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The Right Filler Metal to Weld Duplex Stainless Steel
Q: We manufacture stainless steel tanks used in various industries and we aren't always informed of service conditions and chemical exposures. We would like to reduce or minimize the number of filler metals we use to cut down on costs and prevent accidental use of the wrong filler metal. The primary duplex materials we weld are 2101, 2304, and 2205 using ER2307 and ER2209 filler. Which filler metal would be best for welding these duplex base materials?
A: Yours is a common question. In general, fabricators are looking for ways to simplify the welding process and minimize the potential for costly rework or, worse, field failures of welded products. While you may already be well-versed in the fundamentals of stainless steels (SS), let's review the basics of this material group.
There are three main categories of SS in reference to its microstructure: martensitic, austenitic, and ferritic. Martensitic is outside the focus of duplex grades, so we'll leave it out of this discussion.
Austenitic SS (the 3XX series) are typically used in applications requiring good weldability. They are highly formable and offer a low-temperature service option. Ferritic SS have good strength, moderate ductility, and are relatively inexpensive.
Duplex SS (DSS) were engineered to be the best of both worlds. DSS gets its name from its metallurgical microstructure made up of approximately 50 percent austenite and 50 percent ferrite. DSS are materials that feature improved toughness and almost twice the strength of austenitic SS and feritic SS, along with improved corrosion resistance, including stress corrosion cracking (SCC).
For a pressure vessel application in which the designer/engineer has selected a specific grade of DSS and determined the required material thickness, you, the welding engineer, need to select a filler metal that is at least a matching filler metal to deliver the same properties as outlined previously. You also have the added responsibility of selecting an appropriate welding process that won't degrade the base metal properties in the heat-affected zone (HAZ).
For example, if the job calls for a 2205 grade of DSS to be welded to itself, an ER2209 filler metal would be a solid match. However, if the base material is 2101, it's important to realize that a lean DSS was selected because the material is less expensive than other DSS but still delivers high strength with good fatigue resistance and good corrosion resistance. The best consumable option is the matching consumable, which is ER2307, but ER2209 would be an acceptable second choice.
If the higher amount of molybdenum in the ER2209 is not a concern for the application of the finished product, it would be a fair match to weld all three of the stated DSS base materials. Because of the higher amounts of alloy in this filler metal, the cost will probably be higher.
The primary factors to consider to select the best filler metal are:
- In-service conditions such as chemical environment and temperature ranges.
- Yield and tensile strength, including whether creep resistance or high fatigue strength is required.
- Pitting resistance equivalent number (PREN) or corrosion resistance.
- Welding process and welder appeal.
This article originally appeared in The WELDER magazine.
It is reprinted here with permission of the Fabricators & Manufacturers Association, Intl.