On a Freightliner FL70 at work I noticed that the front rotors have a large number of spots on them. They are concentrated in the middle of the swept area of the rotor and you can feel the roughness when you run your finger over them. Each rotor has about forty spots on each side. Truck brakes normally and the roughness cannot be felt when the brakes are applied. What are these spots and do the rotors need to be replaced?
Spots on Brake Rotors
- Thread starter George7941
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I will add that the spots average 3/8in dia and do not look like rust pitting. It is more like the metal has flaked away.
It could be some porosity in the original disc casting. If the truck is still under warranty I might take it back to the dealer and see what they say.
Otherwise the answer will be the same, you wouldn't want to change the discs without changing the pads (A.K.A. a complete brake job) so just run em till they wear out, then do a brake job. Porosity might marginally increase pad wear (and improve braking performance!) but it shouldn't effect the strength of the disc. The part would have broken up during dimensional machining if it was weak.
Otherwise the answer will be the same, you wouldn't want to change the discs without changing the pads (A.K.A. a complete brake job) so just run em till they wear out, then do a brake job. Porosity might marginally increase pad wear (and improve braking performance!) but it shouldn't effect the strength of the disc. The part would have broken up during dimensional machining if it was weak.
It definitely looks like a casting defect. These are Gunite rotors, hopefully made in the USA. Truck is a 1999, long past warranty. I will replace both rotors and pads. I will see if I can post some photographs,it will definitely explain the flakings much better than I can do in words.
Probly heat checks.
Here is a picture [image]http://rides.webshots.com/album/556086953gYWOOO[/image]
Kestas
Staff member
Casting porosity often looks spongy. It's either porosity or spalling from the surface. I'd be interested to see what the pattern looks like in the pictures. Try to get good, sharp close-ups.
Are these rotors vented? I often find vented rotors where the vents are filled with corrosion scale and don't have any effective venting any more. This opens up the possibility of damage from heat.
You may consider to continue using the rotors. Are these defects that much different from cross-drilled or slotted rotors?
Edit: After looking at the picture, I'd say the indications look like spalling or casting defects called "cold shots".
Are these rotors vented? I often find vented rotors where the vents are filled with corrosion scale and don't have any effective venting any more. This opens up the possibility of damage from heat.
You may consider to continue using the rotors. Are these defects that much different from cross-drilled or slotted rotors?
Edit: After looking at the picture, I'd say the indications look like spalling or casting defects called "cold shots".
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The link did not work out the way I wanted it to but if you copy and paste the URL you can view the picture.
Would heat checks cause a rough surface? The indentations feel about 5 to 10 thou deep.
What would cause spalling?
The rear brakes are the same setup as the front and those rotors are very clean, just shiny metal. Truck is driven nearly every day, so I do not think rust pitting is an issue. Also it is all city drivng at low speeds (moving company truck), so, unless some drivers are abusing the truck, I don't see excessive heat generated in the brakes
Would heat checks cause a rough surface? The indentations feel about 5 to 10 thou deep.
The vents do not look any more rusted or clogged than normal.
What would cause spalling?
The rear brakes are the same setup as the front and those rotors are very clean, just shiny metal. Truck is driven nearly every day, so I do not think rust pitting is an issue. Also it is all city drivng at low speeds (moving company truck), so, unless some drivers are abusing the truck, I don't see excessive heat generated in the brakes
Kestas
Staff member
Excessive heat would have shown up as blue (heat) discoloration on the metal.
The cast iron used to make rotors is inherently brittle on a micro level. Why your rotor would spall, yet thousands of other rotors are trouble-free is a mystery. I'm afraid that only a good metallographic analysis will tell you exactly what's going on. Anything else is pure speculation.
The cast iron used to make rotors is inherently brittle on a micro level. Why your rotor would spall, yet thousands of other rotors are trouble-free is a mystery. I'm afraid that only a good metallographic analysis will tell you exactly what's going on. Anything else is pure speculation.
http://rides.webshots.com/album/556086953gYWOOO
I fixed the link, now you can just click on the link.
I fixed the link, now you can just click on the link.
Looks like it may be "cementite". Its caused by localized hot-spots that convert "grained" rotor iron to an iron carbide. May be due to a casting flaw and why some of the rotor material is flaking off. Read here for some insight:
http://www.stoptech.com/tech_info/wp_warped_brakedisk.shtml
http://www.stoptech.com/tech_info/wp_warped_brakedisk.shtml
Kestas
Staff member
Cementite is incredibly hard. If it was cementite, the areas would stand proud from the rotor surface instead of in relief.
Cementite is caused by fast-cooling grey iron during the casting process. Localized hot spots during braking could result in a martensitic microstructure. Though both are hard spots, the author goofed on his interpretation of what goes on metallurgically.
Cementite is caused by fast-cooling grey iron during the casting process. Localized hot spots during braking could result in a martensitic microstructure. Though both are hard spots, the author goofed on his interpretation of what goes on metallurgically.
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I think that these are casting voids that are small enough not to be an issue of too much importance. I had OEM rotors on my Caravan wear to the point where a quarter size void was exposed. Way too deep to be cut out. I'm sure that mine wasn't just an air bubble and probably had something like what you're seeing on the margins.
Kestas is da man on casting.
Kestas is da man on casting.
IIRC, then the cementite is actually there since casting and not as a result of braking (conversion). They are hard and don't wear like the surrounding iron, thats why they are raised. Am I close?
What is the martensitic microstructure phenomenon and how/why does it occur?
Kestas
Staff member
Yes, shorty, you've got it. When grey iron is poured, you can either get the regular grey iron everyone is familiar with (rotors, engine blocks), or chilled cast iron that is high in cementite. It all depends on the cooling rate in the mold during cooling. Both use virtually the same metal chemistry.
Hydraulic tappets are popularly made of chilled cast iron. The matrix has around 50% cementite. They are cheap to make and highly wear resistant. 25 years ago I made some prototype gray iron camshafts that were selectively chill-cast at the lobes. It made for a camshaft that was hard where needed (cementite-rich at the lobes), yet the rest of the camshaft was soft for high-volume production machining.
Martensite results from a hardening process where carbon steel is heated to an orange-hot temperature, then quickly cooled (quenched). It differs from cementite because cementite is produced by fast-cooling from liquid, whereas martensite is generally produced by fast-cooling orange-hot steel. Each has a unique microstructure.
There's a lot more to this, but I'm trying to be as simplistic as I can to explain this difference.
Hydraulic tappets are popularly made of chilled cast iron. The matrix has around 50% cementite. They are cheap to make and highly wear resistant. 25 years ago I made some prototype gray iron camshafts that were selectively chill-cast at the lobes. It made for a camshaft that was hard where needed (cementite-rich at the lobes), yet the rest of the camshaft was soft for high-volume production machining.
Martensite results from a hardening process where carbon steel is heated to an orange-hot temperature, then quickly cooled (quenched). It differs from cementite because cementite is produced by fast-cooling from liquid, whereas martensite is generally produced by fast-cooling orange-hot steel. Each has a unique microstructure.
There's a lot more to this, but I'm trying to be as simplistic as I can to explain this difference.
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Kestas-Thanks for the info! Always learn something new around here.
Not to get too far OT, is the quenching of carbon steel normally refered to as "case hardening"? I understand how that can densify steel. What happens to iron when the same process is applied? Does it become brittle or stronger? I ask because we have issues with rotors on our track cars sometimes. When they get orange/red hot from high speed braking and we have standing water on the track, sometimes they get splashed. We have only had a few show stress cracking and I'm wondering if its coming from the rapid water cooling.
Not to get too far OT, is the quenching of carbon steel normally refered to as "case hardening"? I understand how that can densify steel. What happens to iron when the same process is applied? Does it become brittle or stronger? I ask because we have issues with rotors on our track cars sometimes. When they get orange/red hot from high speed braking and we have standing water on the track, sometimes they get splashed. We have only had a few show stress cracking and I'm wondering if its coming from the rapid water cooling.
Kestas
Staff member
I'm not sure if the rotor cracking can be solely attributed to standing water on the track. It certainly isn't good to splash hot rotors with water. The cracks could be from thermal cycling (I've seen it on aircraft brakes), or it could be from martensitic transformation on the brake surface from splashing. Only analysis in a met lab would tell you for sure.
Case hardening is popularly done two different ways... Induction harden the surface, then quench and temper, or carburize the part, then quench and temper. Carburizing enriches the part surface with carbon for enhanced response to the quench. Both are usually done to provide a wear-resistant surface.
Some hardening is done to the whole bulk of the part. There are also variations of hardening that include the bulk and case, which is the most popular form of heat treatment.
Any time a part is treated to make it harder and stronger, it also becomes more brittle. Tempering takes away much of this brittleness without losing too much hardness and strength, but this is always a balancing act for the designer.
Nobody uses the term "densifying". Quenching carbon steel is simply called "hardening" or "quench & temper (Q&T)".
Case hardening is popularly done two different ways... Induction harden the surface, then quench and temper, or carburize the part, then quench and temper. Carburizing enriches the part surface with carbon for enhanced response to the quench. Both are usually done to provide a wear-resistant surface.
Some hardening is done to the whole bulk of the part. There are also variations of hardening that include the bulk and case, which is the most popular form of heat treatment.
Any time a part is treated to make it harder and stronger, it also becomes more brittle. Tempering takes away much of this brittleness without losing too much hardness and strength, but this is always a balancing act for the designer.
Nobody uses the term "densifying". Quenching carbon steel is simply called "hardening" or "quench & temper (Q&T)".
Well, I'm definately "nobody" when it comes to metallurgy
Never did like getting water on hot brakes. They used to kill several more rotors when the recruits would wash the cars before the brakes cooled. Told the higher-ups and they stopped the practice. I was just trying to figure if it was solely thermal shock from the water as it effected just the cars run at high speed and splashing. The cars that aren't run this way don't see nearly the cracking. Maybe it just the cheap rotors we have to put on the cars and they are more sensitive to rapid thermal variations
Again, very interesting and informative. Thank you
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