Selecting the Proper Filler Metals for Duplex Stainless Steel

Suppose a welder needs to use GTAW to weld SA-790 4 in Schedule 40 duplex stainless steel. What filler material should he use? Can he use it on the root through all the way out? Is there anything he should watch for during welding? Duplex stainless steel (DSS) has the corrosion resistance of austenitic stainless steels, such as 316L, with the strength of ferritic steels (or carbon steels) because it has a dual microstructure that is approximately 50 percent austenite and 50 percent ferrite. The most popular DSS grade is 2205 (nominally 22 percent chromium, 5 percent nickel), which is used extensively for process piping, especially on offshore platforms. Because of its higher strength, a DSS pipe can have thinner walls than an austenitic stainless steel pipe, which reduces the weight of a piping system.

While DSS is readily welded using all arc welding processes, the trick is to maintain the 50 percent austenite and 50 percent ferrite microstructure in the base metal, heat-affected zone (HAZ), and the weld metal. Here are several important considerations to achieve this:

• Filler Metal Selection? For most applications, austenitic filler metals like 304L or 316L should not be used to weld DSS. Using grade 2209 (22 percent chromium, 9 percent nickel) filler metals will ensure that even the weld metal has a microstructure that is 50 percent austenite and 50 percent ferrite. There is more nickel in the filler to ensure austenite formation in the weld metal. If more nickel is not added, the weld may solidify with more ferrite than austenite, and it may be more susceptible to cracking.
• Heat Input Control? Preheating will slow down the cooling rate of a weldment. Typically, preheating is not required when welding DSS. In fact, a cooling rate that’s too slow may promote more austenite in the weld metal and HAZ than the 50 percent target. However, a cooling rate that’s too fast may promote too much ferrite, which may be more susceptible to cracking especially in a root pass of a pipe weld.
• Welding Process? When pipe welding, the GTA (TIG) welding process with filler metal is typically used for root pass welding. A maximum two percent nitrogen can be added to argon shielding gas for GTA (TIG) welding to promote the formation of austenite in the weld metal, especially in root pass welds on pipe. In addition, a back purge with argon is recommended to displace oxygen in the pipe and to reduce the formation of oxides, which can be detrimental to corrosion resistance. Although you can use GTAW for filler/cap pass welding, GMA (MIG) welding is a more productive option. Flux core welding wires (FCAW) are gaining in popularity due to the ease of out-of-position welding.
• Post-Weld Testing? Many DSS fabrications require post-weld testing for hardness and corrosion resistance. Typical hardness requirements for ?sour service? have a maximum of 32 Rockwell C (350 Vickers). Look for ASTM A923 (for 2205) or ASTM G48A references on your drawings or purchase orders. These specifications, which test for reduced corrosion-resistance as a result of welding, need to be considered prior to quoting and welding.

On another note, welders that use E9018-B3 stick electrodes (2¼ Cr-1Mo) sometimes see an “X-factor” referenced in certain electrode brochures and certificates, but do not understand exactly what this “X-factor” represents. Often, chromium molybdenum (or CrMo) steel alloys are used for elevated-temperature service. They are susceptible to temper embrittlement, which is a reduction in toughness when the material is held at, or slowly cooled through, a certain temperature range, typically 800 deg F to 1,100 deg F (425 deg to 600 deg C). Certain impurities should be minimized to avoid temper embrittlement. X-factor measures the effect of four impurity elements on the temper embrittlement susceptibility of 2¼Cr-1Mo steel weld metal:

X = (10P + 5Sb + 4Sn+As)/100
where:
P = phosphorus (in parts per million); Sb = antimony (ppm); Sn = tin (ppm); and As = arsenic (ppm)

X-factor is also called X-bar, or the Bruscato formula, and it is used with many chromium-alloyed steels. P, Sb, Sn, and As migrate at high temperatures and, given sufficient concentration and time, may accumulate and weaken the grain boundaries in the weld metal, embrittling these regions. Higher manganese and silicon also increase temper embrittlement. However, these elements are necessary for good weldability. X-factors vary depending on the filler metal type and manufacturer. Some critical applications may specify a maximum X-factor of 15. Note, too, that “X-factor” applies only to weld metal. For base steel, “J-factor” characterizes the temper embrittlement susceptibility, using Mn, Si, P, and Sn.