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

Abstract

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.

Effects of Welding Parameters on Sigma Phase Precipitation in 25Cr-5Ni-1Mo-2.5Cu-1Mn-0.18N Duplex Stainless Steel

“…In the previous work, the effects of Mo and Cu contents on the nose time and temperature of C-curve of sigma phase precipitation for UNS S82551 was discussed, which indicated UNS S82551 has less sensitivity to sigma phase precipitation than UNS S31803 and UNS S32750 mainly because of lower Mo content. 12) TTP diagram has effective to understand how the microstructure developed during isothermal states. However, it is reported that the microstructure changes after thermal cycles during welding are more complicated, which is different compared with isothermal heat treatment.…”

“…According to the previous report, sigma phase is confirmed in 600-900°C by the phase diagram for UNS S82551. 12) Consequently, the cooling time between 900 and 600°C is considered an important parameter in the determination of sigma phase formation in UNS S82551. Figure 2 shows the relationship between heat input and cooling times at FL portion for 900-600°C (Δt 900-600 ).…”

Sigma‐Phase Distribution in 2507 Super Duplex Stainless Steel Continuous Casting Slabs

“…The precipitation behavior, influencing factors, and effects of the sigma phase on DSS have been widely studied. [20][21][22][23][24][25][26][27][28][29][30][31][32][33][34] Kazuhiro reported the effects of alloying elements on the precipitation of the sigma phase in SDSS. [26,27] The authors found that Cr and Mo, which are useful for improving the corrosion resistance, can promote sigma-phase precipitation, but the phase does not always grow rapidly.…”