Corrosion at the Weld Nugget of the Friction-Stir-Welded Medium Strength Steel: Effect of Microstructure

Abstract

In the present investigation, friction stir welding (FSW) was carried out on C-Mn steel using various welding parameters. The microstructural examination of the weld nugget (WN) region revealed different area fractions of cementite, pearlite, bainite, and martensite within the ferrite matrix. The average hardness in the region of the WN was higher than the base material. The corrosion behavior of the WN and base metal was investigated. The base metal underwent uniform corrosion. Localized pitting corrosion was dominant in the region of the WN. The primary corrosion products were c-Fe 2 O 3 (maghemite), c-FeOOH (lepidocrocite), and Fe 3 O 4 (magnetite). The corrosion rate of the WN was significantly higher than that of the base metal. However, the same was considerably lower than the conventionally fusion-welded specimens. Further, the corrosion rate of the WN increased with the increment in the rotational speed of the tool. The change in the corrosion rate at the WN for various specimens was influenced by the matrix grain size, quantity of second phase, area fraction of special boundary, and relative quantity of low-and high-angle grain boundaries.

Effect of the microstructure on the corrosion behavior of dissimilar friction stir-welded 304 austenitic stainless steel and Q235 low-carbon steel joints

“…In addition, the microstructure characterization results showed that, compared with BM 304 , the proportions of TBs in HAZ 304 and TMAZ 304 were significantly lower. Husain et al [46] reported a decrease in the proportion of TBs and the tendency of the sample to corrode. Therefore, the increase in the LAGBs proportion and the decrease in the TBs proportion were the main reasons why RS 304 had worse corrosion resistance than BM 304 .…”

Hydrogen-induced cracking and corrosion behavior of friction stir welded plates of API 5L X70 pipeline steel

“…Behjat et al [27] showed that the severity of attack BM was higher than that observed in SZ due to the high deriving force of galvanic corrosion because to the presence of the coarse microstructure and ferrite/pearlite structure, unlike of the SZ in which that the grain refinement, in the ferrite in bainite structure, during the FSW process decreased the galvanic corrosion. However, Husain et al [28] reported a lower corrosion resistance in the weld nugget than the base metal in medium strength steel due to ferrite grain size, second-phase fraction, the relative fraction of high and low-angle grain boundaries, fraction of Coincidence Site Lattice (CSL) boundaries, and residual strain. In which different corrosion mechanisms occurred, in the MB the corrosion attack was uniform, in contrary the weld nugget that occurred corrosion for pit.…”