Duplex and Intermetallics 1

[EuroWeld]

Hello all,

Duplex and super duplex grades are prone for the forming of intermetallic phases that lower corrosion resistance and mechanical properties like impact strength.
These phases grow in a temperature range between 950-600 degC during heat treatment and/or welding. After metal fabrication rapid quenching is needed after solution annealing to avoid intermetallics to grow and to get the correct phase balance.

Now I am wondering which phase fraction of intermetallics like Sigma, Chi and Laves is still acceptable in base materials (plates or forgings)?
In large object forgings intermetallics cannot be avoided completely but what is the amount that still can be accepted? What phase fraction do I have to think of 0,5, 1, 2%? Already knowing this is strongly related to the corrosion & mechanical test results but in order of magnitude!

Kindest regards,

Euroweld

 

[SJones]

Excellent question since most standards, for example NORSOK M-630, are predicated on the basis that acceptable performance in the Charpy and corrosion testing is the ruling criterion even when an anomalous microstructure has been identified. One way to firm up this approach is to tighten the requirements of the ASTM A923 tests as discussed in NACE Corrosion 2012, Paper 1096, EEMUA 218, and the new supplementary requirement of A923 2014. The only volume fraction requirement that I have seen is from Shell who work with 1% total maximum intermetallics and precipitates, with a 0.5% maximum of intermetallics, determined by point counting.

Steve Jones
Corrosion Management Consultant

http://www.linkedin.com/pub/8/83b/b04

All answers are personal opinions only and are in no way connected with any employer.

 

[EdStainless]

There is another aspect to this situation. Often people are testing the raw material, and then welding and expecting consistent results.
I h ave been involved in a couple of cases where the original plate was clean, but welds did not perform well.
We discovered that even though there were not secondary phases in the plate there was still significant segregation that made the re-precipitation of intermetallics much faster than would be expected.

Finding some of these intermetallics by micrography is very difficult, such as with alpha phase.

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Plymouth Tube

 

[TOVmania]

If existing phases are detrimental to the material, it will yield deficient impact test result.

 

[EdStainless]

The corrosion tests are more sensitive to intermetallics than the low temperature impact test.

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Plymouth Tube

 

[TOVmania]

EdStainless is right about the result of corrosion test is a better representation of intermetallic phase of duplex stainless steel. The recommended 25x50(mm)size is used for corrosion test coupon and the weight of test specimen is required to be measured down to 3 digits in decimal. In most cases, change of 0.00x gram can yield pass or fail of the corrosion test. This weight measurement is very sensitive that sometimes with the same scale, one result can give you the result satifies the minimum corrosion rate or fail on another. Therefore, scale should be precise down to at least 4 digit decimal with error range withing 4th digit in decimal to mitigate possible errors from this in my personal view.

 

[EdStainless]

And it is important that all of the surfaces are freshly ground.
This is to reveal any intermetallics that may have been removed from the original surface by processing or treatment.
No chemical treatment of the test samples is allowed.

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Plymouth Tube

 

[EuroWeld]

To all,

Thanks for all answers!
When we talk about corrosion testing do we mean a ferric chloride corrosion test like ASTM A923 method C? Does this also mean that method C of ASTM A923 is more usefull than method B for detecting intermetallics?

I am curious about the fraction that is still allowable for dss! Is this a volume fraction of 0,5, 1, 2%. For instance: what is the reduction of toughness and corrosion resistance with a volume fraction of about 1%? I am not looking for accurate numbers but just an impression based on expierence. I am aware that this a very complex matter but in order of magnitude.

Besides that:
At this moment it seems that the thickness for large dss objects is limited because of the cooling rate of the core from the object during quenching. In the market the limit seems to be somewhere around 250mm. Is this true and are there any developments regarding this topic?

 

[EdStainless]

There was a paper given this year at NACE that addressed this issue.
It depends on the specific alloy, and the quench method.
Some people have moved to chilled water for quench just to get a little more advantage.
C2014-3651 Francis and Hebdon

200mm bar is pushing the limits, if you have a casting or forging that has holes in it then maybe you can get to 300mm.

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Plymouth Tube

 

[SJones]

Yes, the corrosion testing being referred to is A923 Method C. A lot of end users deploy all three test methods, but have acceptance criteria only for B and C on the basis that an unacceptable volume fraction of detrimental phases, whatever that unacceptable amount is, will cause failure in B, or C, or both. The question then becomes: "what Charpy energy/expansion and ferric chloride corrosion testing acceptance criteria are appropriate to maximise the likelihood of rejecting material susceptible to failure in service. This question is discussed in NACE Corrosion 2012, Paper 2012-1096. A 1999 paper by TWI stated "...the common stipulation that welding procedure qualification testpieces should display a maximum intermetallic content of, say, 1% is of doubtful value. A similar situation would arise also for weld area toughness insofar as cleavage resistance...... It can therefore be recommended that acceptance or otherwise of a welding procedure should be based primarily on direct measurement of properties of interest rather than on metallographic examination for intermetallic phases." (B Ginn, T Gooch, Proc. Conf. Stainless Steel 99 - Science & Market, Sardinia, Italy, 6 - 9 June, 1999)

Steve Jones
Corrosion Management Consultant

http://www.linkedin.com/pub/8/83b/b04

All answers are personal opinions only and are in no way connected with any employer.