Post heat treat stain problem, ATS 34

jonnymac44

jonnymac44

Joined
Sep 27, 2007
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I just got some blades back from heat treat and got a tupperware ready to soak the blades in vinegar to remove some of the scale before finishing like I've done with previous knives. Well, two of the knives in the vinegar bath got overlapped, one on top of the other. When I went to start finishing work on one of the blades the outline of the handle holes from the knife on top appeared on the blade from the knife on bottom the blade is sanding fine except for these dark circles where the other knife sat on top. Sanding at 280 grit has not made any dent.

Does anybody know why this happens? I guess I've learned not to let two blades touch in the vinegar? Should I only soak one knife at a time? Is there a known way to remove the stain. It's really detracting from an otherwise decent looking blade. I don't want to do any more grinding if I don't have to and Im not sure how deep these stains go.

I'll get a picture up in a bit......:(
 
I have never had that happen or heard about it before but you could try a cotton buffing wheel and some ruby dust, or ruby compound, or whatever its called (The red block of polishing compound for your hardest steels). My dad said it would take out a lot of hazing from sanding so I assume it might take the stains off or at least lighten them up. Can't hurt to try because it'll just shine the knife up even more. I also don't think they would penetrate very far without eating away at the blade, but then again stranger things have happened. Good luck!

Also: Have you tried a medium de-burring wheel? It only takes a little but off at a time and won't change the surface unless you hold it in one spot for a while.
 
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Thanks, I went ahead and tried gently grinding the blade with a 120 grit J-flex belt. It got the stains out. I'll have to remember to not let blades touch when in the vinegar. I'd still like to find out why that happened, what causes it.
 
Well I can come up with two theories off the top of my head.
1. The steel was dissolved at different rates becase of the overlap and didn't cause a stain per-say but didn't dissolve the scale completely. This is a grotesque and really a bad example but it's like how the A-bomb at Hiroshima burned people's shadows into the sidewalk because the light had different intensities (my apologies to anyone offended by this reference, it's solely meant for a scientific example).
2. (The more tame one) The scale build up inside the holes settled and while the vinegar was eating away the metal it leeched in before the acid was neutralized.
 
Its probably due to a difference in electrode potential between the two blades even though they are the same alloy. One blade will corrode faster than the other if they are touching each other because one will be sacrificed while the other remains uncorroded.
 
What you have is one that etched and one that didn't ( partially). That is not a problem, and happens all the time. I usually just re-soak the spotted/stained blade by itself for a while.

What you are doing wrong is expecting blades to be ready to fine sand when they come back from HT.
ALL blades should to go back to 120-240 grit to remove a few thousandth post-HT. This is why you don't take them to final dimensions pre-HT.
On carbon steel blades it is removing the layer of scale and decarb. On stainless blades it is removing the oxides and very thin decarb layer. In both cases the good steel is a few thousandth of an inch below the skin. If you don't remove it, you will not make the knife reach its potential for looks, and in some cases, performance.
 
Stacy,
Thanks for the info. I started realize that a little bit myself while working on the stain with the grinder at 120. I started saying to myself, Now that's looking a whole lot better than I'm used to!:D I appreciate the advice:thumbup:

bladsmth said:
What you have is one that etched and one that didn't ( partially). That is not a problem, and happens all the time. I usually just re-soak the spotted/stained blade by itself for a while.

What you are doing wrong is expecting blades to be ready to fine sand when they come back from HT.
ALL blades should to go back to 120-240 grit to remove a few thousandth post-HT. This is why you don't take them to final dimensions pre-HT.
On carbon steel blades it is removing the layer of scale and decarb. On stainless blades it is removing the oxides and very thin decarb layer. In both cases the good steel is a few thousandth of an inch below the skin. If you don't remove it, you will not make the knife reach its potential for looks, and in some cases, performance.
Click to expand...
 
you should not need to soak ATS-34 after H.T. if it was done properly . I never have in 12 years . There should be NO scale .
 
That's what I was thinking Jack. I usually have only a little tarnish that comes off easily with 400 grit paper. Even on D2 and we know how hard it is to grind/sand that stuff after H/T.
 
Scale - No
Staining and oxides - Often.

I normally don't soak them unless there is a reason to etch the blade, as the post-HT grinding/sanding removes all surface colors anyway.
 
I have an issue along these lines. I Heat Treated the D2 blade I'm working on in a shop furnace and someone left Aluminum in it, which I didn't see until it melted to the blade, and I'm noticing that in my hollow grind the oxidation and mild pitting seem to go on forever. I assume because the two metals have a different charge the Aluminum ate into the D2 (I have one nasty spot I think is just to deep to get out, so I'll have a part Aluminum knife); so, I feel your pain. Looks like I'm going to be spending a few long days hand sanding because I still can't make a sanding wheel that stays together for more than a few seconds ; to poor to buy one or buy a new cotton buffing wheel to make one. Just an FYI, if you use polishing compound on a buffing wheel, forget about gluing sand to it, its just pain messy and a waste of time.
 
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