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Low temperature Salt pots


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Does having a Nitrate based low temperature salt pot in the shop increase corrosion in the shop? I am reading conflicting things about this online, but I wonder if some of this is nitrate vs chloride  type salts?

Now that I have some more space in my shop I want to bring some of the heat treating I currently send out back into the shop.  The one I want to bring in the most is heat treating the pry bars I forge in the shop.  I want to austemper  them in salt right after forging them.  The reason I want to do that is they are forged from Pre hardened 4340 (around 35RC).  The end that is forged has to be hardened to 50-55 RC.  I keep a short heat while forging so I don't  further temper the length of the bar.  I  have to certify that they are between 50 and 55 on the end so I need a consistent tempering process and I have to buy a Rockwell tester.   A Ht furnace is not a good solution because I am only tempering the ends of 6' and 4' bars. Keeping the internal temperature of the furnace consistent would be difficult while keeping the door open.  This is why I am thinking Salt is the best solution.  But if it will cause corrosion in the shop I cannot do this as I store a lot of raw castings in the shop that I don't want rusting .  

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I'm not sure what nitrate salt you plan to use.   As a child I used to amuse my self with melting potassium nitrate and dropping pieces of saw dust into the melt . I liked the mini explosions,

  In general nitrates are very dangerous to have in the molten form. I know folks do it industrially but I don't know the safety precautions.  The property of nitrates that makes them good for  black powder makes them dangerous when heated.  I've worked in a couple of places where disaster was two steps away and relied of the operators self interest to prevent it from happening. 

You could  use a sand bath I think.  Check with your local industrial suppliers.  A couple of bags of sugar sand like that used for sand blasting might do the job.  Sugar sand is normally quartz sand so would be stable, not affect the metal, and conduct heat well.

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It appears that you might not have a big problem with outside corrosion.

From:  https://www.dynalene.com/Molten-Salts-s/1831.htm

"Additionally, these salts are non-toxic, non-flammable, and exhibit negligible vapor pressures even near peak operating temperatures, minimizing safety hazards and the need for high-pressure equipment. Dynalene’s molten salts have excellent corrosion resistance to stainless and alloy steels and are compatible with carbon steel below 350°C."

The low vapor pressure implies they don't outgas much in terms of nasties.  Compatible with carbon steels implies that even that which escapes probably won't be too awful.  Some other positive things are also implied about the nitrate/nitrite salts on the site.

But---a KISS solution is always best and what Charlotte mentioned about a sand bath sounds well worth a try before going to more complex solutions.

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Excellent information in the MSDS.   Basically it is potassium nitrate and Sodium nitrate  with a little other  nitrates added in depending.   Making sure that your bath never gets contaminated and stays covered all the time would be key to safety.  They are a little close to the vest about the ratios of the four different nitrates.  One thing about those salts is that they tend to accumulate moisture around them and stray particles will encourage rust carbon steel.  Kind of like last years fertilizer left in the shed next to the garden spade.

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You might consider giving Jerry Lairson a call. He is an ABS Master Smith and a great guy to talk to. He has been using molten salt baths, high temp and low temp, for hardening and tempering his blades for years. He would be a good resource as he is the only guy I know doing it in his smithy.He has a website with contact info. Google him. Nice guy.

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Theoretically there is a small section in the transition between the hardened tip at 50-55 RC and the rest of the bar that  is pre hardened to 35 RC  that is tempered softer but effectively it has not caused any sort of problem in the over 2000 bars I have forged for a steel mill using the outside heat treater.  Using the pre hard material with the forged and hardened tip was specified  by the head metallurgist of a large multinational steel manufacturer.  Grant Sarver also used to talk about keeping a short heat on breaker points when re sharpening them so as not to mess up the heat treat on the rest of the bar.  http://www.iforgeiron.com/topic/11861-paving-breaker-forging-amp-heat-treating-101/#comment-153804     

 

Thanks for all the replies it sounds like I would be OK but I am really hoping to talk to someone who is using Low temp salts.  I  will look up and try calling Jerry Larsen a call once the holidays are over, thanks for the name. 

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3 hours ago, John McPherson said:

Kozzy: always read the fine print, Grasshopper!

"Due to the oxidative products that are released upon decomposition of the salts when used near peak operating temperatures, airtight containment is recommended to reduce safety hazards."

Does not home sound shop friendly to me.

Well yes that is why they included a MSDS.   As a retired lab supervisor I recommend searching for the Manufacturer's Safety Data Sheet for any product that is new to the user.

The particular oxidative product are poisonous oxides of nitrogen.  At one point in my checkered career is spent a lot of time figuring out how to analyze them in the pure form without killing my self or my staff. :rolleyes:  

Can it be done yes.  Would I use Nitrate salts for heat treat?  Not only not but.....NO  A nice well insulated sand bath  makes so much more since and requires much less investment for heat control and maintenance.  K type thermocouples would work I think

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