WPC and Cryogenic Treatment for Forged Aluminum Components 1

[drakem4]

Hello everyone!

I am building a small displacement high output 2-stroke engine right now and the goal is for it is to have excellent abrasion resistance (low wear) and be very reliable. The components that I will be treating are mainly the piston and rings, cylinder lining (aluminum with Nikasil coating). I will also be treating the bearings and crank.

I have heard excellent things about cryogenic treatment. I know that cryogenic treatment is being done to the brake rotors on fleet vehicles (such as police vehicles) and the rotors are lasting at least double the life.

I have also heard excellent things about WPC Treatment (sonic velocity shot peening with nano-sized media). It acts like a heat treatment process at the surface by compressing the outermost layer of the material.
Go to the 55 second mark for demonstration in this link

https://www.youtube.com/watch?v=VvKH-BYQcoM

Now for the question. Which order of processes would benefit the materials better? To cryo treat, then WPC treat? Or to WPC treat, then cryo treat after? Any advice would be greatly appreciated, many thanks!!

Myles

 

[tomwalz]

As near as I can tell, after following cryo for decides, sometimes it is beneficial and sometimes it is not.

To the best of my knowledge, it is still not predictable.







Thomas J. Walz
Carbide Processors, Inc.

http://www.carbideprocessors.com

Good engineering starts with a Grainger Catalog.

 

[drakem4]

Could poor or negative results possibly be the cause of improper processing? I know that the proper processing requires lengthy (even 72 hours) for the complete gradual deep freeze and return.

When you say sometimes it works sometimes it doesn't, by doesn't do you mean adverse effects or just neutral effects (as though nothing had actually happened)?

Thanks for the input!!

 

[tbuelna]

With the application you describe, I don't think the cryo or shot peen treatments will help in terms of abrasion or wear of your engine parts. Shot peen would help with fatigue life of some components, and the cryo treatment might improve the fatigue life/durability of certain steel components. But if your goal is to improve the wear (or abrasion) resistance of the engine components subject to sliding contact, it would be better to focus on improving the lubrication conditions present. You should look at optimizing things like surface geometry & roughness, oil film thickness, heat transfer, contact area, etc. Improving these things will give much better results for what you want to improve than cryo treatment or shot blasting. Plus, these improvements will likely cost far less than cryo treatments or shot peening.

Good luck to you.
Terry

 

[drakem4]

WPC is not just a shot peening procedure, it has nano sized particles that make a surface that can hold oil to help with lubrication. That is its main benefit which should help with abrasion resistance. And for reliability of this engine I will most certainly need materials that resist fatigue/warping. Am I still off on the idea of this WPC and cryo treatment?

 

[tbuelna]

drakem4-

Based on the questions you are asking, it would probably be helpful if you did some reading on how shot peening works, and also about the fundamentals of sliding oil film contacts. With a bit of reading you could probably answer most of these questions yourself.

If after reading up on these subjects you still have some questions, I'd be happy to try answering them for you.

Regards,
Terry

 

[FennLane]

The basics are quite simple:

Shot peening provides compressive surface stresses which help to improve resistance to fatigue crack initiation.

The level of residual stress is important to monitor and the size and shape of the media is also important.

The usual method of assessing the residual stress is by using Almen Strips.

It also seems that anything with the name 'nano' must be better than any conventional treatment.

Some of the claims for this process may be true but I find it difficult to accept that 'micro thermal reactions seal minor surface fractures'

The photomicrographs showing the surfaces are in my view unrealistic and shouldn't be believed.

As regards cryogenic treatment I can accept that there is some benefit in improving the dimensional stability and the performance of steels that retain austenite after heat treatment and that these is some influence on the precipitation of epsilon carbides in tool steels but I find no compelling evidence that DCT has any benefit for Aluminium Alloys.

Some of the claims made for the treatment of cast iron discs are surprising and always start with the disclaimer that metallurgists don't understand.

How does wear improve when there is no change in hardness or indeed structure? is this simple Black Magic?

 

[Frederick]

Interesting. People still believe the only thing Deep Cryogenic Treatment(DCT)can do is to convert retained austenite to martensite.

There are more than just "claims" that brake rotors last longer. There is proof that when they are properly DCT treated there is a substantial increase in wear resistance. A presentation was made at the last ASM Heat Treating show covering the subject. It covered both laboratory tests and practical field tests. There is a real increase in wear resistance.

DCT is not Black Magic. Anyone claiming so must have slept through Metallurgy 101 class. If you remember from Metallurgy 101, metals are crystal structures. Crystal structures react to changes in temperature. As the temperature is reduced, the solubility of alloying elements in the matrix is reduced. As the temperature is dropped, point defects are affected. It is less well known that the atom to atom bond is also affected. DCT causes movement of point defects such as vacancies to the grain boundaries. It also causes the formation of very fine carbides. It reduces the excess energy in the metallic bonds creating a much more uniform crystal structure. All these work to increase wear and fatigue resistance. All these are products of basic metallurgical concepts taught in the most basic metallurgy classes. Metallurgists are so used to working with microstructures that they forget about crystal structures.

As to hardness, DCT causes the hardness to become more consistent. It is not unusual for the standard deviation of the hardness to drop to one half of what it was before treatment.

As to whether DCT works on aluminum, papers by places such as NASA say it does. The Chinese have published several papers on aircraft grade aluminum. DCT also works on carbide as shown by multiple papers. Before you go out and say there is no compelling evidence, at least do a google search on the subject. You will find multiple papers from credible sources.

There are some caveats. DCT must be done properly just like heat treat. Dipping something into liquid nitrogen is not DCT. We have a lot to learn about the process, but the knowledge base is growing quickly. Labeling it as Black Magic or Snake Oil is irresponsible and slows the rate of research.

 

[tbuelna]

The OP asked specifically about the benefits of DCT and WPC in terms of "abrasion resistance" when applied to certain components (piston, rings, liner, crank and bearings) of a 2-stroke engine. Normally these parts are designed to operate with lube oil film contacts that minimize the potential for conditions that would produce abrasive wear of the surfaces. In this regard, I don't think either the DCT or WPC process would help much.

As FennLane noted, when applied properly the residual surface compressive stress from shot peening will improve resistance to fractures that initiate at a metal component's surface loaded in tension. However, the surface finish/roughness profile produced by shot blasting with "nano particle" media probably produces the same result in terms of oil fluid film performance as any conventional grinding/honing/lapping process would. There is also the issue that shot peening usually requires the media to be applied perpendicular to the surface for best results. This would be difficult to achieve on the inside surface of a small diameter cylinder bore.

As for what Frederick noted about using DCT on aluminum components, I checked for any relevant references in the NASA tech database and only found one. It stated the DCT process was not very effective with most aluminum materials. I linked the relevant page of the tech paper below.

 

[Frederick]

First a word of disclosure. I own a cryogenic processing firm. I am co-chair of the ASM Cryogenic Committee. I have hosted ASM webinars in concert with a NASA engineer and a metallurgical engineer from Air Liquide. I think I can speak with authority on the subject.

The paper that tbuelna cites is named Effects of Cryogenic Treatment on the Residual Stress and Mechanical Properties of an Aerospace Aluminum Alloy, by PO Chen et al. The conclusions the researchers came to were:
1. Residual stress was reduced by up to 12 ksi in the HAZ of weld specimens and by up to 9ksi in the parent metal.
2. Significant improvements in SCC (Stress Corrosion Cracking) performance were seen for weld specimens.
3. Minor increases in tensile strength and hardness were noted for parent metal.
4. No significant changes were found in the tensile properties for weld specimens or in fatigue properties for parent metals.

Nowhere does this paper state that DCT is not very effective with most aluminum alloys. The research only covered one alloy and the results were listed for "this particular alloy."

Effect of Cryogenic Treatments on Mechanical Properties of 2A11 Aluminum Alloy, Wag et al, Advanced Materials Research Vols 146-147,(2011) pages 1646-1650 states: "The influences of different process parameters on mechanical properties of 2A11 aluminum alloy were compared, and the results showed that cryogenic treatment could improve mechanical properties of 2A11 aluminum alloy. The dimensional stability increases after cryogenic treatment once, and increases further after cryogenic treatment again."

More are available on Google. The point is that the DCT has been tested and found effective on aluminum alloys. The practical tests in use on engines is also significant. Our customers have reported 5 times the life on racing engines. One was a national champion go kart racer racing a Briggs & Stratton engine.

By the way, shot peening inside a cylinder is not that hard to do.

 

[tomwalz]

Dear Frederick;

I agree with all you have said.

However I still have the question as to whether the performance is predictable or not. If I send you parts can you guarantee a definite, quantified increase in performance for every part, every time.

Thomas J. Walz
Carbide Processors, Inc.

http://www.carbideprocessors.com

Good engineering starts with a Grainger Catalog.

 

[Frederick]

Tomwalz:

DCT will give consistent results if the parts treated are consistent. The variables are the components being treated and the processor being used. I cannot control the quality of what is sent for processing. Nor can any heat treater. But our processing machines are microprocessor controlled, have vacuum insulation, and I can control how the parts are placed in the machine. Our machines use heat exchanger technology so your parts are never hit with liquid nitrogen. We can put a thermocouple on the part to record it's temperature over time. Given this, yes I can give you very consistent results. I cannot speak for other processing companies on this though. Please forgive me for getting very close to advertising behavior here, that is not my intention. My intention is to say that with proper equipment and proper procedures the results will be consistent if the components being treated are consistent. Deep Cryogenic Treatment is a form of heat treatment (it is all heat above absolute zero), and metal crystals obey the laws of physics. But if you vary the treatment via inadequate equipment or procedures all bets are off.

 

[tbuelna]

Frederick-

Like I said, I did a search on the NASA tech server to find some of the numerous papers you said were published by NASA regarding this subject. The only one I could find was the one I linked. The conclusions noted by the authors were that DCT was beneficial for reducing the residual stress in the HAZ and local parent material of some fusion weld samples produced from the "aerospace aluminum alloy" used in this study (I did not see where the specific alloy was noted in this paper). While the paper noted that DCT definitely improved the mechanical properties around the location of a fusion weld made in the test specimens, it also stated that they saw no real benefit from DCT in a sample of the same material that had not been welded.

Most of the aluminum components used on 2 stroke engines are made from sand or permanent mold castings. None of these components use any fusion welding processes.

I totally agree that DCT processes are very beneficial for alloy steels.

I'd like to see the data behind the claims of an increase of 500% in fatigue life of B&S cart engines.

Lastly, I disagree with your claim that it is not hard to do a proper job of shot peening on the inside surface of a 2-stroke cylinder bore.

 

[Frederick]

tbuelna:

I never said numerous papers were published by NASA. I said "places such as NASA." I quoted the papers conclusions. Release of residual stresses were significant. YOU stated that you doubted that there was any benefit for aluminum alloys. My claim was that there were effects on aluminum. DCT clearly had an effect on the welded aluminum alloy. It reduced the residual stresses and it reduced the stress corrosion cracking.

Furthermore, from my October 3 post, "Effect of Cryogenic Treatments on Mechanical Properties of 2A11 Aluminum Alloy, Wag et al, Advanced Materials Research Vols 146-147,(2011) pages 1646-1650 states: "The influences of different process parameters on mechanical properties of 2A11 aluminum alloy were compared, and the results showed that cryogenic treatment could improve mechanical properties of 2A11 aluminum alloy. The dimensional stability increases after cryogenic treatment once, and increases further after cryogenic treatment again." (emphasis added) Drakem4 was looking for reduced warping, IE: dimensional stability.

DCT has definite affect on most metals, compacted carbide, and some plastics. Read Cold and Cryogenic Treatment of Steel ASM Handbook Volume 4A, pages 382 to 386.

Regarding the B&S engine, we treated the various engine parts for a customer. The customer was a serious kart racer. He normally rebuilt his engines after 3 races as they were starting to lose power. In this type of racing power loss results in losing races. He ran his treated engines 15 races and rebuilt them because he was nervous about possible part failure, not because of power loss. That year he won the national championship in his class. That is about as much data as you can get out of racers. They are not laboratories or scientists, but on the championship level they do not take chances on losing.

Regarding shot peening inside a cylinder, been there and done that. You make a special nozzle hooked up to a pressure blaster. The nozzle has a 45[sup]o[/sup] plate at the outlet. The shot strikes the plate and is turned to the cylinder wall. And yes it does reduce the intensity of the shot peening, but with the pressure tank there is more than enough.

By the way, It probably be best to cryo treat after the peening process. I am not quite sure why yet, but we have been getting about six times the life on valve springs that were treated after peening. These are on Top Fuel cars, but circle track and road racers are seeing the same benefits. Of course life is defined by the spring retaining a certain percentage of its pressure at a given deflection. Different racers use different percentages.

 

[drakem4]

Fantastic information in this thread, thank you to all who have responded. The shot peening company (WPC) has recommended to use cryo treating before their process, and Frederick you are recommending to use it after? My question to you would be have any of the valve springs that you treated been cryo treated before being sent out for peening? Or have you just tested with cryo treating after? Again many thanks, my plan is to build a very high performance (45hp from a 167cc 2-stroke) engine that is able to last a decent amount of time before needing to be rebuilt, and I believe I have found much of the information that I need on this thread.

 

[Frederick]

I can easily see where your shot peening company would come to their opinion, and on the face of it they are right. Shot peening is done to create a high compressive residual stress on and slightly below the surface. This raises the fatigue life because a crack cannot form in the compressive layer until it pulls that layer out of compression and into tension. (It's a bit more complicated than that, but I don't want to put people to sleep.) So your shot peening company does not want a lot of the compressive residual stresses that it put in the surface to be relieved by the DCT. That is logical to a point.

All I can tell you is that we have repeatedly seen huge (six times) increases in fatigue life when we DCT treat shot peened springs. Treating springs that receive peening after DCT is harder for us to do in that it would have to be done in conjunction with a valve spring manufacturer and they do not seem interested. I can only tell you what we have done that works. It would be interesting to research in that DCT may make the interface of the shot peened layer with tensile layer underneath more "compatable." Like I said, it would be interesting to research, but we know that DCT after shot peening is successful.

 

[FennLane]

http://www.cryogenicndt.com/docs/Reaserch/DeepCryogenicTreatment.pdf

 

[FennLane]

I just don't understand how reducing temperature causes diffusion of vacancies to grain boundaries. I always thought that diffusion was temperature controlled and reducing temperature reduced diffusion.

It is also obvious that vacancy dislocations reduce in number at low temperatures and increase at higher temperatures.
Cooling metals does reduce the number of vacancies but the number increases again when the temperature returns to ambient.
At a materials melting point its structure is around 7% mobile vacancies. The DCT argument seems to be that the reduction in vacancies that occurs at low temperatures does not return when the material warms up. An argument I don't generally accept.

It would also be interesting to know in what manner the atom to atom bond is affected by DCT. Are you suggesting that there is a permanent change in the Fermi level? I am also not sure what you mean by the excess energy of the metallic bond and how DCT removes this - could you please explain.

The sixfold increase in fatigue strength of DCT treated coil springs is also interesting and a detailed explanation of the mechanism that gives this improvement would be useful.

If DCT removes the residual compressive stress how does it help fatigue resistance?

I would also be interested to know how DCT treatment on shot peened springs influences the benefits that are observed in terms of Bauschinger effects.

While I do agree that many engineering steels do benefit from DCT I am still unhappy with the lack of explanation offered for many of the claims made by the Industry even though I clearly slept through Metallurgy 101 and all my subsequent lectures and research.

 

[Frederick]

Vacancies and other point defects are temperature dependent. Reduce the temperature and you reduce the vacancies, providing that you reduce the temperature slowly enough. This is why real DCT has very slow cool down rates. The equation that describes the number of point defects at equilibrium is N[sub]d[/sub]=N exp (-E[sub]d[/sub]/kT) where N is the total atomic sites present, E[sub]d[/sub]is the activation energy necessary to form the defect, k is the Boltzmann constant and T is the absolute temperature.

If you induce a large number of point defects such as vacancies through various processes and "freeze them in" by a sudden temperature change, reducing the temperature will cause many of these non-equilibrium point defects to be reduced, and subsequent warming up will not induce the non-equilibrium point defects to return unless you repeat the processes that induced them in the first place. This is more than theory, as research from Honeywell on thin film magnetic memories has shown that holes in metallic layers several atoms thick have disappeared after DCT.

In regards to the bond strength, Dr. Mark Eberhart at the Colorado School of Mines has written several papers which indicate that at a given temperature there is a specific distance between atoms in a crystal structure where the energy in the metallic bond is a minimum. If the atoms are closer or farther from each other the bond has more energy. We postulate that by bringing the atoms closer together through extreme cooling, we are removing some of the energy which is not re-introduced when the piece is warmed up slowly, thereby making the crystal structure more perfect.

In regards to the sixfold increase in fatigue strength in valve springs, we wish that we understood it better also. We don't know exactly why, other than in practical tests on actual racing cars we have been able to reliably increase the valve spring life six times. A detailed explanation indeed would be useful, and probably very costly.

Regarding the increase in life of a shot peened spring, again I do not know. Sure is handy though. If you are a racer and paying over $400 for a set of springs, why it works is kind of irrelevant. But we would like to find out why so that we can understand the process better. Trouble is, research is expensive.

Regarding Bauschinger effects, we would also like to know.

There is a lot that is not known about what extreme cold does to metals and other materials. There has not been extensive research into DCT. Remember, humans have been heat treating metals for over 6000 years, heat treating other materials for over 100,000 years and have had heat for millions of years. We have had extreme cold in industrial quantities for only a little over 100 years. Up until about ten years ago most of the research into DCT was to prove that it worked. We are finally getting research now that is telling us what temperature profile give optimal results on what materials. Very little has been done on why it works. Many metallurgists will tell you that there is no affect on anything but steel. Yet the use in the audio industry is growing very quickly.