Some recent reading i have done say that to cryo treat something it has to be brought down in temp gradually so as not to induce stress caused by different temps withing the piece. same as to raising the temp back to normal room temp.
if not done gradually the parts may crack.
Frederick= I meant that the kinetics are too slow to support reasonably timed diffusion reactions. Thus any change in state must be due to a diffusionless x-formation. I've never had any other than the matinsite Rx described or shown to me.
However if there is a state change (thus either reducing H {wich if its a chnage like freezing would tend to reverse upon heating} or reducing S {lower entropy arrangement}) then G would be lower.
Swall- I couldn't tell but for $20 more each it was worth it to see if the anecdotes were proved anecdotally by me.
Swall2- I dont know what cooling could do to the flake graphite, it forms interconnected networks around the dendrites. I always understood that after solidification the morphology of the graphite is generally fixed.
Kenre- The change in dimension associated with the cooling of the different components of the microstructure does not induce enough strain to cause failure... Also thats why Grey Iron (A-type, medium size distribution, I could give you the fractal dimensions if I pull out my thesis too.....) is used for brake rotors, the graphite absorbs some of the strain induced by uneven heating and cooling.
Too all:
I dont really understand how the rate of cooling or heating back to room temp is going to change the effect of the process.
I also dont understand how "stress relief" is caused by cryo-treatment. Are there XRD studies to prove it?
Here is supplier of Cryogenically treated brake rotors for patrol cars. In the testimonials they claim some incremental increase in performance. It looks like the numbers might fall within the margin of error of the experiment.
Unclesyd--I suspect what might have set off this search for brakes with improved longevity by law enforcement was the experience with the Chevrolet Impala police cars circa 1993 or so (it was the bloated whale design that looked like a '49 Hudson). I lived in South Bend at that time and the local paper reported all the trouble the S.Bend police department was having with brakes wearing out. It was an order of magnitude worse that the experience with the previous cars. Sounds to me that GM just put a really poor lining and rotor combination on these cars. At this period in time, lots of the engineering talent was leaving GM (early retirement buyouts) and that included people in the Delco Morraine brake group. Additionally, there were lots of new linings being compounded to deal with some materials that were being phased out (not just asbestos), NVH and cost issues. So, I suspect that a loss of engineering talent and a poorly executed material change in the brake linings led to the problem of poor brake life. Suddenly police departments with Chevy Impalas have a big problem and big problems bring salesmen knocking on your door.
NickE--my musings on graphite at cryo temperatures come from my brake industry experience. In lining compounds, both aircraft and automotive, graphite is frequently part of the formulation. The type of graphite, synthetic or natural can have an effect on lining performance. We used to think that there were only two allotropic forms of carbon--diamond and graphite, but with the discovery of Fullerenes, we know that there are more. So, I was hypothesizing that a cryogenic treatment of rotors could possibly change the graphite structure in the cast iron.
Traditional teaching indicates that cryogenic processing only results in the conversion of retained austenite to martensite. That goes entirely out the window with the fact that studies show it has an effect on non ferrous metals such as copper, silver, aluminum, etc.
That cryogenic processing reduces residual stresses has been demonstrated for years. There is a study by NASA on welded aluminum that states there was a reduction of residual stresses, among others.
In use on brakes, our customers typically and consitently get two to three times the life on their cryogenically treated rotors. This is way beyond any experimental margin of error, and occurs in the harshest of all experimental arenas, that being real life. These people have the data to support this. They keep track of their fleets and what type of brake life they get. Some even have chips in their cars to track the driving habits of the drivers. One large Chicago suburb went from 8000 miles/rotor change to over 30,000. When they get a new car, they just figure after about 8000 miles they will need to change rotors, and then it will be ok until about 40,000. Now we do their fire engines also.
You ask what reactions are taking place at low temperatures. Typically, the solubility of elements in the matrix falls as the temperature falls. Also, the number of vacancies in the crystal lattice structure falls as the temperature falls. This can cause a redistribution of associations in the lattice structure. This is evidenced by the research of Dr. David Collins where he states that the eta carbides formed in steels are initiated by the cryogenic process, but they form on the rise in temperature.
If these theories are correct, the slow drop in temperature typical of a cryogenic process is there for several reasons. The first is that quick drops in temperature can crack metals. It is also well known that if you drop the temperature quickly, you can "freeze in" vacancies. Evidence that vacancies are removed or moved in cryogenic processing comes from the fact that sound acuity in stereos is increased by the use of cryogenic processing of circuit chips used in them. Vacancies in chips can and do cause changes and reflection of the frequencies flowing through the chip which changes the sound. In a study by Honewell, we found that cryogenic processing "healed" vacancies in the thing film magnetic memory chips that were processed.
Other things that happen in materials at low temperature are superfluidity, and superconducting. Both of these happen at temperatures considerably lower than cryo processing temperatures. Superfluidity happens in a narrow temperature band at about 2 degrees Kelven.
By the way, one reason for the graphite in brake rotors is that it helps conduct heat away from the surface, as the heat transfer coeficient is higer for graphite than it is for iron.
Thermal conductivity of graphite can be from 25 to 470 W/mK. Steel and iron are in the 44 to 55 range. The graphite helps conduct the heat into the interior of the brake material. Without it the conductivity would be worse.
The graphite flakes provide three main functions: 1) They reduce heat checking. 2) They reduce thermal shock. 3) They provide dampening. Now, if you look at the bulk thermal conductivity of gray iron, you will not find much difference from steel. Some of the papers I have read on drums and rotors indicate that graphite flakes play some role in the thermal conductivity but don't attempt to quantify it. With type A graphite (the favored type) you have flakes as a discontinous phase in a pearlitic matrix. Consequently, if the thermal conductivity of the graphite was a factor, it would be in the near surface layer of the rotor, i.e. the depth the longest flake reached to. At that shallow depth, it could be argued that the thermal gradients are more important than graphite flakes in driving the heat into the rotor.
"Also, the number of vacancies in the crystal lattice structure falls as the temperature falls. This can cause a redistribution of associations in the lattice structure. This is evidenced by the research of Dr. David Collins where he states that the eta carbides formed in steels are initiated by the cryogenic process, but they form on the rise in temperature."
1. True; dislocation (and vacancy) density is reduced at lower temps, however wouldn't those vacancies return when the temperature is raised again? Also how do the vacancies get removed? It seems the movement of atoms would be too slow at cryogenic temps.
2. So Dr. D Collins' explanation is: That the reduction to cryogenic temps provides the driving force, then as the temp comes back up the kinetics required to form the eta-carbides are satisfied. Or did I not understand your above quote?
3. Grphite in Grey Iron is naturally occuring. It provides for many benficial properties including: dimensional stability under heat cycling, ease of machining, castability, damping, etc... Although the conductivity of the high graphite grades (class20) is higher than that of the lower free graphite grades (Class40 etc...). It seems to me that these things are very well understood, the cryo process is not well understood.
4. I have access to an XRD machine set up for residual stress measurement on martensitic carbon/stainless steels. Do they also show a reduction in residual stresses? then I could easily evaluate the claim that LN2 reduces/redistributes the residual stresses in metals other than welded aluminium. I will see these significant difference in residual stress due to cryo-treatment, correct? (I ask that question in all honesty. Because I still see no mechanism. But that, or so I've been told by other proponents of cryo treatment, is because I have a traditional education in metallurgy and materials science.
The figures I have seen in the brake drum and rotor papers in my possession cite a thermal conductivity of .08-.12 cal/cm2/cm/s/C for gray cast iron suitable for these applications. Steel was given as .07 and graphite as .39.
Flesh:
I think one company has tried to patent cryo treatment of brake rotors recently, but since it has been commercially available and reported in the press for over 20 years, they are just wasting their money and time. Even if a patent is granted, it would be worthless. As far as cast iron in general, again, the treating of cast iron engine blocks has been common for over 25 years. Our company was started because a racer wondered if it would work on an engine block, tried it, and found it would.
NickE
1.The concentration of vacancies is temperature dependent except they can be "frozen in" by a sudden drop in temperature. Going down slowly can force them out. Our experience with the thin film memory chips indicates but doesn't prove that they can be forced out at low temperatures. Remember that the temerature of the material is lowered slowly so that there are times when the material is at intermediate temperatures.
As far as the mobility of the atoms/vacancies at those temperatures, why not? I do not know too much about crystal lattice structures at this point as I am an ME and not a degreed metallurgist.
2. My iterpretation of Collins is that the drop in temperature reduces the solubility of solute atoms and frees them to combine with available carbon once the temperature gets to a point where there is energy to drive the reaction. I could be wrong here.
3. You are right, we do not fully understand cryo, especially where it varies from austenite to martensite shifts. That is why these discussions are so valuable as they spark new thinking on the subject and push us into creating theories that could explain what we repeatedly observe.
4. I assume XRD will detect the reduction in residual stress in cast iron, but you will have to be very careful because in some ways cast iron is more a composite material than a pure metal. Cryogenic processing has been shown to reduce (but not eliminate) residual stresses in many metals and other materials. I can only speculate on the mechanism.
"Short of eliminating any potential for warpage, I can't see how stress relief (from cryo or otherwise) could improve rotor life."
I don't either. That is why I believe that the true benefit from cryogenic processing of brake rotors comes from the increase in abrasion resistance imparted to the rotor by the cryogenics. We've found that cryogenic processing will increase the abrasion resistance of most if not all metals. There is a lot more to cryogenic processing than stress relief and conversion of retained austenite. We just have not proven exactly what that is.
One big problem cryo companies face is that the first thing a metallurgist will do is a cross section of a treated part and an untreated part. He (or she) will then claim there is no difference, and that the process is a fraud. We've had this happen after imparting huge life increases in the part that are outside any natural variation in the material or part function. We need to steer the research away from microstructure and really get someone who knows the ins a outs of crystal structure. That is where we will most probably find the answers to the questions that are continually asked about the process.
The life increase in brake rotors is one of the best things that have happened to cryogenic processing as this use is growing rapidly. It puts a very successful use of the process in front of the public. Your government vehicle fleets are strapped for cash because of the higher prices of fuel and the presure to keep taxes down. They don't care why it works, just that it is saving them a lot of money. Maybe this increased use will spur some real research.
I really appreciate the input from the members of these forums, because it provides new insights into the process and its mechanism. We will find the answer someday.
