No edge on AUS6 blade

[crispy]

I bought this knife because it looked interesting, but Im a little dissapointed. Its a CRKT F4 neck knife in AUS6. I sharpen all my knives free hand on an oil stone, and I can get them all shaving sharp. My 154cm mini grip is easiest to sharpen. But try as I might, I CANNOT get an edge on my CRKT F4.

It seems sharp at first, I can pop of one or two hairs off my arm. Then 5 minutes later it can't cut butter. Im sure its not my technique as all my other knives are a breeze to get sharp, could it be the AUS6 steel?(actually, it probably is my technique:foot: )

Any ideas???

Thanks

 

[Cliff Stamp]

I have found the softer stainless steels, an in particular the AUS series to be the hardest knives to sharpen. Steels with very similar compositions like 420HC and 12C27mod don't have the same extent of problems usually. It could be the vanadium in the AUS series, or just that they are found often on really inexpensive knives and maybe it is a heat treat issue related to grain size.

In any case I would bet there is a significant burr. If you examine the blade under a decent light by turning the edge until it catches the light the burr will stand out as a bright line. Under even low magnification it is easy to see as well. Radio Shack sells small hand held microscopes which show that type of problem easily. The burr will be visible as a strong shadow.

To start slice the edge directly into the sharpening stone, a couple of light passes. This grinds off any weakened metal and gives you a known point to start. Hone as usual until it can slice a piece of fine paper. To remove the burr increase the angle and use *very* short strokes and alternate sides. The worse the burr problem the higher the angle needed, it could easily be double the sharpening angle or more for really burr heavy steels.

The amount of pressure is critical as well. Too low then you won't get enough cutting action, but too high and the edge will just fold over. How much force you need to apply depends on the steel and the shape of the stone and the curvature of the blade because the actual relevant issue is the pressure on the edge. This might sound like a lot of variables but in general it only takes a few attempts to figure out.

As a further complication, it isn't uncommon for knives to have problematic steel on the edge initially and the sharpening characteristics can improve with time. I have seen some people claim this is a sign of "quality" steel but this is false, it is just due to problems with heat treatment. As a user it just means that you should wait until a knife has seen a few sessions of sharpening and heavy use before you judge the ability of the steel.

-Cliff

 

[DGG]

Isn't that a form of "work hardening" when the metal hardens through use? I really don't understand how this happens except in maybe the case of a hammer or something under high pressure contact.

 

[Cliff Stamp]

You can work harden just with cutting because the edge will bend back and forth. You can see this with a piece of paper, fold it down the center and see the difference in the effort to fold it initially and then after the crease was made. Now fold it again but now perpendicular to the first fold and note that is harder to do than on the uncreased paper. So you made it stronger in one direction but weaker in another. Steels will also get brittle when they work harden and the strain can transform austenite to untempered martensite which is a really bad thing to have in steel.

In general what is being see with edges changing in time is just the removal of steel which is either burnt with grinding, decarbed through heat treatment or possibly over soaked or cracked in quenching. It tends to happen very quickly, just a few sharpenings will see stable behavior and you don't need to use the knife at all. Most people who reprofile knives initially don't notice any change in behavior over time because then they reprofiled the edge then removed any damaged steel.

One of the good things about working with steels which do burr heavily like the softer stainless is that once you are no longer frustrated with them, sharpening almost anything else seems trivial.

-Cliff

 

[crispy]

Thanks for the reply Cliff. I did feel for a burr with my thumb, but couldn't really detect anything. I will have to buy a little microscope or a really good magnifying glass.

I'll let you know if I get a decent edge on it.

 

[kel_aa]

Are you sure your CRKT F4 is Aus6? They have been made with 420J2 for a while. As well, I suspect the hard plastic sheath may be contacting and dulling the edge as well.

I have one but gave up on it and haven't touched it in a long while. The little knife and huge sheath wasn't worth the hassel carrying or trying to sharpen compared other choices.

 

[STR]

Maybe you should see if you can get one of the makers or even a supplier like Texas Knifemaker supply to cryo treat that blade for you and then sharpen it. It might take it to a much better level for you. Cryogenic treatment would take it up a notch in hardness so it may be enough to help keep the edge once you get one.

STR

 

[crispy]

Are you sure your CRKT F4 is Aus6? They have been made with 420J2 for a while. As well, I suspect the hard plastic sheath may be contacting and dulling the edge as well.

I have one but gave up on it and haven't touched it in a long while. The little knife and huge sheath wasn't worth the hassel carrying or trying to sharpen compared other choices.

The guy at the shop told me it was AUS6. I hope its not 420.

Maybe you should see if you can get one of the makers or even a supplier like Texas Knifemaker supply to cryo treat that blade for you and then sharpen it. It might take it to a much better level for you. Cryogenic treatment would take it up a notch in hardness so it may be enough to help keep the edge once you get one.

STR

Not worth it STR. The knife was so cheap to begin with. I'll have to look around around for something similar in better steel. Any suggestions?

 

[Cliff Stamp]

As I note, I have emailed CRK&T on several occasions and always found them helpful. If you really can't sharpen it I would suggest discussing the matter with them. Since they don't have a forum here they are not going to be aware of problems and it would be more productive in general if more emails were sent to them linking to such threads so they could get some responce to their steel choices. However how much of their market Bladeforums represents is likely not great.

-Cliff

 

[STR]

I don't think it costs that much to have a blade cryo treated. I know it only adds $3.50 to a blade cost if you buy one from Texas knifemaker out of their catalog and ask them to in house treat it for you before shipping it.
Some of the guys here say it is a very good treatment and makes a difference. It was just a thought.
STR

 

[crispy]

I don't think it costs that much to have a blade cryo treated. I know it only adds $3.50 to a blade cost if you buy one from Texas knifemaker out of their catalog and ask them to in house treat it for you before shipping it.
Some of the guys here say it is a very good treatment and makes a difference. It was just a thought.
STR

I had no idea it was that cheap. Anybody know of heat treat guy in South Africa??

 

[Jeff Clark]

I don't think that the cryo treat will make a difference, but you could do it yourself. Just buy some dry ice and sandwich the blade between a couple pieces for a few hours. Do this in an insulated cooler. A cheap styrofoam cooler is OK.

I suspect that you just aren't sharpening adequately. Burrs normally don't prevent you from shaving, they actually make it easier. To see if you have a burr just strop the blade lightly on some leather or a pad of paper. If you have a burr you should be able to shave at this point. You will be shaving with the burr.

Your real problem is probably that you have not honed the edge thin enough and completely enough to have a fully accute edge. How coarse is your oil stone? A fine hone that does a good job of maintaining nicely ground thin edges will work slowly on improving an edge that is ground thick. Use a coarser hone or a lot of time to hone the blade to under 15 degrees per side all the way to the edge. At this point go back to your usual honing style. Don't really worry about a burr until you have the blade cleanly cutting paper and shaving somewhat.

 

[STR]

I don't think you can get to -185 to -305F with dry ice and a cooler Jeff? Can you? I find it hard to imagine thats possible.

STR

 

[kel_aa]

You are right, the coldest it gets is -109°F.

But I guess it'll work psycologically, as I've put SAK's in the fridge freezer trying to sqeeze a few more Rockwell's out of them (damn the retained austentite!)

 

[Cliff Stamp]

Martensite conversion can be 100% with dry ice, liquid nitrogen is argued for other reasons than hardness, usually specific carbide precipitation. Liquid nitrogen can also be a lot cheaper than dry ice.

-Cliff

 

[Jeff Clark]

Dry ice gets down to -78.5 C, or -109.3 F. That is cold enough for this purpose. Since I am not proposing that he dismantle the blade from the handle I wouldn't want to go any colder. I expect that he can find the dry ice a lot easier than he can get liquid nitrogen.

 

[gud4u]

The problem sure sounds like a wire-edge problem to me - initial sharpness, but dulling almost immediately due to collapse of the wire edge.

Even AUS6 or 420 stainless should be able to handle a 30 degrees included main bevel with a micro-bevel of 40 degrees included.

If the blade is as soft as it sounds:
- Form the main bevel until the burr/wire-edge forms.
- Completely collapse the wire-edge (cut something really hard).
- Form the more obtuse micro-bevel with light honing pressure.

Hard-pressure slicing into hardwood, cross-grain slicing into pine or slicing a hard plastic should completely collapse or fracture a wire-edge. Then form the micro-bevel, removing enough metal to repair the damage.

Hope this helps!

 

[Cliff Stamp]

The problem sure sounds like a wire-edge problem to me - initial sharpness, but dulling almost immediately due to collapse of the wire edge.

As an open issue, when doing edge retention trials I have noticed that the results are fairly random to a much larger degree than you might expect. For awhile I thought it was just cardboard variances, user influences, differences in sharpening etc., then lately after some discussions I started refining my sharpening and aiming for a higher initial sharpness. As a side effect of being very demanding on the initial sharpness the variance in the trials dropped down to 5% readily.

Now the main difference I noted when rejecting edges were burrs, by this I specifically mean edges which were uneven in shaving ability, cut well on one side and not on the other, and in each case I could see the edge bent even under light mag. Now I don't know what else you would call this if not a burr but it has some differences than usually described. For example the edge didn't often fold over immediately. On a 50 m cardboard run the edge would only crap out after say 10 or even 20 m, it was fine initially and cut close to expected.

Hard-pressure slicing into hardwood, cross-grain slicing into pine or slicing a hard plastic should completely collapse or fracture a wire-edge.

I tried some of these things and the burr described would remain, you could still see it but with extended cutting the edge retention would be low as in 1/10 of optimal. I think there are variations of burrs, some which immediately fold, but some which are smaller (formed differently?) and can resist most common methods of removal. I also think in reflection that one of the main reason some steels like 52100 are praised for edge retention is simply that they form clean and thus you end up comparing optimal results for sharpening that vs a burr on another steel.

Note I have used the cross cut thing on other burrs, to crack the edge off a machete for example. Just noting how the problem may be a bit more complex and of wider influence than generally thought. In regards to dry ice and liquid nitrogen, any university with a science program should have liquid nitrogen, you don't need much to cool a blade down. Precool the blade as much as possible as room temperature is really hot in comparison. Dry ice can be found in breweriers as a waste product you can usually get it for nothing.

-Cliff

 

[kel_aa]

Dry ice can be found in breweriers as a waste product you can usually get it for nothing.

Is this to combat global warming, condensing it rather than venting the gas to the atmosphere? Or do you mean "by product", and they actually sell it for money?

Do you want to test (in all or just the sharpening aspect) an F4, Cliff? I know you have done CRKT 420J2 before, so it might not be that interesting to you. You can have the whole knife and sheath rig, minus the beaded chain (transfered that to a Mora, much more deserving). Everything is like new except two light sharpening attemps with a Sharpmaker.

 

[Cliff Stamp]

Is this to combat global warming, condensing it rather than venting the gas to the atmosphere?

I doubt it, I never asked about the details. They didn't sell it, they just gave it away, they thought it was amusing. I meant to actually thank Labatt's in the papers published.

I know you have done CRKT 420J2 before, so it might not be that interesting to you.

Verification/duplication is always worthwhile just to verify that you are not seeing something far outside the expected range in either way. If you were inclined it would be appreciated.

-Cliff