Effects of reducing atmospheres of bright annealing on the surface and corrosion characteristics of super duplex stainless steel tubes

Abstract The influence of the gas composition in the reducing atmospheres of bright annealing on the surface and corrosion characteristics of super duplex stainless steel tubes was investigated using a variety of experimental and analytical methods. This study showed that the addition of N2 to the reducing atmosphere increased [N] pick up on the surface, promoting the phase transition to a single γ phase with a fine-sized δ remaining along the grain boundary of γ. The increase in partial pressure of N2 in the reducing atmosphere resulted in a thicker single γ phase with Cr2N precipitates along the grain boundary of γ. The corrosion resistance decreased substantially as a result. On the other hand, after annealing in 100% H2 at 1100 °C, some cracks with phase transformation to ɛ-martensite were formed mainly on the top surface region. The corrosion resistance of the annealed sample could be recovered only when the degraded surfaces had been removed, exposing the original phase structure with equivalent phase fractions of δ and γ. Based on the experimental results, more desirable bright annealing conditions for steel tubes with superior corrosion resistance are proposed.