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Sigma Phase Precipitation In Hyper-Duplex Stainless Steels

Research Scholar

Doris Ivette Villalobos Vera, Materials Science and Engineering (Mexico)
John C. Lippold, Faculty Mentor

Biography

Doris Ivette Villalobos Vera was born in 1981 in Minatitlán, Veracruz, México and started in engineering in high school. She earned her bachelor's degree in industrial engineering in 2004 and then she completed her master's and doctorate degrees in materials science with an emphasis in welding metallurgy of stainless steels. She is currently working on her postdoctoral research focused on the metallurgical behavior of hyper-duplex stainless steels under the guidance of John C. Lippold in the Welding and Joining Metallurgy Group at The Ohio State University.

What is the issue or problem addressed in your research?

Hyper-duplex stainless steel is the new generation of duplex alloys, which have been developed with the purpose of satisfying the needs of the offshore and petrochemical industries related to corrosion resistance and mechanical properties, offering a relatively low cost and a long service-life compared to super austenitic stainless steels and nickel based alloys. In order to achieve the properties that can support those aggressive environments and extreme conditions such as hot/cold deep-seawater, they have a well-balanced Creq/Nieq ratio and particularly, a high content of Cr and Mo. However, the addition of those elements, promotes the formation of intermetallic phases at high temperature such as sigma phase, which can be formed in duplex alloys either during welding or high temperature-service. Moreover, sigma phase is hard and brittle which implies that its formation will reduce the final mechanical properties and the corrosion resistance since is a Cr-rich phase, causing catastrophic failures during service in marine environments such as in umbilical's subsea or in the petrochemical industry such as in vessels.

What methodology did you use in your research?

In this work, the performance and behavior of duplex, super-duplex and hyper-duplex stainless steels at high temperature have been evaluated and compared in order to establish the susceptibility of sigma phase formation based on the Creq/Nieq ratio. To accomplish this work, samples were heat treated at 800°C and 900°C under different dwelling times followed by water quenching. All the heat-treated samples were grinded, polished and electrolytic etched in order to reveal the sigma phase in the austenitic/ferritic microstructure. The Scanning Electron Microscope (SEM) in secondary electron and backscattering modes were employed in order to identify the morphology, size and chemical composition of the sigma phase. The chemical composition of sigma phase was obtained by means of Energy Dispersive Spectrometer (EDS). Moreover, Orientation Imaging Microscopy® analysis helped to identify any trace of sigma phase present in the matrix microstructure.

What are the purpose/rationale and implications of your research?

The purpose of this research is to study the microstructural behavior of the newly developed hyper-duplex stainless steels at high temperature in order to establish the susceptibility of sigma phase formation based on the Creq/Nieq ratio. These results will provide an overview of the scopes and limitations of this alloy to determine its feasibility for industrial applications to prevent catastrophic failures.