As I'm taking my daily browsing of Mbworld. I came across a statement about mild steel use in headers and the claim that its better than stainless steel for headers, Or something along those line. So I was inspired to try and set the record strait about mild steel vs stainless steel and its aspects of use.

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.

 

So what grade of steel is your mid-section made of?

 

Also from a steel manufacturing standpoint mild steel is much more inconsistent bc of the large amount of variance in the spec. That being said there are specs for all types of steel so knowing what you have and what it's properties are is important. I've had mild steel headers on my S10 for 8ish years. Haha all about application. Why would I spend SS money on a $2-3k car? The AMG only gets the best SS ftw in any spec over mild steel.

 

Hooley, I'm curious as to what you might think about what Brad says with the Evosport headers from this recent thread: https://mbworld.org/forums/w211-amg/...-now-what.html

He said this:

"There is a reason we use mild steal [steel]. For it to crank [crack], 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." https://mbworld.org/forums/4702056-post19.html

It's all interesting stuff and I'm curious about it. But now we have two different views going on here it seems.

Thanks for your post, it was informative.

p.s., I'm assuming what you posted was in response to that thread, but I'd like to hear more......

 

i don't know that i've ever heard or read anyone generically claim mild steel to be "better" than stainless. regardless, mild steel has been successfully used on exhaust systems for decades and for most people it will outlast their ownership of the car, even when left uncoated. ironically, most reports around here of headers cracking are of the stainless variety (but i would surmise that has more to do with workmanship rather than material).

 

I've been hear a while, Its best to let people say things in threads that relate to them. If I was to reply to that thread it would have taken away from their customers issue. Something I was not interested in doing. Carbon steel, weather it be mild, medium, or heavy is harder than stainless steel, in general. When steel gets harder it in turn becomes more brittle. The easiest way to think of it is... When carbon is added to steel the more brittle if becomes. Carbon steel can be heat treated. Where as 304 stainless can not. Some types of stainless can be heat treaded, but not 304. it comes down to the contents inside the stainless, in turn dictates the stainless steels grade. 304 stainless has an element of elasticity to it where as carbon steel does not.

@mikey, You have to ask yourself, are any of the headers that are stainless steel made from stainless steel from the USA? Granted I've seen 409 stainless crack all over the place, but thats a total different beast than a 300 series US made stainless steel. I personalty think 409 is junk. some shops and manufactures like it simply for the selling point that is considered stainless steel.

 

nice write up!

 

i hear you mike and i fully agree...but i also recognize that for many exhaust applications, mild steel is quite adequate.

 

Hooley, et all

We have had this debate before and it is searchable, but I want to counter a number of points so that the viewers get a more accurate picture of the facts. Although we have been making headers for 10+ years and sold over 1000 in that time, I will actually quote an independent and give comment in this post. The individual that I will quote is Jack Burns (whom I think we will agree has a fair bit of experience).

From this page: http://www.burnsstainless.com/stainlesssteel.aspx

Jack starts near the top with this (which I am sure to most would be the end of the debate, but it really is the start:

This is a true statement, however, once you coat mild with a true aerospace 2-stage lifetime ceramic coating, it nullifies this statement. We will see more as we go...

So as we read, we see that low carbon content steel (such as 304) is not as hard as high carbon content steel (mild) as to yield strength. (This does not impact elasticity or tensile strength) We also see that stainless such as 304 will start to break down above 800° (such as on high EGT S/C or Turbo applications).

OK, so Burns acknowledges that coated mild steel and uncoated stainless give the same benefits as to exhaust velocity. This is one of the 2 major reasons we have always coated ALL our mild steel headers.

Pertaining directly to 304:

Burns states quite clearly that 304 is a great choice for NA applications. However, due to the reasons posted above, does not make a great choice for high EGT applications.


Looking at this chart, again from Burns, we see some very interesting facts (not opinion as was the earlier quoted text).

• Tensile Strength - Stainless IS higher than mild, not the reverse
• Modulus of Elasticity - There is more flex in Mild, not less
• Coefficient of Thermal Expansion - Mild is lower
• Coefficient of Thermal Conductivity - uncoated mild is much higher

So now is evosport stance and a comment on why I posted this. To start with the second, I posted this as I have a very big problem with anyone posting opinion as FACT. Hooley, your entire post is coached as absolute fact, and it supports YOUR product. What you are really doing is posting a position statement as to why you manufacture a certain way, with an effort to paint any mild header as inferior. That is not ok. Feel free to post your opinion or your position, but please for the betterment of the community, post it as such so people understand.

Now to the first point. Evosport makes headers out of Mild, 304, 321 and Inconel (depending on the application).

Mild Steel304321InconelSo the net/net is simply this: There are multiple choices for header material. Evosport had very clear and supported reasons for our choices of material for each. This is not to say that another choice for another manufacturer is wrong, it is to state why we have selected the material we use and why.

Hopefully this helps.

Thanks
Brad

 

damn you brad... good job

 

Although I have to say that when you said "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" then there was also the implication that 304 was simply inappropriate material, period. So you can see the confusion that a reader might have about it.

I do realize of course that the context was within an example of a specific set of headers that cracked, but for the uninitiated the assumption could be that 304 is inferior no matter what. And I think that's why Hooley addressed it. So both of you were stating a specific position within a certain context.

Good info and all is very much appreciated This certainly helps clear up the concerns. Thanks.

I'm looking for a bolt-on mod that makes sense for the n/a 6.2 motor in respect to bang for the buck and headers make sense. So it's all a good education for me and helps a lot.

But one thing all this implies to me (in conjunction with the other thread) is that if you're getting Evosport headers (or other comparable quality) then be certain to keep an eye on the motor mounts otherwise you'll be susceptible to cracks. And with 304 steel used in the n/a 6.2 liter headers (Evosport or MBH) then it's an even bigger issue. And if it's an issue even if the mounts are just beginning to fail, then it's a problem with any header despite the material, but worse with 304. On the other hand without concerns over high EGT heat cycles, maybe the 304 won't suffer the brittleness you mention So there's going to be a potential problem with cracking if you ever have mounts failing (even beginning to fail) no matter the material. And with 304 being even worse (potentially speaking.)

Maybe I'll stay stock

 

Not interested in debating. But I will say this lastly, Brittleness in steel basically mean the lack of plasticity. When carbon is added to steel that increases its strength, However with increased strength from the added carbon, it in turn decreases the plasticity.

Plasticity in metal means how much the metal can move, shape, or distort without breaking.

More carbon content the less it can move without breaking, Less carbon content in steel, the more it can move without breaking.

We use 2 grades of stainless steel in our headers. On the part that sees the most heat (header flange) we use T-321 stainless steel plate and T-304 tubing for the primary. If this was a turbo application that sees much higher exhaust temps than a supercharged engine. 321 through out would be the way to go. However since the supercharger is sitting on the top of the motor. The 304 stainless is not exposed to two glowing red turbos. That make 1400 or so degrees of heat on the manifold.

 

I love it when topics are debated in a civil matter....

To the experts: what about ceramic coated T304? Also, can you shed some light on titanium headers/exhaust?

 

+1...That was quality reading...and I'm not even being sarcastic in the least. This is what MBWorld should be about...informative without anyone pissing on each others remarks.

Good job...

PS I'm also interested in the last part of MB_Forever's comment...your 411 on the matter would be appreciated. All the GTR guys seem to be headed down the titanium route mainly for weight savings (I believe) but I ended up going with an electronic valve exhaust system that is made from 304 steel (mainly for the ability to switch from quiet to loud - I've already been given notice for noise violation from my homeowner's assoc. on my E55 straight pipe). The more I can learn the better...thanks again.

 

I'm afraid I have some misgivings about your fundamental assumption here. The ultimate tensile strength of AISI 302 stainless steel of 860 MPa is much higher than the 400 Mpa figure of mild steel. AISI 302 stainless steel also has a yield strength of 502 MPa, higher than the 250 Mpa figure for mild steel. How then is stainless steel softer than mild steel?

 

F@#CKING HOA's; while I understand the desire for a standard set of rules to protect property values, I know of way too many instances when the wrong person gets on the board and miraculously starts to channel Stalin. . . my favorite story was when my cousin was the president of his HOA, he led the charge to "gate" his community. . .one of the home owners took issue with his tactics and the punchline was him (as well as the home owner who took issue with the gate) had to place calls to their respective wives to bail them out of jail. . .I couldn't resist telling him "well it seems the gate wasn't necessary because it appears the criminal element already lives with in the community" rant over. . .Hooley, does your X-pipe mid section work with Kleemann headers with no cats?

 

You know what you are right, I was wrong. I went down to the steel yard today and basically got a crash course and master class all rolled into one. The guy kept telling me I was thinking about chromoly. And thats why people choose chromoly for cages. If someone want to correct me as to why the NHRA does not have a spec for stainless steel cages in their book (the real reason why)? Other than it would cost a boat load to get it in a safe gauge. knowing chromoly has to have a wall thickness of .083 and Mild Steel cages have to have a wall thickness of .120 inches. We use 16g (.060) stainless in our header and that costs a boat load.

I also was corrected about the hardness thing that Brad stated. Where stainless is more dense than mild steel. Interesting development I also found out was there are mild steels out there that can be stronger than 304 stainless. As there are many different mild steels out there. I picked up some 16g mild steel pipes to do some self testing vs stainless. In the end, I guess it all depends.

Off the top of my head we can swage out stainless pretty far without cracking. So I want to see If I take the mild steel out as far before it cracks. Part of me things the mild steel will crack on its rolled seem or at least before the stainless will. We will see.

The last piece I will add to this thread is... There is a wealth of information out there. In my spare time I found a bunch of information that supports both arguments. In one article I find info that supports my stance. Then the next one says the opposite. It can be really tricky to make heads or tails of it all thats for sure. Hopefully I can get some free time this week to crack some metal.

Best Regards,
Mike

 

great info in here!

 

GJ mike

 

My 2 cents..... Steel. For one, durability. And aluminum does cost more to repair. My guess is the performance gains would be deathly minor and the extra $100.00 would be better spent on other toys for your car.

For a DD... Steel... For a weekend racer, aluminum.

 

Dont see the relevance of your comment this is a debate over which STEEL is better for headers ,.. aluminum would not be a good choice for any exhaust material

 

I read somewhere that mild steel headers sound better than stainless