Is scorched blade trash?

R

Ricky-O

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Oct 30, 2021
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While grinding bevels on some AEB-L after heat treatment I created some areas with brown scorch marks. These brown areas disappeared as I continued grinding the bevels, but I don't know how it affected the hardness of the blade (and I don't have a means of testing it). So, should I bother making a handle for it, or should I just trash it and chalk it up to a learning experience (passes too slow or too high a belt speed)? Although I certainly also need practice in making handles, I don't want to waste time and material on a knife that won't work well.
 
The discoloring means those areas got too hot and are softer. The surface of the steel reacts and shows the colors, so even if you grind the colors off, the steel is still soft underneath it. It would need to be re heat treated to be a viable knife. You may be able to reprofile the knife to get back into harder steel, but it's hard to know exactly how deep the discoloration and softening went, especially after grinding the colors off. If it was the tip, you could grind back a bit and shorten the knife, or narrow if it's only along the very edge.
 
A file should give you at least an idea if it's dead soft. Hit an area you know to be full hardness, then the suspected soft area...there will be a difference in the feel/material removal if there's an issue.
 
Color is VERY had to tell exact temps, but the color charts indicate a brown color is around 500°F - per the Alpha Knife Supply AEB-L charts that temp will give around 56 to 58 Rc. While it's softer than desired, for sure not a "trashed" knife. Even a purple to blue color "should" keep hardness above 55 Rc which is still harder than most factory knife.

Understand, this is based on colors which are VERY unreliable, but color is what old time blacksmiths used all the time for HT'ing.
 
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Here's a picture of the colors, if you don't have this already:
EKRdMi8.jpg
 
Looking at the color chart, my guess is that my scorch marks were about the brown listed at 400 degrees F; it was pretty light brown. So maybe it's not as bad as it could have been????
 
Depending on the Rc hardness before grinding you might not have lost much at all. How what the AEB-L HT'd and what temp it was tempered at. Here's a link to a HT hart for AEB-L and you can see where the typical Rc is for AEB-L - look way down at bottom of page for chart. Using the good color chart Weo posted and this chart you can get a good estimate as to cRc hardness.

www.alphaknifesupply.com

AEB-L Stainless Steel • Alpha Knife Supply - AKS

AEB-L Stainless Steel For Sale • Largest variety of thickness, lengths and widths of AEB-L Billets and AEB-L Bar Stock • Fast Shipping & Excellent Prices!
www.alphaknifesupply.com
 
The source of the AEB-L is also a factor. I get mine from NJSB and 1975 + Cryo got me 66 to 68 HRC on my Wilson Hardness tester (Dead on a 64 HRC Magnacut blades from Bos heat treating and a 63.6 test block) after 2 tempers at 305 degrees. Had to temper it down a bit more to get to 62-63. So it will also depend on the starting hardness, too. Lower tempers means it's easier to overheat!
 
Taz said:
The source of the AEB-L is also a factor. I get mine from NJSB and 1975 + Cryo got me 66 to 68 HRC on my Wilson Hardness tester (Dead on a 64 HRC Magnacut blades from Bos heat treating and a 63.6 test block) after 2 tempers at 305 degrees. Had to temper it down a bit more to get to 62-63. So it will also depend on the starting hardness, too. Lower tempers means it's easier to overheat!
Click to expand...
That's really high for AEBL. Is there anything different in the composition to to other AEBL providers?
 
I was pretty surprised myself, so I checked the tester against my test blocks and other known blades and it was reading correctly. I got this batch of steel back in December of 2012, so I am not sure if it was a different supplier back then? Buderus vs Uddeholm VS chinese import of the same recipe, or variance in batches could be the reason? I've gotten 66 out of the quench on 52100 following KSN's recipe and just over 66 with 80CRV2 as well. The hardness on the AEB-L went up after a couple tempering cycles at 300 (it was around 64-66 out of the cryo before tempering). Got up to 66 with Magnacut recently as well after 2150, cryo and tempering at 305. I have a digital thermometer in my kitchen oven when tempering so I can monitor the temp more accurately until I get my tempering oven built.
 
It would be interesting to see a study of tempering (hardness, perhaps, and microstructure, other tests?) versus time at a given temp. How quick is the transition to tempered martensite? Is the few second duration at elevated temp that scorches sufficient duration to through-temper that area of the blade?
 
I have not done those tests, but my experience has been that a second at 450°F does not ruin the edge. Maybe you have to gerund it back a millimeter, but ruined - NO.

Not knowing the OP's HT parameters, AEBL source, and hardness readings pre and post temper make any speculation about the current hardness just a guess.

Aside-
I don't get the new trend for Rc64/65 blades. They are not really superior to Rc61/62 blades except in the lab. In a home kitchen I doubt most people could tell one from the other.
 
You have to consider how quickly the heat transfers through the steel. You can overheat a spot on the edge but there's not enough heat or time to affect much beyond that spot

Stacy E. Apelt - Bladesmith Stacy E. Apelt - Bladesmith i agree. I think the trend for 64-66rc blades is a marketing thing, plus they often are quite wear resistant steel. Which sounds like a pain in a kitchen knife, i would rather something i can touch up the edge quickly
 
I was surprised at the hardness and had to temper them back a few times.

I do like Magnacut at 64, especially on a kitchen knife but run most steels around 60-62, depending on the steel and useage.
 
weo said:
Here's a picture of the colors, if you don't have this already:
EKRdMi8.jpg
Click to expand...
Those colors are from surface oxides typically on carbon steel. AEB-L is a stainless steel formulated in part to resist oxidation. Temper colors on stainless likely don’t compare well to colors on carbon steel.

Finish the knife and test the areas that changed colors by cutting. Likely you won’t see much difference.
 
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Good point - I'd not considered the difference in temperature color between carbon and SS like AEB-L
 
me2 said:
Those colors are from surface oxides typically on carbon steel. AEB-L is a stainless steel formulated in part to resist oxidation. Temper colors on stainless likely don’t compare well to colors on carbon steel.
Click to expand...
Good to know, thanks.
I learned my one thing today, now I can sleep easy.
 
It sure would be nice to find a similar color chart for SS. Perhaps the different SS alloys would give different colors at each temp? This is a whole new ball game I expect.
 
It might be the same. There are good reasons why it wouldn’t be, but to my knowledge, no one has done a test on it. Also involved are any oils or contaminants on the surface. A blade with some quenching oil still clinging to the surface, even after being wiped down, will show way different color patterns.
 
If color is all you have, use it. It’s a poor substitute for a thermometer though. I’m not one to think a very brief time at a slightly elevated temperature would do much to the bulk of the blade, but I’m also one to believe sharpening on an uncooled belt sander will nearly always cause overheating of the very edge, even though you can’t see it.
 
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