Microstructural characterization and mechanical behavior of 9%Ni steel pipes processed by friction stir welding were studied. Two welding configurations, thickness 8 mm and 12 mm, were used. 12 mm-thick pipe was used for Crack Tip Opening Displacement (CTOD) tests and the remaining tests were carried out using 8 mm-thick pipes. Butt joints were welded at a constant rotation speed of 300 rpm, travel speed between 50-65 mm.mim~(-1). force control of 32 kN and tool offset in Y-axis of -9 mm. Defect free butt joints were produced with full penetration (for 8 mm-thick pipe) and good surface appearance. The microstructure of the affected regions by the process were characterized as martensite with prior-austenite grain (PAG) size variations. The results showed good mechanical behavior for cryogenic applications.
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