| With high power carbon dioxide laser system, C-Mn structural steel for automobile application was welded. Bead-on-plated welding process was applied with 6㎜ thick hot rolled steel plates, and the welding conditions such as travel speed, position of focus and laser output power were 1.2m/min, l.0㎜ under the sample surface and 2.1㎾, respectively. Microstructural examinations and calculation indicated that the microstructrue of the laser weld, which experienced very high heating and cooling cycle during welding process, may not be predicted from the founds in the conventional arc process. Test results also demonstrated tha the austenite nucleation takes place at the pearlite-ferrite boundaries instead of the interface between ferrite and cementite in the pearlite. In the laser welding, the initiation and completion temperature of austenite transformation are much higher than that in the arc welding. Hardness of martensite in the HAZ varies from 635 Hv to 350 Hv because the homogenization of austenite is not complete even at the temperatures far above the A₃. |
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