304 vs 316 in an exhaust manifold aplication 1

[chad86tsi]

For the record, I am NOT a metalurgist, just a hobbiest that tries to understand things with research and "good" advice.

I have been fabricating automotive turbo exhaust manifolds for some time using .065" wall 304 and 321 ,and some inconel 625 tube, and now similar models using sch 10 pipe of 304 and 316 grades.

Question is, am I really making a better product using 316 vs the 304 on the pipe aplication? the exhaust temps peak at 1800*F for periods not likely to exceed 15 seconds, but normal operational temps may be as high as 1400*F sustained. this product is subjected to vibration stresses, weight of the turbo and exhast, and the normal motion of the engine.

I can't totaly remove these stresses, so I need the manifold to be capable of handeling them. With the tubing, I've had several fail using the 304, I belive it's just too thin and the tubes reach a temp nearly that of the exhaust stream, even during the transient peaks. They generaly fail in the tubing, or near the welds, but not directly in the welded areas. It appears to be a funcion of vibration and tube movement (which I can't avoid) so I belive it's jsut weakened metal structure, and carburization, hence the use of 321 (at 2-3 times the cost), or inconel when I can afford it (aprox 10 times the cost).

I tired adding bracing, but this just bound the flanges into one position so the tube elongation just caused thermal expansion stresses instead (tube movement).

Now I'm focusing on using pipe instead of tube, it's more cost effective, easier to find, and the added thickness of the walls yileds greater strenght all by itself. I've been told by an ehaust manifold fabricator that I can go back to using 304 in this pipe aplication, but I'm leary of using it after the failed tube units I had before. Do you think I should continue to use 316 for it's margionaly better thermal properties at a cost premium of ~20%?

The pipe body will be less likely to reach the temp of the exhaust stream just due to the added thickness and poor conductivity of stanless in general, but I can see it reaching pipe tems of up to 1600*F occasionaly.

 

[MikeHalloran]

The 20 pct premium for 316 material pales relative to your total cost, and especially relative to the cost of a warranty replacement.

You may occasionally be forced to use 304 anyway. Keep good records and that will give you a limited set of data points on which to base a decision at some later date.





Mike Halloran
Pembroke Pines, FL, USA

 

[EdStainless]

I wouldn't use 316 for this. It does not have better high temp oxidation resistance. And with todays molybdenum prices 316 is almost 1.5x the price of 304.
You need to make sure that you use 304L, and low carbon weld filler.
Yes, 625 is probably the best material for this application (though alloy X would work great also), but the cost will choke a horse.
You have learned the hard way about high temp systems. If they are too light they will fail, and if they are too stiff they will tear themselves apart. You have to allow some flexing since they will expand when hot.
I don't like the option of making these heavier. You will be adding stress from both the weight and the added resistance to expansion. You need to be selective about which parts you make thicker. The other thing that you will run into is that stainless pipe is a lower quality product than tubing. You may have issues with bending and dinesions.

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Rust never sleeps
Neither should your protection

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[chad86tsi]

Yes, I am in a bit of a corner here, I can hardly sell them if I use a propper product like 321 SS or 625 inconel because cost is a major factor in my consumers decision making process. Also, the 321 and 625 are terribly difficult to source and I use a lot of material in the product.

Many of my competitors have used "black pipe" elbows with good success, but I know mild steel manifolds tend to embrittle with time regardless of design. I'd like to make mine last as long as possible. One competitor uses a chromoly tube of an unknown alloy that seems to be a very strong product initialy and lasts several years, but they all seem to have cracked in the base metal eventualy.

I am aware that the cast pipe is inferior in grain sturcture vs. tubing, and may have inferior alloy control, but it was my belif that the wall thickness would offset this problem.

I had read in an aplication chart that 316 had a wroking temp ~200* higher than 304, which is why I switched. Is this statemenet not true realitive to my aplication? It will not be subjected to high pressures or anything other than atmospheric oxidation on the exterior, and a high CO2/CO/C content on the interior. It's moslty the thermal stresses to the grain structure, prolonged mild vibration, thermal expansion/contraction, and some carbon precipitation over time that make these fail.

If 304 is equal in strength at temps of ~1400*F, and has the same longevity over prolonged use in this range, I may as well use it, but I will pay a premium for the 316 if it is indeed better, just for the peace of mind.

What about ceramic coating to protect the metal from the various gasses it is exposed to? This insulating barrier tends to keep more heat in the base metal which can weaken it, but this protects is from oxidiation.

Thaks for any advice you guys can offer

 

[MikeHalloran]

Try ceramic coating only the inside.



Mike Halloran
Pembroke Pines, FL, USA

 

[EdStainless]

Yes, coating the inside may be a good trick. There are some coatings that go on as a slurry that you then bake harden.
You want it on the inside to minimize the temperature swings of the metal. I would expect that the corrosion is worse on the outside, but they probably fail from thermal fatigue first anyway.
It is tough that 321 is so much more expensive than 304, it really should be just a little more expensive. I am guessing that it has to with the fact that most 304L is comercial tubing and the 321 is aircraft.
What sizes of tubing do you use?
As alloys go that are easily formed and welded the move to 309 or 310 would give you better strength and corrosion resistance. The catch is that with todays Ni prices you probably can't afford it. Though if you could find some 309 in sch5 or sch10 pipe you might have the ticket.
Actually a cast alloy manifold is a great product. The problem is that you need patterns and tooling for each shape. It is expensive if you only need a few. The cast structure works very well at high temp and you don't have welds to worry about. There are a whole spectrum of cast high temp grades that are commonly used in heat treating furnaces and other applications.

There is another old trick that I have seen used in comercial furnace burners. They put a very thin liner inside the main tube. It is not tight, and maybe only 0.025" thick. It serves as a heat shield and keeps the hot gas from directly hitting the main tube.

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Rust never sleeps
Neither should your protection

http://www.trent-tube.com/contact/Tech_Assist.cfm

 

[kenvlach]

Why hasn't anyone suggested one of the higher Cr ferritic stainlesses?
Advantages: CTE only 2/3 that of austenitics and little or no expensive nickel.
I suggest a low carbon (or stabilized), > 17% Cr grade.
From Allegheny Ludlum:
"Hot end exhaust applications may require an 18 percent chromium alloy such as Allegheny Ludlum Type 439 (18 Cr-Ti, UNS S43035), AL 468™ (18 Cr-Ti + Cb, UNS S46800), AL 436S™ (18 Cr-1.2 Mo-Ti), or AL 441HP™ (18 Cr-0.7 Cb-Ti) alloys."

http://www.alleghenyludlum.com/Ludlum/pages/products/xq/asp/G.3/qx/ProductLine.html

AK Steel has 434, 439 & 439LT grades that would be good in my opinion.
"Like Type 439, AK Steel 439 Stainless Steel exhibits considerably better oxidation resistance than Type 409 at 1700°F (927°C) after 1022 hours of cyclic testing."

http://www.aksteel.com/markets_products/stainless_ferritic.asp

 

[EdStainless]

While the ferritics would have better oxidation resistance and lower cost there are some issues.
The biggest is "885F embrittlement". After exposure these alloys will loose all ductility. They will still be ductile when they are hot, but when ever they are cold they will be brittle.
There are also issues with forming and welding.

The 18Cr stabilized ferritics (especially the ones with Al) are fantastic in this service, if they are well supported and protected from thermal shock.
Don't get me wrong, I like these alloys and work with them a lot, but I am not comfortable with using them in this application.
Now using them for the thin wall internal heat shield liners, that might be the way to go.

= = = = = = = = = = = = = = = = = = = =
Rust never sleeps
Neither should your protection

http://www.trent-tube.com/contact/Tech_Assist.cfm

 

[chad86tsi]

Yes, I too belive it is the thermal effects and unavoidable vibrations that kill these, not the gasses present. Coatings that are common in ths aplication can be as much as $200 per item, so it's pretty cost-prohibitive. any tips on what coatings are available? the only one I know of (jet hot) is rated at 2200-2500*F

Exactly Edstainless, it is truly a shame that 321 is so scarse, it's not that different in the alloys used to make it vs. 304, so cost of manufacture and raw materials are not that much more. It's just that 304 is so heavily used, and thus available.

I'm using a 1-3/4 OD tube, 16 gage walls (.065").

Any good sources for 321 tube in bulk? I have a mandrel bending shop near me that would bend 321 for me if I can supply the material, his vensor only has 304, and a variety of mild steels.

I prefer to go back to tubing, but this pipe is the most reliable version I offer.

 

[chad86tsi]

This is what the 321 ss version looks like:

http://webpages.charter.net/catsamuel/DCP01017.JPG

http://webpages.charter.net/catsamuel/DCP01022.JPG

the runners are tuned in length to this motors power peak, and all 4 runners are within 3% of equal length.

 

[EdStainless]

For coatings you may want to talk with ZYP.
Let me look into tubing.

= = = = = = = = = = = = = = = = = = = =
Rust never sleeps
Neither should your protection

http://www.trent-tube.com/contact/Tech_Assist.cfm

 

[unclesyd]

I think you should look at 409 SS either plain or Aluminized. You might want to give Atlas a call.

Other tubing that you might want to look at is available from Middletown.

http://www.atlassteel.com/

http://www.middletowntube.com/steel-tubing-home.htm

 

[NickE]

I'm going to suggest looking at the 4nn series. Along with others.

I have a 439 downpipe on my car and it has survived 67,000 miles of daily driver, "brisk" TSD rally, RallyX, screwing around in the snow, etc... Temps are a bit lower than pre-turbo, but not much from anecdotal internet data. (my EGT is in the manifold at the hot cylinder.)

It did scale quite heavily, however it has not flaked off anywhere. I'm sure a good header paint would have helped.

 

[chad86tsi]

I've heard some 4NN allowys are prone to rust with temp, 409 in perticular. Also, my EGT's after the turbo are ~ 900-1100, but up to 1400+ before it. part of a turbos "free" energy comes from the energy in the temp conversion.

Is 439 a commercialy available grade? I have not seen it.

 

[EdStainless]

There is 439 and then a whole bunch of 43x or 46x grades, each auto company specifies their own stabalized version.
409 is junk at maifold temps, you need more Cr.
The aluminized material works fine for pipes, but cutting, fabricating, and welding can be problematic.

Have you ever built using thinner tubes? I have seen high perf manifolds built with light gage tubes in order to allow more flexing and less stress.

= = = = = = = = = = = = = = = = = = = =
Rust never sleeps
Neither should your protection

http://www.trent-tube.com/contact/Tech_Assist.cfm

 

[NickE]

chad86tsi- When I referred to scale I meant rust. Sorry... 439 does red rust under temp and standard midwestern salt/water/crud. However unlike rust on some steels the rust is relatively hard and doesn't flake off causing trouble.

 

[chad86tsi]

Thakns for the distinction NickE, this product is something that has to look good in service, it's an aftermarket performance item that people would want to show off. I understand there is a distinction between minor surface rust vs severe structural damaging corrosion. In my aplication any rust is too much.

It seems from this discussion and my previous research, there are no "magic alloys" for this aplication that don't also come with a "magic" price tag. Lots of choices and all have pro's and con's in suitability and cost/benifit factors.

Thank you all for for your valuable input !

 

[NickE]

welcome to materials selection.

I totally get the appearance perspective. Exhaust is however a whole nother story at least to me. Although a well polished equal length header is quite beautiful.

 

[EdStainless]

Chad,
Are you interested in some lighter gage 321? I have some 0.049" wall if you are interested.
ed

= = = = = = = = = = = = = = = = = = = =
Rust never sleeps
Neither should your protection

http://www.trent-tube.com/contact/Tech_Assist.cfm

 

[chad86tsi]

What is the OD and length/QTY of tubing you have? What I'm looking for is 1-3/4" OD in 16 gage, but I could probalby use 18 gage and not loose much strength.

I'll have to ask my bender if he can do that ID, his setup is for 16 gage, but I suspect the 18 gage will bend OK on the 16 gage toling, but he'll have to make that call, not me...