I often receive questions on the correct parameters for welding duplex stainless steels. There seems to be a lot of confusion on which practices and quality checks are best. I also notice that each industry treats duplex stainless differently. These materials do take care and proper procedure to maintain corrosion and mechanical properties.
First, it is important to mention that Rolled Alloys and other suppliers have welding guidelines with details on shielding gases, amps, volts and other settings you will need to have a successful procedure. Here are links for welding 2205 duplex and for ZERON 100 super duplex. The notes below are shortened rules of thumb, so please consult the full welding guides for details.
Most duplex stainless steels have an over alloyed filler. For example, 2205 uses 2209 and ZERON 100 uses ZERON 100X. These filler metals are nearly identical in chemistry to the base metal, except for an extra 2% nickel. The increased nickel content aides in forming the 50/50 mix of austenite and ferrite phases. The benefit of these over alloyed fillers is that you do not need to post weld heat treat the fabrication. There are some variations that can be used, for example, LDX 2101 is often welded with 2209 filler metal because it is more readily available. Using this filler metal with higher nickel, chromium, and molybdenum increases the corrosion resistance and makes a sound weld.
Heat Input and Interpass Temperatures
One of the most common errors I see when welding duplex stainless steel is the heat input and interpass temperatures are not followed correctly. In short, duplex’ are welded with relatively high heat input and low interpass temperatures. This does not make the welding process more difficult. The reason these parameters are important is because duplex stainless rely on a nearly equal balance of ferrite and austenite . If the weld deposit does not see enough time at temperature, it may have high ferrite and not enough austenite. If the weld deposit sees too much time at temperature, the austenite may be too high and there is an increased chance of forming nitrides and sigma phase in the base metal. These changes will decrease the corrosion resistance and toughness.
Because welding duplex stainless steel is not fool-proof it is important to perform quality checks. We normally suggest that when developing a weld procedure it is critical to test the corrosion resistance, the impact toughness, and the ferrite/austenite mix in the weld qualification. The welding guides have more detail on which test is appropriate for each material. Once your WPS is written to provide good corrosion, toughness, and ferrite the final production weld should also yield good results. Since it is often impossible to fully test the final product, we usually suggest testing the weld bead for ferrite content. Typically incorrect ferrite levels will be the first indication of a problem. If a problem is found, further testing should be performed.
Hopefully this helps. If you have any other questions, please contact us at Metallurgical-Help@RolledAlloys.com