When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
why do noobs bump old *** threads with irrelevant, one sentence posts?
304 is a bitch to machine, it's really hard and loves to chew through cutters and end mills... Which is why most flanges are made from mild steel. It's a much softer metal and much easier/cheaper/quicker to machine. You have to use relatively slow feed rates with any stainless steel.
It's also corrosion resistant, where as mild steel will form a surface oxide... This oxide normally protects the metal from further degradation but over time can become a full blown material dissolving rust. This can be avoided with things like ceramic coatings, etc.
As previously stated SS also keeps more heat in the exhaust system vs. mild steel... This in turn equals hotter, faster exhaust gasses which means quicker spool and more power overall.
As far as coatings, the average turbine housing can be coated for roughly 50 dollars plus shipping each way, pistons around 45 dollars a piece, ehaust manifolds typically start at 150 for small items and go up from there
It's been more than 2 years since I have last wrote over here.
Just as a quick refresh.
I have made this manifold for a turbo CRX (D16 engine) and this guys runs his car for approx. 3 years now without any issues.
Therefore I think that Mild Steel is a good option for your manifold!
don't powder coat it. Usually when we've done powedercoating, you can reheat it and repowder it. Making me think it gets "soft" again so the other layer of powder will attach. Stay with ceramic.
I have been spectating on this forums for a long time but finally joined.
Being from Australia this makes things a little different for material choices for me. Carbon steel butweld elbows and seamless pipe as very cheap compared to stainless steel products. A DN40 STD WT elbow costs me about 1.5-2.0 AUD where as stainless versions are very limited. I cannot find 304 Short Radius elbows as all the local suppliers only stock in 316 and charge $4-6 each for sched 10. The other thing is unless consuming large quantities of Argon the price is very high also which makes a purged manifold more expensive.
I have been building manifolds for many years and apart from one K20 manifold i did in stainless, all my manifolds have been from carbon steel. Some have been ceramic coated and some have been just painted in a high temp paint. I have also had 4 different turbo manifolds on my old car and experimented with designs and materials. One set of manifolds i built for a customer were involved in a collision with a tree which pushed the turbo into the engine and left marks where all the runners were pushed into each other. There was a small crack where the wastegate take off was welded into the collector. I welded this up and with a new turbo the manifold was re-used on that car after it was all pulled straight. I was impressed with the durability of the manifold.
One misconception is that carbon steel is softer than stainless. It is infact harder as the yield strength is higher. The ultimate strength is lower which is what makes it easier to machine but one thing i found when building the stainless manifold is i didnt need any heat to massage the collector to match the turbo flange. This would have been impossible with carbon steel and i need to get the material red hot before i have any hope of massaging it.
Also previous research has indicated that stainless has carbon come out of solution when exposed to elevated temperatures for periods of time. This is what can cause the cracking after heat cycles. I would hazard a guess to say that back purging and full pen welds will reduce the stress riser at the root of a non full pen weld. I guess this is why you guys do not have the issues that i would expect from using stainless in this application. The carbon steel has enough strength at high temps that means this type of weld detail is not required for the strength requirements of a turbo manifold.
Below are some photos of some of the manifolds i have made in the past from carbon steel:
P.S. I have always been inspired by contributors on this forum and have pushed myself to achieve better quality welding in a hope to someday come close.
I aim for single pass but if i get a section of weld which has sunk below flush i will put a second pass over that area.
The larger wall thickness does need substantially more filler than the thinner stainless. If i planned to do it in 2 passes i would probably end up with a neater weld but that would take more time and i have the ebay generation to deal with who want everything for Chinese prices cause they can get one of ebay for that price.
I'm nearly graduated M.Sc. and I've been studying welding and welding metallurgy, so I have bit knowledge to back my sayings.
As far as strength goes, mild steel (S355) and 304 are similar. Yield strengths are 355/205MPa and ultimate tensile strengths are 470-630/515MPa. As you can see S355 is a tad stronger and especially yield strength is important when building manifolds. You really dont want your manifold to yield.
Thermal expansion coefficent for S355 is 11,1 (10^-6/K) and for 304 its 17,2. So 304 will expand roughly 1,5 times more as said before, which will result more stress in curves. Thermal conductivity on other hand goes for 304. Conductivity for S355 is 42,7 (W/m*K) and for 304 its 16,2. So 304 will need more time to warm up, but it will stay hot longer than S355.
Big thing with 304 is corrosion resistance. As everyone knows, mild steel will rust. Stainless steels wont rust as easily, but austenic stainless steels will loose their corrosion resistance when they get hot enough. For 304 limit for continous temperature is 870°C and above that temperature it will rust quickly. That is the reason for inconel manifolds. Inconel can handle way higher temperatures without loosing its corrosion resistance.
To summarize a bit:
S355 pros:
-Tad stronger
-Can handle heat better
-Easy to weld (Both GMAW and TIG)
-Easier to machine
-Price
S355 cons:
-Rust (Needs thermal coating)
-Higher thermal conductivity (In changing exhaust temperatures. Although some ceramic coatings have very low conductivity, so they will solve this problem)
304 pros:
-Corrosion resistance to 870 °C
-2,5 times lower thermal conductivity
304 cons:
-Price
-Harder to machine
-Needs back purge
-Tad weaker
-More expensive
I have been building carbon steel turbo headers for some time now, sch 40. Mainly to be able to use a evo 8/9/10 turbo. Have not had one come back cracked.
I have noticed that on thinner tubing without bracing will crack due to the weight of the turbo or even the wastegate. Sch 10 holds up well, I just prefer carbon steel sch40 as i can buy it any hardware store on the cheap, an easier on my bandsaw and beltsander belts
Thats good to know. Looks like you got a good header. From my experience with 16g tube headers on a DD car that is diven hard is that they will start to crack at the merge collector and the wastegate cutout pipe. Maybe its the exessive vibration from poly mounts or just that they were driven hard
Could be for sure. Most builds in the shop are rwd cars either single or twin turbo. Beings they are rwd, most have engine plates instead of motor mounts.
I don´t know if the exhaust gases are harsh to mild steel and there fore stainless is better. BUT.. what happens to the turbo if you get pices of coating or corosion in the exhaust turbine. That is why I went with stainess and have never looked back.
Also I have seen people have trouble with threads ripping out of their heads when using thick stainless flages. They expand too much. Thats why its better with mild steel flanges..
I have not encountered issues from exhaust affecting the carbon steel. It will depend on how often the car is used and where it is stored though as if the car is not used that often and left out on grass it can promote condensation forming in the manifolds which will promote rust.
I have always had my manifolds ceramic coated and when i have removed them in the past you can wipe out the carbon with a rag and the coating is still nice and smooth. A bit of polish and it would probably come up well.
With regard to the pipe vs tube debate it depends a lot on the design. The thicker pipe will likely have lower stresses when compared to tube of the exact same design so you can get away with short runners and stumpy designs much easier. With tube you will need longer runners with more distance between head and collector to keep the strain low when the runners move around and expand and contract with the heat. I have made a manifold before from thin tube with short distance between head and collector and it just cracked very quickly and near instantly after each repair. I remade it with steam pipe and it never cracked for the 3 years i had it before removing to try another design.
Stainless does not seem to have carbon build up like mild steel does from what I've noticed. I can't remember the last time I made a pipe manifold. Everything in our shop is constructed from tubing whether it's a single t3 turbo fwd civic to a twin turbo 90mm mustang
I'm so happy to see there are a variety of solutions here. When I researched this I found lots of negative comments on mild steel manifolds vs stainless, but after looking at the cost, ease of machining, welding etc, I decided to take a chance on the mild.
All I can say is that I'm very glad I did. I was able to locally source cheap sch40 pipe and els. This was important to me as my design was evolving as I was making it. It's easy to tig, gas mig, or even stick weld (I'm not a fan of flux core). In fact, for tacking, the stick was the easiest process, then tig after that.
No coating for me....a non issue thus far.
I will say that there was a lag in getting the piping hot...I would actually notice a spool difference for the first few minutes of operation. This was on a V engine though, which had a crossover pipe etc, so lots of surface area.
If carbon was a big deal, there would be a lot of diesel pickups with issues....
08-22-2012, 09:59 PM
Re: Stainless steel vs mild steel Manifolds!!!
I dont see what the big deal is about old threads.
