Ferrite - Sigma phase & Duplex s.s. 2

[hvdesai1284]

Dear All,
We all know that welding of duplex stainless steel is somewhat critical because of the formation of brittle phases like sigma phase,alpha prime,chi phase etc.the formation of theses phase normally happen in between 300°C to 1000°C.now, during welding there is a strong possiblity to achieve this temperature range sometime. so how can we make this welding comfortably?

My second question is that we all know that higher ferrite content(higher than 10%)in the steel is strongly responsible for the formation of brittle phase like sigma in case of Austenitic stainless steel,then in case of Duplex the ferrite content is around 40%-50%.then how can we protect the duplex from Sigma phase formation??????????

 

[metengr]

during welding there is a strong possiblity to achieve this temperature range sometime. so how can we make this welding comfortably?

By limiting the interpass temperature (temperature of the weld region between passes) to as low as possible. I would recommend 300 deg F maximum.

then how can we protect the duplex from Sigma phase formation??????????

By limiting the service temperature to 600 deg F and below. Sigma phase does not form during welding, this phase forms as a result of elevated temperature exposure (thousands of hours).

 

[EdStainless]

1. And by controlling the heat input. It isn't that hard to do, after all it is how you should weld most materials, but you need to pay attention.
2. Duplex grades are engineered to solidify with that structure. The chemistry is balanced for that. The secondary phases are a result of transformation not solidification. In some of the super duplex grades they can form quickly (2 min at 1700F), but the time frame is still well beyond what you see in welding (if you do it right).

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Plymouth Tube

 

[TomEun]

The most critical issues in DSS welding are to sustain the phase balance (ideal : 50:50 ferrite-austenite), chemical composition control (PRE), and minimizing of the intermetallic compounds during welding. Therefore the control of heat input, interpass temperature, and shielding gases may be the most important factors for the welding. The ferrite, 35-65% will be a good structure to prevent/mitigate the SCC in chlorides containing water.

Please find the following references for more detail.

References;

1. API TR938-C
2. DNV-RP-F112
2. Welding Guidelines for DSS and/or SDSS, Metrode
2. Welding Guidelines for DSS and/or SDSS, Avesta
3. Welding Guidelines for DSS and/or SDSS, Aalco
4. Welding Guidelines for DSS and/or SDSS, OutoKumpu
5. Welding Guidelines for DSS and/or SDSS, ATI
6. Welding Guidelines for DSS and/or SDSS, NiDI
7. Practical Guidelines for the Fabrication of DSS, ISBN 978-1-907470-00-4

Thomas Eun

 

[hvdesai1284]

FOR "Thomas Eun" - As you said earlier in case of ASS -"The ferrite contents of ASS (austenitic stainless steel) after successful solution heat treatment may be nil. The purpose of ferrite control, 3-10% is to avoid the solidification crack during welding. It means the ferrite control, such as 3-10% is not applicable to the solution heat treated products. Please be aware the best corrosion resistance of ASS as a final product can be obtained from 100% austenitic structure (zero ferrite contents)"

now in case of DSS, also we are performing heat treatment(above 1000°C) after welding (in case of pipe fittings)according to client requirement.then in case of DSS also, above 1000°C some amount of ferrite transformation may occur in to AUSTENITE? and in that case it will not disturb the whole required DSS structure?

- Sigma phase is mainly a problem occured in ferrite phase,then in case of FSS(Ferritic s.s) it will not creat a big issue during welding?
- For ASS, somewhere i red that a weld solidifies as 100% Austenite is crack prone because sulfer in the steel is not soluble in austenite & it is last to solidify,thereby forming a weak liquid fracture path along the austenite grain boundaries,that is easily pulled apart by thermal weld stresses.sulfer is soluble in ferrite & thus a small amount of ferrite forming during solidification holds the sulfer in solution and prevent the liquid sulfer segregation.Is it the correct reason for which we can see the 3 - 10% ferrite in ASS?

- And my last but not the least question is that according to my knowledge, in ASS there is basically no heat treatment required,but in our all clients'(Product - pipe Fittings)specification, heat treatment for fittings at 1040 is required.now i can not understand the necessity of the same.it is required only in order to dissolve the chromium carbides in case if any produced during welding or reason is some else???

 

[EdStainless]

Alloys that solidify 100% aust are difficult because a number of issues. One of the worst is the tendency for hot shrinkage tearing. It is difficult to keep them from self destructing.
Think of some of the 6% Mo alloy, alloy 20, 904L.

You re-anneal Aust alloys after welding to restore the micostructure. You want to get rid of residual ferrite in the welds, any carbides in the HAZ, and minimize the segregation in the welds as well. After anneal there should be no dendritic structure visible.

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Plymouth Tube

 

[Guest102023]

While we're on the subject, I have a physical metallurgy question:

When welding at too interpass, which is more of a problem, sigma phase or chromium nitrides, and which appears first? I would appreciate a good basic reference on the subject.

 

[metengr]

If you are welding at too high of heat input with duplex and super duplex stainless steels, the precipitates will form within minutes, and not sigma phase. The precipitates will ruin the corrosion resistance and if enough form will alter the mechanical properties of the weld region. I have seen it.

TomEun has provided some excellent references to review. Spend the time to search and download them.

 

[Jandyr13]

How can I avoid the porosity in duplex steel?

What is the best deoxidation for duplex steel?

 

[metengr]

How can I avoid the porosity in duplex steel?

Keep the weld region dry and clean before welding.

What is the best deoxidation for duplex steel?

In terms of.....?

 

[Jandyr13]

In terms to avoid porosity. My question is about melindre not welding.

 

[metengr]

You need to start a new post. Also are you interested in duplex steels or duplex stainless steels, there is a difference.