PNNL

Abstract

Severe plastic deformation of 304L stainless steel plate was carried out by friction stirring under an isothermal condition by maintaining the tool temperature at 825 ºC. The friction stirred zone showed a grain size of 3.4±1.2 µm, 34.2% of low angle grain boundaries due to dislocation rearrangement, 10.2% of (S3 and S9) special grain boundaries, and discrete Fe-Cr-Mn rich particles. No other detrimental phases such as martensite, and sigma were observed. Electrochemical polarization measurements were carried out on the friction stir welded (FSW) specimens in the neutral and acidified 3.5% NaCl solutions and the results were compared with those of mill annealed wrought material (referred to as base metal (BM)). The BM specimens showed marginally better pitting resistance in the neutral chloride solution than the FSW because of a large fraction (50.4%) of special grain boundaries, and larger grain size (47±16 µm). When tested in 1 N H2SO4 + 0.6 M NaCl solution, the FSW showed better corrosion resistance than the BM. The impedance analysis indicated that the surface film of the FSW had a lower impedance and also a lower defect concentration than those observed in the BM pointing out higher diffusivity of point defects in the FSW.