305SS Sticking to each other during heat treat 3

[305ss]

Here’s a problem that has puzzled me for years,

I heat treat 305 series stainless steel the parts are quite small approx 0.2500 squares x 0.017 thick, my problem is that they stick (fuse) to each other.

I use a temperature of 1850f and a hydrogen atmosphere (9999% pure) the same problem accurse using a vacuum furnace.

I understand way the SS sticks but cant help put think that there’s a cure for this.

I have tried mixed percentages of atmospheres such as Nitrogen and Argon blended
with H2 but this only creates a discoloration that cant be tolerated, I have also tried with some success Aluminum Oxide power but this Alo2 tends to embed itself in the base material and this is another problem.

I have been in the heat treat industry for over 35 years and have not solved this one yet.

Comments from readers are welcomed.

 

[metengr]

Some general thoughts on your process;

How long do you keep these pieces in the furnace at 1850 deg F? Perhaps you need to change how you load or arrange these small pieces (I presume you use a batch-type heat treat operation where you just place these small pieces into a bin and heat and quench)?

If you were to change to a continuous, bright anneal heat treatment furnace your residence time might be less. A single stacking layer with the pieces spread out laterally would prevent them from sintering together?

 

[NickE]

metengr- nice, i was just about to post similar in the other thread.

 

[EdStainless]

We used to run water injection on our bright anneal in order to keep the DP no lower than -50C. Fusing was theonly reason that we did this. If we ran dry we could reach -80C and everything would stick together.
This is normally worse when you have rough endges or burrs. Are these parts stamped? How are they de-burred? Maybe they are being cleaned too well.

= = = = = = = = = = = = = = = = = = = =
Corrosion never sleeps, but it can be managed.

http://www.trenttube.com/Trent/tech_form.htm

 

[btrueblood]

We used to use boron nitride spray and/or powder as a "stop" in brazing in H2 and vacuum atmospheres. Was told it is effective in preventing diffusion bonding also. You may want to talk to Scarrot (Scarott?) Metallurgical Services in California, they know about both creating/enhancing diffusion bonding as well as preventing it.

Alumina powder is very hard, abrasive material, and I can see how it would tend to penetrate ss at high temps. BN powder (NOT cubic BN or crystalline form) is very soft, and forms a slurry when wetted. The spray form (from McMaster-Carr) goes on in a nice, even coat, and rinses away afterwards in water.

 

[unclesyd]

Here is another source of the BN powder as suggested by trueblood. ZYP also makes a dip that will do the same thing. You need to give them a call.

I have used Borax solution, before BN to prevent the same thing from happening. My parts were a little larger so I was able to coat every other one.

http://www.zypcoatings.com/

 

[metman]

The BN mentioned above might be the same thing we used as a stop-off to prevent braze metal or braze alloy from adehering to areas of the the substrate that we did not want the braze metal to attach to.

See also the comments about fiberfrax and fiberglass in the other thread.

 

[unclesyd]

I wouldn't use anything with fiberglass as it can start melting at these temperature. If you use any of the refractory papers i would stick with Zr, Al, or Si in that order, of course you know the price is the reverse of same.

metman,
The BN materials from ZYP and others are excellent stop offs.

 

[rorschach]

Wall Colmoloy makes a stop paint for nickel brazing (in hydrogen atmospheres) that if memory serves was basically milk of magnesia that might work.

 

[EdStainless]

You beat me to the MgO comment, it works fine.
I would bet that you could use TiO also, very common as a white pigment.
We got tired of paying the price for BN slurry, so we bought BN powder (I believe that it is made by Saint Gobain) and mixed it into pure isopropanol. We found htat if you microwaved it the BN would stay in suspension. We sprayed it on hardware in a vacuum furnace.

= = = = = = = = = = = = = = = = = = = =
Corrosion never sleeps, but it can be managed.

http://www.trenttube.com/Trent/tech_form.htm

 

[btrueblood]

In a former life in the rocket biz, we used to use magnesium oxide (MgO) to coat waspalloy fasteners in high temp. vacuum service, with the intent of providing an anti-gall/anti-diffusion bonding coating. We had better luck when we switched to boron nitride (BN). MgO powder is not as water-soluble as BN, tends to clump, and doesn't wet to, or stick to clean stainless/nickel/chrome alloy surfaces as well as BN. It is, however, a fairly soft oxide that shouldn't cause the abrasion problems that alumina could.

Oh, btw, "Milk of Magnesia" is magnesium hydroxide, Mg(OH)₂. It converts to MgO (by losing water) at temperatures above 350 C, according to my CRC Handbook, thus the info regarding MgO given above.

 

[unclesyd]

Use an analytical or reagent grade MgO or Mg(OH)₂ the milk of magnesia sold over the counter has some chlorides in it.

 

[305ss]

Thanks to all for your interest and feed pack, I will try and address your response questions.

To metengr
I use a continuous, bright anneal heat treatment furnace @ 1850f the time each basket is in the heat zone is 5.33 min this is the min time to yield the correct hardness Vickers of 135hv + / - 10hv.

Parts are approx 7 to 8 parts thick, I realize that if I was able to process them to where they didn’t touch they cant stuck but due to production requirements this is not possible.

To Edstainless
I’m somewhat familiar with what we called a water bubbler used to run the H2 through to create a wet atmosphere, I have not tried this due to the DP in the furnace is approx –23C.
Yes the parts a stamped and tumble de-burred then cleaned in an ultrasonic cleaner, the parts have little to no burrs viewed under a 30X microscope. Your comment about the parts being cleaned to well may be correct, I have run tests in the past on parts that where not US cleaned and had mixed results, perhaps I need to revisit this.

To btrueblood
I have not tried Boron Nitride power but have tried many materials, as a buffer media the trouble is that they all tend to remain on the substrate material even after a US cleaning I use a SEM and LPS method to detect them. I refer to them as hard partials and during the use of the parts these partials (micro size) can dislodge and cause catastrophes failing
I’m familiar Scarrot Metallurgical I haven’t seen Dave S, for years and will consider contacting them.


To Rorschach
I have tried using the stop off materials from Wall Colmoloy and they very hard to remove I have talked with Mr. Bob Peasley in the past but nothing worked.
You are correct about the milk of magnesia I recall my Dad using this many years ago as a brazing stop off.

To Edstainless
Can you tell me more about the BN process and the microwave used, is the final product in a dry power form.

Thanks to all
Regards 305SS

 

[rorschach]

one thing, if you skip the cleaning step, you'll run the risk of leaving oil on the surface which will decompose to carbon in the furnace and result in chrome carbide on the surface of the parts.

yes removing MgO is tough. perhaps holding the tumble until after anneal might be an answer.

 

[305ss]

rorschach
You are correct about the oil becoming carbon, I would use a mild detergent.

The small parts are to soft after anneal to tumble they distort.

Regards

 

[EdStainless]

Perhaps you could clean after anneal in a vibratory with corn cob or a plastic media.

We bought fine powder BN, I don't remember the particle size. Simply added some to IPA, roughly 30% by volume. We just stired the heck out of it then cooked it in the microwave for a few min. Stired and re-cooked. Eventually the BN became very slow to settle out. We either applied it with a brush or we sprayed it with a hand can using N instead of air.

= = = = = = = = = = = = = = = = = = = =
Corrosion never sleeps, but it can be managed.

http://www.trenttube.com/Trent/tech_form.htm

 

[305ss]

EdStainless
You applyed the BN in a past form, on the small parts that we use I was thinking that perhaps we can apply it as a light powder and do a DI water clean.

Comments please