uraniummetallurgist

The properties of steel are derived from a combination of alloy and heat treatment. In the heat treatment, the steel is held to a particular temperature to ensure the desired phases are formed as per the diagram kindly provided by Adk46. Then the steel is quenched at a specific cooling rate to "lock in" the desired phases which occur only at high temperature once the steel is at room temperature. The phases have different atomic structures and chemical composition and therefore unique physical properties. This overall process is tracked in what is called a TTT or time, temperature, transformation curve.

Cyrogenic treatment involves not only cooling but often also cycling heating and cooling to achieve the desired outcome. This is the "secret" part.

All the best,

joe

MainePorsche

So,

Taking a rotor out of a box and putting it in a freezer will not quench at the specific cooling rate required for the properties desired.

Adk46

In the more usual context of certain steels for demanding applications, it would be done after a first quench into oil or water, but before tempering. Most transformations happen only a elevated temperatures, where atoms are jumping about and can move fair distances to find happiness according to the phase diagram. The strange thing about steel is quenching doesn't allow time for this, but instead there are coordinated short mechanical shifts along certain planes in the crystal lattice - producing "martensite" from the parent "austenite". It's very hard stuff, and brittle. Tempering brings back a certain amount of normalcy, from which you get some toughness and ductility.

But the martensitic transformation may consume only some of the parent austenite. The remainder might be tolerable, or not - cooling to below room temperature can reduce it.

More pedantry: Shape memory alloys depend on this martensite business, which is brought on by mechanical deformation. As chaotic as the result appears to be under a microscope, in the titanium-nickel system the transformation is magically reversible with a little heat - all the little shifts spring back, in the order they occurred.

Edit: For reducing retained austenite, the quench rate below room temperature isn't important, just the temperature reached. If I'm guessing correctly, your rotors have not received just a quench, they've been tempered also - too late to do any more quenching.

Wisconsin Joe

Just like heat treating, it can happen at any time. "Cryo-treating" is somewhat common for gun barrels. It has shown to gain some benefit. Usually in the single digit percentage for accuracy.

And that "some wear benefit" is the key.

How much is "some"?

Obviously, if the rotors are expensive and the treatment isn't (I have no idea how much it costs) and it extends the life, then it's worth it.

If not, not.

uraniummetallurgist

Sorry all that will do is make the rotor cold. Cryo treatment uses liquid nitrogen at a temperature of -320F and follows a specific holding time at a variety of temperatures.


All the best,

joe

MainePorsche

Thanks Joe.
That's what I thought.

wpgshark

I had real trouble with rotors on my Volvo V70, the car was always overloaded screaming up and down hills on ski trips, the front rotors just weren't up to the task. Anyway after trying everything else, I ended up with slotted cryos. I was so impressed that I now have them on virtually everything I own that isn't a Porsche. When the time comes of the 928 I'll put them on that as well.

RKD in OKC

In 1997 I heard about and found cryotempering. And it's not just getting them very cold. It is a procedure of lowering and raising the temperature at specific rates depending on the metals much like heat treating and quenching. They way it was explained to me was the process more evenly distributed the crystalline structure in the metal.

When I asked my mechanic about it (an old guy that had been doing stuff racing for a very long time). He told me about how he used alcohol and dry ice to cryotemper the push rods on high performance 911 motors. Said if they did not cryotemper them at the rates and performance they were running them the push rods would bend. He recommended we try the cryotempered rotors on my 944TurboS with the bigger brakes that I was having problems with the brakes getting too hot despite the different pads we tried.

When I asked the engineer/founder of the manufacturing company of oil & gas controls I worked for about it, he said it was a bunch of BS. That year for Christmas I gave him two Bic disposable razors. One was cryotempered one was straight from the drug store. I put a note in with them asking to try them both and see if he found any difference in them. He came back to me amazed because one lasted almost a week as usual but the other stayed sharp for over a month!

Also gave the razor test kit to all those at the company that had influence over engineering and tooling. The company went thru a huge number of drill bits and the cryotempering company guaranteed the drills would last 5 times longer for each time they were sharpened. That would have saved the company a huge amount in not only tooling, but time spent sharpening. They all thought the razor test was amazing but believed the tooling company they bought drills from that it was BS, despite the cryotempering company guaranteeing drills would last 5 time longer or their money back.

When I sold the 944TurboS and got the 90GT I started having the front brakes overheat. Added rear braking bias, then higher temp pads, then cryotempered slotted rotors. The overheating stopped after getting the cryotempered slotted rotors.

On my current GTS I have only added the higher rear brake bias. I still have the stock rotors and pads, with no fading due to overheating. But, after a day of autocross the rotors do have a nice blue tint to them and I had to disconnect the front wear sensors because i was melting them and they would go off.