Automotive header/manifold alloys 1

[yoshimitsuspeed]

If you listen to aftermarket car talk there is a ton of back and forth and many opinions on what materials are best for automotive exhaust systems. Once you get past the downpipe you can get away with just about anything. Some have even done aluminum with surprising success.
But at the manifold especially on a high performance motor there is a lot of heat and over time heat cycles and fatigue will make just about anything crack eventually. I have formulated opinions of my own but I don't know enough about materials engineering to feel I am any more qualified than most the other opinions out there and I would like to have a better understanding of things on that level and be able to develop better products as well as explain things better when asked.
I have a basic understanding of materials properties. Understanding tensile failure of a bolt and how that changes relative to heat and things like that are pretty simple stuff. But when it comes to exhaust systems if it's designed remotely properly yield and tensile strengths really shouldn't be a concern unless you get the metal hot enough to eliminate it almost entirely. Ductility could be important but across a wide range of temperatures. I don't imagine it's a big factor though it could play some role in fatigue which I figure has to be in the top three along with I believe thermal cycling and possibly carbon absorption. High performance motors can definitely get into the range of heat that should allow pretty quick and easy carbon absorption but it's my understanding that metal can absorb carbon at lower temps over time. I would like to know a little more about how that could play into the longevity of a manifold. I have always believed the best answer for any alloy is something like a ceramic coating on the inside that would prevent carbon absorption.

Now a lot of pipe resellers and a lot of fabricators will swear up and down that stainless is the best material for manifolds but I have always had a bit of a hard time believing this. Largely because of Stainless's very high CTE and also the fact that even the most high temp Stainless is usually specced for very close to what high performance exhausts can hit. 304 is the most commonly used but for high performance applications they will try to sell you on the benefits of 316 or 321 but are these benefits applicable to headers? Tech sheets brag about things like creep and rupture resistance but rupture isn't going to be a big concern. Would creep help lead to fatigue cracking? Would 321 be less prone to fatigue cracking than 316, 304, or mild steel?
If the manifold is hard anchored in more than one place it seems the CTE of stainless would definitely be a huge disadvantage and make it more likely to fatigue in areas that flexed more. Even in a system that allowed more movement I still imagine that moving and flexing must fatigue the material faster, especially in something like a manifold where the temps can change drastically across the surface and over time.

Aside from it's corrosion resistance is there anything that would make stainless better than mild steel in terms of longevity and crack and fatigue resistance?
What about mild steel with thermal barrier inside or in and out vs uncoated stainless? Would coated mild steel not be at least as good if not better?

One other thing I have heard that seemed odd to me were a couple claims that thinner was better. This goes against all my materials understanding and I would always want to go as thick as could be justified in terms of weight. But other than weight is there ever any advantage to going with thinner piping?

 

[EdStainless]

When metal gets hot it expands, either you try to hold it in place and generate very high stress, or you allow some deformation and spread out the the stress. This is why thinner material is usually better for high thermal cycling applications.
316 does have slightly better oxidation resistance than 304, and if both of them are very low C they are as stable as 321 is. We have supplied thin wall 625 Ni alloy tube for use in racing headers. Stainless headers do look nice.
Aluminized steel has been used for decades and works well for straight pipes, but welding into headers is an issue.
Low carbon steel with interior ceramic and exterior coating just to minimize scaling works fine. But that is the limitation of steel, scaling. This why people use coatings and alternate alloys.
In modern production cars the pipes ahead of the CAT are usually aluminized steel, the CAT housing and pipes back to the muffler are 409 stainless, and the muffler and tail pipes are 439 stainless. You need more corrosion resistance in the colder sections to resist corrosion from the acidic gasses.

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P.E. Metallurgy, Plymouth Tube

 

[yoshimitsuspeed]

EdStainless thanks for the response.
I don't understand why thinner material would be better in regards to thermal cycling. Both thin wall and thick wall would move just as much during that heat cycling assuming the temps were the same though I would expect if anything thick wall would keep a more uniform temp and in most situations stay slightly cooler overall. I would think that thick wall would be better just because you are dissipating any movement forces through a greater sectional area of material. We have been making our turbo manifolds out of 11 gauge mild steel for years and have never heard of any one cracking while many thinner stainless manifolds have much greater reputations for failure. I would love a more detailed explanation on why thinner would be better in this application.

Stainless manifolds look great until they get hot but a high performance motor will most likely take them above the corrosion resistance temp anyway at which point a coated manifold will look much better. When I did my car which is a high comp low boost turbo I chose to go Burns 304 with Cerakote turbine coat on the inside as a thermal barrier. I left the outside raw to show off the expensive stianless I bought and months later it looked horrible. Even with the thermal coating inside it was moderately oxidized and didn't look anything like stainless. Granted it's only surface oxidation and a bare mild steel mani would have rusted a lot more but a coated mani would look much better. I haven't tried 316 or 321 but I have always been skeptical since their max recommended temps and oxidation temps really aren't much higher than 304. I am sure that little bit of difference is huge in industrial applications where that spec puts it in a more acceptable range for the application but do those attributes make a big difference on a manifold? Does 321 stay looking nice at temps a high performance motor sees or would a coated mani still look better?

Aside from looks will the 321 last longer or perform better than mild steel would uncoated or coated?

Does carbon absorption make a manifold more likely to crack over time? If so is the effect the same on stainless as it is on mild? I assume so since high carbon stainless can still be more likely to crack in other situations.

 

[EdStainless]

The thin wall tube will be slightly cooler, the metal itself is the greatest resistance to heat transfer.
This tubes will distort at very light loads, thus keeping the stresses lower. In thicker walls there is much more variation in temp (even from inside to outside) and chances for greater stresses. Where the tubes connect to the flange is very tricky with light wall tubes. They are usually flared and then welded to a small raised hub. I have also seen a short tube (slightly smaller diameter) inside the main tube just to keep the hottest gas from directly impinging on the joint area.
We used to have parts trays in heat treat that were rapidly cooled from 2350F to 1200F, over and over. If we used thicker material or tried bracing them we would get cracks, making them thinner resulted in distortion, but they didn't try to tear themselves to pieces.
The Ni alloy tubes (alloy 625) that they used on motorcycles were only 0.035" wall. These heated from about 500F to 1600F in 1sec, and this happened every time they accelerated.
The reason that people use 321 is to minimize the carbides that form in welding. If corrosion resistance was a issue then I could see using it, but in exhaust I don't know why you would. You get just as good of carbide free welds in low C 304.
The discoloration of stainless may not be pretty, but it sure better than scaling and flaking.
There are other high temp alloys, with 309 being one of the best for the price.


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P.E. Metallurgy, Plymouth Tube

 

[SwinnyGG]

You are, in my opinion, drastically overthinking a materials selection application that isn't very complicated.

304/304L is the most common stainless steel used for aftermarket headers because it is cheap.

Aluminized steel is extremely common in OEM exhausts.. because it is cheap.

 

[yoshimitsuspeed]

1. I am trying to understand things better so that I can improve our designs.
2. There are constantly arguments in groups, forums, and such where a lot of opinions are given without a very good understanding of the science behind it and I don't currently know enough to give an opinion but know enough to believe that a ton of misinformation is being thrown around.

So yes an uncoated mild steel manifold can rust and scale but if coated is stainless structurally any better? If oxidation is not a factor would mild steel or stainless last longer without fatigue or other failures?
A number of stainless companies seem to use mild head flanges specifically because of the better CTE. Is the lower CTE not a notable advantage in terms of fatigue? Is that outweighed by some weaker performance characteristic of the mild steel?

I am also having a hard time wrapping my head around thinner material being better or heating more evenly. The thermal conductivity of steel is much higher than air. Yes a thicker cross section reduces the thermal conductivity from inside to outside but it should help even out the temperature along the length by conducting more heat along the length. It will also act as more thermal mass meaning it will heat and cool slower and more evenly. It is claims like this that I have a hard time understanding without knowing why that would be true.
The thinner tube distorting would cause stretching and stress on the metal that should in theory lead to fatigue failure. The example of the flat sheets isn't a fair one because a thin flat sheet can warp and potato chip and this movement will allow it to dissipate those stresses over a larger area but the shape of tubing will not allow this kind of distortion. That expanding and contracting would just continue to stretch and compress the metal which should lead to fatigue. Especially at a weld or joint where movement is more constrained. Thicker tubing would have much more similar strength in the area of a weld as it does on the tubing as well.

 

[SwinnyGG]

it should help even out the temperature along the length by conducting more heat along the length

A thicker tube will definitely conduct more heat down the length of the tube before that heat can exit the tube wall into the air.. but that's not a good thing.

For maximizing component life (by reducing thermal stres) you want heat to pass into the air as soon as possible, not travel down the tube.

The same logic applies to mechanical deformation. A thin tube and a thick tube will both deflect the same amount due to thermal expansion- but a thin tube is more flexible, meaning that the deflection due to thermal expansion imparts less stress into the tube.

The thinner tube is also lighter (good), easier to manufacture (good) and less expensive (good).

Car forums are, in my experience, just about the worst possible source for engineering analysis or OEM or aftermarket parts.

 

[yoshimitsuspeed]

Which is why I come here for answers and not car forums.

So say we have 900C EGTs, the header flange is going to stay much closer to say 200C pretty much no matter what. A thinner tubing will heat up to it's max temp furthest from mass like flanges, motors, turbos, etc much faster and will fluxuate a lot more than thicker pipe that would transfer more heat to the flanges, head, turbo etc right? SO wouldn't the thicker material keep a lower more even temp?
If you want the heat to pass through to the air as fast as possible exterior thermal barriers would be the worst thing you could do for longevity right? That is not a claim you will hear made by many thermal coatings companies that's for sure. And if that is true then Mild steel should be even better because of not only a lower CTE but also a higher TC.

As for strength I still don't get the thinner being better. How much the exhaust flexes will be determined solely by the temp, length and material. A 2 foot section of 11 ga will grow just as much as a 2 foor section of 20 ga. Now it's true that the thicker material would apply more force to the anchor points but it would also be stronger and more resistant to failure. For example if we had 2 10 foot sections of straight pipe at 700C and hung weight off of each the 11 ga piping would take a lot more weight before failing than the 20 ga would. If say that long section of pipe were constrained at the end both pipes would bow the same amount but the larger cross sectional area would be able to take more force applied to it right?

 

[EdStainless]

The other thing with heavier tube is that the ID surface can be 100's of degrees higher than the OD, so you have stresses from temp differences around the tube, along the tube, and through the tube.
Your example of a hanging weight isn't what happens. It is more like having a length of tube between fixed locations, now when you heat the middle the thicker one will try to push harder, but when the ends are fixed it will just generate more force and internal stress.
While the thicker tube may have a lower average temp it will conduct more heat further away and it will have a greater temp difference though the wall , which at some point will become the limiting factor.
Mild steel will work as well or better than SS, as long as oxidation and scaling isn't a factor.
An external coating will minimize cooling, the tubing will run hotter, but fluctuate less, and also lessen external corrosion. So it could be helpful.
Strength isn't an issue in headers, I am not sure that you can make them too weak unless they can't support their own weight since there isn't much other load.

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P.E. Metallurgy, Plymouth Tube

 

[yoshimitsuspeed]

Yes the thicker tube will push harder but it has more cross sectional area, strength and rigidity to resist that force. The thicker you make the walls of a soda can the more weight you can put on it before failure and the less likely it will be to fail. That is not really applicable to our fatigue failure but I still don't understand how thicker walls would make fatigue failure more likely. Especially and much more so when the tubing is attached to thicker metal using thicker hard welds. The more I think about this the less plausible thinner is better seems to me.
The runners are going to be welded to a thick flange at one end and a collector or flange at the other end. Any mounts, brackets or bungs will be thicker material welded to the tubing. Cracks pretty much always start at the weld where the thinner tubing meets the thicker and where thicker harder welds meet the tubing. Thicker tubing should make it harder to crack in these areas as the strength, rigidity, and flexing properties should be more similar to the weld and thicker materials. It should be more likely to deform over a larger area instead of flexing and applying most of the stress right where it meets the thicker material. Being thicker it's also much easier to make weld beads thinner and closer to the tubings cross sectional area instead of a big weld with a rapid transition from tubing cross sectional area to much thicker. I'm not sure how much difference that would make but it should make some. Especially in a production environment when speed may start taking priority over precision.
But the big thing in my mind is that I can't think of how having 11 ga material welded to a 1/2 flange couldn't be better than having 22 ga welded to a 1/2 flange. The flange will stay much cooler, especially at the head. The thinner material will get much hotter much closer to the flange making the temp transition band much smaller than thicker tubing which would have a slower transition from the hottest to the coolest. If you want a bracket made out of 1/8 x 1 strap in a place where it has to be welded to the tubing the 22 ga will have a huge transition from thin to thick with a big weld to get the structural characteristics you need of your bracket. Welded tubular manifolds almost never fail in the middle of the tubes. They always fail at welds and usually when the tubing meets a thicker or less movable component.

https://www.miataturbo.net/attachme...73d1309839857-cracked-ebay-manifold-crack.jpg

http://img.photobucket.com/albums/v209/tomt64/Tomei010.jpg

https://goo.gl/images/ZagIrm

So if it almost always cracks where it meets a thicker less movable component wouldn't thicker tubing make cracking in those areas less likely?

 

[EdStainless]

If you have decided that you know best then fine.
I am just telling what I have seen in race cars, motorcycles, and aircraft. They all choose to go as thin as possible. It is better to have the tubes bow or distort than try to tear welds.

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P.E. Metallurgy, Plymouth Tube

 

[yoshimitsuspeed]

If I am wrong I am looking for technical understanding of why which you repeatedly fail to give.
Welds usually tear where the tubing meets the weld. Sometimes it tears in the weld. Either way thicker tubing will have a thicker weld with more cross sectional area which should be more resistant to tearing. Unless you can explain why that wouldn't be true but so far your reasoning doesn't seem to explain it very well.
I'm not telling you that I know. I am trying to provide counterpoints and reach a better understanding of this.

 

[EdStainless]

The stress is higher in thick material because of both thickness and stiffness.
In both materials the welds will be proportional to the walls, so the strength will be relatively the same.
Thinner welds are actually helpful because often the welds are stronger than the base material which does not help the situation as it only increases local stresses further.
Going lighter lowers the stiffness, and then things can deflect rather than just push.
A lot of the weld joint design does require special attention, especially when connecting thick material to thin. But as long as the thicker material is cooler it is not difficult.
I have watched engines on test stands and seen the headers start to glow in a couple of seconds and you can see the tubes moving as they heat. With thicker headers the inside surface would be trying to expand, but the outer material wouldn't let it. that creates a lot of stress.
In many cases people fight these issues by using heavy tubing or castings and then insulating the outside of it. This will minimize the thermal cycling issues by keeping the header hot all of the time. the only real drawback to this that it moves more heat further from the exhaust. This requires more shielding and insulation further from the engine. it also traps a lot of heat right at the block which will put a load on the cooling system.

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P.E. Metallurgy, Plymouth Tube

 

[Screwman1]

I agree with Ed; thinner tubes have increased flexibility and result in reduced peak stress at the welds. When we were welding up the inconel headers for the indy cars they had to be as thin as possible to keep them from cracking in multiple locations.

 

[Tmoose]

Geometry easily beats metallurgy much of the time.

Gross geometry, like the length of the tubes between clustered or restrained connections to provide flexibility.

And then local geometry. The stress concentration at as-welded weld "toes" is brutally high.
If cracking is an issue, improvements can be made if The toe of the fillet weld at a tube junction is dressed with a sharp burr or ground with a stone to improve the shape, and remove undercut or other discontinuities.
Published tests results show Improvements in fatigue strength are between 50% and 200%. Not bad at all.

A fillet weld joining a tube to a relatively stiff header flange can also create a pretty ugly stress profile on the back side of the weld, in the weld "root." Some folks call the area a "crack plane" with good reason.

 

[yoshimitsuspeed]

Thinner tubes will have less stress but how much stress metal can take is directly related to it's cross sectional area.
I can see how on longer piping like say a long runner header having thinner tubing being more flexible could allow movement over a long enough section of tubing could allow stress to be applied over enough area that it would be less likely to over stress a certain area. But I still can't help but feel like there would be advantages to thicker material like it staying a more consistent temperature across the whole length instead of being much hotter further from flanges or changing in temp much more rapidly. For example it's pretty rare for full throttle acceleration to last more than 20 seconds and usually far less than that. So on your more typical say 10 second full throttle pull if the center section of thin wall tubing got up to 700C and thick wall tubing got up to 500C while the flanges remained closer to say 200C wouldn't the latter be significantly better?
Isn't it also better to have thicker material attached to the necessarily thick flanges? Wouldn't it be better to keep that change in thickness as small as possible? Wouldn't it be better to go from a 3/8 flange to 1/8 tubing instead of .040 tubing?
Maybe if I present more specific examples you guys can help me understand better where stresses would be applied and how thinner tubing would be better.

http://cdn3.volusion.com/pa9ja.37rfp/v/vspfiles/photos/1046002-2.jpg?1486109685

Starting with a long runner header. Now for typical fatigue anchoring this header firmly at the collector would reduce vibration and forces from the exhaust system moving it around. But anchoring it hard would increase thermal stress and fatigue.
So for example 1 let's say that the header is not anchored but able to float freely, or maybe anchored on a rubber mount or bowed metal hanger allowing it to move pretty freely.
Thicker tubing would bend and flex less. It would move the collector end more and transfer more movement into the rest of the exhaust system. A downstream flex pipe, rubber exhaust mounts etc. But the manifold would stay cooler and a more consistent temp. So wouldn't thicker tubing apply less overall stress to the header and more movement in the rest of the exhaust system? Wouldn't that reduction in internal and localized movement and stress mean the header would last longer?

Now let's say the header is mounted hard to the block. I always try to avoid this. I like to use a bowed thin hangar that allows movement, but let's just say it's hard mounted to the block. In a case like this I could see how the tubing being able to bend more freely across the entire length would put less stress on the header flange and collector but there is also less cross sectional area where they meet, a much more drastic transition in area from the pipe to the weld to the flange, and the tubing would be changing temp much more and much more rapidly. Thicker pipe would stay cooler and more even in temp, and being stiffer would transfer more force into the collector, flanges and other components that can handle more force. If we think in terms of localized stress and let's say the head flange is 200C and the thin wall tubing 20mm away is 500C and 40mm away 700C vs thicker material being 300C and 500C shouldn't that help reduce localized stresses near the welds?

So especially if it's allowed to flex and move a little at one end I still can't see how the thicker tubing would be harmful. It still seems to me thicker should be better especially if it's not constrained hard at more than one point.

Next if longer more flexible tubing is actually better if you have to go short is there a point at which thicker would be better trying to get the whole thing to move as a single body?
This is our turbo manifold.

https://scontent.fapa1-2.fna.fbcdn....=9310bfd596a9ad02690d9664067eaae7&oe=5AA7F784

In our application longer runners are not possible. The lengths of tubing between flanges is short and the length of tubing between welds is extremely short. We have been making them for years without one reported failure. I use 11 gauge tubing. I like the fact that it's easy to weld and also easy to make welds that are very close to the cross sectional area of the tubing as opposed to big high welds which I always felt would be more likely to create a failure point. When I am making the downpipe I make a flexible hangar mount to the block that helps with some of the weight, vibration and exhaust system flexing but I know some customers have run them for years with only the flexible chassis exhaust mounts supporting the rest of the exhaust system and the turbo hanging off the manifold with no issues. Over those years I have seen hundreds of pictures of other thin wall manifolds cracking. I don't know about the manufacturing of all of them. Many and most common are the cheap Chinese manifolds and such which could be more of an issue with metalurgy or QC or something but any way you cut it in my experience thin wall manifolds have a much higher reputation of cracking just like in the pics above.

I am also curious how this ties into cast manifolds. Naturally the cast iron or steel will behave a little different and due to it's nature I'm sure thin wall cast would be a horrible idea but thick wall cast manifolds usually last hundreds of thousands of miles before cracking. If thinner is better then why do some thin wall welded manifolds fail after 10k miles while a cast manifold may just outlive the car?

 

[SwinnyGG]

Thinner tubes will have less stress but how much stress metal can take is directly related to it's cross sectional area.

Stress appear to be confused about the difference between stress and force/load.

Failure STRESS is determined by material.

Failure LOAD is determined by geometry- by taking the stress and multiplying by cross sectional area, as a simplistic example.

Thinner tubes see less stress because they are flexible enough to not pick up load as they move.

 

[yoshimitsuspeed]

jgKRI
That only addresses one of the points I was trying to make but it still does not seem all encompassing. If we were able to imagine that header fixed absolutely completely hard at two or more locations then stiffer tubing would definitely apply more stress to areas in the component. But that stress may be applied to heavier thicker components that can take that stress better.
But if the header is only fixed hard at one end then thicker tubes will just cause the header to move more transferring forces to other components instead of building as much internal stress right? What about the fact that thicker tubing will stay more consistant temp? What about the fact that 1/8 tubing welded to a flange should distribute forces more evenly than thin wall tubing?
I can't help but feel people are cherry picking the points they want to make that they think defends their stance without addressing all of the variables as much as possible or explaining this in a way that a broader range of people might actually understand.

Isn't stress a form of internal force/load? If we fix a pipe at both ends and heat that pipe that pipe will apply force to the walls and that force will equate to a given force per sectional area. If we put a solid bar in it's place the force it will apply to the wall will be much greater but the material sharing that force is equally greater. The solid bar should have a better chance of moving the wall more but I don't see how that results in a faster failure. If exposed to temp changes the thin tubing would heat and cool faster. If say we heat the room to 1000C the center secion of that tubing will quickly heat to close to that temp. The tubing will expand a lot and bow a lot and if we imagine that tube welded to each wall that angle of bow would add a great amount of force, strain, load to that area where the tubing meets the weld.
On the other hand a solid bar the same diameter would heat much more slowly. The center of the bar would stay much closer to the temp of the walls as the whole bar conducted heat into the walls. The bar would then expand much less. It would apply a ton more force on the walls but it would be much less bowed because of the temperature and because of the cross sectional area giving it more strength. Especially if we imagined it welded through it's full thickness to the walls. The force on the walls would be much greater but the angle of the bow would be less, the temp would be less, and the strain on the weld should be less. Especially strain per unit area. Should it not?
But more importantly in just about any real world situation and certainly with exhaust there is no magically fixed wall. Everything moves and everything should be designed to move. That solid bar would push one end of the wall away more giving the bar less bow and less deformation. Does everyone here agree that the solid bar would see a thermal fatigue failure before the tubing?

 

[EdStainless]

The point at which materials of construction really matter (alloy and thickness) are largely driven by energy density.
A stock engine making <50hp/l can use just about anything for headers and be fine. Neither flow nor temperature is an issue. Considering that most standard cars are rarely driven at WOT, and when they are it is for very short times.
If we have an engine that is mildly modified, 75hp/l configuration is still very tolerant.
If you are talking a performance engine >100hp/l then you are in a different world. Racing you are at WOT a high percentage of the time and you are generating 2-3 times as much heat as in stock configuration. A heavy cast manifold will never survive this thermal cycling and you need better flow.
It all depends on the application.

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P.E. Metallurgy, Plymouth Tube

 

[SwinnyGG]

everything should be designed to move

Yes.

Which is why thinner tubes are better.

Does everyone here agree that the solid bar would see a thermal fatigue failure before the tubing?

Your analogy in nonsensical. The reason no one is arguing your points directly is that you're making it pretty clear that you think you're right and are here to get confirmation. That's not conducive to people answering your questions.

The concept that you're neglecting in your thought process is that when parts are heated they don't heat at a uniform rate throughout.

A tube containing hot gas and exposed to convection on the outside is experiencing a thermal gradient.

The thicker you make the wall of the tube, the more slowly heat travels from the gas inside to the gas outside.

The more slowly gas travels from the inside to the outside, the steeper the thermal gradient through the cross section of the tube.

The steeper the thermal gradient through the cross section of the tube, the higher the internal stress generated by prolonged thermal exposure.

The higher the internal stress, the less likely for a part to have long fatigue life.