Ferrite - Sigma phase & Duplex s.s.
Ferrite - Sigma phase & Duplex s.s.
(OP)
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??????????
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??????????





RE: Ferrite - Sigma phase & Duplex s.s.
By limiting the interpass temperature (temperature of the weld region between passes) to as low as possible. I would recommend 300 deg F maximum.
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).
RE: Ferrite - Sigma phase & Duplex s.s.
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).
= = = = = = = = = = = = = = = = = = = =
Plymouth Tube
RE: Ferrite - Sigma phase & Duplex s.s.
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
RE: Ferrite - Sigma phase & Duplex s.s.
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???
RE: Ferrite - Sigma phase & Duplex s.s.
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.
= = = = = = = = = = = = = = = = = = = =
Plymouth Tube
RE: Ferrite - Sigma phase & Duplex s.s.
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.
RE: Ferrite - Sigma phase & Duplex s.s.
TomEun has provided some excellent references to review. Spend the time to search and download them.
RE: Ferrite - Sigma phase & Duplex s.s.
What is the best deoxidation for duplex steel?
RE: Ferrite - Sigma phase & Duplex s.s.
Keep the weld region dry and clean before welding.
In terms of.....?
RE: Ferrite - Sigma phase & Duplex s.s.
RE: Ferrite - Sigma phase & Duplex s.s.