Does sand blasting damage a blade?

[Atlas Knife Company]

Yes, I understand that sand/media blasting warps thinner metal due to the micro-heating from the friction caused when the sand hits the metal. I discovered this by accident on a blade and the next day saw an explanation on a history channel show. I have several pieces of slightly warped Titanium that are 1/16" thick. I put one in the cabinet and sure enough I was able to get it almost perfectly straight. This was far more effortless than trying to straighten Ti by bending it. Like they say, if it's that easy then there must be a reason everyone doesn't do it.

Here's my question: Assuming I do the blasting to a HT blade before I sharpen, what , if any, damage is caused to the HT/structure of the blade? If enough expansion in the surface is created to warp the blade, surely it must be affecting the metal beneath the surface. Perhaps tempering or stress relief needs done afterwards? I'm not talking about blasting to straighten a finished blade, just enough to give that satin finish.

 

[Justin King]

Good question. My guess, straight out of my butt, is that on HT'd steel in the upper 50's/low 60's, the effect is so shallow as to be negligible given the non-directional nature of the finish, but I would be interested to hear an informed answer. It might well depend on the size/density and velocity of the media, get those values high enough and you can probably damage anything.

 

[Stacy E. Apelt - Bladesmith]

I would say that unless you have a super powerful blaster, there is little or no damage to the steel or the HT.

 

[Nathan the Machinist]

I don't think it is moving due to heat, regardless of what the TV show said.

Shot peening is used to cause localized yielding in order to put a surface into compression, which reduces tendency for crack propagation. I suspect the same mechanism is at work with bead blasting.

When you bend metal with heat, you're causing the metal to expand on one side, and reducing the yield point on that side due to heat. So the surfaces yields under compression and, as the metal cools, pulls that surface into tension. So the metal bends towards the side the heat was applied. (This is the force that causes welds to crack in non ductile materials like cast iron.)

When you warp metal using a needle scaler, shot peen, wire wheel, grit blast, buffing, you're mechanically stretching the surface which ends up in compression as a result. So the metal usually bends away from the side the work was done. Which would be the opposite reaction that you would get with heat.

( the exception to this would be when the forces are high enough to directly bend the work piece, as you would see grit blasting thin aluminum, or beating thicker steel with a hammer)

What direction was your work moving in relation to the blast?

 

[Atlas Knife Company]

It was moving away from the blasting. I'm most worried about a weakened structure as if the blade was always being held in a flexed form.

 

[JCaswell]

I don't think it is moving due to heat, regardless of what the TV show said.

I agree.
... Never heard the 'micro-heating' hypothesis before.

Anyway, I think you're much more likely to alter your material's condition at least, in grinding (by overheating thin sections) rather than sand blasting.

 

[A C Richards]

I have never had any problems with bead/media blasting knife blades. I have done a few dozen and not had any warping or weakening that I can readily see. I have not done micro analysis or anything, just working with them.

 

[Atlas Knife Company]

Has anyone done micro analysis before/after blasting? I wouldn't have given it a second thought, except I was able to nicely straighten the piece of Ti with blasting alone.

This seems to be an exceptionally easy and accurate way to straighten thin warped metal so there must be a down side to it. Since I can sand out the satin afterwards, is there any reason I shouldn't do it this way.

 

[Justin King]

On softer materials you are essentially drawing the surface out with the impacts of the beads. This will cause work hardening but in the case of softer metals this often causes an increase in tensile strength so this is not usually considered a bad thing.
I am guessing that on hardened steel it's effect will be shallower and probably much less pronounced.

 

[pennerknives]

I have been making knives for 30+ years and do my own heat treating. I use 440C, ATS-34, and S30V, steels. Due to high demand I began making kitchen knives about 8 years ago. I began flat grinding 440C and attempted to make thin blades. I found that very few if any of my blades came out straight and most had compound bends or waffle taco shell edges. It took me three years to build and heat treat a blade that is 10-12 thousandths of an inch thick at the cutting edge before sharpening. This is what I have discovered about finishing thin blades:
1. After heat treating, any sanding or finishing that causes heat will warp your blade
2. Keeping a blade flat is about SCALE management. Clean steel does not warp and all steps of blade formation are thought out to reduce blade stress caused by scale. (this is the short answer)
3. I have abrasive blasted approximately 500 knives to remove scale after heat treating and have found this the best way to refinish a wafer thin blade and keep it flat.
4. Abrasive blasting appears to have a beneficial effect on the toughness of the blade. I recently sandblasted a number of blades outside when the outside temp. was around -45 celsius. At colder temp. it takes 2x as long to clean off scale and the hardened steel exhibits unusual surface structures that show up when polished.

 

[Overmountain]

Off topic.

 

[Omega Leather Works]

I've *glass* blasted thin ti and the only thing I've noticed is lots of sparks.

I've *glass* blasted a few hardened blades and have noticed nothing at all other than the obvious finish difference, as well as a propensity for rust (on stainless blades).