quenching aluminium in salt baths 2

[etch]

I was thinking today about another post, when someone asked if aluminium was corrosive. I replied that salt water is coroosive to aluminium.

I am now thinking if this is so, why do people use oltem salt baths as a quenching medium, surely this promotes corrosion, especially along grain boundaries. Could it be that there is little water for the salt ions to attack the metal, or the length of quench is so short?

 

[ornerynorsk]

I might have missed something in my years, but generally salt bath is used on ferrous heat treating. All the aluminum I've ever had treated has been done atmospheric. I believe molten salt is too hot to be useful for any aluminum applications.

 

[TVP]

etch,

Can you elaborate what you mean by "oltem salt bath"? Surely you did not mean molten salt baths, such those used in ferrous heat treating (ornery norsk mentioned this).

Quenchants used for aluminum alloys usually fall into one of two categories:

1. Water - room temperature to boiling
2. Aqueous solutions of a polymer - polyalkylene glycol or polyvinyl pyrrolidone

 

[etch]

hey this is why im asking, i just read that molten salt baths are used as quenching mediums, I guess you have answered my question, THERE NOT USED ON ALUMINIUM.

 

[trytrue]

I hope this is not too late and somebody can read it.
On 05-11-02, ETCH posted about short cycles in aluminum heat treat.
After some replies, TVP realized that it is possible to have very short heat treat cycles in aluminum alloys and he (TVP) posted titles of three excellents technical report about short heat treat cycle.
One of them "Effect of a short solution treament time on microstructure and mechanical properties of....." in the 2nd page states "The approximate dimensions of the bars were 12x15x55 mm. The bars were solution heat treated in high temperature salt bath held at 540 or 550 degrees C. The advantage of using a salt bath was that the heating rate was high..."
The quench was made on hot water but the solution in molten salt bath.

 

[kenvlach]

People sometimes make the mistake of equating salt with sodium chloride.
From
Heatbath

http://www.heatbath.com/Products link page.htm

ALUMINUM SOLUTION HEAT TREATING

Aluminum solution treating is commonly conducted in molten nitrate baths in the 850 - 1050 deg F. Maintenance of neutrality to avoid attacks on aluminum is critical. Heatbath Park Metallurgical nitrate salts are formulated for maximum purity and neutrality for use in aluminum solution treating.

PRODUCT IDENTITY MELTING POINT WORKING RANGE
Tempering C-ALHT 430 deg F 480-1100 deg F
AL-2 Salt 500 deg F 550-1100 deg F

Tempering C-ALHT A granular mixture of sodium and potassium nitrates that is formulated per MIL-S-10699-B, Class 11 and MIL-H-6088. The salt is colored pink for identification purposes.

AL-2 Salt A granular mixture of sodium and potassium nitrates that is formulated per MIL-H-6088. It is less costly than Tempering C-ALHT with only a slight sacrifice in working range. The salt is colored pink for identification purposes.