Effect of multiple thermal cycles on microstructure evolution and properties in 25Cr-5Ni-1Mo-2.5Cu- 1Mn-0.18N duplex stainless steel

Duplex stainless steels are widely applied to industrial sectors because of their mechanical properties and corrosion resistance. Whilst duplex stainless steels have excellent properties, they are vulnerable to precipitate sigma (σ) phase during welding. Recently, a new duplex stainless steel UNS S82551 (25Cr-5Ni-1Mo-2.5Cu-0.18N) has been developed to achieve lower material cost than UNS S31803 (22Cr-5Ni-3Mo) and UNS S32750 (25Cr-7Ni-4Mo) by decreasing Mo content, and which can be utilised in slightly sour conditions. However, it is not clear the effect of chemical composition and welding condition on the microstructure and sigma (σ) phase precipitation for the new material.

In this project, the effect of chemical composition on sigma (σ) phase precipitation was first investigated. Time-Temperature-Precipitation (TTP) diagrams for sigma (σ) phase were generated, which showed that sigma (σ) phase precipitation is accelerated by the increase of Mo content for duplex stainless steel. The effect of weld thermal cycling on sigma (σ) phase precipitation behaviour of UNS S82551 was investigated, and it was found that the amount of sigma (σ) phase increased with increasing the number of thermal cycles for UNS S31803 and UNS S32750, but sigma (σ) phase was not observed for UNS S82551. Girth welding was performed to study the effect of welding parameters on microstructure transformation and toughness and corrosion properties of UNS S82551. Phase balance was shown to be acceptable using point counting and EBSD (electron back-scattered diffraction), and sigma (σ) phase precipitation can be avoided at up to heat inputs 2.0kJ/mm (nominal) and 200- 250oC interpass temperature for UNS S82551 pipe, to result in good toughness and corrosion resistance. To complement the experimental work, computational welding simulations were performed. The model predicted that no sigma (σ) phase would be present in UNS S82551 welds. The above findings provide improving knowledge about the effect of chemical composition and welding condition on microstructure evolution and properties in duplex stainless steels.