Deep Dive: Duplex Stainless Steel

Field installed duplex stainless steel fermenters

Enerfab works hand in hand with our customers to understand how fabrication and welding affect the corrosion resistance of their process equipment. These proactive steps in research, development, engineering, and project planning improve reliability, safety, and profitability. Our ongoing research and development efforts with Duplex grades help us better understand the material’s failures in the field and improve the material performance by analyzing the current weld procedures and pushing fabrication techniques beyond the guidelines set by the ASME code.

What is Duplex Stainless Steel?

Duplex Stainless Steel (DSS) combines high corrosion resistance with excellent mechanical properties and material strength to offer advantages when fabricating critical equipment for process industries. Like most stainless steel, its corrosion resistance affords longer service for fabricated process vessel equipment. But the high allowable stress of DSS materials allows for wall thickness of ASME Code pressure vessels and storage tanks to be thinner than 300 series stainless steel and other steel materials.

Microscopic image of duplex stainless steel composition

What We Must Consider When Welding Duplex Stainless Steel

Owners, engineers, and fabricators need to understand the unique properties and composition of Duplex Stainless Steel because incorrect welding and forming procedures can compromise the corrosion resistance and strength of the material during the fabrication process.

The unique properties of Duplex Stainless Steel are attributed to the compositional formulation and thermo-mechanical processing of a two-phase microstructure consisting of equal proportions of Austenite FCC and Ferrite BCC. DSS depends on a partial solid-state transformation to austenite from ferrite. Phase balance can be altered by cooling rate and composition. Too much ferrite in the microstructure leads to loss of toughness and ductility and promotes nitride precipitation or nitrogen super-saturation in the ferrite. On the other hand, extensive austenite in the microstructure leads to a loss of material strength.

So, when welding DSS materials, we must consider particular weldability concerns that can lead to changes in composition and improper phase balance. If the cooling rate is too fast, it leads to excessive ferrite and possible loss of corrosion resistance. If the weld is too hot, it can lead to detrimental intermetallic phase precipitation and loss of material toughness.

The Benefits of Expertise and Experience

Solution annealing duplex stainless steel

Enerfab has been fabricating DSS process vessels and tanks since the 1980s. Our weld procedures have been refined over time to ensure proper voltage/amperage, travel speed, and interpass temperatures to prevent rapid cooling or excessive heat. Using the proper filler metals retains the material strength and promotes austenite formation. Our skills and experience allow us to implement DSS where it can be most beneficial.

But for our customers and us, the possibilities for DSS go beyond the weld procedures and current fabrication techniques outlined by the ASME code. We work closely with our customers to understand where equipment from other fabricators has failed in the field to develop a plan to correct those issues moving forward. We regularly contribute weld samples for industry corrosion studies and have assisted in setting industry guidelines for the selection and use of FCAW and GMAW welding processes. We have participated in R&D studies with fellow industry experts to develop solution annealing processes that eliminate the risk of detrimental phase formation and subsequent cracking in DSS and SDSS.

Duplex stainless steel vessel fabrication

Duplex Stainless Steel offers unique possibilities for process equipment, but it requires experience and expertise to ensure fabrication upholds the strength and corrosion resistance DSS provides. From the shop to the field, Enerfab works with you to reduce the risk of equipment failure and provide the best value and outcome for your process.

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