| Abstract Scope |
The use of duplex stainless steels (DSS) is well established across different industries where materials are subjected to harsh service conditions. The need for excellent corrosion resistance and elevated strength at a reasonable cost is nicely fulfilled by the DSS family, which presents a balanced microstructure formed by austenite and ferrite. The balanced microstructure provides excellent mechanical properties, whereas the phase's chemical composition dictates the material's excellent combination of generalized and localized corrosion resistance, as well as stress-corrosion-cracking resistance.
The pitting resistance equivalent PREn is usually the classification criterion for the stainless steels, accounting for the presence of Cr, Mo, and N, formulated as PREn = %wtCr + 3.3%wtMo + 16%wtN. The Hyper Duplex Stainless Steels (HDSS) present the highest corrosion resistance among all DSSs, with PREn values exceeding 45, while also reaching yield strengths exceeding 700 MPa. This performance is provided by the elevated content of Cr, Mo, and N. However, when increasing the alloying content sigma phase, a brittle Cr-rich phase becomes stable. Even a low volume fraction of the sigma phase can severely reduce toughness and corrosion resistance. Therefore, developing processing methodologies to avoid sigma phase precipitation in such alloys is critical. Understanding the kinetics of this phase formation is the best way to find the process conditions that will render better performing components.
The data available on the kinetics of sigma phase formation on HDSS is quite limited. Time-temperature-transformation TTT and/or continuous-cooling-transformation CCT data is very limited in the literature. Therefore, the research program supported by the Manufacturing and Materials Joining Innovation Center (Ma2JIC) at The Ohio State University has developed a kinetic model of sigma phase formation on DSSs and further developed it for HDSS. However, models are only meaningful if they properly describe and predict real phenomena. Hence, firstly a series of isothermal transformation experiments were performed using a Gleeble® thermomechanical physical simulator to measure the kinetics of sigma phase formation on HDSS. The sigma phase quantification was performed using SEM. Such high-quality experimental data was used to develop a TTT precipitation model using the CalPhaD-based software ThermoCalc. However, as most manufacturing processes impose complex continuous cooling/heating thermal cycles instead of simple isothermal conditions, the additive rule was used to calculate the sigma phase CCT data from the formerly developed isothermal precipitation model.
The CCT kinetic model was validated using three controlled cooling rates, 1, 2.5, and 4 oC/s imposed on the material using the thermomechanical physical simulator. The validation testing cycle used rapid heating to 1200 oC to minimize any sigma precipitation during heating. The samples were held at 1200oC for 90 s to ensure complete sigma phase dissolution. After the patamar, the samples were controlled cooled at the defined cooling rates from 1200oC to 600oC. The resulting sigma phase volumetric fraction was also quantified using SEM.
Application-relevant validation was obtained using the HDSS corrosion-cladding process. Automated cold wire GTAW was used to deposit three layers of HDSS (UNS S32707_ on A516 Gr.70 carbon steel plate. The cladding process heat input was 1.6 kJ/mm with a maximum interpass temperature of 100oC. The shielding gas used was a mixture of 98% Ar - 2% N to minimize N loss. The cladding process thermal history was recorded and was used to calibrate an FEA model, which was developed to reproduce each bead’s thermal history. The FEA-based calculated thermal history was used to run the developed kinetic model. In addition to the kinetic model prediction, microstructural analysis was performed using optical microscopy and SEM. Charpy-V-Notch (CVN) testing was used to verify the thermal history and potential sigma phase precipitation effect on the deposited overlay.
The microstructure found was mainly ferrite, austenite, secondary austenite, and sigma. Some additional chromium nitrides were formed on the third layer. Sigma phase volume fraction varied from 0% to 31% volume fraction in 600s. The TTT precipitation model presented a high agreement with the experimental sigma precipitation. Furthermore, the calculated CCT kinetic model shown a sigma precipitation threshold at 4oC/s cooling rate; only lower cooling rates will precipitate sigma.
The cooling rate validation test presented an excellent agreement with the model. At the cooling rate of 4oC/s only 0.02% ±0.06% sigma volume fraction was found, whereas a cooling rate of 1oC/s would produce 5% ±2.67% of sigma. Also, on the application validation, the automated GTAW cladding slowest cooling rate was 20oC/s, and, as such, the kinetic model does not predict sigma precipitation. The SEM microstructural analysis did not found sigma precipitation, which agreed with the kinetic model prediction. Also, the hardness levels were all below 350HV0.2, indicating sigma phase absence.
The developed kinetic model and research show that an HDSS sigma-free cladding procedure is achievable if the cooling rate threshold of 4oC/s is not exceeded. More importantly, it shows that the UNS S32707 material can be processed by the current manufacturing processes such as welding and cladding producing sigma-free components. |