direct heat of the element. If you could take temperature readings at different places in your oven, you would find considerable differences in temps.

Last edited by Ed Tipton; 01-04-2014 at 05:33 AM.
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Old 01-07-2014, 07:57 PM
metal99 metal99 is offline
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Quote:
Originally Posted by Sam Cro View Post
Scrap it and start over! It is trashed ! Chalk this up to a loss and lesson learned (Never go to bed with a knife in the oven .)

Sam
Why so quick to say its garbage Sam? He could have scrapped a perfectly good blade man!

Tempering colors shouldn't be something you depend on. A good thermometer and a hardness test are something that can be depended on.


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Last edited by metal99; 01-10-2014 at 09:33 PM.
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  #8  
Old 01-10-2014, 02:02 PM
samuraistuart samuraistuart is offline
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Yep.....knife is more than likely A OK.
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Old 01-11-2014, 08:21 AM
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Gary Mulkey Gary Mulkey is offline
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Quote:
Originally Posted by racinca View Post
Thanks guys. I think it's ok. I sharpened it and chopped into a 2x4 pretty good with no edge deformation. I also rolled the edge over a 3/32 rod - it flexed as it rolled over but left behind no deformation.
GOOD FOR YOU!. We would probably all be better off to test our blades more often. The 2x4 chop & brass rod test are two that will give you a good idea of what you have. I'll occasionally test one to destruction just to know that it's where I want it. It's often hard to trash something with extensive time invested in it but without doing it once in a while, you can't be confident in the quality that you are turning out.

Nice job.

Gary


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5160, art, back, blade, blades, brass, common, edge, forged, handle, heat, hot, knife, man, materials, rod, steel, temper, whittling, wood, wrap


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