Duplex Stainless Steel Storage Tanks & Pressure Vessels

Duplex stainless steel is a subset of stainless steels. The name “duplex” is derived from the material’s metallurgical structure consisting of two phases: austenite and ferrite in near similar proportions. The material provides better corrosion resistance, particularly chloride stress corrosion and chloride pitting corrosion, as well as higher strength than typical 300 series stainless steels, including 304 and 316 grades. 

Compared with austenitic stainless steel, duplex stainless steel has a higher chromium content (20–28%), higher molybdenum (up to 5%), lower nickel (up to 9%), and .05–.50% nitrogen. The unique microstructure of duplex stainless steel, with its balanced austenite and ferrite phases, provides exceptional strength and corrosion resistance, often at a lower cost than comparable alloys.

Tate Metalworks utilizes duplex stainless steel in both field-erected and shop-fabricated tanks to provide enhanced durability and reliability for a variety of applications.

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Benefits of Duplex Stainless Steel

We have been fabricating API storage tanks and ASME Pressure Vessels from duplex stainless steels for decades. Our clients choose this material grade for the following reasons:

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Tate's Expertise in Duplex Steel Welding

Tate Metalworks fabricates and erects API-650 and API-620 storage tank sand ASME pressure vessels from both lean duplex materials—S32101 (2101), S32304 (2304), S32202 (2202)—and duplex materials—S31803, S32205 (2205), S32750 (2750). With decades of experience fabricating and erecting these tanks and pressure vessels, we have developed dozens of proven weld procedures for joining duplex materials.

All of our duplex weld procedures adhere to ASME Section IX Welding Code. Key components to maintaining the material’s strength and corrosion resistance properties include careful control of heat input, travel speed, and interpass temperature. During the development of our dozens of duplex welding procedures, Tate Metalworks has utilized post-weld impact testing, tensile testing, corrosion testing and ferrite testing as per industry and code standards. Our high standards and expertise assure our clients that the investments they have made for API storage tanks and ASME pressure vessels will serve them well for years to come.

Our duplex weld procedure includes:

• Ferrite content measurement: Duplex stainless steels require a balanced ferrite-austenite ratio to achieve optimum properties. Ferrite content measurement using techniques such as ferritescope or magnetic induction is critical to ensure the weld has the desired ferrite content, which affects mechanical and corrosion properties.
   
• Corrosion testing: Duplex stainless steels are tested for their resistance to various corrosion types, such as pitting corrosion and stress corrosion cracking. Testing methods include exposure to corrosive environments (e.g., salt spray testing), electrochemical methods (e.g., potentiodynamic polarization), and immersion testing.
   
• Mechanical testing: Mechanical testing evaluates the strength, toughness, and ductility of welds. Tensile testing, impact testing, and hardness testing are common methods used to assess mechanical properties and ensure they meet specified requirements.

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Steps to Successfully Welding Duplex Stainless Steels

Step 1) Pre-weld preparation

Proper cleaning and preparation of the base metal are crucial to prevent contamination. Remove any oil, grease, scale, or surface oxides that could affect the weld quality.

Step 2) Welding process

Duplex stainless steels can be welded using various processes such as gas tungsten arc welding (GTAW/TIG), gas metal arc welding (GMAW/MIG), shielded metal arc welding (SMAW/stick), and flux-cored arc welding (FCAW). The choice of process depends on the thickness of the material and the specific requirements of the application.

Step 3) Filler material

Selecting the correct filler material is critical to achieve the desired properties in the weld. Typically, matching or overmatching filler metals with high chromium, molybdenum, and nitrogen content are used to ensure corrosion resistance and mechanical properties similar to the base metal.

Step 4) Welding parameters

Control welding parameters such as heat input, interpass temperature, and travel speed to minimize the risk of sensitization and distortion. Duplex stainless steels are sensitive to thermal cycles, so maintaining proper heat control is essential.

Step 5) Post-weld heat treatment

In some cases, post-weld heat treatment (PWHT) may be required to reduce residual stresses and improve the corrosion resistance of the weldment. However, duplex stainless steels generally require less PWHT compared to austenitic stainless steels.

Step 6) Avoiding sensitization

Duplex stainless steels can be susceptible to sensitization, where the formation of chromium carbides reduces corrosion resistance in the heat-affected zone (HAZ). Proper welding procedures and filler materials with low carbon content help minimize this risk.

Step 7) Testing and inspection

Conduct thorough testing and inspection of welds to ensure they meet specified requirements for mechanical properties and corrosion resistance. Non-destructive testing (NDT) methods, such as ultrasonic testing (UT), dye penetrant testing (PT), and Ferrite testing, are commonly used.

Step 8) Welding sequence

Plan the welding sequence to minimize distortion and control the cooling rate to avoid cracking. Back purging with inert gas (e.g., argon) during welding can help prevent oxidation and maintain the correct composition in the weld.

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At Tate Metalworks, we are committed to excellence, safety, reliability, and innovation in our craftsmanship. No matter what type of material is used, you can trust that our tanks are engineered to meet the highest standards of quality, performance and safety. Tate Metalworks offers peace of mind and long-term value for all your storage tank needs.

Contact us to learn more about our proven expertise in your industry, or request a proposal to get started on your duplex stainless steel project.

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