The harsh environment from heavy industry, such as oil and gas, presents a need for corrosion resistance combined with high toughness. These challenges are mostly fulfilled by duplex stainless steels, which present a balanced austenite-ferrite microstructure, hence the duplex name. The austenite presence enhances the toughness, whereas the chemical composition alloying provides the pitting resistance.
The pitting resistance equivalent (PREn) is the classification criterion for the stainless steels, accounting for the presence of chromium, molybdenum, and nitrogen formulated as PREn = %Cr + 3.3 % Mo + 16 %N wt%. The hyper duplex stainless steel (HDSS) presents the highest corrosion resistance with PREn values exceeding 48 while also reaching yield strength exceeding 700MPa. This performance is obtained by alloying with high levels of chromium, molybdenum, and nitrogen. However, when increasing the alloying content sigma phase, a brittle cr-rich phase might become stable. Even small volume fractions of sigma phase severely reduce toughness and corrosion resistance. Therefore, modeling the sigma phase kinetics in such alloyed material is critical to controlling and avoiding the intermetallic presence.
An experimental precipitation kinetic model was developed through a series of isothermal transformations using a Gleeble 3800. Optical and electron microscopy were used for sigma phase quantification and to develop an experimental-based interpolated time temperature transfortmation (TTT) map. A calphad sigma phase precipitation model was developed based on experimental data and the additive rule was used to calculate the sigma phase continuous cooling transformation (CCT) curves. The kinetic model was validated on a Gleeble, obtaining a cooling rate of 4oC/s as the threshold for sigma phase formation.
Automated GTAW deposited three layers of HDSS wire overlaying an A516 Gr.70 carbon steel plate using a shielding gas mixture of 98% argon and 2% nitrogen to compensate for nitrogen loss during the arc welding. The overlay experiments produced a minimum cooling rate of 20oC/s meaning no sigma would be formed. An initial mockup using heat input of 1.65kJ/mm with a maximum interpass temperature of 100oC was built and tested, since no sigma phase evidence was found, finite element analysis (FEA) was used to maximize productivity while still avoiding the 4oC/s threshold. From the FEA model, an ideal heat input of 2kJ/mm with a maximum interpass temperature of 200oC was used to build an optimized mockup. The microstructural analysis, mechanical, and corrosion testing did not indicate sigma phase presence.
Due to the absence of the sigma phase presence, as-deposited samples were heat-treated on the gleeble, using the cooling rates obtained from the kinetic model to develop a controlled amount of sigma phase. Impact toughness specimens with sigma phase artificially induced were used to establish ductile to brittle transition temperature DBTT curves in the as-deposited condition and with sigma presence. Sigma brittleness caused a severe reduction of the upper shelf energy USE values and a high increase in the DBTT value.
The increase in heat input and interpass temperature reduced the welding time by 33% while also increasing the deposition rate by 20%. Increasing the heat input produced higher austenite content which increased the average DBTT and the ASTM G48 CPT by 10oC.
The developed research and kinetic model show that a sigma-free overlay is achievable if the colling rate threshold of 4oC/s is not exceeded. More importantly, the combination of the kinetic model with the FEA optimization revealed that not only the HDSS material can be processed using the current manufacturing processes, but also that significant productivity improvement can be obtained while avoiding sigma phase formation.