How detrimental is secondary austenite in Duplex Stainless Steel?

[Naruwan]

Hi,
I had been performing lots of ferrite count (in accordance to ASTM E562)on super/ duplex stainless steels. Besides ferrite count, we also had been performing ASTM G48 Method A pitting corrosion test on these stainless steels. Some specimens did failed the corrosion test. They normally will failed at the weld root.

We did a sectioning of the failed corrosion tested specimen and examined it's microstructure under an optical microscope. What we found out was that the microstructure at the weld root is full of secondary austenite. Is secondary austenite one of the factors which caused the test specimen to fail? How detrimental is secondary austenite? What is the mechanism behind it which, cause the test specimen's susceptibility to pitting corrosion to be decreased?

Thanks!

 

[metengr]

I am not sure I would call it secondary austenite in the weld root region. Instead, you had an improper phase balance with possibly intermetallic compounds within the weld root region. What was the filler metal used for the weld root versus fill and the welding process? It sounds as though improper weld metal chemistry or poor gas shielding gas was used when the weld root was deposited.

 

[EdStainless]

First I would move to practice C, it is a more repeatable test.
Second, what do you mean by failed? The pitting resistance of as welded samples is expected to be rather low. Are you sure that you are not expecting too much.
Third, Metengr is spot on. Phase balance and the presence of secondary phases (especially at the austenite/ferrite interface) is most likely the driver if you are having corrosion issues.

There were welded with the correct over-alloyed fillers, right?
You do use nitrogen in your shield gas?
You are very careful about heat input and interpass temperature?

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

 

[jwhit]

Are these castings by any chance? Poor or no solution anneal after welding may be the cause of the issue. If they are castings, ASTM G48 Method A for 24hrs at 50C is usually called out for corrosion testing superduplex castings and 40C for duplex (A995 4A). It is critical to have controlled heat input during welding and over alloyed filler metal.

 

[Naruwan]

Hi Guys,
according to the details from the customer, the fillet metal is SANDVIK AWS ER2594 and the parent material is of ASTM A790 UNS S32760. The shielding/ purging gas is a mixture of 98%Argon+2%Nitrogen. Process is GTAW and position is 5G.

Ferrite balance of these failed corrosion samples is within their specification range: 35%-55%. I understand that in theory, the corrosion should be low if the phase balance is within the range, however it is not the case in reality. What our customer eventually did was to experiment with purging with 100% nitrogen. The corrosion test passed, however this time the ferrite content was too low, it was around 33% It was clear that nitrogen, an austenite former, had contributed to the forming of more austenite than needed. (Slow cooling rate of the weldment and content of Nickel will also contribute to austenite forming too).

No one seemed to know what had contributed to the failure of the pitting corrosion samples. Some had blamed on the blue tinting, which is visible on both HAZ next to the weld root. (Pickling was not allowed) While other blamed on the purity of the purging gas. Some did mention that it might be the turbulance of the purging gas, which had introduce oxygen in during the purging process. Other 'more experienced' customers had put the blame on the secondary austenite.

For the above reasons, I think they each had a role in contributing to the failure of a pitting corrosion test.

 

[jwhit]

We use ER2594 with welding grade Argon and have had no issues with passing the corrosion tests. We do, however, solution anneal after welding to eliminate any 3rd phases/intermetallics. What is your heat input? We keep ours to <35kJ/inch which is critical, I believe Avesta's magic number is <38kJ/inch.

 

[EdStainless]

A heat tint would be enough to lower corrosion resistance.
Better shielding maybe all that you need, though checking heat input is a good idea.
Did they test the castings after anneal and prior to welding? Both A923 and G48 are needed.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube