Effects of Mo and Cu Contents on Sigma Phase Precipitation in 25Cr-5Ni-Mo-Cu-1Mn-0.18N Duplex Stainless Steel

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.