Product Code: ICAL06_104

Effect of Alloying Elements on Porosity Formation in Laser Welding of Heavy Section Steel Plates
Authors:
Susumu Tsukamoto, National Institute for Materials Science; Tsukuba, Ibaraki Japan
Goro Arakane, National Institute for Materials Science; Tsukuba, Ibaraki Japan
Kazuhiro Kojima, Nippon Steel; Futtsu, Chiba Japan
Tadayuki Otani, Nippon Steel; Futtsu, Chiba Japan
Shigeru Ohkita, Nippon Steel; Futtsu, Chiba Japan
Presented at ICALEO 2006

Effect of some alloying elements on the porosity formation has been investigated in full penetration laser welding of 12 mm thickness C-Si-Mn steels using a 20 kW CO2 laser. Formation process of the porosity was directly observed using a micro-focused x-ray transmission imaging system. In the back surface atmosphere of N2-Ar or N2-O2, nitrogen is absorbed from the back surface and then supersaturated nitrogen forms the bubble in the molten pool. Some alloying elements affect the porosity formation. With increasing C and Ni contents in the base metal, the porosity is less likely to occur. This is attributed to change in the solidification mode from full ferritic to ferritic-austenitic or austenitic-ferritic with increasing both elements. Higher solubility of the nitrogen in the austenite phase suppresses the porosity in high C or Ni steels. On the other hand, S enhances the porosity formation. This must be caused by reduction of surface tension in high S steels. The critical nitrogen pressure to form the bubble in the molten pool decreases with increasing the S content.