Multipass Autogenous TIG Welding ofSuper Duplex Stainless Steel

Multipass welding of super duplex stainless steels (SDSS) needs further characterization due to their growing applications in petrochemical and offshore industries. This study, as a result, is aimed at investigating the effects of the number of passes and the arc energy on the microstructure and properties of 2507-type SDSS (UNS S32750). From one to four TIG weld passes were autogenously applied on a plate using two different arc energies and with pure argon gas as the shielding gas. Chemical analysis showed increasing nitrogen loss with an increasing number of passes and increasing arc energy. Microstructural analyses revealed formation of nitrides in the weld metal and heat affected zone, and sigma phase at some distance from the fusion boundary. Thermal cycle analysis in combination with Thermo-Calc calculations indicated that excessive reheating cause degradation of corrosion properties of multipass weldments, by reducing the pitting resistance equivalent number of austenite to less than 40. Multipass welding resulted in a more ferritic weld metal microstructure and an increased hardness. Recommendations, based on the present study, are as follows: 1) Corrosion attack can occur not only in the weld zone and next to the fusion boundary, but also in a location at some distance from the fusion zone due to reheating in the sigma phaseformation temperature range. This should be considered in inspection procedures 2) Nitrogen loss degrades the mechanical and corrosion properties of weldments even when welding with a low heat input. Using filler metals with higher nickel contents and nitrogen containing shielding gases are therefore recommended. 3) It is often recommended to use a heat input in the lower end of the recommended 0.3-1.5 kJ/mm range in multipass welding of super duplex stainless steels. However, welding with a higher heat input and fewer passes, in some cases, can decrease the risk of formation of secondary phases.

Keywords:

Super duplex stainless steel , multiple thermal cycle , nitrogen loss , pitting resistance equivalent , ferrite number