Suitability of 2304 lean duplex vs 316

[Asteryx]

I'm in the process of spec'ing a cast stainless for a marine product which will be submerged during use in salt water in a wide variety of locales, operating temperature range of say -5 to 35 deg C, and exposed to salt atmosphere outside when not in use.

Most/all competing products use 316/L, this is proven more or less adequate.

For better strength and improved corrosion resistance we have been working on using CD3MN / 2205. However, it has occured to me that 2304 might make a more cost effective alternative.

I understand it's not as good as 2205, but the mechanical properties are adequate and the corrosion properties (in the above environment) need only be at least as good as 316. I can find nothing in my research specifically concerning this latter comparison/context, so my request is for comments on that. Queries to steel suppliers so far are frustrating.

Any input?

 

[MikeMet]

You can find comparative data for 2304 vs 316 sst in many places. Try Nickel Development, Outokumpu Website, Allegheny Technology website, Sandvik Website, and a few others.

You may simply want to spec out CD4MCu. An old, weld known, duplex stainless steel casting that is a standard for almost every casting shop in the world, and better strength adn corrosion resistance than CD3MN. I don't know if many vendors cast 2304 if it is a special product there is not cost advantage.

 

[EdStainless]

The pitting resistance of 2304 is low enough that I wouldn't risk it.
If you make the part in CD3MN you will make it significantly thinner than in 316, correct?
I would stick with the clearly superior alloy and not try to shave it too close.

There are newer chemistries (2003 and 2404) that might fit your bill better. But as Mike said, if they are specials they may not save you any money.

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[Asteryx]

Thanks for the replies above.

Mike, you are correct there is much info re 2304 vs 316 but I am having trouble translating it practically to the marine environment. We don't care about conditions outside this or extremes. I have already spoken to Sandvik for one.

Of course 2304 is not a casting standard so I've probably started off with the wrong vocabulary here and should be asking for input re cast duplexes with cost advantages over the 2205 equivalent CD3MN.

I am curious you suggest CD4MCu. Should we expect this to be cheaper than CD3MN? Obviously we will need to talk to the foundry but it helps to know beforehand.

Ed, pitting resistance - worse than 316? It's 316 that's the baseline for us. The critical crevice and pitting temps for 2304 are above those of 316, and the PREN is the same, so I had thought we were safe.

Re section thickness, no, the design of the particular product we're talking about is essentially solid and shape is defined by its function. The weak spots are comparatively small so extra bulk to make up strength where it's needed won't affect the cost much. 316 could be used, we'd just prefer a step up particularly in terms of SCC and pitting, if it can be done economically.

 

[pwtomlin]

I'm not sure how much cost savings you are going to see trying to transition from cast 316 (CF8M) to cast 2205 (CD3MN) or CD4MCuN. Obviously, there is less alloying content but the duplexes are harder to cast well so there is less competition. I would guess it might be easier to find a supplier of CD4MCuN since it is an alloy with a longer history than CD3MN. Of course, pricing also depends on what sort of size and quantity you are talking about. If it's a large number of parts you should get a better price than something you order one or two a year.

 

[Guest102023]

Duplex will be generally superior, but it is vulnerable to crevice corrosion, so it depends on the the design. If you want a better comfort level you can have salt spray testing done at many laboratories.

 

[cloa]

The salt spray will give him some benefit but lots of that data kicking around- he needs submerged conditions testing too.

 

[mcguire]

2205 will be free from corrosion in all seawater unless crevice corrosion is permitted by design. 316 and 2304 are inadequate unless you can tolerate pitting. If you have crevices, you must go all the way to 2507 or 6Mo grades to be corrosion free.

Michael McGuire

http://stainlesssteelforengineers.blogspot.com/