Help needed: burned edge

Revolverrodger

Revolverrodger

Joined
Jul 23, 2007
Messages
3,880
While I was grinding the bevels of my knife (nitro v steel) I unfortunately overheated the edge and it changed color (burned)

What can I do to fix that?
Can I directly heat to 1900 and plate quench or do I need to add prior steps

Thanks for the help
 
If it never got past a dull red in color, you did not do anything that needs fixing (I haven't looked into/used nitro-v, but unless it is wildly abnormal this is true). Burning steel, in the proper sense of the term, happens well into the 2000s F range. What you are properly refering to is overheating/overtempering the blade, and refers to a reduction in hardness to a hardened blade that would hapen from overheating a section past your intended temper temperature.

If you had already heat treated the blade, such an error may require heat treating the blade again, grinding the overheated section out, etc. That all being said, I have tested this on a couple coupons of steel, and while I can't give precise HRC reductions, I can say that in the case where the steel was heated momentarily to a temp sufficient to create a blue area on the edge, I couldn't observe a reduction in hardness. Tempering is a temp vs. time process, and while temperature is the larger factor, I imagine that tiny area of the edge only held that temp for half a second at the longest. This may be the reason. I did the test on 1084, where a blue color should have equated to an HRC in the low 50s. A file should have chewed right through it and did not. Either way, I am not precisely sure why it didn't lose as much hardness as it should have. It could have been errors in my testing methodology, and as I used files for the test, it likely did lose a cople points of HRC in that area.

No matter my results, in this case, since it was prior to HT, you are OK.
 
that color is just a very thin oxide layer on the surface. comes right off with sand paper
 
You are over-worrying this. I often get the steel red hot in grinding. Minor blue and other colors along the edge are pretty normal. When getting the blade ready for HT, just sand the surfaces clean and the colors go away.

Post-HT is a different thing. While a slight blue spot on the edge ( usually referred to as burning the edge) that happens by accident while taking the blade up the grits may be survivable, any large burn is bad. It would usually require grinding the edge back a good bit, and can be as bad as requiring a new HT.
 
What Stacy said.
One thing that can be a problem pre-HT is warping in quench. I have found if I get a bunch of the tip blue, it almost always warps more in HT. Normalizing fixes this.
 
Low grits can go higher speeds.

High grit 100 and up slow it down. Often the burn comes from to high of a speed on a higher grit belt and probably the belt is dull and causing to much friction.

A fresh 36 or 50 grit belt can get the majority of the work done. After that use slower speeds and fresh belts to refine the grind.

Trying to get to much metal off with higher grit belts will often lead to burning your steel.

Also remember to use the lateral 1/3 of the platen more and do not drag the tip, in contact, across the entire platen. If you grind with the blade totally flat on the platen the tip spends more time in contact and gets more friction then the rest of the blade.

When you are in the refining stages then slow down the grinder and then carefully grind the blade flat with no gloves on to feel the heat to avoid burning the blade.
 
A very rough and basic look at the technology of grinding as it pertains to thermodynamics:
As grit size increases, the number of friction creating cuts increases by a power of 2 ( squared). That means that the difference between a 36 grit and an 80 grit belt is roughly 4 times the amount of cuts on the steel. Go from 100 to 400 and the increase in action goes up 16 times. This is because there are 16 times as many pieces of grit in a square inch. Even though the cuts are smaller as the grit size goes up, this leads to higher friction because of the vastly greater number of cuts.
The coefficient of heat of the steel is constant regardless of the belt grit, thus the steel heats much faster at higher grits.

You have to significantly slow the belt to keep the friction down to a controllable level. While slower belts help, wetting the belt/blade junction is the best way to lower the friction the most. Making a wet or moist grinder is the way to go for keeping the blade cool in post-HT grinding. Dipping in a bucket of water will help somewhat, but that water is gone in a second, and then the friction returns and the blade edge temperature rises rapidly.
The most efficient system for knife makers is a fine mist sprayed at the belt just above the blade by something like a Cool-Mist spray. Be advised that all belts are not made with waterproof glue.
 
Stacy E. Apelt - Bladesmith said:
A very rough and basic look at the technology of grinding as it pertains to thermodynamics:
As grit size increases, the number of friction creating cuts increases by a power of 2 ( squared). That means that the difference between a 36 grit and an 80 grit belt is roughly 4 times the amount of cuts on the steel. Go from 100 to 400 and the increase in action goes up 16 times. This is because there are 16 times as many pieces of grit in a square inch. Even though the cuts are smaller as the grit size goes up, this leads to higher friction because of the vastly greater number of cuts.
The coefficient of heat of the steel is constant regardless of the belt grit, thus the steel heats much faster at higher grits.

You have to significantly slow the belt to keep the friction down to a controllable level. While slower belts help, wetting the belt/blade junction is the best way to lower the friction the most. Making a wet or moist grinder is the way to go for keeping the blade cool in post-HT grinding. Dipping in a bucket of water will help somewhat, but that water is gone in a second, and then the friction returns and the blade edge temperature rises rapidly.
The most efficient system for knife makers is a fine mist sprayed at the belt just above the blade by something like a Cool-Mist spray. Be advised that all belts are not made with waterproof glue.
Click to expand...
Have you run anto any water soluble ceramic grit belts? I have only noticed the cheaper AOs decomposing. Then, below 120 in ceramics ceramics, heat never seems to be a problem regardless of belt speed or pressure.
 
Using a fine mist, there seems to be little problem with any good quality belt. IIRC, all the 3M ceramic and CZ belts are Wet-or-Dry use. When I bought directly from Klingspor, I just told the rep I wanted waterproof belts.

The cheap AO belts do seem to be done with fish glue or something soluble. Obviously, all paper backed belts are dry use only.
 
I run my VSM ceramics wet at 120 grit and they seem to do fine, but if they are not stored perfectly flat to dry, they warp and will never run true again. I did once make the mistake of getting an original Trizact wet. Basically ended up with grit goo oiled up all over the place and a ruined belt. Should have paid more attention to the belt info.
 
Stacy E. Apelt - Bladesmith said:
Yup, I did that, too.
I let the belts dry on the grinder if possible to prevent the curling.
Click to expand...
Grayzer86 said:
I run my VSM ceramics wet at 120 grit and they seem to do fine, but if they are not stored perfectly flat to dry, they warp and will never run true again. I did once make the mistake of getting an original Trizact wet. Basically ended up with grit goo oiled up all over the place and a ruined belt. Should have paid more attention to the belt info.
Click to expand...
With the mist I have been considering eliminating 36/40 grit and just kicking it off at 80 (or even 120) It seems to extend the life of belts enough to make this worth consideration, and I feel I can run the belt a bit harder (more pressure) with the added lubricity.

Has anyone tried running one of the cutting/grinding fluids in lieu of water? If so, is there a benefit? As I see it, the heat control is the major benfit from running water. It doesn't seem to make a huge diference in friction reduction, I figured one of the water based lubricants (may) perform better.
 
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