SynopsisA ferritic filler submerged arc welding (SAW) process has been developed and applied to the manufacture of UOE pipes.In order to achieve sufficient cryogenic toughness of ferritic weld metal, the welding wire and flux compositions are recommended to be designed so that the oxygen and silicon contents in the weld metal become approximately 250 ppm and 0.25 %, respectively. To prevent cold cracking, it is suggested that the difusible hydrogen content should be less than approximately 3.5cc/100g of the weld metal. The combination of a fused type flux (CaF2-A1203-Si02 system) with a ferritic welding wire of low Si 12%Ni was found adequate for satisfying the above two conditions. Next, rapid quenching and tempering conditions to maximize both the cryogenic toughness of the weld metal and the productivity of UOE pipes were determined. A unique quenching thermal pattern having a holding period at 600 °C before quenching from 790 °C was found crucial to satisfy the above requirements. The subsequent tempering was carried out by raising the temperature continuously to 600 °C, and then immediately water cooled. Under this thermal pattern, a heating time as short as 6 min is required to reach the target temperature of quenching and tempering, and to yield a toughness o f as high as 10 kg f m. This rapid heat treatment technique enables the use of a high frequency induction heating system on the production line of UOE pipes. The satisfactory toughness is believed to be a result of the austenitic grains being refined and the carbide precipitation being accelerated by holding the welds at 600 °C.The UOE pipes manufactured by the ferritic filler SAW process have satisfactory tensile properties as well. The strength o f the ferritic weld matches that of the 9 % Ni steel plate, so that the expanding strain applied to the pipe wall for improving its dimensional accuracy does not concentrate in the weld metal as it does in the case of an austenitic weld. I. IntroductionWith the growing demands for LNG and other liquefied gases, the welding of cryogenic structures has become increasingly more important. 9 % Ni steel, a typical material for cryogenic use, has usually been welded with Ni-based alloy welding fillers (austenitic fillers) because of the excellent cryogenic toughness of the resulting weld metal even in the as-welded condition. However, the drawbacks of this welding material are : the high material cost, the high hot crack susceptibility, and the yield strength of the weld metal undermatches that of the base plate.In manufacturing 9 % Ni steel UOE pipes with the austenitic filler SAW, the undermatching in strength is apt to be magnified, because the circumferential strain exerted in the mechanical expanding stage is concentrated in the weak weld metal, and, in the worst case, the strain concentration results in the fracture of the weld seam especially when there are hot cracks in the weld metal.

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