Brake rotors cryogenically treated
Brake rotors cryogenically treated
(OP)
Can someone state the mechanism by which a cast iron brake rotor, when cryogenically treated, becomes more wear resistant?
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Brake rotors cryogenically treated
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RE: Brake rotors cryogenically treated
RE: Brake rotors cryogenically treated
RE: Brake rotors cryogenically treated
RE: Brake rotors cryogenically treated
RE: Brake rotors cryogenically treated
RE: Brake rotors cryogenically treated
RE: Brake rotors cryogenically treated
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.
Regards to all,
F. J. (Rick) Diekman
RE: Brake rotors cryogenically treated
RE: Brake rotors cryogenically treated
RE: Brake rotors cryogenically treated
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.
RE: Brake rotors cryogenically treated
TVP mentioned XRD - this technique should be performed at cryogenic temperatures to prove/disprove the atomic spacing theory.
Regards,
Cory
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RE: Brake rotors cryogenically treated
RE: Brake rotors cryogenically treated
RE: Brake rotors cryogenically treated
Problem:
G=H-TS
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.
Nick
I love materials science!
RE: Brake rotors cryogenically treated
RE: Brake rotors cryogenically treated
As a side note, I did find this phase in my samples. Although my initial estimation put it at less than 1% by volume.
RE: Brake rotors cryogenically treated
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?
Mike Halloran
Pembroke Pines, FL, USA
RE: Brake rotors cryogenically treated
RE: Brake rotors cryogenically treated
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.
regards
FJD
RE: Brake rotors cryogenically treated
if not done gradually the parts may crack.
Ken
RE: Brake rotors cryogenically treated
What REACTIONS occur in steel at - 300 °F?
I suppose terminology is important here. What is the definition of reaction, precipitation, phase change, etc.?
Regards,
Cory
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.
RE: Brake rotors cryogenically treated
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?
Nick
I love materials science!
RE: Brake rotors cryogenically treated
http://www.commandautomotive.com/pages/8/index.htm
One of our local law enforcement agencies was trying this type rotor but wrecked both test cars in non brake related incidents.
RE: Brake rotors cryogenically treated
RE: Brake rotors cryogenically treated
RE: Brake rotors cryogenically treated
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.
RE: Brake rotors cryogenically treated
RE: Brake rotors cryogenically treated
What's your point?
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.
RE: Brake rotors cryogenically treated
RE: Brake rotors cryogenically treated
RE: Brake rotors cryogenically treated
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.
Nick
RE: Brake rotors cryogenically treated
RE: Brake rotors cryogenically treated
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.
Regards to all of you,
F. J. (Rick) Diekman
RE: Brake rotors cryogenically treated
RE: Brake rotors cryogenically treated
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.
Regards,
F. J. (Rick) Diekman