Discoloration of 347SS parts in brazing cycle 3

[jsokal]

Greetings,
I have an odd problem, and I hope someone can steer me to a solution. I am vacuum brazing some 347SS machined parts with AMS4787 (gold-nickel) alloy in a vacuum furnace at 1825°F with a partial pressure of argon at 1torr. The 347SS detasil sometimes come out quite blue,with the brazes still being very good. The leakup rate is good about 10microns/hr. Now the odd thing is other details made of other alloys do not discolor in the same load. I ran some tests with the same cycle and furnace using IN718,410SS, Greek Ascoloy, and 347SS coupons. I personally cleaned each one and wiped them down with alcohol. After the cycle the test pieces were pristine, except for the 347SS which had blued. I would expect the IN718 with Al and Ti would blue long before the 347 if it was a leak problem....Any ideas>

 

[tomwalz]

Don’t think this is your answer but why use alcohol which leaves a residue?

Thomas J. Walz
Carbide Processors, Inc.

http://www.carbideprocessors.com

 

[unclesyd]

tomwalz,

We use laboratory grade reagent alcohols, mainly methanol to clean components for welding and brazing. We never had any problem with residue.

Generally blue indicates an oxide. We force oxide on SS components by using steam/air oxidation. In this process we get varying results on different alloys in the same batch.

How dry is the Argon gas?

 

[EdStainless]

The 347 only? That does not seem right. I would expect the 410 to tint first. Dose this come and go? Do you know who made the material? What if you passivate in hot 20% nitric acid first? I am suspecting residue on the parts. It could be a function of the surface texture.

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[jsokal]

Hi,
Just to answer some of the questions and give some more info.
The actual parts are acid cleaned prior to brazing,.. the test pieces I used were cleaned with scotch brite and wiped clean with alcohol (reagent grade).
Yes only the 347SS turns blue, the 410SS and IN718 are bright.
I have considered the surface contamination but am at a loss after my tests. The 347SS parts are fully machined parts, while the test panels I am using are wrought sheet. Surface texture may have something to do with it. I am trying to get a hold of some wrought 347SS to see if it happens on sheet. The dewpoint of the argon used in the partial pressure is -60°F or better. All other brazing performed in this furnace is impecable.

John

 

[EdStainless]

This is happening on the cooling part of the cycle. I still can't figure out why only 347.

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Rust never sleeps
Neither should your protection

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[jsokal]

Ed,
I agree,..will be running a few more tests cleaning the parts in a various number of ways to determine if once and for all if it is a residue issue or not.
Will re-post with findings.
John

 

[tomwalz]

Unclesyd,

I can see why reagent grade, laboratory alcohol would make a difference.

In Washington State reagent grade, laboratory ethanol is Everclear®. You get your permit to use it then you go to a state liquor store which sells you a brand name.

Fusel oils?
1-propanol
2-propanol
butanol (various isomers)
amyl alcohol
furfural

We like a caustic based cleaning using a commercial compound as they generally are formulated to rinse well.

In situations such as this we would recommend a can of Easy Off oven cleaner as a test.

Where we do use alcohol we use a denatured, commercial cleaning blend in an ultrasonic bath.

Tom


Thomas J. Walz
Carbide Processors, Inc.

http://www.carbideprocessors.com

 

[unclesyd]

What is the acid being used for acid cleaning?

Scotch Brite pads have been known to leave a residue on machined surfaces. The pads were made from Nylon.
We use Nylon polymer in our steam/air oxidation process instead of oils to control the formation of undesirable oxides.

We would normally use SDA formula #3A with Ethanol + Methanol. The one we used had a very low moisture content and as I recall there was there was no ash on ignition and no fluorescence on the wiped surface. A look at your surface under UV light be a quick test.

We also used alkaline cleaners in either soak or ultrasonic baths. If we used an alkaline cleaner in a ultrasonic bath it was always follow with an ultrasonic rinse due to poor rinse ability of caustic based cleaners. On some very delicate jobs we would use an alcoholic/ KOH bath for it's better rinsing properties.

The two oven cleaners that came to our attention both contain way too many chlorides to be used around SS.

 

[unclesyd]

One thing to do is accomplish all the mechanical operations, polishing, buffing. etc, prior to any chemical cleaning to lessen the chance of any residue.

 

[jsokal]

Hi,
Here are some of the results of my tests:

1- Wrought sheet 347 does not discolor, only machined parts

2- It does not happen 100% of the time, took an old part which was not cleaned at all, and it came out clean, except an area where I had used a grinder to identify the test part,..the mark tuned very blue.

3- When processed in another newer furnace which reaches a much higher vacuum level (10-5 torr)before introduction of partial pressure, no parts ever come out discolored. The furnace normally used reaches 10-3 maybe 10-4 torr before gas argon is introduced at 150°F.

4- Tried parts which were purposly contaminated with machining coolant,..they came out clean,..no effect whatsoever.

5- Had a part and a sheet metal tube acid cleaned (60%HCl then neutralized in 3% sodium hydroxide then rinsed in H2O.
Tube came out perfectly clean,...part very discolored.

6- It seems if I get a part that had gone through the fuance cycle and came out clean,...it will no longer be suseptible to this blue discoloration.

There you have it one furnace doesn't cause this to appear, the other does. The ONLY difference is that the smaller furnace reaches a higher ultimate vacuum. In terms of leak-up, the "good" furnace has a worst leak-up 39 vs 10 mics/hour.
It seems ovbious that material condition, machined vs wrought makes a difference, and perhaps vacuum level. Does anyone think that the higher vacuum in the 10-5torr level could be vapourizing something on the machined surfaces at very low temperature (150°F and lower) before the argon partial pressure is introduced?

John

 

[unclesyd]

It is exposure of a part with an active surface to O2. The oxide your are seeing is probably a monomolecular layer so the amount of O2 required is very small.

The part that you ground had a very active surface in the affected area that is why it blued. The part could have been passivated in 20% HNO3 and thoroughly rinsed or left sitting for the weekend and the blue oxide probably wouldn't have formed.

HCl is the best and the worst cleaner for SS. After cleaning with HCl the surface is very active and would normally be passivated prior to introduction into the furnace. A quick dip in 20% v/v HNO3 and thoroughly rinsed will passivate the surface.

 

[EdStainless]

One detail may be the fact that 347 deliberately has inclusions in the material. It may be that material (solvent, oil, air?) are more easily trapped in this surface than others.
A hot nitric acid passivation with hot DI water rinse may be enough to prevent this.

= = = = = = = = = = = = = = = = = = = =
Rust never sleeps
Neither should your protection

http://www.trent-tube.com/contact/Tech_Assist.cfm