mild steel or ss

By DongeR Date 02-02-2004 19:53

Hey there I am a newb to the welding world but my friend is somewhat of an amateur but better in welding.

We are doing a custom exhaust manifold setup with a turbo hanging off of the exhaust manifold. The turbo is roughly 18lbs +/-5lbs. we are debating the type of steel we should use. The exhaust manifold should be economical but also sturdy. We have access to a tig welder that can weld SS but not aluminum.

Flange issue:
I can buy the exhaust to valve train flange and exhaust to turbo flange. Flanges at http://www.roadraceengineering.com/flangesandgaskets.htm I believe these flanges are mild steel. OR We have a 304SS plate to make the flange but we have to weld two pieces together to get the flange long enough.
Picture ---> http://www.roadraceengineering.com/parts/tweekerparts/manifoldflange.jpg Exhaust Manifold Flange
1/2" thick steel. All holes drilled. Port size is correct to match a slightly ported exhaust port. $30.00
Picture --->http://www.roadraceengineering.com/parts/tweekerparts/flange-tdo5-turbininlet.jpg Turbine Inlet Flange
1/2" thick steel, laser cut to match the 7cm gasket size. Bolt holes are now drilled1/2" thick steel, laser cut to match the 7cm gasket size. Bolt holes are now drilled $20.00

Piping issue:
The temperature of the flanges and piping will read 800 degrees Celsius (1500 degrees Fahrenheit) cruising at 80 mph with a reading from my Exhaust Gas Temperature Gauge(EGT). We can get Mild steel at http://www.mcmaster.com or use SS. Which mild steel or SS do you recommend? Can I mix and match mild steel flanges with SS piping or should I use all mild steel or all SS.

I really want something economical and STURDY because the turbo is heavy, there are many shakes due to roads, and the exhaust manifold will be hot due to the exhaust. The main issue that has been on my mind is the cracking of welds due to the heavy turbo, heat and roads.
Thanks in advance,
DongeR

By OSUtigger (**)

Date 02-03-2004 03:17

DongeR,

My suggestion for the flanges would be to purchase them. First and probably foremost, the amount of time and money necessary to fab up those flanges would probably cost a lot more than $30, and would probably be more headache on top of that if a hole was off just a little bit. In addition to this, if the ports aren't exactly right, you will probably get some weird turbulence or, at the least, some weird sounds (A buddy of mine has aftermarket headers on his 350 and the danged thing sounds like it has a perpetual exhaust leak, but it's just the turbulence inside the pipes combined with the thin walls). A manufacturing laser is more trustworthy and proven, in my opinion, than a hand machined job.

I know that SS will look awesome, but save your money on that stuff and get some mild steel pipes ceramic coated at your local powder-coater (lower coefficient of friction and can handle a lot of heat) after you get it all welded up. They will look just as good when you're done, and it will not radiate as much heat, but I will warn you that this may up the turbo inlet temperature.

If you do go with SS, it is easily weldable to mild steel. I think 309L is the rod you are looking for, but don't quote me. Just about anyone else on here should be able to point you in the right direction on that one, as well as what grade of SS is the best for high temps.

As for sturdiness, I know its probably not possible, but if there is a bolt that you could mount a bracket to on the turbo and then tie it in to a bolt on the engine, that would be the sturdiest. If this isn't possible, weld a bracket on the exhaust pipe pre or post turbo and tie it into a mounting bolt or similar. Having the ability to fab doesn't mean that you have to do it like a major manufacturer would do it--build it simple.

I do have a question though- Is that 1500F the peak temp? I was under the impression that a turbo worked the hardest when it was winding up instead of when cruising (this is in reference to a CAT or a Cummins, and not a gasoline engine, so there may be a difference). In either case, your EGT doesn't mean that the flanges and pipes are at that temperature as well, just that they are somewhere between that and the engine compartment temperature depending on how much airflow is in there.

Enough of my rambling, and hope that helps!

G. L.

By DongeR Date 02-03-2004 08:20

1500 -1600 would be peak temp.

By flatjwl (*)

Date 02-03-2004 13:25

You can use mild steel flanges as the components to which they attach will help dissipate heat from them, but the tubing needs to be SS from head to turbo.

By DongeR Date 02-03-2004 16:35

so what kind of ss steel would you recommend?

By airweld (**)

Date 02-03-2004 17:49

For aircraft exhaust systems, 321 s/s is the standard choice, welded with 347 rod.

http://www.burnsstainless.com/

http://www.powerflowsystems.com/

http://www.aircraftexhaust.net/exhaust_comp.html

Try these links. Hope they still work. Don't have time to check them myself. Good Luck, Scott.

By kam

Date 02-03-2004 18:28

Great links airweld!!!!

kam

By Bill A

Date 02-03-2004 15:25

For expected service temperatures anywhere near 800 deg C I would not weld stainless directly to mild steel. The coefficient of thermal expansion for stainless is different enough from mild steel that the heating/cooling cycles associated with engine on/engine off will cause thermal fatigue. I'm not an engine builder, but in the petrochemical process industries it is common practice to use flanged connections when joining stainless to mild steel in high temperature applications because of the high susceptibility of the disimilar metal welds to thermal fatigue.

Also, FYI, the common temperature limit for steel (used in process industries) beyond which you can get unacceptable rates of oxidation is around 1050 deg F and even lower than that for cyclic temperatures. Ceramic coating would minimize oxidation if the coating is resistant to disbonding caused by the thermal cycling.

By kam

Date 02-03-2004 16:12

I thought thermal fatigue might be and issue as well but I see it done everyday (welding ss to low carbon steels). Not really sure what temperatures they are subjected our application is for your plan old everyday cars and trucks. Havent seen any returned due to cracking around the flange weldment. We ship 100's of them out everyday to the big 3 and others. In all cases 309 is the wire we use. I do agree with the other post which suggests building some type of support to take the load off of this joint. Common types of ss joined to low carbon steels would be: 409 and 18crcb.

kam