Pitting During Salt Bath Heat Treating

[Metalhead97]

Hello,

I'm developing a Pd-based alloy, which forms a thermal oxide due to the alloying constituents. I'm investigating salt bath heat treating as a possible avenue for heat treating rather than vacuum heat treating. Preliminary studies using reactant-grade NaCl as the salt and at 1700F, 15min resulted in very little oxide formation, but the grains were pitted and intergranular attack took place

Is pitting normal for salt heat treating baths or was the pitting a result of using a halide-based salt? What other salts and/or processing steps can be used to avoid pitting? Thanks for any help.

Metalhead

PS. I quenched in water. Is that a no-no?

 

[cloa]

There's no normal for salt heating treating baths as always it depends on the alloy involved. By that's not pitting, pitting is a localised macro corrosion action, like you said you had intergranular corrosion.



Your chances of finding someone who understands specialty Palladium alloys is pretty unlikely, better to consider a consultant.

 

[unclesyd]

I would get with someone like Heatbath and discuss your requirements with them.
Most salt baths are somewhat reducing in nature to prevent what you are trying to do. There maybe some oxidising baths that operate in your temperature range but most oxidizing salt baths I'm acquainted with operate at a much lower temperature.

http://www.heatbath.com/park

 

[EdStainless]

I would suggest that you look for non-halide salts.
Sounds also like you need something that is oxidizing, or at least oxide stable.

You need to do some metallography and determine what phase has been attacked. You may have a sensitive grain boundary phase or some such thing.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

 

[macmet]

Metalhead,

I am not able to help you with your issues, but I'm curious what sort of uses a Pd alloy will have. What are they typically used for?

 

[Metalhead97]

cloa,
As I said, the interior of the grains had holes in them (I call that pitting) and intergranular attack also took place. I'm sure I'm not going to find somebody who understands my alloy since I'm the person that is developing this specific system.

unclesyd,
A reducing bath would be great. I just need to be reducing enough to prevent the formation of Al-oxide. I'll give heatbath a call. Thanks for the info.

EdStainless,
I think most non-halide baths operate at lower temperatures than what I need. I do have a two-phase system so that's likely a reason for the attack.

Do salt baths need to be conditioned before use to remove contaminants? Also, is quenching in water safe for alloys that are attacked by the salt?

 

[Maui]

To what depth did the attack take place? It is not unusual to see a relatively shallow decarburized surface layer where the salt attacked the part being heat treated. The severity of the attack depends on a number of variables such as the alloy, the soak time and temperature, and the type of salt that is being used. Most parts that are salt bath heat treated are intentionally oversized for the removal of this affected surface layer. I suggest building in an appropriate amount of oversize to be removed by finish grinding or polishing after heat treatment is completed if this is a possibility for your application.

Maui

 

[Metalhead97]

macmet,
The potential applications for this Pd-alloy are biomedical and/or for electrical contacts. However, the majority of the Pd-alloys my company develops are for electrical contacts.

Maui,
I cannot give you an exact depth since I haven't sectioned the material. However, the attack looks very severe under the SEM and would be a no-go for wire applications due to fatigue issues.

Metalhead

 

[CalvinKelly]

While you may have started with pure NaCl, it picks up oxygen from the air, and entrained oxygen when you submerge the part.
No heat treat salt bath will prevent oxidation of aluminum.

Out of curiosity, how did you contain the salt, and how did you heat it?

In my opinion, you ought stay with vacuum, otherwise prepare for a lot of misery.

Commercial heat treat salt baths have a myriad of problems, again in my opinion, relating 95% to maintenance proceedures.

Neutral salts are never neutral.

James Kelly

http://www.rolledalloys.com

 

[Metalhead97]

CalvinKelly,

The wire actually came out of the bath thermal-oxide free. The problem was that the material was attacked: the material underwent corrosion in the salt pot.

I'm actually ditching the salt pot idea. The problem of going with a vacuum furnace is that they are so damned expensive - even for used ones. It makes no sense to buy a furnace if the demand for the product doesn't justify it. "Canning" works for ingot and large rod sizes, however it is the intermediate sizes that are going to need a batch vacuum furnace.