Comments on Stainless Steel Exhaust Manifold

[clarkoh]

I started to make Stainless Steel exhaust/ turbo charger manifolds for the Cummins Turbo Diesel found in the Dodge pickups. I saw a prototype and the comments were that the use of this piece vs. the stock cast iron piece was a drop in exhaust gas temperatures by several hundred degrees F. The fabricator had used Schedule 10 stainless steel 1 1/2 inch elbows and pipe to make his piece. I have done the same on the manifold in the attached picture. Schedule 10 SS comes in 304 and 316 alloy. I have both available and made my first prototype from 304. So far so good. I have it on my test mule (2000 Ram 2500 2WD, with bigger than stock turbo and injectors and custom air box, 271,000 miles). One of the performance ss exhaust material suppliers suggests using 321 alloy for applications like mine. Exhaust gas temperatures can exceed 1400 F. I am wondering if using readily available, and reasonably priced schedule 10 304 or 316 alloy elbows makes sense? I have purchased some 2 inch .065 wall 321 tubing from the supplier. It allows me to build a much less restrictive manifold, is lighter in spite of being larger and seems to hold up fine. Comments?

Flying Hammer LLC a hot rod fabrication facility in Lincolnton, NC

http://www.flyinghammerllc.com

 

[DVWE]

One of the major issues with 304 or 316 stainless is a phenomenon called sensitization. Which basically means the chromimum in the steel is lost at the grain boundaries because it has an affinity for carbon. They form chromium carbides and will happen in the temperature range from about 800 - 1500 F. When exposed to a corrosive environment, which I'm not sure whether or not diesel exhaust is, the steel will begin to corrode and lose strength at the grain boundaries. So 321 is alloyed with titanium to help alleviate this, because chromium will form a carbide with titanium before it does with carbon. The problem with welding is that titanium doesn't transfer across the arc very well, so there are alternatives such as 347 filler metal which is alloyed with Niobium and Tantalum because they transfer across the arc better, and have similar effects as the titanium. But after welding, these benefits are not fully gained until the entire weldment is Post Weld Heat Treated at 1650 usually with a minimum time of 4 hrs. So you will still get some corrosion at the weld joints unless this is done, which by your attachment, there's quite a bit of welding done. The choice is yours, but 321 is a bit harder to come by. It's definitely not going to hurt anything to use it, and if you are Tig welding, use either 321 or 347 filler, even if you don't PWHT. In the long run, you're probably better off using the 321.

 

[DVWE]

"So 321 is alloyed with titanium to help alleviate this, because chromium will form a carbide with titanium before it does with carbon."

Well I completely typed that sentence wrong! What I meant was that carbon will form a carbide with titanium before it does with chromium. Sorry for the misprint.

 

[clarkoh]

Thanks for the explanation. I am Tig welding and using 347 filler rod on the 321. That was the advise from the tubing supplier. I am still experimenting with the 304 and 316 combination and have the manifold in the picture installed on my test truck now. So far, there have been no problems. But, I have yet to really put some heat in it. I will hook up a trailer and put the Bobcat skid steer on and find some serious grades to really stress the prototype by exposing it to some 1200 plus temperatures. When welding we also back purge the tubing with another argon supply or use Solar Flux B when it is impossible to back purge. When I get some of these together, I will seek the services of a heat treatment company.
Clark

Flying Hammer LLC a hot rod fabrication facility in Lincolnton, NC

http://www.flyinghammerllc.com

 

[EdStainless]

1. If the 304 is really 304L with low carbon then there is little risk of sensitization. I am not sure that sensitization would hurt this application anyway.
2. The lighter the walls the better. The tubing may distort a bit more, but there will be less stress and less risk of thermal fatigue cracking.
If there is a way to open up the center bridge between the flanges so that it is not in contact with the tubes it would further help. You don't care if the tubes distort.

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Plymouth Tube

 

[clarkoh]

I have 2 unique designs. Each design uses the same flanges for the exhaust ports and the turbo mounting. I had the exhaust ports cut into pairs to allow some movement and at the same time keep them aligned. When I weld these things together I have a jig which maintains the alignment of the bolts and flanges on the cylinder head side and supports the turbo flange in proper alignment.
1- The first design uses the Schedule 10 304/304L SS 1 1/2" elbows and pipe. The OD is 1.88, with 0.109 wall thickness. I can tolerate some small amounts of warpage or distortion. I simply resurface the exhaust and turbo flange on a Zirconium belt. In the orignal configuration, limited distance between the exhaust mounting surface and the turbo flange, this smaller pipe is all that will fit.
2- The second design uses 321 alloy 2" tubing with a .065 wall. I use the 347 filler rod when tig welding 321. I use this design when I can move the turbo flange away from the cylinder head. This allows the use of non-stock larger diameter turbo compressors. In many of these applications, spacers have been used to move the turbo away from the manifold.

Flying Hammer LLC a hot rod fabrication facility in Lincolnton, NC

http://www.flyinghammerllc.com