[unclebilly]

Ah hah! I do have experience with this.

I did some experimentation with this as one of my 4th year mechanical engineering projects. If I dig, I should even be able to find an E-Copy of my report.

Here's what I did:

Got a used brake rotor, and cut in into several specimens.

Specimen 1 was my standard.

The remainder of the specimens were vapour cooled with LN2 (liquid nitrogen) to about -100°C and then were placed into the LN2 (I'm going from memory on the temp there) in an insulated container.

The next morning, the specimens were still below 0°C but the LN2 had all evaporated. I removed one of the samples from the batch (Sample 2).

I then normalized the remaining samples at about 100°C for one hour in air and set one aside (sample 3).

The remaining samples were then heated slowly to 600°C (extreme brake rotor temperature - I developed technology to measure brake rotor surface temperature on a previous project in instrumentation class).

The samples were then cooled (I can't remember if I air cooled them or quenched them - maybe I did both).

I then polished and etched all of the samples with nital. And looked at the grain structure with an optical microscope.

Here's what I found:

Sample 1 (untreated) had graphite all through the sample - nothing special.

Sample 2 was pretty neat - carbide had been precipitated into all of the grain boundaries (hence the long wear characteristics of cryo treated tool bits).

Sample 3 still had the carbides but some had turned back to graphite (very small amount).

The heated samples - these were different from sample 1. The carbides were all gone but the graphite was not randomly scattered throughout the sample - it was only in the grain boundaries (where the carbides had been). The metallic structure was much more regular than sample 1. This can all be clearly seen in the micrographs in the report.

Upon conducting hardness testing on any of the samples, I didn't see any significant difference in any of the samples.

So there's my 2 bits - cryo treatment does change cast iron. Would I pay $650 to have it done? Probably not, you can get LN2 and all you need is a big enough cooler to use as your cold chamber. Be creative.

If anyone want's my report, let me know and I'll try to find it any email it to any intertested parties.