Heat treated blade warps after sandblasting

[KNelson]

I just recently finished some S35VN blanks. HT was foil wrapped in 309, throw in warmed kiln that's been at 1550 for 30 minutes and let sit for 10, ramp to 1975 hold for 25m, plate quench with compressed air, into -100 dry ice slurry for 1 hour, temper at 375 for 2hr X2.

I then started to hand sand a finish on them, decided to try one of them in the blast booth of sieve 3 glass and after blasting one side I took it out and noticed it went from perfectly straight to a banana.. freaked out a minute then blasted the opposite side and the blade curved back.. kinda wonky straight but mostly straight.

Anyone know what happened? Thanks

 

[Stacy E. Apelt - Bladesmith]

Glass bead blasting is a microscopic peening process. Each peen stretches the surface a bit. Thousands of peens happen every few seconds in bead blasting. This will curve the blade away from the blasted side. This is far worse on thin blades.
Blasting both sides alternately to equalize the stretching may help.

If anyone hasn't tried hammer curving steel, tale a piece of 1X1/8" stock and hammer only one side with a ball peen. It will slowly roll up into a circle if you keep working it.

 

[KNelson]

Glass bead blasting is a microscopic peening process. Each peen stretches the surface a bit. Thousands of peens happen every few seconds in bead blasting. This will curve the blade away from the blasted side. This is far worse on thin blades.
Blasting both sides alternately to equalize the stretching may help.

If anyone hasn't tried hammer curving steel, tale a piece of 1X1/8" stock and hammer only one side with a ball peen. It will slowly roll up into a circle if you keep working it.

Aaaggh your spot on with the direction of the curve.. darn me never again. Wasn’t sure if that brought out a flaw in my heat treat.

 

[P.Brewster]

If you search, you'll find some old threads that discuss media blasting as a straightening technique.