Problem finishing A2

M

Mark Redmon

Joined
Nov 7, 2007
Messages
451
Hi All,
I'm having some difficulty with an A2 blade of mine. I resanded and re-buffed this one several times and I keep getting these little pock marks on the blade. I haven't had the issue with the ATS-34 blades or 304 bolsters I've been making at the same time using essentially the same process. I really wanted this one to be a mirror finish. It was pretty hard to get a good photo of the issue, so I apologize in advance for the photography. Any thoughts?

You can see it around the reflection from the light, but it's like that all over the blade.
A2Prob1.jpg


This was through my stereo microscope at 10X

A2Prob10X.JPG


Thanks for any help/suggestions!

-Mark
 
That looks like impurity's in the steel or you have a piece of crap stuck in your buffing wheel?
 
Although it is nowhere what you are looking for, I think it is kinda cool. Exactly when does it appear? When sanding up to a high grit, or only after buffing? What grit do you sand up to before buffing?
 
I'm pretty new at this, so I can't rule out anything in my method. (I'm open to suggestion on that too). I try to be careful about keeping the wheels seperate in their own ziplock bags, but I guess it's possible something coarse got on the wheel. I'm starting to wonder if it has something to do with the compund. I hand sand up to 400 then go to a green blending compund. I didn't go to white yet on this one. I don't notice it after sanding, but my shop lighting is not that great. I might sand it back again and see if I've got a new wheel around still...
 
I had a condition like this last winter when I quenched an unknown steel in oil that wasn't preheated. There were little pock marks all over the surface. It appeared as though thousands of tiny little explosions had occurred during the quench. There weren't any marks on the tang, which wasn't quenched. I didn't notice them until I got down to about 400 grit sand paper.

I've never made a knife from A2, but I do know that it is air hardening. Is that how you hardened it? Quenching may cool it too fast, which in turn could induce problems. But don't quote me on that. I'm sure the experts will chime in.

Although, all the marks in the photo are oriented in the same direction, which indicates your sanding/buffing process is the best place to start troubleshooting.
 
It looks like there are little soft spots in the steel, which are removed faster than the surrounding metal. Did you soak it at the correct heat for at least 15 min? Did you protect it from oxygen while at heat or grind away the decarb? Did you wildly overheat the metal?

Throwing darts here...
 
Thanks for the input guys. Those are some interesting thoughts... I should mention that I don't do my own HT yet. I've been sending them out to TKS. I can't seem to do anything to get rid of the little dimples... makes me think Nathan might be on to something with the small soft areas. I started back at 220 tonight, but maybe tomorrow I'll try going down to 120 and working up again. I guess if that doesn't work, I might just put an edge on it and put it through the paces to see if it's worth finishing...

-Mark
 
One goofy thought I had (since I have heat treated over a few hundred blades in A2) was that, is it possible Texas Knifemakers Supply got the steel confused with another steel and heat treated it incorrectly (because they thought it was a different steel type?)

Possibly too many orders to heat treat that day !

I'm 100% positive they would heat treat their A2 blades in a kiln (Evenheat I think) and unless your steel you're using isn't actually A2, the results should not have that after a proper heat treat, even after an improper heat treat with a torch i've yet to ever see that in A2. Are any of the other blades like that or was that the only one?
 
I'm thinking a steel or HT problem,too. Wrong steel? Wrong HT?
If the HT was soaked long enough to allow the alloy elements to distribute and combine properly, it can make a sort of hard-soft mix, with little hard balls of carbides. These micro balls look like gum-balls under the scope. They catch the polish and buff,and may pull out in polishing, leaving behind a little crater .

Try this.
Sand it back to 220 , then go up through the grits slowly until you get to your finest grit (about 2000-2500 is good). Now go back and wet sand from 1000 to 2000/2500 again. After the wet sand, clean everything well with soap and water. Hand polish with Flitz a bit and see if the spots are there ( you can hand polish to the finish if desired). If all looks good, machine buff ,starting with black compound, then move to gray, and finally to pink/white/green ( which ever you use. I'd suggest white or green for A-2). The pock pattern that develops in polishing is called orange peel . It can come from little balls of polish building up and pulling out a small amount of metal as they separate from the metal. Some metals are more prone than others. The more complex the alloy (more carbide balls) the easier it does this. Using a CPM steel can avoid this

Some tricks to avoid orange peel are: Charge the buff often ,but lightly with compound.It should not have a hard coat of polish. Use a buff rake if needed to remove any caking of the polish on the buff. Avoid having the compound build up on the blade. Don't apply too much pressure when buffing, let the compound do the work. A carrying/flowing agent can help a lot.The tallow and other carriers in many types of buffing compound are there as lubricants, but may not be enough. The old guys used a brush with kerosene, but WD-40 will work easier. After charging the wheel, give it a very short burst of WD-40 and then polish. The results can be phenomenal. Be advised that your grip will not be the same with this method, and if you are not skilled on the buffer, I would skip it. Finally, don't bear into the blade when buffing. It can heat the surface much hotter than it feels. Smearing and pocking are signs of too much pressure.
Hope this helps.
Stacy

Note on safety:
Buffing is inherently dangerous. Using any compound that make a good grip less secure just makes it worse.I included the lubricant trick for information, but don't recommend it to newer polishers. If you use it, just lightly lubricate the buff. Polish slowly and keep a firm grip on the blade.

Think ahead ,and avoid placing the blade to the wheel in any way that it might catch.

Polish with gentle pressure, and unless you are really skilled, avoid high speed and large wheels. I throw a 10-12" buff at 1800RPM - but I polish things for a living, and have for a long,long time. A 6" wheel is a good starter buff, especially if you only have a 3600RPM buffer.

Stacy
 
I guess anything is possible since the blades were out of my control.... I sent out some other A2 blades, but haven't had a chance to work on them to see if they exhibit the same problem. I did notice that they cryo'd my entire batch (instead of just the ATS-34)... I hope they didn't HT them all as ATS-34 since the blades were clearly marked.

Stacy, thanks for the advice. It's pretty cold in the garage/shop, but maybe I'll layer up and give that a shot tonight. I'll keep you posted. I'm pretty comfortable buffing these days, and I always try to be careful, but you can never be TOO careful. My "buffer" started out as a variable speed bench grinder that runs from about 2k to 3500. I've been running it towards the slower end for most things.

Thanks for all the help guys!
-Mark
 
I do quite a bit of A2 some I have HT myself and other are done by Paul Bos, have not seen the issue you are having. Would like to hear if you find the cause.
 
It almost looks like a high sulphur ,free machining grade of steel where the sulfides have been pulled out.
 
It's generally called "orange peel", and usually the result of
over buffing.
It can usually be avoided by keeping the buffing wheel clean,
using the right compound, speed, pressure for the steel you're
using.
Not all steels respond well to the same compounds/techniques. It's
sometimes necessary to buff in two or three stages. I used to
go from gray to white to green to get an acceptable finish on
D2.
 
This may have been cause by buffer wheels having steel dust particles on them which will cause nice shiny scratches. But when you run 250 knives through a oven at one time who knows what you will get. That's one reason to do your own heat treat and temping yourself, in doing so you know what's going on. But this is just my opinion. :yawn:

Terry
 
I will agree with Russ. Wrong speed on the buffer with green compound,too much pressure will cause that look in almost all steels. Sand her down again and go to your white compound and skip the green. Hope this helps.
 
I have had a hell of a time with ATS-34. It does not seem to sand or file down.
I either use to fine a grit of sandpaper or file and just burnish the scratches or with
to heavy a grit which with the file especially just put huge gashes in the steel.
 
The tear-drop shape of the pocks all in the same direction suggests you may be buffing too much from one direction, also. Buffing should be done from all (safe) directions, moving constantly.
Some very good diagnosis and advice in this thread.....

Unsub, "large gashes" with filing can be caused by galling from not keeping the file clean enough. Hand sanding is best accomplished by working ones way through the grits sequentially (different direction each grit), carefully removing all the scratches from the previous grit before proceeding to the next. ATS34 has a lot of chromium and molybdenum, making hand finishing significantly more difficult than with lower alloy steels.
 
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I really appreciate everyone chiming in on this... I'm feeling a little burnt on knives right now, so I'm not going to force it. I'll probably get back to this one sometime this weekend. I'll let you know what I find. Thanks again, everyone!
-Mark
 
I seen something like this years ago on a part that I made but I am not sure that this would be the problem but more of a thought...The part was 420 ss and it called out for passivation (vinegar soak?) but it looked almost identical to this...Anyway....I'm sure I am wrong...Perhaps Mete can inform me a bit better?

Bill
 
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