Help me understand steel better!

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Frozy

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Apr 3, 2010
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Hey girls and guys, i have very little information and knowledge on steels so i need your help.
Ok as we all know, steel is harder than plastics (most of them to be precise). So why does my blade of CPM S30V gets scratched by cutting up pingpong balls? The scratches are not deep, but i wonder how can a softer material scratch a harder one?
 
Think about those new water jet cutting machines, they can cut through steel. I would suppose that with enough force a soft material can be used to cut a harder material.
So, when you are forcing your knife through a softer material, there is friction, due to the force applied upon the two mateials coming together.

Maybe someone will chime in with more expertise, or a physics degree.
 
I don't know exactly what ping pong balls are made of, but "hardness" isn't always exactly what you think it is. You can run a metal file across a S30V blade and it'll skip across without a mark, but cardboard will scratch it. While the overall composition of a material might be soft, there could be something microscopic that's harder or abrasive. I mean, you can scratch plastic with toothpaste, and toothpaste is pretty soft, right?

Everything we cut with a knife should be softer than the blade, yet blades dull. There's more going on than you can see.
 
foxx said:
Think about those new water jet cutting machines, they can cut through steel.
Click to expand...

From what I've read about water cutters they don't use only water; there's a particulate of some sort (I forget what) in the water.
 
Frozy said:
Hey girls and guys, i have very little information and knowledge on steels so i need your help.
Ok as we all know, steel is harder than plastics (most of them to be precise). So why does my blade of CPM S30V gets scratched by cutting up pingpong balls? The scratches are not deep, but i wonder how can a softer material scratch a harder one?
Click to expand...

Depends on what is in the plastic.

There can be materials in things like plastic, cardboard and wood that can be harder than the steel, or hard enough to scratch it.
 
I have a titanuim coated scissors (Clauss) which i use to cut the same pingpong balls and it has no scratches at all. The FRN sheath of my spyderco schempp rock scratches the blade upon just the first time sheathing it; but FRN is fiberglass, which i assume is harder than steel. However i find it ridiculous for the soft pingpong ball to be able to scratch steel.
 
Ping pong balls aren't made in the cleanest of conditions. They aren't a food item, so they aren't subject to the high regulatory restrictions and standards that are required for food manufacturing.

Sand, dirt, dust, metal particulates and all manner of stuff can get into ping pong balls and as long as it performs reasonably well, it passes QC. Remember that steels like S90V were developed for blades to cut material from extrusion machines--and little pieces break off of the edge all the time and end up in the plastic.
 
Something else to consider. You can use leather, or even a fingernail buffing block to get a really fine hone on a knife. What that does is remove tiny microscopic bits of metal, and those are generally considered soft, especially in comparison to steel. No steel is going to get away with no damage; its just going to be less damage than the ball receives.

Another aspect of it is the sharpness of the edges on the ping pong ball, and the angle at which it is being pressed against the blade, not to mention the geometry of a sphere. There's a lot of structural integrity, and even plastic can cut if there's enough force being applied at the right angle. Heck, Cold Steel makes some pretty decent plastic blades, and you can stab and cut with those just fine.

Finally, it's also possible that you're mainly marring the finish, not really getting any kind of real scratch. If it's plasma coated, or has a satin finish, the plastic will remove the finish, or even leave some of itself on the steel. So it may not actually be a scratch, but rather just some excess material left on there, or the soft finish removed.
 
There is hardness. Hardness is great, we all love it.
There is wear resistance too. Wear resistance is much less known, but it is great too.
Life is like that, made with many things we can't see but are there to take part on it.
 
It isn't just some excess material left there- i used metal glo on my blade and the scratch marks can't be removed.
I'm cutting pingpong balls into half spheres for art decoration stuffs.
Probably u r right about the presence of other particles...
 
My Military got scratched all up to be damned when cutting some cardboard. It didn't surprise me though. I just figured that on satin finished knives it would be similar across the board. You can remove the scrqtched though, but you would need some sandpaper and time.
 
There's a lot of different physics involved in cutting. A kind of off topic example: my physics prof (worked on weapons in the military for many years) blew a whole through a tank with a nalgene bottle. High speed plastic can do a lot of damage (which was a mistake btw :))
 
Many plastics, cardboard, paper, and other things have very abrasive particles in them. Plastics have various oxides added for color, the most prominant being titanium dioxide (titania) which gives a very white color. Others are aluminum oxide (also white), chromium oxide, iron oxide, etc. Stabbing your blade through a piece of cardboard is similar to stabbing it into the ground, at least in terms of the abrasives encountered, if not the magnitude of scratching. Some of the high wear steels we like for knives were in fact developed as polymer injection molding die steels for plastics that contain high volumes of abrasive pigments, as well as being corrosive in their own right. The combination of corrosion, abrasion, and the high pressures involved w/ injection molding demand a steel that is stainless, wear resistant, and strong (hard).
 
me2 said:
Many plastics, cardboard, paper, and other things have very abrasive particles in them. Plastics have various oxides added for color, the most prominant being titanium dioxide (titania) which gives a very white color. Others are aluminum oxide (also white), chromium oxide, iron oxide, etc. Stabbing your blade through a piece of cardboard is similar to stabbing it into the ground, at least in terms of the abrasives encountered, if not the magnitude of scratching.
Click to expand...

Great point me2. I hadn't even thought about the additives.
 
The waterjets I was familiar with used tiny garnet stones and were recaptured in a carbide catch can on the under-side of the workpiece. I've seen ads for water additives to increase cohesion for machines just using directed extremely high-pressure water to cut. Not sure what those are intended to cut though.
 
I sometimes deliver skids of refined garnet to places that do waterjet cutting. I didn't realize that a particulate was used in conjunction with the water until I asked them what they used the garnet for.
 
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