| Low carbon steel was welded with martensitic stainless steel by CO₂ laser and microstructures, mechanical and corrosion properties were compared with those obtained by GTA welding method. In laser welding process, since more heat was transfered through the low carbon steel side compared with the stainless steel due to the difference of thermal conductivities between two steels, the size of grain growth region in the heat-affected area was larger in the low carbon steel side as compared to that in the stainless steel side. Meanwhile, since more heat was accumulated in the stainless steel side of the weld pool compared with the low carbon steel, more clear dendritic structures were developed in stainless steel side, and its cooling rate was calculated to 6500˚K/sec. When as-received steels were welded by laser beam, porosities were formed in the weld pool and in the boundary region between weld pool and low carbon steel side. Meanwhile, if steels were heat-treated before laser welding, porosities were not formed. In tension tests, laser welding resulted in a little larger tensile strength and elongation compared with those of GTA welding. In Rever bending tests, laser welding resulted an increase of about 10-50% in bending repetition number compared to that in GTA welding, and fractures were occured in the grain growth region of low carbon steel side for both welding processes. Immersion tests of welded specimens in ferric chloride solution resulted that pitting corrosion took place in the boundary region between low carbon steel and weld pool. Corrosion rate was smaller in laser welded specimens compared to GTA welded ones for non-heat treated specimens, and the results were reversed for heat-treated specimens. |
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