Duplex alloys are quite different from standard stainless steels. They contain roughly 50% each of ferrite and austenite. If not properly welded, this class of alloy can be susceptible to formation of embrittled phases or formation of precipitates which are susceptible to pitting. By recognizing this, and properly following recommended procedures, mechanically sound and corrosion resistant fabrications are easily accomplished.
Sandvik provides welding guidelines to successfully join duplex base materials.
Generally speaking, the following parameters are required to be followed:
Due to the sluggish nature of ferritic materials, weld pool flow tends to be sluggish. To prevent lack of fusion it is recommended to use wider joint angles and larger root openings than commonly used in stainless steels. See Sandvik Welding Guidelines for more specific information.
SHIELDING AND BACKING GAS SELECTION
Due to the nature of ferritic materials, weld pool flow is sluggish. This can be compensated for by the proper shielding gas selection, which can also benefit the proper austenite and ferrite balance. The selection of backing gas can have a beneficial affect on the corrosion resistance.
See Sandvik Welding Guidelines for more specific information.
In order to achieve the optimum ferrite to austenite ratio, the heat input must be properly controlled. The recommended heat input range is dependent on the grade of duplex stainless steel being fabricated. See Sandvik Welding Guidelines for more specific information.
Duplex alloys have specific interpass temperatures recommended, in order to prevent formation of brittle intermetallic phases. The proper interpass temperature is dependent on the grade being welded and the base metal thickness. See Sandvik Welding Guidelines for more specific information.