Recently, a new duplex stainless steel UNS S82551 (25Cr-5Ni-1Mo-2.5Cu-0.18N) has been developed to overcome the drawbacks in super martensitic stainless steel, conventional 22Cr and 25Cr super duplex stainless steels in terms of the productivity and the cost. The characteristic of the alloy design of S82551 is to use Cu, instead of Mo, addition to ensure the corrosion resistance and strength. In addition, S82551 is expected to be less sensitive to sigma phase precipitation during single or multi-pass welding compared with conventional duplex stainless steels and super duplex stainless steels due to the significant decrease in Mo content. There is a trade-off relationship between achieving better properties and avoiding sigma phase precipitation when increasing alloying elements such as Mo, Cr and Cu. In order to utilize the new material S82551 in industry by welding in a similar manner to conventional and super duplex stainless steels, the prevention of sigma phase precipitation is an important subject.This work investigated the effects of Mo and Cu contents on sigma phase precipitation in S82551 using time-temperature-precipitation (TTP) diagram generated by experimental work and calculation. The effects of Mo and Cu contents on the nose temperature and time of C-curve were clarified and compared with S31803 (22Cr-5Ni-3Mo) and S32750 (25Cr-7Ni-4Mo), which indicates the critical heating condition to be free from sigma phase precipitation in duplex stainless steels.

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Recently, a new grade of duplex stainless steel, UNS S82551 (25Cr-5Ni-1Mo-2.5Cu-0.18N), has been developed to overcome the drawbacks in super martensitic stainless steel, conventional 22Cr and 25Cr super duplex stainless steels in terms of productivity and cost. The characteristic of the alloy design of UNS S82551 is to use Cu, instead of Mo, to ensure comparable corrosion resistance and strength. In addition, due to the significant decrease in Mo content, UNS S82551 is expected to be less sensitive to sigma phase precipitation during single or multi-pass welding compared with conventional and super duplex stainless steels. There is a trade-off between achieving better properties and avoiding sigma phase precipitation when increasing alloying elements such as Mo, Cr and Cu. In order to utilize the new UNS S82551 steel in industry for welding in a similar manner to conventional and super duplex stainless steels, the prevention of sigma phase precipitation is important. This work investigated the effect of weld thermal cycling on sigma phase precipitation behaviour in UNS S82551 welds. During thermal cycling in the tests, the amount of sigma phase for UNS S31803 and UNS S32750 increased with increasing the number of thermal cycles, and lower cooling rate, but it was not observed for UNS S82551. Based on the isothermal kinetics of sigma phase precipitation, the amount of sigma phase precipitated during thermal cycle can be predicted by applying the additivity rule to the physical model. The area fractions of sigma phase calculated have a good fit to the experimental ones.