Can anyone explain this?
I am finding alot of companys using Mild Steel Flanges BUT stainless runners on turbo manifolds.

 

im pretty sure that the stainess flanges warp.

 

...and so do mild steel, aluminum, titanium, etc flanges.

 

Stainless warps less than mild steel. This is a factor along with cost.
I use mild most of the time due to cost. I like a thick flange with certain design criteria and making them out of stainless would triple the cost. Benefit of said cost: they would stay shiny longer. That's it.
Stainless is also twice as strong as the mild steel, which, if there is some minor distortion of the flange, will make it impossible to get a good seal.
As long as 309 is used to weld them together, it's fine.

 

^ False

Just because Stainless steel is harder that does'nt make it distort any less..

Thermal conductivity of ss is .33 times that of low carbon steel.

Coefficient of thermal expansion;
- Austenitic ss is 9.6x10 to the -6
- Low Carbon Mild steel is 6.7x10 to the -6

that bieng said...

Mild steel flanges are used on high end manifolds such as Full-Race because it controls distortion better and less residual stress is left after the weld cools.

 

Yes, it does. I was talking after the fact. If it is not perfectly straight, it is less likely to bend straight while tightening the bolts due to it's higher strength. Hardness was not mentioned.
The thermal conductivity is much less, meaning the heat stays and will not spread out like mild steel. The definition of distortion is uneven heating and cooling. Because the heat stays localized, there is also more residual stress, correct.

 

Your first statement was "Stainless warps less than mild steel." in ether cases that is incorrect... before or after welding

And now you just said: "The thermal conductivity is much less, meaning the heat stays and will not spread out like mild steel".
^ Thus making stainless warp Easier.(while welding) and thats exactly why i already mentioned the coefficient of thermal expansion rates.

But most Flanges get fly cut after they are welded anyways, Ss or Lc.

So in my opinion Low Carbon Mild Steel should be used due to less distortion.
Stainless Steel can also be used but an additional machining step will have to be added to completing a perfect seal

 

It's almost always a cost compromise since low carbon steel is less expensive to work with. It works just fine when used on manifolds/headers w/ ss runners, fabricators have been using this combo for a while...even though the runners expand at a different rate than the flanges! From my experience, the only real downside is the lack of corrosion resistance. Depending on the environment, compare a ss flange to a ms flange, and the ms flange will look a lil funky after a while

 

Stainless warps more and isn't as strong as mild. Not that strength is really an issue on manifolds. As bells mentioned cost is a big factor. My cost on stainless is about 2x what I pay for mild.

 

Sorry, totally missed that typo. My point was the opposite. I now see the confusion. My bad.

 

No worries.

 

I use stainless flanges in n/a headers every time except for a mild steel headers (very rare for me) as they stay looking nice and with the lower heat from welding the thinner material warpage is less of a factor.

But when it comes to a thick wall turbo manifolds i prefer mild steel hands down.

 

Mild is also easier to machine flat afterwards.