Cryogenic Quenching

[uel23]

Studying the effects of various heat treatments for Aluminum alloy A319, one idea was to change the quenching medium after heating into solid solution. I decided to quench in liquid nitrogen and dry ice. After quenching and aging for two hours I conducted tensile tests. For the two quenching mediums, I recieved similar results for maximum tensile strength and ductility, however the yield strength for the dry ice quenched samples was significantly higher. I was wondering what would be the cause of this.

 

[swall]

Were the specimens separately cast test bars or were they bars machined from castings?

 

[EdStainless]

Dry ice plus what? What was the liquid that you used?

Liquid gases will give rather slow cooling rates because of the surface boiling film.
My guess is that the quench rate in dry ice was faster. You could use a thermocouple and see.

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[uel23]

the bars were cast test bars, machined bars from a cast are to be used for seperate testing. The dry ice was made from acetone + CO2, so the liquid was the acetone.

 

[Compositepro]

Actually boiling conditions usually result in the highest heat transfer rates. Latent heat is used and the boiling creates lots of turbulence and convection so there is still plenty of liquid contact. With no boiling the liquid must warm in order to cool the metal. Then the warm liquid must move away so cold liquid can continue to absorb heat - a much slower process.