SS parts that turn green in an furnace Braze cycle??

[Capntom]

We have had a few furnace over the years that haved turned green after the furnace braze cycle. The heat treat facility thinks it is moisture but I am not sure. He says he pulls a hard vacuum and has good atmosphere. he does a furnace clean prior to our runs. He is mainly a Heat treater, not a brazer. Can it be the alloys??? The alloy is 18-FM Chrome core. In the brazement the crabon steel and nickel filler are "clean" but the 400 ss is tinted. Any suggestions?

 

[unclesyd]

Does he use a flux and if so what is the flux material?

18-FM Chrome???

Do you have a standard designation for the alloy?

 

[Capntom]

He is using Argon, I am not sure what flux, if any. I will check. The alloy is as follows:
0.015 C, 0.40 Mn, 0.90 Si, 0.02 P, 0.30 S, 17.5 Cr, 0.2 Ni, 1.75 Mo, 0.25 Cb, Bal. Fe

Chrome Core 18-FM stainless is a soft magnetic ferritic material designed for operation in more corrosive environments than tolerated by 18 percent Cr-Fe Type 430 stainless. This new alloy has corrosion resistance superior to that of Type 430 FR stainless with generally similar magnetic properties.

 

[Capntom]

I have seen this condition duplicate itself with other suppliers and Alloys. It is always with SS parts though.

 

[unclesyd]

Where all the alloys that show the discoloration of the higher sulphur variety?

 

[kenvlach]

Without knowing the flux, just a guess that chromium chlorides or fluorides are forming.
If the corrosion is water-soluble, this will ~verify.
If not, the flux may be extracting Cr at high temperature, but then the corrosion decomposes to Cr[sub]2[/sub]O[sub]3[/sub] on cooling with trace amounts of oxygen present.

 

[EdStainless]

I agree with Ken, most likely to be Cr vapor that then forms oxide/hydroxide. The vapor pressure of Cr is rather high.

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[tripleZ]

Vapor pressure as Ed stated seems like a likely cause. My vacuum furnaces will get chromium build-up over time just because of the sintering temps and pressure we're capable of pulling at those temps (they hover around 300 microns). It also likes to build on the carbon boards that I use for support. I will say that it's usually silver however. I've gotten green parts before, and that usually happens when I have a small seal leak with allows atmosphere to enter and oxides to form. It's only on the surface however.

The vacuum furnace setup can be made to alarm-out when a cleanup cycle hasn't been performed in "X" number of runs. Our cleanup cycle goes rather high (around 2500 degrees F) at 300 microns, but it does an excellent job of blasting scale and buildup out of the furnace.

Here we use continuous brazing furnaces under dissociated ammonia atmosphere. I'm normally only getting green oxide layers on the parts or belt when someone has a genius idea to place a fan near the exit end of the furnace and blows air down through (despite the doors and angel hair).

 

[EdStainless]

The source of oxygen and/or moisture does not need to be at high temp to get this condition. We used to get it when we opened the furnace warm in humid weather.

(zzz, we have an overhead door at one end of our bright anneal furnace. Some days I just want to kill someone for that)

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Rust never sleeps
Neither should your protection

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