why are the headers being made out of stainless steel vice mild steel- is there any problems with making then out of mild steel- then have them coated ? I see post saying that mild steel would be fine for a street kit but not a race kit- why? thanks for any input

 

m/s can't stand up to the heat and has a bad tendency to crack. that's what i've heard anyway. i've just gotten into this whole turbo thing and i'm hooked already!

Dave

 

think you heard wrong.
mild steel's thermal expansion is less then stainless.
stainless tends to crack overtime, while mild will hold better. problem with mild is it will rust, unless ofcoarse you have it coated, then it will be fine.

 

Read this http://www.burnsstainless.com/TechAr...s_article.html

 

that was a good read metalman. glad i bought 321 stainless.

reading this paragraph from Burns seems to support what i thought was explained to me. i read it as saying that the m/s would not hold up to the abuse that stainless can.

You've probably seen Indy cars with their enclosed engine compartments and thermal clam-shell enclosures around their turbocharger headers. They must thermally wrap their exhaust pipes just so the radiant heat off the tubes won't cause fires or melt any critical systems. In this case headers made out of mild steel would completely fail and break apart due to the severe heat retention, let alone scale and send death particles into the turbocharger, ruining the turbine blades. 321 stainless steel has excellent high temperature fatigue resistance in this enclosed application and does a darn good job of living in this hostile environment better than any other material except the ultra-high nickel content steels ( such an Inconel ), which are hard to find, very difficult to work with and extremely expensive.

i'm always willing to learn so anyone can chime in who is in the know.

thanks,
Dave

 

I've got a few thoughts. Whether they're right or not is a different story.

1.) Burns Stainless is trying to sell stainless tubing, not mild steel. Of course their website will paint SS in the best light, and will try to convince you that the competitive products aren't any good. Thats good marketing. Is their website wrong? I can't say, I'm not a metalurgist.

2.) A turbocharged street truck and an Indy car are radically different animals. Heck, I don't think current Indy cars are using turbos anymore anyway, so I'm not sure it is a useful comparison. A race vehicle of that type is going to see high exhaust temperatures for extended periods of time (for the Nascar guys the same thing applies). How often are you going to drive like those guys (80% throttle or more for several hours)? They expect headers to last at least a race weekend or more, how long do you want yours to last?

3.) There chart does provide some useful information, but raises some questions too.
a.)They're comparing CREW tubing (cold rolled electric welded) tubing to annealed stainless. The condition will be annealed, so why compare to cold rolled tube (should have used the values for annealed mild steel tubing). No matter, the only real differences are the Ultimate tensile and yield strengths for the mild steel will go down.
b.) And why provide info on aluminum? Do they think many people build aluminum exhausts?
c.) They don't really highlight that the thermal expansion is 33% higher for SS than for the mild steel. May not sound like a whole lot but it is important, especially if you're building a header with limited ability to accommodate that change in dimension.
d.) They try to highlight the fact that steel's heat transfer rate is higher. Stainless having a lower heat transfer rate can be a blessing and a curse. Its a blessing in that the stainless takes longer to transfer the heat in to make them heat up. It also takes significantly longer for it to transfer the heat out to cool the pipes back off again. Adding a thermal barrier to the steel pipe probably makes the two fairly comparable, but without some data, that is merely speculation on my part.
e.) They mention high temperature fatique and such like that, but don't give any information at all about it. Why not? I'm sure the data is available to them, why not provide it to us?


I'm not saying that 321 (or some other special alloy) is NOT the best stuff to make headers out of. What I am trying to say is that there are 3 sides to every story, (in this example Burns Stainless, the mild steel guys, and somewhere in between the truth). In engineering, there is always a tradeoff, there is rarely if ever a clear cut answer. Here it is cost vs claimed life expectancy. For 4 times more money will they last 5 times longer? I don't know, I don't have any real data.

Just some thoughts to stir the pot a little.

'Dreamin'

 

yes 321 will be less prone to cracks the 304, that is a definate, CAS and turbotech used 304 on there kits. THey have had problems with cracking by several members.
321 is very pricey, if you got the cash then go for the 321
the main downfall to mild is its corision resitants, but with todays coating technology its an affordable solution.
you can buy one 2.5" U-bend of mild steel for $16, 321 from burns will cost you $78

i think burns was a bit biased and untrue when stating this
several people have wrapped mild steel pipes with exhaust wrap and have been fine.

 

hmmm....great discussion going on here. nice to have a calm conversation about something once in a while.

Dave

 

Manifold/Header design plays a huge part in preventing cracks. Material type helps but you hang any turbo that weights 40+ pounds off a tube that's .063 and add 1500+ degrees and something's going to crack sooner or later. I highly recommend adding gussets or as much support to the T- flange area. It also seems like any log manifold design always cracks due to the lack of expansion room. I believe that at least a little runner length is required to add in expansion and contraction. I fabricate manifolds from 304 and 321 all the time. Never had any problems with either to this day. I do however always recommend 321 but most don't want to spend the extra cash for the small amount of security it offers over 304.

Brett/METAL MAN

 

Mild works great for turbo manifolds as long as you can keep it from rusting. 304 will precipitate it's chromium carbides when elevated to the temps that a manifold sees under load...that's when intergranular corrosion occurs...basically the now unprotected steel will rust and corrode within the metal grain...this is what leads to the inevitable cracking. 321 is similar to 304, but adds titanium for greater strength and corrosion resistance. Of the 300 series stainless alloys, I'd say 321L would be the best choice...the L designation meaning lower carbon content then regular 321.

But, just as the article that was posted stated, Inconel would just about be your best bet for an "end all be all" turbo header. I could make all of the 45 and 90 degree fittings you'd need to make it, but I doubt any of you would/could afford it

 

I'm sure Inconel (or some other high nickel ultra alloy) would be great for exhaust work.

But you're right, I doubt any of us could or want to afford it. My guess is that it would be about 2x the cost of the 321 SS, just for the materials. And then its harder to fabricate with (welding is more specialized, etc).

Ceramic would be a nice option, but I doubt they'd take water splash very well, or the weight of a turbo hanging off the end, or the vibration that would be inherent in an automotive application. They probably wouldn't be cheap either.

Cast iron is a great choice. Low thermal expansion, reasonably good corrosion properties, decent strength. But fabrication is the killer there. Back when steam locomotives ruled the earth, there was a lot more casting going on. Cast iron, cast steel, etc. Now manufacturing is in a "billet" mode, where you take parts and carve them out of big chunks of steel with a CNC mill or lathe. We have "lost" (most of the folks with the skills retired 40 years ago) the ability to do castings inexpensively. Don't get me wrong, there are still shops out there that can do castings, just they're few and far between. There was a time that any big machine shop had a pattern department (guys responsible for making the forms for the molds) and a casting department. Guess that is the price of progress (and labor costs don't help any either).

The carbide precipitation in SS is an interesting thing. I don't have much experience in metalurgy, so its interesting to hear what's going on inside the material. Most people probably don't realize that many metals (like steel, SS, etc) change grain structure as you heat them up. Like precipitation hardening of 17-7 SS.

'Dreamin'

 

On street cars for daily use, mild steel with coating is the way to go. 304 flat out sucks for daily use. The test Supra that we used for a 74GTS, has had the header welded 3-4 times so far. If you go stainless, then go 321. We offer our kits in all three metals, but I recommend mild steel any day with coating.

Jose

 

ok so now that i own a set of stainless 304 turbo manifolds.... whats the thoughts of jet hot coating the outside and inside of these things to make them last longer? say the JET-HOT TDC™? im not asking for a mericle that 304 will become better than 321 but for those of us who already own ss304 manifolds that are part of our projects....

Thanks, Will

 

You can coat them and it will help some to reject heat, but thats not the problem with 304. The problem is its thermal expansion. Those are internal properties of the metal that just can't be remedied.

 

coating 304 wont stop it from cracking, my TTi log header cracked even though i had it coated

 

hey smokinhawk did you coat the inside too?