Post Heat Treat Scale Removal

Hi Perk.

When you say heat treat scale, do you mean after a normalize or anneal cycle straight out of the furnace?

Or do you mean after an austenitize, oil quench, then temper?

milo

Hey Milo,

Forgot to mention that I was hardening the parts so yea, after austenitize, oil quench, and temper. Sorry bout that.

No problem.

The issue with acids after the oil quench is they can't break the oil film.

Plus, enough time with an agressive acid and Hydrogen embrittlement can result.

I don't know anyone trying detergents and aqueous after oil quench either.

Shot blasting is more aggressive than sand blasting, and I believe that a vibratory or tumble with the correct media would be a great bulk second step for final clean up after a rough shot blast to get the worst / heaviest scale off. I used a mix of rough shot (grit) and round shot to assure good cutting and to allow the grit to round up as it wore. I don't recall my exact blend.

As other posters have advised, using inert gas furnace or vacuum furnace will not allow oxygen to contact steel at high temperature, thius preventing formation of scale.

Remember, acid don't cut through oil, thats why we put oil on our guns and tools, to protect the iron from acids.

Thats why I asked.

milo

Ultrasonic chisel.

I assume that you want to remove traces of iron oxide before polishing with fine abrasives. Sulfuric acid has never worked very well at reducing iron oxide in comparision with other reducing agents. Hydrochloric acid (commonly named fuming muratic) that is easily available at most stores to use in the removal of mortor from tile and brick is much better but also much more agressive against the base metal. You may want to consider using phosphoric acid that is easily obtainable at your local sewage plant where it is used to fertilize microorganisms. Another more desirable but more costly approach would be EDTA or EthyleneDiamineTetraAcetic acid. This organic acid is biodegradeable and less harmfull to the skin and can be purchased as an anhydrous powder from industrial chemical suppliers.EDTA can be (cation) metal specific if you want to preferentially remove iron before other metals. Also, you can blend EDTA and Phosphoric and obtain pretty good results without damaging the substrate.

Hey inventorman, thanks for the response. I've been weary of using HCl after heat treat because I'm not sure if it could cause hydrogen embrittlement? Not sure if I'm just being paranoid, but I've preferred to play it on the safe side. I tried using phosphoric acid some time ago and was more partial to the results from the sulfuric acid. As far as EDTA, could it be equally beneficial to just use Acetic acid? Thanks again for your help.

EDTA is much more preferable to the use of acetic acid, (and much more expensive)However, if you choose to try acetic acid, add a little table salt to the acid and it will work better. One thing to remember about the use of any chloride is,-- that when the chloride concentration exceeds 400 parts per million the chloride corrodes (pits) all steels

Another Question Perk, Do you use inert gas in your heat treating ovens to limit scale or are you using a liquid bath? And what type of quench materials are you using?

GA by the way inventorman.

Brad

Have not tried the inert gas but have been interested in giving it a shot. As of now, parts are sealed in a stainless foil bag, pulled at appropriate time, and oil quenched. Thanks.

Hey Perk, cheap test for inert gas if you are using stainless foil bags. Fill the bags with argon opening up, close and place in heat treat, closure up. Gas will expand so expect it. Oil quench as normal and check the scale for improvement.

Brad

I would look up Vacuum De-Carburizing. I believe that this type of heat treat furnace will keep it from forming on the part at all. Even Using Argon and other inert gases, though they help, it still has scale and you will work yourself to the bone to remove it. You may be able to eliminate several processes if the Vacuum De-Carburizing works for your application.

Heart Cooks Brain,

Thanks for the suggestion. I'm not familiar with Vacuum De-Carburizing. The name suggests the removal of carbon which I believe would decrease the strength, right? Not sure if this method is what I'm looking for?

I would recommend Hydrochloric acid solution at about 6-7% (vol) and 180 F. If possible stress the surface before processing.

I agree HCL works best for removing scale, and blasting beats everything, don't forget to passivate!!

You may be able to avoid blasting all together, try the following

1)Thourogh de-grease, with perc or whatever is being used these days, oil and grease cause terrible wetting problems, air dry (oil free air-hose with a nozzle, make sure to get all the nooks and crannies).

2) HCL bath

3) rinse,rinse,rinse

4) 5 or 10 minute Anodic soak in a Caustic electrocleaning solution.

5) rinse,rinse,rinse

6) passivate or plate or black oxide or whatever!!

Good Luck