Heat Treatment Salt Sourcing and Maintenance

[Gtyler]

Hi all,

 

I’m building a large high temp salt pot kiln which is nearing completion. Despite my searching I haven’t been able to find answers to some questions I have about the salt itself and I hoping that the incredible knowledge base here may be able to help. I also hope that this information may be helpful to others in the future to safely maintain their systems.

 

1. “Neutral” is a term often seen used to describe commercial salts. “Balanced” is another one. I haven’t found much information on what this means except some references to metallic oxides and other oxides building up in the pot which can be cleared by stirring with a graphite carbon rod or other rectification methods. Neutrality is something that seems to get out of whack over time. What I don’t understand yet is how salts can be unbalanced or out of neutral to begin with. Is it simple due to oxide impurities present in the salt to begin with? I’m curious about this because I can’t find a good source for commercial salts and I may be forced to go with a DIY solution. Can I simply rectify my salt bath as soon as I melt my DIY salt? 
2. For those who DIY their own salts I frequently see a 50/50 mix of sodium and calcium chloride used but I haven’t ever seen this mix recommended in any official documentation (e.g. Mil-10699). My current plan is to do a class 5 salt (50/50 potassium and sodium chloride) from cheap table salt (non-iodized) and potassium chloride water softening salts. I haven’t looked into the purity of these sources yet, so that’s something I need to do, but I’m curious about the choice of calcium chloride over potassium chloride. Can anyone shed light on the differences/pros and cons of these two options? 
3.  Rectification is another topic I haven’t been able to find a ton of information on. I’m trying to find out if stirring with a graphite rod is enough to neutralize my salt or if there are other methods I should be employing (methyl chloride perhaps?) to keep the salt in good condition, or is a carbon rod enough? 

 

Thank you for your help and/or interest. 

[Steve Sells]

Do not use table salt. The most commonly used salts are nitrate-based and these can be used from temperatures of 300°F through 1100°F (150°C – 595°C). see here https://www.globalspec.com/learnmore/materials_chemicals_adhesives/industrial_oils_fluids/heat_treatment_salts

[LeeJustice]

The only input that I can give is that Calcium Chloride is hygroscopic, it absorbs moisture (water) from it's environment.  It is also deliquescent (I just learned that one word, but have seen the result.), it will absorb moisture until it becomes a liquid brine.  Ever use Damp-Rid?

[Gtyler]

Just wanted to give this a bump to see if anyone can shed a little more light on this topic. 

 

On 9/20/2021 at 1:40 PM, Steve Sells said:

Do not use table salt. The most commonly used salts are nitrate-based and these can be used from temperatures of 300°F through 1100°F (150°C – 595°C). see here https://www.globalspec.com/learnmore/materials_chemicals_adhesives/industrial_oils_fluids/heat_treatment_salts

Steve, thanks for the link. I am planning to do a molten salt quench oven at some time in the near future which will definitely require nitrate salts, but right now I am looking for information on high temp salt baths for austenitizing. 

 

On 9/20/2021 at 1:54 PM, LeeJustice said:

The only input that I can give is that Calcium Chloride is hygroscopic, it absorbs moisture (water) from it's environment.  It is also deliquescent (I just learned that one word, but have seen the result.), it will absorb moisture until it becomes a liquid brine.  Ever use Damp-Rid?

Lee, thanks for this info. This make the choice of calcium chloride over potassium chloride even more curious. Calcium chloride seems like something to avoid in a salt mix.