Defective Castings in Brake Rotors

This would be a question for Milo, and maybe a few others. Manufacturing brake rotors should be a mature industry with few defects.

"Manufacturing brake rotors should be a mature industry with few defects."

If we are speaking about domestic sources, I'd agree. However the new replacement disk had raised cast lettering "Made in China". I've cleaned the deffective disk thoroughly, I will see if there is a country of origin on it somewhere.

The disk in question was replaced about 18 months earlier by a repair shop who I no longer use because of poor diagostic skills and poor workmanship. The disk had only seen one set of pads and was never machined. In fact when the thickness was measured, it was close to that of the new disk. I estimate 12,000 miles since replacement.

I tested aircraft engine and racing crankshafts 40 years ago using UV dye penetrants and black light or magnetic flux generators and steel powder.

Compared to what is available today, those are the technologies of dynosaurs.

With product liability concerns and legal sharks everywhere, surely something better should be available in the modern QA process to detect such defects, at least for domestic manufacturers.

Laughing Jaguar

My guess is that sometime being at a VERY hot state it hit road (?) water. Near surface created cracks on the rapidly cooled area, after some use stressing, joined randomly to create fly-away chips of arbitar shape and gave these cavitations. Sign of a bad, contaminated material, possibly recycled, and not bad casting. S.M.

" My guess is that sometime being at a VERY hot state it hit road (?) water. Near surface created cracks on the rapidly cooled area. . . . . gave these cavitations."

The defect only occured on one side of the brake rotor which is one of the ventilated types.

The damage is on the inside surface, the dust shielded one. The outside surface which is more exposed to water and debris is perfect. While the shielded surface might run a few degrees hotter, the difference should not be significant.

"Sign of a bad, contaminated material, possibly recycled,. . . . ."

That's a more likely possibility. Still, my experience with perfectly round inclusions in castings of large optical glass disks (borosylicates) suggests that these were spherical gas pockets that were trapped below the surface during casting.

Laughing Jaguar

It is an interesting problem, could you by any chance send a picture of the disc ?

". . . . any chance send a picture. . . ?"

I can easily take images of the entire surface and will also attempt macro closeups.

Thanks for the suggestion.

"I'll be back!"

L.J.

As promised. I've brought you some images of the damage.

Using a fine wire digital probe, I measured the depth of the pockets to be as much as 1mm deep and some with diameters as large as 1mm across.

The depth of each pocket was almost always half the diameter of the opening and all of the openings appear to be circular.

This evidence furthers my suspicion that we're looking at inclusions, spherical voids caused by gas entrapment during the foundry process.

The other side of the rotor is perfect! The overall disk thickness is well above the minimum threshold for safe machining.

It occurs to me now that if these were gas pockets that were once spherical and below the surface, then the wear rate on this side of the disk had to be exceptionally high to cut deeply into those voids, higher than the other side.

That triggered a recollection that the wear on the inside friction pad was much higher than it's kin on the outside.

A case is building, not just for gas entrapment but for a concentration of a foreign substance in the casting material as was suggested earlier by Simple Mind

I wish I still had a Rockwell Tester

Thanks for the support.

the Laughing Jaguar

Sounds like you've got it figured out.

Maybe one of our metallurgist friends can confirm?

What's bugging me about bubbles or not is that this relatively complicated (ventilated) shape shouldn't be natural flow but rather pressure casting. Anyway the contamination issue reminds me the story of a company's effort once to prototype a CVT box targeted to hit 100.000+ miles. Although they ordered the best quality steel money could buy at the time for the parabolic contact rotors it stil broke, and analysis showed it had some contamination. It turned out to be floating iron oxides that was sucked together with the perfectly good metal in the final casting process (and they solved it with a simple trick...) S.M.

Simple mind wrote: ". . . . this relatively complicated (ventilated) shape shouldn't be natural flow but rather pressure casting."

Here in the States, yes. Who knows what goes on on the other side of the Pacific.

I am not well versed in casting metals but have done a fair amount of designing of parts that have been injection molded from thermoplastic materials.

Same church. . . different pew.

I find myself wondering what would happen if a liquid metal gets too hot. Might it not vaporize and cause bubbles?

If the mold was supposed to be cooled prior to ejecting a previous molded part, so as to speed up production, and it was not cooled enough, the high temp of the mold surface might elevate the liquid to where it starts to create vapors.

If the part was not cooled properly and the pressure of the casting process was vented, the liquid metal could react spontaneously and create vapors. No?

I'm not a metallurgist and I've admitted a lack of experience with casting metals so I'm simply thinking out loud.

Still SM, I suspect entrapped gas. How it got there is for swifter minds than mine to figure out.

Thanks for the dialogue. . . . everyone.

L.J.

Generally, steel castings would be either cast in sand, shell. or investment. Permanent molds are not used to the best of my knowledge to cast steel parts since most permanent molds are made either from cast iron or steel and neither would be suitable to cast steel into.

I would be willing to bet sight unseen that your brake drum were either sand cast or shell mold cast. This is not a cast to size part requiring the expense of investment casting.

As mentioned before it could very well be a poor gating system or it could even be just a lousy pourer who allowed air to be aspirated into the sprue, forming oxides as it mixed with the steel. This often happens if he slows down or interrupts the pouring as he fills the mold. It also could be that there was excess moisture in the molding sand at the sand-metal interface and steam was given off from the drag side face and traveled up into the metal again forming oxides inside the casting. Another possibility could be a sand wash where pieces of the molding sand broke away and washed into the mold cavity with the metal and was entrapped inside the casting just below the machined surface.

If it was a gas issue a magnification of the defects would show the cavities to have smooth walls whereas a sand defect would have irregular surfaces. A foreign material such as an oxide formation often has grayish powdery areas exhibited although they might disappear due to the nature of the brake drum service and the weathering of the surfaces. As to steel "vaporizing and causing bubbles" it is true that water vapor does break down over a hot surface of molten metal and aluminum and bronze will absorb the hydrogen and cause gas porosity. I am not sure that that also happens with steel but I do know that it is not the metal itself that vaporizes and causes the gas bubbles.

In any of those cases "real time" radio-graphing (digital x-rays) of the parts, magnetic particle or eddy current tests would find such defects and allow the defective parts to be removed. As to the quality of the parts produced in China, well as the man said, "You get what you pay for" and if you pay cheap you get cheap. While there are many well funded and quality producing foundries in China, there are probably ten times more that are still pretty basic and antiquated and producing "cheap and dirty castings" at cheap prices.

I have seen this on way to many autos of late. My grandson is a very good mechanic and does several brake system repairs a day. He has come to the conclusion that China is making is house payment with the junk parts they make and sell here in the US. He is absolutely correct in his remarks.

I spent a little time in China 15 or so years ago (company I worked for was going to have castings made there, they went belly up after all the law suits from bad materials hit the fan.) saw my job and others lost to cheap labor and junk parts. So as others have stated, you get what we pay for. My suggestion is not to use the brakes unless you have an emergency. Then go to the auto parts store and replace all before driving again.