Relief of residual stresses seems to be the current mechanism offered up by proponents in which to explain the benefits of cryogenic processing cast iron brake discs. The latest issue of Heat Treating Progress from ASM has an article on the cryogenic industry. Even shows two images of brake discs (with and without cryo processing).
Hmmm...if the brake rotors are tested for wear, and one is at cryogenic temperature before the brake material makes contact, and the other is at room temp...then one brake sees a lower average temperature-vs.-time...could reduce wear...
TVP - I thought it was a review article when I first read it but the more I look at the affiliation of the author the more I think it is advertizing blurb!
Well, I was given two rotors free and was asked to perform a metallurgical analysis. I've completed the simple basics (microstrucutre, chemistry, hardness) and it looks like a standard grey iron. Any ideas on what other analytical techniques I should be using? Or is physical/performance testing necessary?
The article on cryogenic processing in Heat Treating Progress is the publication of a paper given at the ASM conference last fall. It is not an advertising blurb. Yes, the author is affiliated with a cryogenic processing company, but so are a lot of other people who know something about cryogenic processing. I find, however, that not many of its detractors know much about it.
I do not believe that the relief of residual stress has a whole lot to do with the increase in wear resistance in cast iron. And, as the microstructure is pearlitic cast iron, and there is very little or no retained austenite, it cannot be the austenite to martensite transformation either. So you have to look deeper.
We believe that the benefits of cryogenic processing are from subtle changes in the crystal structure. As the temperature drops, solubility of elements in the matrix changes, vacancies move or are eliminated. We have seen some indication of this in the electronics industry. In a project done for Honeywell on thin film magnetic memory chips, Honeywell thought that they detected the movement of atoms in the structure that healed vacancies in the chip layers. The program ran out of time and money before this could be confirmed. But the removal or moving of vacancies could explain the increase in sound acuity in stereo systems after cryogenic treatment.
We know for a fact that these effects are responsible for the formation of carbides in hardened steels. (See the work by Dr. David Collins.) Nobody that I know of has proven this in cast irons, but it is plausible that carbides are formed. There is also a theory that the atom to atom spacing in a crystal structure has an ideal distance where the bond energy is at a minimum. Reducing the temperature allow this spacing to become more even and closer to ideal, creating a better crystal structure. These are unproven theories, but the do give an explanation as to why pearlitic cast iron responds to cryogenic processing when there is little or no austenite to transform. It would also explain why cryogenic processing works on brass, aluminum, silver, titanium, etc.
Cryogenic processing consistently increases the life of brake rotors two to three times. There is no doubt to that. Why? Some day we will find out. Until then there is no reason not to use this amazing process to reduce costs and increase efficiency.
Having worked for an auto brake supplier,I would like to point out that there is considerable variation in linings and rotors. Linings are a sintered product, a MIXTURE (i.e. non homogeneous) of different ingredients. Even with the same recipe, linings will vary from batch to batch. Cast iron rotors will vary from ladle to ladle of iron. This variation in the materials can be expected to show up as a variation in performance, so this has to be taken into account when comparisons are made of cryo vs non cryo product. To his credit, I believe Frederick has had some brake dyno testing done on cryo treated rotors.
Ok, so what I'm reading here is I need to perform physical testing of my samples if I want to prove to myself these cryogenically treated rotors are "better". Thank you all for your input.
If you have access to x-ray diffraction, I would recommend performing some testing on as-cast and cryo processed parts. Measure the residual stress from surface to core in several steps. If there is no substantial difference in residual stresses, then it is likely a different mechanism at work, such as one of those described by Mr. Diekman.
We use lots of LN2 here at work. After an official process is complete the vat just sits there for days at a time. What would I do to my rotors to cryo treat them?
Well, I can't speak for the cryo processors, but from what I've read in the literature, both theirs and third party, the cool down cycle, time at cryo temp and ramp up back to room temp are considered to be some of the important process paramaters. In other words, "don't try this at home, kids". But, if I were you, Tmoose, I would throw a couple in for a couple of days just for fun. May not/probably won't do any good, but at least you won't screw them up.Might want to measure rotor runout before and after, though.
"There is also a theory that the atom to atom spacing in a crystal structure has an ideal distance where the bond energy is at a minimum. Reducing the temperature allow this spacing to become more even and closer to ideal, creating a better crystal structure."
So if the crystal structure is more ideal, there must be a reduction in G.
With no change in H, since there are no reactions occuring at this low of a temperature, and no change in S (since its still pearlitic iron) there is no permanent change from lowering T to a small value, then after X time raising it to 293K again.
(That said I bought cryoed Rotors on my last change and they did last longer than the stock rotors with far more aggressive pad compounds.)
I have heard of different carbides and such precipitating, but they would be very very small since the driving force would be huge and kinetics are non-existant. Thus highly homogenous neucleation.
NickE, Swall, on that note -- What if those rotors contained a slightly elevated level of primary carbide phase (which formed during solidification). Wouldn't you expect to see better wear resistance?
As a side note, I did find this phase in my samples. Although my initial estimation put it at less than 1% by volume.
Say I have a population of brake rotors, some of which are destined to crack from the thermal stress associated with normal braking.
If I process them cryogenically, some of those doomed rotors will crack from the thermal stress associated with cryogenic processing, and will never have the opportunity to fail in braking service.
Did the cryogenic processing improve the rotors that survived it?
Flesh--As to primary carbides enhancing the wear resistance of a rotor, I would say yes, no and maybe. In theory, one could argue that primary carbides would be indicative of a rotor with higher hardness/better wear resistance. But, they are not uniformly distributed and in service could lead to hard spots and uneven rotor wear. In a cast iron rotor, you want a good graphite distribution to enhance dampening and increase the resistance to heat checking. Primary carbides are an indication that the carbon equivalent of the iron was a little too low and that an optimum graphite structure was not achieved. Now graphite--and the effects that cryo treatment might have on its structure--that would be an interesting subject for research.
Tmoose
If you throw a couple rotors into the vat, do me a favor and don't drive around here. Its a good way to induce cracking, and all available research indicates that that won't work anyway.
Nick
Who says that there are no reactions occurring at -300F? By definition, things START to happen at absolute zero. If you want to get a good laugh at a Cryogenics Society of America meeting, tell them that reactions do not happen at -300F.
