LDX 2101 and other "lean-duplex" 2

[ponchoak]

I remember hearing about these new lean duplex stainless steels coming out a couple years ago, and how they were going to take the world by storm. I have also heard that much time of the Stainless World Conference was spent on these materials.

Now, I can read the websites as well as anybody, the new tank farms, and floodgates, etc., but has anybody used, discovered, or heard of any other applications these lean duplex material are good for?

I have a supplier jamming this LDX 2101 down my throat as a general replacement for 316L in most environments at temperatures below the blue brittle range, and as a general tool-room type grade. The science seems to be there, but I am curious for real world experiences.

 

[gr2vessels]

ponchoak,
The UTS for 2101 is 95 ksi, but 316L has only 75 ksi. The Yield for 2101 is 60 ksi and for 316L is only 30 ksi. The corrosion resistance is similar for both of these materials and the price of 2101 is less than of 316L. You gain 25% weight reduction for the same application and probably a further 10% on the material purchase. It is tough and suitable in slurry application, mildly corrosive environment. Many minimg refinery applications...also lots of info on the internet...
cheers,
gr2vessels

 

[moltenmetal]

...and it's no easier to weld than 2205? Sorry- still not interested! A 10% savings in material wouldn't pay the extra labour involved with welding in such a way that the corrosion resistance is maintained.

 

[ponchoak]

ok, but what about applications?
I have had two very successful users, one was a paper mill, which the steel is designed for, and the otehr wa in a detergeant plant. The guy used it as a conveyor roller where detergeant components were dripping down and corroding the 303 that was previously in use. His part life increased dramatically.
Anybody else with successful uses?

 

[strider6]

This is a list of successful cases of use of LDX by Outokumpu.

http://www.outokumpu.com/pages/Page____41031.aspx

i think it's a source on which you can trust

S

Corrosion Prevention & Corrosion Control

 

[EdStainless]

Welding?? the duplex grades are easier to weld than 316 in terms of maintaining corrosion resistance. There is no way that 316 welds will come even close to the base metal, in the duplex grades if you use the right filler you will get good corrosion resistance. I have had good experience simply telling welders that it is SS and XXX is the filler. Good gas, the right filler and minimized heat input are the keys to all stainless welding.

No, LDX2101 does not have better pitting resistance than 316, but it sure is close. AL2003 will be clearly better than 316.
In either case, the applications that I have personally seen were all large, tanks and silos. Here you can save at least 30% of the pounds of metal, erect faster, and get better corrosion resistance.
For small stuff I don't see that the engineering involved is justified.

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Plymouth Tube

 

[moltenmetal]

EdStainless: our results have been quite different than yours. We did ferrite content tests on 2205 welds and found that it was very difficult for us to maintain the duplex structure despite careful TIG welding (ie. careful control of heat input, 90 C interpass temperature etc.). We never did qualify a procedure for 2205 because of this.

Had we merely told the welders to weld as if it were 316, I can't imagine the results would have been better...

Any suggestions on what we might have been doing wrong?

 

[GRoberts]

Was the ferrite content too high or low? If it was too high, your welders may not have been adding enough filler metal. If it was too low, the filler metal could be too high in nickel. With GTAW, the gas composition can also be tweaked with N to reduce ferrite.

 

[moltenmetal]

GRobers: ferrite was (way) too high. Filler metal was added as necessary to complete the welds with the required geometry (this was small pipe (1/2-2") with a standard butt weld prep), so I'm at a bit of a loss in relation to how you could add more.

We didn't try adding N2 to the gas, which was the one thing noted in Outokumpu's notes on welding the duplexes that we weren't aware of. How much N2? How does the N2 content affect the ferrite content? I know that N is one of the key alloying ingredients for pitting resistance in this alloy (but I'm not a metallurgist and didn't understand how this works). I was unaware it affected the ferrite results- does N have a role in stabilizing the duplex structure?

 

[GRoberts]

What were your ferrite requirements? Sometimes people get ferrite % and FN confused. Don't know what type you were measuring though. Was the ferrite too high in the weld, HAZ, or both? The Nitrogen is an austenite promoter (as well as good pitting resistance). There is plenty of literature out there on adding Nitrogen, but I think I usually see it at 3-5% or so? I haven't had to try it myself as we hadn't usually had problems with ferrite content on GTAW welds.

For starters, I would check the filler metal to make sure it is properly over-alloyed with nickel. What is your "standard" butt weld prep? That means different things to different people. Were you using a backing gas I assume? One of the keys with GTAW pipe welding is to make sure you add plenty of filler on each pass. That means not just adding enough filler on the root pass to get the two sides to fuse together, but enough to get a substantial thickness to the root pass.

 

[weldtek]

Holding % ferrite in the 30-60 range shouldn't be a problem using the GTAW process without adding N to the shield or purge gas.
The only time we've added N to gas is when welding 308 with a requirement to limit ferrite to around 4%. This is a challenge even with carefully selected filler. In these cases we added only 1/4 of 1% with dramatic effects.

 

[EdStainless]

And you were using 2209 filler right?
in our mill we get fine results with autogenous welds using 5% N in the shield gas (none in the backing).
Was your problem in the weld or in the HAZ?
A little more heat input may not hurt your application.

In general the corrosion resistance of as-welded duplex is near the base metal value, while for Mo bearing austenitic grades you will loose a significant amount of pitting resistance.

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Plymouth Tube

 

[moltenmetal]

It was a few years ago already- I'll have to dig the met lab reports out of storage to find out for sure whether it was a weldment or HAZ area problem- but we definitely had ferrite % levels well beyond acceptable.

I think GRoberts has it right: we were too concerned about controlling heat input, such that we probably deposited a very thin root with too little filler metal in it. The procedure we'd been given had a 90 C interpass temperature which made the welders paranoid about heat input.

The next time we get slow, we've definitely got to take that challenge: weld a piece of small dia 2205 pipe with correct filler metal but weld it otherwise as if it were 316SS- in all except true interpass temperature (ie. let it cool right down between root and cap passes)- then send it out for ferrite content and see what we get. Are you saying that we shouldn't really care how hot we get the parent metal or how large the HAZ is during each pass?

I know there's plenty of ferrite in a 316 weld (to keep it from cracking during cooling?)- you can tell that with a magnet- and that this should negatively affect the weldment's corrosion resistance.

 

[TVP]

moltenmetal,

Just to clarify, you asked about the metallurgical N has on stainless steel. N is an austenite stabilizer, just like Ni. This means it promotes the formation of austenite instead of ferrite, thereby reducing the amount of ferrite in the metal. This pertains to any molten metal, whether it is during the original melting/refining process or during the welding procedure.

 

[EdStainless]

too little filler could be the biggest part problem.
the temperature during welding can be an issue, but you really need to get it hot. The normal temperature rise is not a concern.
The lean duplex alloys are very resistant the formation of secondary phases. This makes them less sensitive than 2205 to welding temperatures.

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Plymouth Tube