Chemical removal of heat treating scale and oxides -- steel parts

A. I would like to suggest electrocleaning the workpiece. It is the reverse from the electroplating process, causing the dirt on the surface of the workpiece to fall off.

A. There are dozens of approaches and formulas. Some based on galvanic phenomena, some based on chemical attack (either highly acidic or heavily chelated), some based on the reducing potential of melt or fused salts. It depends very much on budget, type and volume of work and established facilities. Get a good book on the subject. A web search, no matter how thorough, will never be a substitute for a good specialized book. May I suggest to you the ASM Metals Handbook [adv: this book on eBay , Amazon, AbeBooks] .

A. I will suggest a 3 in 1 degreasing solution before tempering; this will remove scales and oil.

A. We've used hot alkaline cleaners for removing that kind of scale, and sometimes it even works. Pickling in hot, dangerous hydrochloric acid can also work, but I don't recommend it: you might remove the scale, and cause the part to crack from hydrogen embrittlement. Since you noted you're a student, I'd start with Naval Jelly or other rust remover that you get from a local hardware store. Those are usually alkaline based, so it should be safer than other formulations I can imagine.

Yet we've found the best thing is to keep it from happening in the first place. Something we've had really good success with is filling the foil pouch [on eBay or Amazon] with a slow stream of nitrogen or argon, which displaces the air and keeps the parts bright and shiny. It's surprising how little you need, and bottled gas, especially nitrogen, is not expensive. Compare it to chemical cleaning, and prevention is cheaper than cure. By far.

Good luck!

A. naval jelly [on eBay or Amazon] is basically phosphoric acid in a jellied alcohol media. I did many types of tool steels with the SS wraps [on eBay or Amazon] with very very little staining. The secret is to eliminate as much air as possible by folding or collapsing the wrap so there is very little air and then sealing it with a triple fold pressed as flat as convenient. Lee's nitrogen suggestion is very workable if you seal the baggie well.

A. For eliminating or substantially reducing scaling, anti scale compound may be used.

Removal of anti scale compound after heat treatment is easy, and surface finish of component is good. Unproductive work of scale removal is either eliminated or substantially reduced.

A. Have you considered chemical methods such as acid cleaning/pickling?

A. Lots of possibilities:

1. Fixture them in your vibe so they don't bang each other.

2. Pickle in inhibited HCl, but be certain to bake after.

3. Alkaline Derust hot.

4. Glass bead blast.

A. Even though it is a high carbon steel, you should not be seeing scale if your furnace were working properly. Just flowing nitrogen in a furnace does not remove all of the oxygen, and certainly not at once. It is better to prevent the scale than to try to remove it afterwards. I would try to find a good consultant. The MFG of your furnace might be able to help you with a better operation cycle or help to reduce leaks.

A. You should look at a Phosphoric Acid based pickling process which will not attack the steel. Or try shotblasting.

A. Are you sure the acid is the problem? It appears to me that your problem is the heat treat scale is too thick. Any method to remove it will affect your close tolerances. Consider a controlled atmosphere furnace. Another option would be a vacuum furnace (absolutely the best but more expensive).

A. Hi Chris,
Sulfuric acid is very common for removing mill scale. For heat treated and weld steel you may want to set up a small pilot tank with the following mixture:
18% by volume of 70% nitric acid
6% by weight of ammonium bifluoride
2% by volume hydrochloric acid

Temp. ambient
Time 15-20 minutes
If your parts are small enough try it in the lab, or in a 5 gal. bucket. As far as reactions the solution will only produce a mild exothermic reaction. Another option would be using HCl with an inhibitor to avoid attack of the steel. good luck!

A. Hi Chris,

The question of which process improvement will give you the most value is one that only you and your company can answer. Here are some suggestions.

1. Make sure that your alkaline cleaners are properly formulated to handle the types of soils that you encounter. Your chemical supplier should be able to help you.

2. Add agitation to the soak cleaner and electrocleaner.

3. Add heat to the electrocleaner. Higher temperatures will allow this product to do its job better.

4. Ask your chemical supplier about a formulated product in the pickling stage. This may help with the removal of the scale that you mentioned.

A. Try using a pickle tank after cleaning. This is the best way I found to remove heat and mill scale.
35% muriatic acid / 65% water
room temp

You may still have to do a little wire brushing or scotch brite rubbing.
Keep this tank covered when not using. It will rust and corrode everything around it.
You will have to re-clean parts after this process before continuing into plating.

A. Also we have found a really good substitute for acids. It is an acid salts from Enthone Actane 340 is the name it is used a lot in place of the HCl. The ratio we use is 1 pound per gallon @ room temp. It can be heated. Also no fumes. This may be something to try.

? Sorry SK, I don't understand your question.
Please introduce yourself and your situation with a paragraph rather than a sentence fragment. Thanks :-)
Luck & Regards,


Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

A. Try sand blasting or shot blasting.

A. Hi Sundara. I think this page introduced a whole bunch of excellent ideas for your consideration. I think at this point the ball is probably in your court for you to ask detailed questions about the many ideas that were suggested. Good luck.

Regards,

A. Mike,
I would say "probably", but you should be able to prove it one way or the other rather easily.

A. Hi Pal. We've posted your inquiry but I'm not sure what will be added after 18 years and dozens of previous suggestions. If you cannot afford to use an inert gas furnace to prevent the scale, but the production is too high to bag the nails & needles, and anti-scale compounds can't practically be applied, you'll have to chemically remove it as suggested. But depending on how hard the components are, hydrogen embrittlement could be an issue. Maybe try specific some followup questions in response to one of the earlier suggestions?

Luck & Regards,