What Thermal Shock ?

One thing you'll often see on cryogenic Web sites or read in articles is a reference to a "safe" process . It refers to a common practice in the early days of processing tooling where parts were just dropped into a container of liquid nitrogen, which sometimes caused them to crack or shatter from the thermal shock. At least one article by a processor manufacturer on the Internet goes to great lengths to explain how their dry process is "safer" than immersion in liquid nitrogen, with the strong implication that any immersion process is "unsafe". However, they misconstrue the actual immersion process used by all except one manufacturer - who does quench in liquid nitrogen.

Our processor cools by evaporating liquid nitrogen through a heat exchanger that exhausts into the chamber, thus producing a pure nitrogen environment. It can be used entirely as a "dry cryo” unit, continuing to hold the temperature at -300F with just nitrogen gas, but we prefered to program it to flood the load with liquid nitrogen two hours after the temperature has bottomed out for several reasons.

1. The rule of thumb is, "the colder, the better”, and the chart recorder shows that the temperature is at least 20F colder with liquid than with just gas.

2. A liquid transfers heat about 25 times faster than a gas, so for big pieces like a cast iron 483 hemi engine block, the cycle time of the typical dry process barely has time to cool the thickest areas before the temperature begins to rise. The extra cold and higher rate of heat transfer from liquid nitrogen does a much more thorough job.

3. We keep the highest item in the load totally submerged for at least 24 hours, which means that the items on the bottom of the load are sometimes at soak temperature for over 60 hours. Much research has confirmed that a soak time of less than 20 hours leaves measurable retained austenite. Many of the "dry cryo”people advertise a total cycle time of less than 20 hours, and if you subtract the ramps down and up, that is far less soak time than is adequate. To be sure, their process helps in many cases of poor heat treating, or especially if stress relief is the only goal, but it’s certainly not an optimal recipe. The extra time it takes for the liquid nitrogen to flash off with our process guarantees a long soak. If 99.5% of all heat treated steel was improved to the limit by a 20 hour total cycle and only 1/2 of 1 % was helped by our recipe, we'd still do the long process. We want to run the absolute best possible recipe for any situation.

4. Liquid nitrogen guarantees a dry (water vapor free) atmosphere in the processor. Most processors cool by injecting nitrogen gas into the chamber like ours does on the rampdown. That also drives out all atmospheric moisture that is in there from having it open for loading, but there are two well known processors that circulate liquid nitrogen through a heat exchanger to just cool the air in the processor. That causes a severe rust problem on any ferrous metal parts in the load due to condensation during the rampup.

5. The processor is not opened until the load has reached ambient room temperature.

So, the bottom line is: