There funny things that happen, metallurgicly speaking, when heat is applied to a complex metal- at temps well below 'softening' temperature, you can get changes in the metal structure, release of stresses, subtle changes in crystalinity. We really don't know. I do know that PROPERLY DONE, you can powdercoat wheels without damage. I also SUSPECT that most powder coaters don't know the critical temperatures they should avoid.

Maybe whomever reassembled these dropped a center by accident. or perhaps had a slight flaw in the masking of the powdercoat such that there was some minute flake of coating on a mounting flange that cause a few thousandths of stress on these two members...

I know enough to know that when it comes to failure analysis there are few groups with the expertise (or interest) to do it right. (I can tell you that had you crashed, your familys' attorney would have done well to contact Exponent in the CA bay area for the definitive answer as to what caused this....) Techniques such as SEM, surface crystalinity measurements, fracture analysis, etc, will tell how a fracture started, why, how it propagates, etc. The information to be gleaned from the faces of the fracture are impressive.

It appears that HRE has determined that these cracks must have been caused by the subsequent work, but I wonder if this is based on actual inspection of the failure zone, or just the conclusion of 'someone modded these, mods can cause failures, this faulre was caused by the mod'.

I appreciate Alan's reply. Clearly they are working with you. I am actually in the process of buying a new set or HREs- impressed by their quality at many levels (product, communication, specs, certifications).

I believe it IS possible to work on wheels and do so safely- the assertion that only HRE can do so must be false. But it is undeniable that if someone does service a wheel then they need to (indeed they DO) take on the responsibility for any problems.

A

 

Actually, for forged aluminum, powdercoating is NOT that common as the cure temp for most is too high. This is the problem and one I have been trying to get more people to understand. This is not cast aluminum and it is not steel. The failure is a typical fatigue failure and if you look at a strength after exposure to temp vs time for 6061-t6 you'll see a very significant dropoff in strength after exposure which translates to reduced fatigue life. You have to be VERY careful powdercoating and even painting for that matter but most paints have a lower cure temp.

Assembly/disassembly should not cause a fatigue failure unless it was really done poorly.

 

what would you consider the maximum baking temp for these forged aluminum wheels so as not to cause damage? You guys obviously do your own in house coating, so I imagine that you have seen this limit. This is good info because I think there are many people powder coating their wheels after the fact.

 

Alan, Thanks for taking time from your weekend to address my problem. That alone speaks volumes.

I was contacted today by HRE. They offered to sell me a set of replacement wheels for $2,000 but I have to ship my wheels to them and I have to pay for shipping them back to me. No idea how much that's going to cost me. They also said that I can choose a different set of wheels and have them finished any way I like. The rep said he will email which wheels I can choose from.

 

WOW, not sure what you paid for the cracked wheels, but that sounds like an awesome deal to me? Most HRE wheels are $5k to $6k a set? A new set for $2K finished anyway you want? Congrats and good luck with your choice.

 

no answer to this?

 

I don't see how you can expect a one size fits all answer to that...6061-T6 will be different than other alloys...temps could vary even between batches of the same alloy...needs to be tested and verified for the particular material and design/shape of the wheel center...that is why powder coating is better done by the wheel manufacturer or accept that you are taking your chances (might work, might not)

 

Dennis always has high expectations and is VERY impatient

 

now when have I ever been impatient, huh? well, huh? I'm waiting for an answer?


I expect it because they are a manufacturer that deals in that alloy and that does their own powder coating. They have the answer otherwise they would not be saying that powder coating caused the crack, as they would not know for sure. All I am saying is that a 6061T6, is a measured material with fully measured properties. The fact that HRE indicated that powder coating could likely have caused the problems only begs the logical question, what baking temp is the limit in powder coating.

Unless your tracking a car 5 years or you've had a wreck a broken wheel that soon is unacceptable, so I think my question is more than valid.

I applaud HRE for giving the good deal, but the customer is still out 2k plus his original cost and all of the shipping. So a look into what caused it is legit.

 

That is an awesome deal on a new set of HRE wheels. Those wheels would normally sell for $5-6K new. You bought them used, with a 3rd party modifying them. HRE didn't really have to offer you anything here. I'd take the offer.

 

+1

Seems like they're going above and beyond, here. After the horror stories that I've read about other manufacturers, I wouldn't hesitate to buy from HRE. (I love the 19" P40, but I still have that article talking about the performance advantage of the factory 18"s stuck in my head.)

 

This is an insane deal and props to HRE for stepping up and offering this. They are basically a brand new set of HRE's for less than $2500 (assuming they are giving you brand new inners and outers). You cannot beat that... if I were you, I'd jump on it in a heartbeat.

Edwin

 

Sorry. I've recently taken on a different position here so I'm not able to be as quick to respond. I see this thread died so at the risk of stirring up trouble for ourselves and enduring the wrath of my team here, I'll try to answer your question.

It isn't just a simple matter of temp. Temps as low as 220 degrees F will degrade 6061-T6 given a long enough exposure (10000+ hours). Temps above 400 degrees will cause almost immediate degradation. Temps in between are a function of time and temp. For proprietary reasons I'm not going to say the time and temp cycle we use to cure our powdercoat, but suffice it to say that we are confident we are being smart about this.

For background, most powdercoaters cure over 400F. Most chemical paint stripping is done at 500F which is VERY common here in SoCal and why we don't do this and only use a bead blaster in-house. At 500F you are causing severe degradation at the surface of the part even if it sits for just a few minutes and since stresses on parts are highest at the surface and where a fatigue crack starts, this is where you're going to have a problem. I actually bet the stripping caused more damage than the recoating.

For the record, since the wheels were taken apart, stripped at an unknown temp and recoated at an unknown temp, the warranty is clearly void (it was void the minute they were taken apart) and technically we didn't need to spend a single minute on this... but that simply wouldn't be in line with our beliefs in how we support our product and customers.

 

Alan, I appreciate the response and it is definitely not stirring anything. I fully understand why powder coating process wold void warranty. I have been checking around have been told that most powder coating processes bake at 320 degreesF for many minutes(up to 20). This would in effect have the potential to cause the problems of the wheels in question. I figured that most wheel coaters would be using a lower temp powder coat process, widely available today. but apparently not. And I at least am aware of it now and can avoid the OP's issue. PS: I someday plan on getting your P40's in matte black.

 

Just wanted to say that I think HRE was wonderful in how they handled my situation. They gave me some really nice wheel options and went over the wheel dimensions with me in detail. Once I had chosen a wheel Vince (my sales rep) tailored the wheel to exactly what I wanted in terms of finish, lip width and even center cap cutout to allow Porsche center caps.

Many thanks to Alan and Vince.