Revamping Charpy Impact Specimen Testing Setup

[Rogue909]

I am in the process of reviewing our Charpy testing procedures & setup. Right now our procedures are very good... our setup? Less so.

Current setup
We are currently using a Nalgene 4150-2000 insulated "bucket" filled 6-8" high with methylbutane. The specimens are loaded 9 (I think) high in a rack and put in the bucket. Liquid nitrogen is poured on top as the cooling media. A thermocouple & mixer are lowered into the bucket to agitate & monitor the concoction.

Testing parameters
All of our tests range from -150°F to room temp.

Problems; The important part..
Our bucket is a pain to get the samples in and out of. The racks don't stand up like they are supposed to and fall over. Then the mixer & thermocouple get knocked around like a pinball machine.
When the operator messes up and need to put a sample back in the bath they cant. The rack needs to be emptied first then refilled.
The specimens, while waiting for the rack to be emptied, get condensation on them. When they are loaded into the rack they freeze together into a block.
Methylbutane is expensive (course' everything is), requires a Hazmat fee, and must be kept in a freezer or it explodes.

Objectives
I want to replace the bath with a shallower rectangular bath. Preferably one that is self-agitated with a warmer. This way they can pour in the liquid nitrogen and let it overshoot a bit; then bring it up to temperature. (We have other equipment that uses the liquid nitrogen. So sourcing the liquid nitrogen is not a concern.) Then we can lay the charpy specimens down on the bottom of the bath (or on a false bottom) and pick them up one by one.
I'd like to switch to something other than methylbutane. Methylbutane was sourced long before I got here; and I have no idea why. (Can anyone answer that??)

I've had a hard time sourcing the bath. I'm beginning to believe that I'm looking it up wrong.
I'm also not entirely sure what to use as a substitute for the Methylbutane. I've read ethanol multiple times; doesn't ethanol have issues with evaporation?
It is probably evident that this isn't my forte. I was hoping I could get some insight in the right direction.

 

[btrueblood]

Isobutane and LN2 gets you -160 F, not sure why you would use it unless you need to get down below about -100 F. Ethanol+glycol in different mixtures, in dry ice, will get you a variety of temperatures.

More here:

https://en.wikipedia.org/wiki/Cooling_bath

 

[btrueblood]

Oh, and methyl butane is just pentane or iso-pentane. And yes, you will need something that freezes at or below the temp you are trying to hit, driven to that point by dry ice or LN2, but there are hydrocarbons that are less volatile, or less toxic, or less flammable than others. The comment about using 2-propanol (aka isopropyl alchohol, or rubbing alcohol) in lieu of acetone in a dry ice bath is one such substitution.

 

[metengr]

If you are routinely conducting CVN impact testing at low temperatures, take the time to talk to the folks below. Your current low temperature impact test set-up sounds like you would not pass an audit on ensuring your specimen temperatures are uniform and reliable.

https://www.spscientific.com/Produc...ystems/Low_Temperature_Baths/CharpyCool_Bath/

 

[EdStainless]

I don't do these any longer, but we cooled them dry.
A chilled Cu plate inside of an insulated box.
We just set the temperature. It took a while to chill samples, but no solvents, and everything was dry.

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P.E. Metallurgy, Plymouth Tube

 

[Rogue909]

@btrueblood
How would you go about storing the chemicals when you're done with testing? Currently our insulated "bucket" is stored in the freezer with the methylbutane. I'm not the biggest fan of the technique.

@metengr
What about our setup do you find problematic with regards to the specimens temperature?
While I am certainly open to admitting mistakes & not to being an expert in the field; we are audited frequently by customers, 3rd party, and regulatory committees.

The SPScientific CharpyCool system does not get cold enough for our samples. I'd seen their system already

@EdStainless
I've not yet looked into dry cooling systems. Are they sufficient to get to -150F?
Do you have one off of memory?

 

[EdStainless]

Our cold plate was capable of reaching -150F.
I think that our refrigeration system went to about -60F, and then it switched over to LN.
I don't recall who built it (this was 30 years ago), but it was a bought system.

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P.E. Metallurgy, Plymouth Tube

 

[btrueblood]

FWIW, I've never done cold charpy testing, most of my experience is in test bunkers (functional testing of various aero systems) and chemistry labs with low temperature reaction rates (rocket fuel ignition delays). Fun stuff. Blew the window out of a fume hood one time...

Glycol and isopropyl alcohol plus water mixtures can be stored in capped plastic bottles/jugs on a shelf at room temperature. I'd probably have several ready-made solutions (to suit various temperatures) on the shelf, or in a freezer, in quantity sufficient to fill the pan, and a tap at the base of the pan to allow draining the fluid back when finished with testing.

Condensation is always a concern when doing low temperature testing. Either have a glove box or similar, possibly vented to outdoors, which is trickle-purged with dry nitrogen, where you can put samples being transferred, or have the test lab be a conditioned space where the humidity is kept very low, both methods should help reduce condensation build up. If you use polar solvents like acetone or alcohols/glycols, i.e. ones that mix with water, you do need good humidity control, or your samples will soak up moisture from the air and slowly drift over time. There are fairly straightforward ways to assay the amount of glycol or alcohol in water mixtures, or you could just dump them when you see the temperature drift off of spec, and mix up fresh solution.

If you are doing this often enough (more than once a month) I would certainly look into the dry sump chillers, if only for the convenience (set a digital controller and walk away). In the end it will likely depend on cost too.

 

[Compositepro]

In my experience with low temperature mechanical testing, the test chambers were kept cold with a spray of liquid CO2 (LN2 could be used). A digital controller activated a solenoid valve for cooling and a blower kept the chamber stirred for uniform temperature. Test specimens were pre-cooled in the chamber and kept there until ready to test. Operation was very simple, clean, and reliable.

The room must be well ventilated.

I would be very concerned about storing a bucket of flammable liquid in a freezer.