Developing a Weld Overlay Specification for Hyper Duplex Stainless Steel

Abstract Hyper Duplex Stainless Steels (HDSS) are dual phase, ferritic-austenitic materials with a remarkable yield strength (≥ 700 MPa) and corrosion resistance (PREN/W > 49). It has been developed as an alternative to super-duplex stainless steels, where higher mechanical and corrosion performance is required. Unfortunately, such highly alloyed materials are susceptible to brittle intermetallic phase formation, such as the sigma phase. Understanding the intermetallic formation is essential to obtain its optimal properties and define advantages and limitations for a welding specification. Overlay experiments on three-layered HDSS deposited over a carbon steel plate using 1.1kJ/mm and 1.65kJ/mm shown a clear austenite/ferrite phase ratio difference between the first and last layer. The last deposited layer has a larger ferrite volume fraction and chromium nitride presence. However, no sigma phase was found on the overlay conditions. A sigma phase kinetic model was developed using Thermocalc Prisma, experimentally adjusted, and validated by physical simulation in a series of isothermal heat treatment tests. The additivity rule was used to calculate continuous-cooling-transformation (CCT) curves from the adjusted temperature-time-transformation (TTT) curves. The kinetic model predicts no sigma precipitation for cooling rates faster than 4°C/s. Physical simulation with controlled cooling rates validated the model. Also, the thermal history analysis of the overlay experiments has shown no sigma was expected due to the total time and temperature transformation and cooling rates not reaching the calculated CCT.