duplex stainless steel

[johnc01]

I am looking for information on backwelding of duplex stainless pipe welds,along with all ASTM identification for duplex stainless steels and a location where i can find a WPS demonstrating this

 

[welding1]

What size of pipe, thickness and diameter. What type of duplex?

What welding process are you intending to use?

What sort of corrosion resistance do you need, what's your fluid?

This material is not like ordinary stainless steel, welding it can be thwart with problems. You have to achieve the correct balance between ferrite and austenite or you will impair corrosion resistance and toughness. You have to use an over-alloyed filler wire.

You can't weld it without applying filler metal; else your weld will be 100% ferrite with no corrosion resistance; unless you do a full solution anneal after welding.

Try talking to a good consumable manufacturer. Or steel maker such as Avesta.

http://www.avesta.com

http://www.gowelding.com

 

[johnc01]

Welding1, following is information.

1.The pipe size is 26 in.
2.Pipe thickness is 17.4 mm
3.It has 2.3 mm thickness over required
4.Welding process is GTAW/SMAW
5.The fluid is natural gas saturated with water and miximum carbondioxide contents in the gas are 1.1% by volume.

The concern is with backwelding on internal of weld joint with relation to sigma failure. We have tried to demonstrate that if WPS is properly qualified,sigma failure will not be a concern, as a comment has been made that backwelding is not permissible under any circumstances as it causes sigma failure and we have tried through various other means to demonstrate that it is permissible if properly qualified and controlled. Any further comments are appreciated.

 

[welding1]

The main problem with duplex is small bore pies in super duplex. The thickness you have should not be a problem to weld, providing its done carefully, keeping heat as low as possible. Backwelding should not be a problem providing you are using filler metal. In fact it's no different to any form of multipass welding and multi run welds tend to promote austenite, thus improving toughness.

Sigma is a compound consisting of 55Fe 45Cr, it lowers corrosion resistance, but its greatest effect is lowering toughness, which is a problem if you are operating at low temperature. However you probably won't get brilliant toughness using SMAW. If you are getting good impact properties then its unlikely you are getting much, if any sigma. You could have it metallically examined, but finding sigma phase is difficult unless its in significant quantities.

Sigma tends to be tolerated more than it used to be. A major North Sea installation was found to contain sigma in pipelines, they justified it rather than repairing/replacing it and as far as I know its still in service today. Modern duplex is far superior to what it used to be as major advances have been made over the last 10 years. This is also true of welding consumables.

To get sigma you need 100 seconds at elevated temperature.

The best source of help is consumable manufacturers.

http://www.gowelding.com