What is the better material for manifolds? 321 or 316L?

I believe that most guys use 304L and 321 but I heared of just one company that is working with 316L.

Through some researches I did in the past I believe that the lower carbon percentage of the 316L makes it the better choise to prevent intercristallin corrosion. Also the molybdenum percentage seems to make it a little stronger when it is hot.

Any inputs on this topics?


Thanks

 

321 works better due to its high heat properties.

 

Concerning the heat properties I read that they are nearly identical.
So that confuses me.

What I read is that 316L got much less carbon than the 321.
Less carbon seem to be important for crack resistance caused by intercristalline corrosion.

Though the low amount of carbon which would mean less strenght they add molybdenum to the 316L.

More ideas? It is an interessting topic.

 

The main difference is 321 is a stabilized stainless steel. There are elements added to help resist carbide-based problems. They use these elements as "scapegoats", or "getters" wanting to join with carbon more than the chromium does. Titanium, and niobium are just a few they can use. 347 is also a stabilized grade, used as a filler for 321 base metals commonly in industry. The general idea is to keep the chromium and carbon from forming chromium carbides, as this renders the metal nearly uselesss, both in ductility, and in corrosion resistance. Adding the titanium, etc. allows the carbon to join with them, creating a carbide of little detriment to the metal alloy.

321 is far superior to 304 or 316 for high heat applications. This is the whole reason that most use overly heavy 304 pipe turns and material. Just using a better alloy will save alot of weight, and time. .065" 321 would be plenty for turbo manifolds, with the weld being accomplished quickly in one pass. Never mind all of the prep time on the fit-up versus pipe. Add in extra filler rod, extra gas, and the cost is virtually a wash for a better product with 321. Less weight is also less stress on the manifold fasteners,etc.

 

real ballers use inconel.

 

Yes, triple the price, but you can use .024". Really thin stuff. I remember using 12 amps last time, went pretty quick.

 

@9bells

I kow about the things you wrote, but it is difficult for me to express it in english :-) But thanks so far.

You were talking about properties of 316. But I wanted to know about 316L.

316L got much less carbon than 316 as far as I know
So I thought 316L would not form chromium carbides because there is not enough carbon in the material to do so.
My idea was that 316L would not have the carbide-based problems and the additional 2% of molybdenum (compared to 321) would raise the strenght?

 

No, the lower carbon content is an attempt at solving the problem, but it is not as successful as 321 is in terms of high-heat strength. Carbide precipitation is an important part of the puzzle, but once that has been minimized, you have to look at the actual strength of the material at the desired operational temperature. Although molybdenum is in fact an additive for creep strength in steel alloys, this amount is intended for corrosion resistance. It actually makes the stainless more susceptible to sigma phase formation in the 1000-1600F range typical of automotive applications. In this sense, 304 is superior to
316L, since 304 is almost completely unaffected by this formation. In this temp range, the formation of sigma phase will lead to failure in 316L. 321 is a much better choice for the temperature range seen in turbo applications because of the long service conditions at the 1000-1600F range is much less harmful to the material in an actual "working" strength aspect. At room temp, 321 is only slightly stronger, but at elevated temps, the gap widens significantly. Add-in service for long duration in the sigma phase formation range, and you are better off with 304L than 316L, and 321 is the best choice overall.