What in the fresh Heck is happening in my tumbler

[Huntsman Knife Co. LLC.]

Got a very frustrating mystery for you guys.

I stonewash most of my blades and have never had any issues.

I have a very nice Mr. Deburr tumbler and I use the highest quality Ceramic media. I run it wet with a little giant pump running a mixture of water and Kool Mist.

Recently Ive had 3 blades develop these insane tiny pits after tumbling.

I clean up my blades post heat treat and give all my blades a 400 grit shining sating finish before being tumbled. There is no way these are voids in the steel as its happened across both CPM 3V and Z TUFF and I would absolutely positively 100% see them during grinding and while inspecting the finish before tumbling. I've found voids in steel before and they don't look like this in my experience.

These pits look to me like some kind of corrosive action. The only time I've ever seen anything like this is during stain testing when aggressive black corrosive pitting occurs.

You guys have any guess as to what the heck is going on in my tumbler?

1. I never have any problems with rust and have often times turned off the tumbler and left blades in there overnight. Never any corrosion.
2. The media is all high quality and I cant reproduce a gouge like this even with a brand new piece of my sharpest media.
3. I have a new little giant pump, maybe it is throwing some kind of highly corrosive metal or something into the tumbler?
4. Maybe some kind of galvanic or corrosive action from a salt or maybe aluminum being liberated from the media as it degrades?

Send help.

Here are photos. I hit this blade with a fresh 220 grit belt for better visibility of the issue.

This one is straight out the tumbler, stonewashed finish intact.

 

[Stacy E. Apelt - Bladesmith]

Are you sure they are pits? Have you used a pin or fine point to see if they are actually deep, or just a surface spots.

 

[ea42]

I had something similar happen on some Sandvik and it wound up being voids in the steel. Had nothing to do with the media or tumbling process although the sludge and liquid may have washed the voids out a bit making them more noticeable.

Eric

 

[Huntsman Knife Co. LLC.]

Are you sure they are pits? Have you used a pin or fine point to see if they are actually deep, or just a surface spots.

I had something similar happen on some Sandvik and it wound up being voids in the steel. Had nothing to do with the media or tumbling process although the sludge and liquid may have washed the voids out a bit making them more noticeable.

Eric

They are little gouges. You can pin a needle into them.

I've dealt with voids before and I think its unlikely here because
1. I check inspect the blades so thoroughly during grinding that I would for sure see something like that so glaringly obvious.
2. Its not showing on any satin blades I've done from the same steel batches.
3. Its happening all over the bevel and flats. For PM steel to be that randomly riddled with voids would be rare.
4. I've finished dozens of blades from these same steel batches and havent had this pop up yet.
5. Its happening to two different steels from different suppliers in the same way.

So all that in mind, I think voids are incredibly unlikely in the first place across two steels, and for me to not notice them at all with such a fine satin finish would be unlikley as well.

I just threw in a test blade that I thoroughly inspected and sure enough, one hour into tumbling it developed two tiny, needle tip sized black dots. What the heck. Its so hot in my shop and with the friction from the stones, it must be 120 degrees in the tumbler. I wonder if these are just pitting out fast.

Hopefully i can get this figured out.

Edit:

For reference, here is a void a found a while back in a different Z tuff blade. What's happening here looks different than the voids I've encountered.

 

[ea42]

That really is strange. The second pic in the first post almost looks like remnant scratches from a coarse grit belt. Really odd. I can't fathom how something from the tumbling process could cause that, especially since nothing caustic is used.

Eric

 

[Kevin Wilkins]

Wash all of your media and your machine and try using only distilled water in the machine in the future.

 

[Stacy E. Apelt - Bladesmith]

It may be that some contamination has entered the tumbling media. Something very hard and pointed. It could be some sharp pointed grains that were in the media, or something floating around in the media, or some media that has carbide contaminates in it.

Suggestions:
Unload the media and wash the heck out of it.
Look for something that may not belong there.
Check and clean the drum carefully inspecting it for and rough spots or trapped media.

How the pits got there is only half of the issue to me. Why they show as black is the other. Is it possible that these have been around before but were not black and didn't show as well? Are the blades shiny ground steel, or black when they go in the tumbler?

Also, do all the blade in a batch get the same pits, or only some of them?

BTW, what model Mr. Deburr do you have?

 

[Crag the Brewer]

Why not in the flats of your saber grind up near the spine?
Why not near the edge?

Are you grinding and tumbling soft?

Or are the blades already hardened?

Is your grinding fixture some how maring your finish if clamping?

If it was chemical or in your tumbler, id think it would be in more spots on your blades, not just that one area

 

[Huntsman Knife Co. LLC.]

Alright yall I think I might have gotten ti the bottom of this one.

There's some kind of electric current or static buildup happening in the tumbler and its creating super charged galvanic action. Might be aluminum or cermic oxides feom the grinding belts that are stuck of the surface of the steel or aluminum coming from threaded handle connectors that are bouncing around in the tumbler.

Either way, the solution is to ground out the tumbler with copper wire.

Ive tumbled dirty blades before. Ive had lost threaded connectors in there before with no issues. None if this is possible without electricity.