Duplex stainless steels are materials based on the Fe-Cr-Ni-N alloy system, where the chemical composition has been adjusted such that the steel’s base metal microstructure consists of nominally 50% ferrite and 50% austenite, hence the term duplex. Due to the higher alloy contents and demanding processing, duplex stainless steels are more costly to produce than other austenitic alloys. Super-Duplex Stainless steels are largely used in critical applications such as oil and gas, chemical, and paper industry. However, there is still demand for higher corrosion resistance combined with strength. Hyper Duplex Stainless Steels (HDSS) are materials with a remarkable yield strength (≥700 MPa) and corrosion resistance (PREN/W > 49), which have been developed as an alternative to super-duplex stainless steels, where higher mechanical and corrosion performance is required or desirable. Unfortunately, such highly alloyed materials are quite prone to brittle intermetallic phase formation. A better understanding of HDSS cladding operations and its effect on the precipitation of intermetallic phases, corrosion, and impact toughness performance is needed.
Sigma phase precipitation modeling was obtained by calculating time-temperature-transformation (TTT) curves using Thermocalc and measured experimentally through physical simulation in a series of isothermal heat treatment tests using a GleebleŽ 3800 thermo-mechanical simulator. Scheil’s additivity rule was used to calculate continuous cooling transformation (CCT) curves from the TTT curves. This provided a kinetic model for intermetallic sigma precipitation in HDSS.
Quantitative metallography was used to quantify the intermetallic presence. One sample had three regions treated at different temperatures, which resulted in three distinct microstructural regions for each applied thermal-cycle.
SEM microstructural characterization has shown that short aging times are required to precipitate intermetallic phases, likely due to the complex alloying of the material. Chi and Sigma, intermetallic phases, were found quickly forming between 850⁰C to 900⁰C. Cellular clusters of lamellar structures composed of sigma and secondary austenite were also observed at aging times as short as 200s at 850⁰C. SEM energy dispersive spectroscopy (EDS) measurements were used to confirm phases characterizations. The high content on Mo in the Chi phase produced a brighter contrast when compared with the Cr-rich sigma phase. Also, for longer heat treatments, a dark precipitate was formed along with the interfaces, which the Fe-Cr-Ni-N equilibrium phase diagram predicts to be HCP CrN2. The EDS measurements, even though not quantitative, confirmed the predominance of Cr and N.
Bead on plate specimens (Hyper-duplex filler metal deposited over carbon steel plate) was produced using the cladding parameters developed for a high heat input, 1.2kJ/mm, and a minimal amount of intermetallic phases were observed.