Can You Weld 2205 to 316?

The question of whether you can weld dissimilar stainless steels, specifically grade 2205 duplex stainless steel to grade 316 austenitic stainless steel, is a common one in fabrication and engineering. The short answer is yes, it is possible to weld 2205 to 316, but it requires careful consideration of the different material properties, potential challenges, and the selection of appropriate welding procedures and consumables to ensure a sound and corrosion-resistant joint.

Understanding the fundamental differences between these two stainless steel grades is crucial before attempting such a weld. Grade 316 is a molybdenum-bearing austenitic stainless steel known for its excellent corrosion resistance, particularly in chloride-containing environments, and good weldability. Its microstructure consists primarily of austenite, a face-centered cubic crystal structure that contributes to its ductility and formability.

Grade 2205, on the other hand, is a duplex stainless steel, meaning its microstructure is a mixture of approximately equal proportions of austenite and ferrite (a body-centered cubic crystal structure). This dual-phase structure gives 2205 a significant advantage over 316 in terms of strength (roughly twice the yield strength), resistance to stress corrosion cracking (SCC), and pitting and crevice corrosion, while still maintaining good weldability, although generally requiring more controlled welding parameters than austenitic grades.

When welding dissimilar metals like 2205 and 316, the differences in their chemical compositions and physical properties can lead to several potential issues if not addressed properly. These challenges primarily revolve around:

1. Formation of Undesirable Intermetallic Phases: Duplex stainless steels like 2205 are susceptible to the formation of brittle intermetallic phases, such as sigma phase, chi phase, and Laves phase, if exposed to elevated temperatures for prolonged periods, particularly in the temperature range of 650-950°C (1200-1740°F). While this is more of a concern during welding of duplex to duplex, the heat input during dissimilar welding can still promote their formation in the 2205 heat-affected zone (HAZ) if not carefully controlled.

2. Differential Thermal Expansion: 2205 and 316 have slightly different coefficients of thermal expansion. During welding, the differential expansion and contraction can induce residual stresses in the weldment, potentially leading to distortion, cracking, or reduced fatigue life, especially in thicker sections or constrained joints.

3. Ferrite Content in the Weld Metal: Achieving the correct ferrite-austenite balance in the weld metal is crucial for optimal strength and corrosion resistance in duplex stainless steels. When welding 2205 to 316 using standard austenitic filler metals, the resulting weld deposit will likely have a predominantly austenitic structure with a lower ferrite content than desired for optimal performance, potentially compromising the strength and SCC resistance of the joint, especially on the 2205 side.

4. Corrosion Resistance: While both materials offer good corrosion resistance, their specific resistance mechanisms and performance in certain environments differ. The weld metal and the HAZ of both materials must maintain adequate corrosion resistance to avoid preferential attack at or near the weld. Dilution of the weld metal with the base materials can influence its corrosion resistance.

To successfully weld 2205 to 316, careful consideration must be given to the following aspects:

1. Welding Process Selection: Several welding processes can be used, including Gas Tungsten Arc Welding (GTAW or TIG), Gas Metal Arc Welding (GMAW or MIG), and Shielded Metal Arc Welding (SMAW or stick welding). GTAW is often preferred for the root pass and thinner sections due to its precise heat input control. GMAW can be more efficient for thicker sections and fill passes. Pulsed current techniques in both GTAW and GMAW can help to reduce heat input and improve control.

2. Filler Metal Selection: The choice of filler metal is critical. Using a filler metal specifically designed for welding duplex stainless steels to austenitic stainless steels or for dissimilar welding of stainless steels is recommended. Common choices include:

• Austenitic Stainless Steel Fillers with Increased Nickel and Molybdenum: Fillers like ERNiCrMo-3 (Alloy C-276) or ERNiCrMo-13 (Alloy 625) are often used. These nickel-based alloys offer high strength, excellent corrosion resistance in a wide range of environments, and good tolerance for dilution from both the duplex and austenitic base materials. They typically result in a fully austenitic weld deposit.

• Duplex Stainless Steel Fillers with Over-Alloying: Some duplex filler metals, such as those with slightly higher nickel content (e.g., ER2209 or ER2594), can also be used. These fillers aim to achieve a more balanced ferrite-austenite microstructure in the weld deposit, even with dilution from the 316 base material. However, careful consideration of the welding parameters and heat input is crucial when using duplex fillers to avoid excessive intermetallic phase formation in the 2205 HAZ.

The specific filler metal selection should be based on the service environment, mechanical property requirements, and the thickness of the materials being joined. Consulting welding material manufacturers and relevant welding codes and standards is highly recommended.

3. Welding Procedure Specification (WPS): A well-defined WPS is essential. It should include details such as:

• Heat Input Control: Maintaining a low to moderate heat input is crucial to minimize the risk of intermetallic phase formation in the 2205 HAZ. Interpass temperatures should be strictly controlled and kept below 150°C (300°F) in most cases.

• Welding Technique: Stringer bead techniques are generally preferred over weave beads to minimize heat input.

• Shielding Gas: For GTAW and GMAW, appropriate shielding gases, such as argon or argon-based mixtures with helium or nitrogen, should be used to provide adequate weld pool protection and promote optimal weld bead characteristics.

• Cleaning: Thorough cleaning of the weld joint and filler metal is essential to prevent contamination.

4. Post-Weld Treatment: Post-weld heat treatment (PWHT) is generally not recommended for dissimilar welds between 2205 and 316, as the optimal PWHT temperature ranges for the two materials differ significantly and could be detrimental to one or both materials. However, in specific critical applications, a low-temperature stress relief may be considered after careful evaluation. Chemical passivation of the weld joint after welding can help to restore the corrosion resistance of the weld.

In conclusion, welding 2205 duplex stainless steel to 316 austenitic stainless steel is a feasible process when performed with careful planning, appropriate filler metal selection, and controlled welding procedures. The use of nickel-based alloy filler metals like ERNiCrMo-3 or ERNiCrMo-13 is a common and often reliable approach due to their excellent corrosion resistance and ability to accommodate the differences in the base materials. However, understanding the potential challenges related to intermetallic phase formation, differential thermal expansion, ferrite content, and corrosion resistance is crucial for achieving a high-quality and durable weldment. Consulting with experienced welding engineers and adhering to relevant welding codes and standards are strongly advised before undertaking such dissimilar welding operations.