Enhancement of corrosion protection performance of SUS304/Q235B dissimilar metals lap joint through fiber laser

Mechanical bonded SUS304/Q235B bimetallic pipes have been widely used as subsea oil and gas transmission pipeline. Due to the complexity of the subsea environment, the thin inner layer of SUS304 might crinkle and swell locally and then separated from the base layer of Q235B. In this work, lap welding inside the SUS304/Q235B bimetallic pipes was investigated with special emphasis on the corrosion resistance of the near surface region of the lap joint. Nothing the significant difference of weld width between its upper part and lower part of typical laser welded “wine-glass” type bead, laser welding was employed here to reduce the dilution of alloy elements in the fusion zone of inner stainless steel layer. The results showed that content of major alloy elements such as chromium (Cr) and nickel (Ni) in the near surface region of fusion zone could be kept as about 14 and 6 wt.%, respectively, by optimizing the laser lap welding process parameters. The laser welded SUS304/Q235B overlap joints had excellent shear strength, and the shear fracture was consisted of numerous dimples. Finally, laser filler wire cladding of SUS316 stainless steel on the top surface of previously achieved overlap joint was carried out to ensure that upper part of the weld zone had enough corrosion resistance.