Mechanical Propertics of Mild Steel Weld Metals Produced in Pressurized Ar-CO_2 Atmospheres

Mild steel sheets with V-groove were welded in pressurized Ar-CO2 gas mixture atmospheres up to 2.0 MPa, using a 1.6 mm diameter solid wire. The effect of ambient gas pressure and gas composition on the mechanical properties of weld metals were studied. The Vickers hardness and tensile properties of weld metals were varied with the ambient gas composition but did not depend on the ambient gas pressure. The Charpy V notch toughness of weld metal was changed markedly with CO2 partial pressure in the atmosphere, and showed a maximum at 0.1-0.2 MPa CO2 partial pressure which resulted in about 0.03% oxygen content of weld metal having a fine ferrite microstructure. This suggests that toughness is strongly influenced by the oxygen content of weld metal. Mechanical properties of pressurized steel weld metal can be controlled by CO2% in the welding atmosphere, because the oxygen content of weld metal is determined by CO2 partial pressure.