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The feasibility of gravity welding and welding phenomena under pressurized argon atmosphere (up to 5.1 MPa abs.) were investigated. The ilmenite type, high titanium oxide type and low hydrogen type electrodes of 4 mm in diameter were used. As base metals, SM41A steel plates of 6 and 9 mm in thickness were used. Main results obtained are summarized as follows : 1) Sound weld can be obtained under pressurized argon atmosphere up to 5.1 MPa by low hydrogen type electrode. Porosity and pits are not observed in the weld, but they occur by the other type electrode in the pressure range of 0.15 MPa to 0.7 MPa, disappearing when pressure is over 1.1 MPa. 2) The suitable welding current to obtain sound welds increases with an increasing ambient pressure. 3) Butt welded joints obtained by low hydrogen type electrodes under the pressure up to 5.1 MPa have sufficient tensile strength and impact value. However, they tend to decrease with an increasing pressure. 4) The maximum voltage, V_<max>, and the mean voltage, V_<mean>, increase with an increasing pressure up to 2.1 MPa. However, they have a tendency to decrease over 2.1 MPa. The minimum voltage, V_<min>, decreases with an increasing ambient pressure. 5) The melting rate of electrode decreases with an increasing pressure up to 0.6 MPa, while it increases when the pressure is beyond 0.6 MPa. 6) The transfer period of droplet from electrode tip increases with an increasing pressure up to 0.6 MPa, while it decreases when the pressure is over 0.6 MPa. 7) The diameter of droplet increases with an increasing pressure up to 0.6 MPa, while it decreases when the pressure exceeds 0.6 MPa.