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    Mild Steel vs. 304 Stainless Steel and the Mercedes Headers debate

    There has been some discussion on these two topics and want to see how our members weigh in on it. I'm not a header or steel expert but seems there are two very conflicting theories here:

    Click here to enlarge Originally Posted by Brad@Evosport
    ou need to replace your motor mounts if you have not already done so. There is a reason we use mild steel. For it to crank, there has been an excessive amount of torquing into the merge collector. That can only happen with the fluid filled motor mounts begin to or have totally failed. This allows the engine to rotate more than anticipated and as the exhaust is very rigid, the forces concentrate on the merge collector in the header. This will happen to ANY header, FYI. In fact, if it was 304, it would be far worse and happen sooner as 304 is more brittle after many high EGT heat cycles.
    vs. :

    Click here to enlarge Originally Posted by Hooleyboy

    First things first, Mild steel is a great metal to make things with. Its affordable its very strong and relatively easy to work with when there is no rust. Mild steel has a bunch of uses in the automotive industry. Roll bars and roll cages can be made from it. NHRA rules state 1.75 .083 wall, Mild steel can be used to construct a NHRA spec cage. With NHRA rules set to protect the driver, Its clear Mild steel is good stuff. In fact if you made a roll cage out of stainless steel, it would offer limited protection.

    The truth is, Stainless steel is a softer metal. Its flexes, it bends it does everything you dont want in a devise that is meant to protect you. Carbon steel comes in a few grades. Mild Carbon Steel, Medium Carbon Steel, and Heavy Carbon Steel. When you see the term "Mild steel" that is referring to that amount of carbon inside the steel.

    We all know that carbon anything is strong stuff. Thats why we use it in things that need to be structurally sound. MBH even uses it in our jigs for headers. We can't afford to have our jigs bend. In this case mild steel is a perfect choice for us. As long as we keep the rust off it. Its very dry here in AZ and the jigs never see outside use, so rust isn't a huge issue.

    Stainless Steel:
    Stainless steel comes in a bunch of ratings from T-409, T-304, T-308, T-316 T-321. The list could go on for days about the many other types of stainless steel. In this case we keep to what you need to know in the automotive industry.

    T-409 meets the minimum requirements to be called stainless steel. In fact we all have it on our Mercedes now. As our stock exhaust system is made from it. Its stainless steel in its cheapest form.
    T-304 has proven to be ideal for header use and exhaust systems. It is non-magnetic, has good flexibility qualities and handles heat well.
    T-308 is basically what you use to weld T-304. Its used as a filler rod.
    T-316 is a stainless steel that has good use in water, Surgical stainless steel is T-316. My Kitchen knifes are made out of T-316.
    T-321 is considered on of the best steels to use in high heat applications. think of it as T-304 stainless steel with some titanium in it.

    As said before mild steel has its uses. Where it is an ideal choice for the job. That is the same with stainless steel. Where I wouldn't make a roll cage from stainless steel. I also would not make a header or exhaust system with mild steel.

    Here are some reasons why I wouldn't make an exhaust system out of mild steel.
    1) Its extremely ridged, due to the added carbon inside the steel.

    2) It rusts. In fact non protected mild (carbon) steel can begin to oxidize with in a few moments of regular exposure to air. Imagine what it would do in a rain storm or through a puddle.

    3) Not having the flexibility of stainless steel, it is much more prone to cracking. So, rather than expanding under heat, the rigidity of mild steel tends to crack.

    4) coating, I have never seen a coating that 100% lasts. It flakes off, it scratches and exposes raw metal, and it makes repairing difficult because you have to remove the coating before you can weld the problem area. Also note the inside coating is one of the first areas to go. This is because, the thermal coating is trapping heat inside causing it to experience much higher temps that the outside coating. Coatings can also contribute to cracking. Due to the stiffness of mild steel coating a header causes the header to be much hotter inside. The headers reaction to the extreme temp difference from outside to inside can cause cracking. Much like putting an ice cube in a glass of hot watter. The ice cracks from the extreme temp difference. Stainless steel resists this because it will expand before it will crack.

    5) Condensation: Have you ever seen a car start in the morning and seen water come out of the tail pipe? I'm sure you have. That water was building up over time and just sitting stagnant on what could possibly be an exposed (uncoated) section of mild steel. Causing rust.


    In comparison, quality US made stainless steel will have none of these down falls. Granted any header can crack no matter what its made of, but the likely hood is far greater with mild steel when used in a heat oriented application. Also take note, that overseas stainless steel is not the same as Stainless steel made in the USA. Stainless steel from China is an inferior product when compared to stainless from the USA.

    In conclusion mild steel is great for structural uses. where as stainless steel is not. Stainless steel is good for things that are exposed to the elements, things that need to be less stiff. This makes it one of the best choices for use in automotive exhaust systems.
    Which is it?

  2. #2
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    I've been doing some research combined with the little bit I have learned about exhaust systems over the past decade and my understanding is this:

    1. Stainless Steel is more expensive to make headers out of. There are several reasons for this including the material cost and that it's harder to form. So, it takes more skill and labor.

    2. Rust. You know this exhaust piece will be exposed to some elements at some point. You know rust can be an issue, mild steel rusts, pretty simple.

    3. Stainless does not require a thermal barrier coating to cope with EGT's.

    4. Stainless steel thermal characteristics lend themselves to better exhaust gas velocity (as in, a better choice for a performance oriented header).

    5. Longer lifespan for 304.

    These would be the main reasons and most competitive header manufacturers choose Stainless Steel although the piece is usually more expensive. You can have great results with either metal though.

    I don't understand the post by Evosport in the sense of 304 being more brittle as it can cope better with thermal stress due to its physical properties. This is why mild steel headers should have and need a thermal barrier coating.

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    The only issue I see with mild steel is rust, and it rusts pretty quickly. I can't comment on the other factors, but what you have said seems logical. I personally would never use mild steel on anything I plan to keep for any amount of time. I'm going to forward this to a buddy of mine that does a lot of fab work to get his take on it.

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    I'd be interested to hear the opinions of some of the experts on this board as well. Also, it'd be nice if someone can shed some light on titanium exhausts....
    2007 E63 P30
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    I don't think I've ever heard of issues for 3, or 4. In fact I can't think of how 4 could be true.

    1, very true, mild steel is softer, and therefor easier to cut, grind, weld, bend, etc. The reason it makes a better roll bar is that stainless steel is more brittle and will just crack on impact vs. absorb energy by bending a bit. 2, yes, they rust, coat them and take care of them and this won't be an issue.

    5, subjective, if its coated and doesn't rust, there is no lifespan difference.

    Personally, I'd rather have mild (And thus made my headers from mild) because it handles heat better and doesn't get brittle/crack as easily.

    I agree 100% with the first post you quoted.

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    Click here to enlarge Originally Posted by Bimmerdude Click here to enlarge
    Personally, I'd rather have mild (And thus made my headers from mild) because it handles heat better and doesn't get brittle/crack as easily.
    It doesn't handle heat better which is why it needs to be coated. Stainless handles higher heat better as well as retains more heat leading to higher exhaust velocity, right?

    Plus, stainless can be coated as well.

    I think the reason Mild is being defended is because it is what they sell and they chose to go cheap with materials. It's amazing considering what they charge. Supersprint uses stainless for their M5/M6 headers for example, Evosport goes mild coated steel.

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    Click here to enlarge Originally Posted by Sticky Click here to enlarge
    It doesn't handle heat better which is why it needs to be coated. Stainless handles higher heat better as well as retains more heat leading to higher exhaust velocity, right?

    Plus, stainless can be coated as well.

    I think the reason Mild is being defended is because it is what they sell and they chose to go cheap with materials. It's amazing considering what they charge. Supersprint uses stainless for their M5/M6 headers for example, Evosport goes mild coated steel.
    Its possible that they are defending it because of this, but stainless is more brittle due to higher carbon content and lower nickel content. 409 is less brittle than 304, and 321 is really the way to go for headers but super expensive and harder to work with.

    Do a few tests, take a piece of mild steel pipe, cut it, do the same with stainless, the stainless takes WAY longer to cut through (with a band saw at least). Fabricators like mild, and I've never heard of a performance difference from it...it conducts heat better, but I don't see that as a real issue if its coated.

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    Best material for headers hands down is Inconel. If you want the best that's the only way to go.

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    Click here to enlarge Originally Posted by Bimmerdude Click here to enlarge
    Its possible that they are defending it because of this, but stainless is more brittle due to higher carbon content and lower nickel content. 409 is less brittle than 304, and 321 is really the way to go for headers but super expensive and harder to work with.
    Yes, stainless is harder to form and not as malleable. It does have higher thermal ranges though. A stainless header needs to be better designed for the application, you can't just bend it into place.

    [center]
    Click here to enlarge Originally Posted by Bimmerdude Click here to enlarge
    Do a few tests, take a piece of mild steel pipe, cut it, do the same with stainless, the stainless takes WAY longer to cut through (with a band saw at least). Fabricators like mild, and I've never heard of a performance difference from it...it conducts heat better, but I don't see that as a real issue if its coated.
    In any exhaust pieces how they handle temperatures will affect performance. I don't make exhausts but people that do praise Stainless in its ability to perform due to heat retention. We are talking about guys who want to squeeze out every last horse, right?

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    Click here to enlarge Originally Posted by Jimefam Click here to enlarge
    Best material for headers hands down is Inconel. If you want the best that's the only way to go.
    Absolutely, and for what some of these guys charge for mild it SHOULD be Inconel.

    Considering the Evosport M5 headers are $4500 for coated mild... they have a nice margin there.

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    I saw a guy pay 12k for custom made inconel headers for a GTR a few years back. When I look into having a new exhaust manifold for the N63 I'm gonna try and see if I can get them made in Inconel.

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    1 out of 1 members liked this post. Yes Reputation No
    Click here to enlarge Originally Posted by Jimefam Click here to enlarge
    Best material for headers hands down is Inconel. If you want the best that's the only way to go.
    We use 718 Inconel and Hastelloy to make turbine blades for aerospace Pratt & Whitney engines and land based power turbines. The $#@! is near indestructible even at 1200F it retains nearly all mechanical properties.

    Ferrous materials such as steels undergo phase changes after about 800 degree F, which means it's not the same material anymore at that temp.

    I'm not gonna go into detail because its sunday and Im tired right now.

    Stainless steels are more brittle after heat cycling, it has a thermal conductivity nearly half that of mild steel. What does that mean?

    Ok, a material with a lower thermal conductivity builds thermal gradients faster. The thermal gradient is a region of VARYING TEMPERATURE and this causes something called pure induced thermal stress. Add to that MECHANICAL stress such as the weight of the header and the engine torque over and you have a crazy high stress probably at or near where the flange and primaries meet up.

    A material with a high thermal conductivity will build less thermal gradient as it warms up, but mild steel is inherently not as strong as stainless. Although, I am not trying to down mild steel either since it is the workhorse of our nation. Mild steel is a very TOUGH material. TOUGHNESS is the ability for a material to absorb energy in the form of either cyclic stress, constant stress, impact, thermal stress etc... But it's overall ultimate strength may be lower than stainless.

    So, for making headers; you can use either and make a good set of headers. The problem is, if the design is flawed either will crack. Stainless will crack first though.
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    Click here to enlarge

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    Click here to enlarge Originally Posted by Jimefam Click here to enlarge
    I saw a guy pay 12k for custom made inconel headers for a GTR a few years back. When I look into having a new exhaust manifold for the N63 I'm gonna try and see if I can get them made in Inconel.
    Of course you can have them made in Inconel, just $$$$$$$$$$.

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    Click here to enlarge Originally Posted by Sticky Click here to enlarge
    Of course you can have them made in Inconel, just $$$$$$$$$$.
    Inconel as a material is actually sort of expensive, but the REALLY expensive part is the processing of inconcel. Machining it is not easy, casting it is not easy, bending it is not easy etc... You get the point.

    That's why they cast turbine blades to near net shape, because going back into a casting with more than one machining process costs a ton of money in tooling and time. The material isnt even the big deal.
    I have never used inconel for anything other than turbine rotors and blades/stators, but I can assure you making an inconel exhaust sounds like a hell of a lot of labor.
    Some people live long, meaningful lives.

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    Click here to enlarge

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    Click here to enlarge Originally Posted by DBFIU Click here to enlarge
    So, for making headers; you can use either and make a good set of headers. The problem is, if the design is flawed either will crack. Stainless will crack first though.
    Will it? This guy switched from mild to stainless, but looks like his problems were caused by rust:

    Click here to enlarge

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    theres too many variables sticky, the stainless headers may have been made from a different thickness material. Maybe the shape of the mild steel header was causing some crazy high stresses due to heavy preload, maybe the stainless header was just a better design?

    You have to remember, no two headers are identical. But assuming they are shaped identically, in theory the stainless headers are more sensitive to heat cycling.

    Rust is another issue, if you live in florida like me, I would go stainless.
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    Click here to enlarge Originally Posted by DBFIU Click here to enlarge
    Stainless steels are more brittle after heat cycling, it has a thermal conductivity nearly half that of mild steel. What does that mean?

    Ok, a material with a lower thermal conductivity builds thermal gradients faster. The thermal gradient is a region of VARYING TEMPERATURE and this causes something called pure induced thermal stress. Add to that MECHANICAL stress such as the weight of the header and the engine torque over and you have a crazy high stress probably at or near where the flange and primaries meet up.
    Ok, what I don't get is doesn't stainless have a higher thermal limit? And it can sustain and retain higher temps better?

    Stainless is more brittle after cycling yet it can handle higher temps?

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    Click here to enlarge Originally Posted by DBFIU Click here to enlarge
    theres too many variables sticky, the stainless headers may have been made from a different thickness material.
    It's just a good example of what can happen with mild.

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    1 out of 1 members liked this post. Yes Reputation No
    Click here to enlarge Originally Posted by Sticky Click here to enlarge
    Ok, what I don't get is doesn't stainless have a higher thermal limit? And it can sustain and retain higher temps better?

    Stainless is more brittle after cycling yet it can handle higher temps?
    Yes.

    Its sort of like this, a paper clip can be bent back and forth more times than a pushrod can. Does that mean the paperclip is stronger than the pushrod? No, it just means the pushrod is more brittle, it can handle much higher stresses when they are applied in one direction, but not back and forth like a paperclip.

    Stainless retains heat better, this is true. It melts at a higher temperature than mild steel, this is true. But it cannot sustain the same thermal cycling that mild or low carbon steel can, it just has too much chromium and nickel in it which are brittle at higher temps after much cycling.
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    Click here to enlarge Originally Posted by Sticky Click here to enlarge
    It's just a good example of what can happen with mild.
    No doubt, mild aint the $#@! when it starts to rust its game over. I would probably go for stainless myself if i went for a set of headers. But just make sure the headers are GUSSETED, $#@! do you know how many times I see a set of headers BEAUTIFULLY MADE and are bolted up to a set of heads with a 100 lbs turbo hanging CANTILEVER off the header? Yea, there goes your 10k dollar header no wonder right? Gussets at the flange and primary is a 2 dollar insurance policy for 10k headers regardless of material.
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    Click here to enlarge Originally Posted by DBFIU Click here to enlarge
    Its sort of like this, a paper clip can be bent back and forth more times than a pushrod can. Does that mean the paperclip is stronger than the pushrod? No, it just means the pushrod is more brittle, it can handle much higher stresses when they are applied in one direction, but not back and forth like a paperclip.
    Ok, so mild cycles better yet it can't hit the same temps so can't mild just melt at high EGT's that say stainless can handle?

    Click here to enlarge Originally Posted by DBFIU Click here to enlarge
    Stainless retains heat better, this is true. It melts at a higher temperature than mild steel, this is true. But it cannot sustain the same thermal cycling that mild or low carbon steel can, it just has too much chromium and nickel in it which are brittle at higher temps after much cycling
    So on paper Stainless should perform better in a header or manifold application where this heat is critical, correct?

    Can't stainless just be coated to offer the best of everything?

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    By the way disclaimer I am by no means an automotive exhaust designer expert, just a guy with experience with exotic materials in the gas turbine industry. Similar, and hopefully it sheds light on some different materials out there.
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    Click here to enlarge Originally Posted by Sticky Click here to enlarge
    Ok, so mild cycles better yet it can't hit the same temps so can't mild just melt at high EGT's that say stainless can handle?



    So on paper Stainless should perform better in a header or manifold application where this heat is critical, correct?

    Can't stainless just be coated to offer the best of everything?
    The coating of stainless will not detract form the fact that it has a low thermal conductivity and a susceptibility to crack IF DESIGNED INCORRECTLY.

    What is the definition of designed incorrectly?

    Having a 100 lbs 88mm turbo hang off the header on the side of the motor with no torque link to absorb engine torque. $#@! like that is bad.

    Stainless should perform better in theory, if one can address its sensitivity to thermal cycling, that is having to get hot and cold often while under heavy mechanical stresses. Remove those mechanical stresses and the stainless should perform ok, assuming it is strengthened at joints such as primaries. Stainless is a good header material dont get me wrong, but its not the materials fault when the header cracks because of inadequate structural support.
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    I cant say I hate mild steel either, because its a great material. Very very resilient to cyclic loading. That is where it shines. It doesnt have the same high temperature mechanical strength, but it can be cyclicly loaded more often than stainless (assuming the same temperature conditions are being met for comparison). Mild steel has good fatigue resistance, like the paper clip vs. the pushrod.
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    Click here to enlarge Originally Posted by DBFIU Click here to enlarge
    Stainless is a good header material dont get me wrong, but its not the materials fault when the header cracks because of inadequate structural support.
    Right, and I think that is the main point. What Evosport acted like was that stainless will crack no matter what due to being cycled. If the design puts a lot of stress on it, it will crack, but likely any setup would eventually if the design is that poor.

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