How to prevent steel oxidation without protective atmosphere or vacuum?

[ihopeitworks]

I need to heat steel up to around 640C for about 30 - 60 minutes through induction heating. At that point I will allow it to come to room temp naturally. I cannot do this in a vacuum nor protective atmosphere (gas purging the system). Are there other ways I can prevent oxidation?

Which steels should I look into? We typically use AISI 4140. Will surface coatings help?

 

I'm sure there are coatings that will survive short-term exposure, but I have no actual experience there.
Suggest you contact a reputable heat treater.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[EdStainless]

There are ceramic based washes that will minimize oxidation.
But to be honest why do you care?
This is a moderate temp for short time, you will have to remove the surface material anyway because of decarb and other surface defects.
And any coating will then have to be striped off later in another step.
Are you trying to HT finish machined parts?

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P.E. Metallurgy, consulting work welcomed

 

[gtaw]

You could try to wrap it tightly in stainless steel foil, but I don't know how that would be affected by induction heating.

Best regards - Al

 

[Dhurjati Sen]

If its like a localized heat treatment, then the insulation protects the steel from surface oxidation. Weld joints in piping are a common example.

DHURJATI SEN
Kolkata, India

 

stainless steel foil - no
the insulation protects the steel from surface oxidation - no

Any temporary protective barrier must be in intimate contact with the surface such that air is excluded.
But as EdS stated, cleaning a bit of scale from the part after HT might not be the worst solution.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[EdStainless]

Wrapping parts in an airtight envelope of SS foil will suppress surface oxidation, the Cr in SS will oxidize first and suck up much of the oxygen leaving only a heat tinted surface on the part.
But these are a pain to wrap, and are nearly impossible when quenching is required because you can't get the part out fast enough.
You really need to think about a nitrogen purge system. It doesn't need to be perfect in order to significantly reduce surface oxidation, it is inexpensive, and does not require additional process on the parts.

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P.E. Metallurgy, consulting work welcomed

 

[bobjustbob]

There are a few manufacturers of "Stop Off Paints" that you can coat the surface with.

Bob

 

[mfgenggear]

or copper plate all over, before heat treat then strip after hardening, embrittlement relief after.

 

[berkshire]

Would a flux like this prevent the heat tint ?

https://www.airgas.com/product/Weld...|Welding Products-Fluxes & Powders|35-70|C:60

B.E.

You are judged not by what you know, but by what you can do.

 

[gtaw]

Well, let's see, the original post said he couldn't use a gas atmosphere. That ruled out heating it in an argon atmosphere. I have a client that heats his parts in a small argon purged oven to prevent oxidation, but sounds like that is ruled out, maybe.

He said it was air cooled, so there is no need to quickly remove the stainless foil in order to quench it.

As noted, some heat tint may form on the surface as the oxygen reacts with the surface, but the amount of air is very limited if the part is wrapped tightly. I suppose if even the smallest amount of heat tint is a problem, a scavenger could be placed in the foil wrapper to combine with the oxygen instead of the chrome.

Best regards - Al

 

[EdStainless]

gtaw, I have seen pieces of Ti, Zr, or Ta foil used as a 'getter' inside HT packing.
If this is a small number of parts then wrapping makes sense.
If this is hundreds then maybe a coating (clean, dry, coat, HT, strip, clean) but it will add work.
If this is thousands of parts then a protective atmosphere is the rational option.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[gtaw]

If the numbers justify looking at a "better" way, then that's what they should do. Tough to tell whether it's one piece, ten, one hundred, or thousands per day. It's like the Paul Harvey radio show, I'm just waiting for "and now, the rest of the story."



Best regards - Al

 

[davefitz]

Back in the 1980's there were developed chromizing treatments that circulated a solution with chrome thru the tube/pipe and caused a surface layer to be rich in chrome, that prevented oxidation. The process was abandoned due to enmvironmental issues with the hexavalent chrome waste product, but I think that can now be addressed with modern membrane systems and electrocoagulation methods.

"...when logic, and proportion, have fallen, sloppy dead..." Grace Slick

 

[DAVIDSTECKER]

This is a bit off the wall, but when heat treating a Samurai sword they mask off areas with a clay mixture I believe.
I wonder id something like that would work?
It shouldn't interfere with the induction heating.