Effect of Pressure on Occurrence of Porosity

On Gravity Welding under High Pressure Atmosphere (The 3rd Report)

Abstract

The effect of ambient pressure on the occurrence of porosity in weld metal of mild steel by gravity welding process was investigated. Welding was executed under the pressure range from 0.1 to 5.1 MPa (abs.). Ilmenite type (D4301), high titanium oxide type (D4313) and low hydrogen type (D4316) electrodes of 4 mm in diameter and SM41A steel base plates of 6 and 9 mm in thickness were used.
Main results obtained are summarized as follows:
1) Pores occur over the pressure of 0.15 MPa by D4301 and D4313 electrodes. Pores tend to increase with an increasing pressure up to about 0.3 MPa and to decrease over 0.4 MPa. Pores are not observed over 0.7 MPa.
2) The contour of pore is relatively short and cylindrical under 0.15 MPa, long, thick and cylindrical under 0.2 to 0.3 MPa, horn type under 0.4 to 0.5 MPa and thin and cylindrical under 0.6 MPa.
3) The solidification distance from weld bond line to pore, Xp, tends to decrease with an increasing pressure up to 0.3 MPa and to increase over 0.3 MPa.
4) Diffusible hydrogen content in weld metal by D4316 electrode increases with an increasing pressure up to 1 MPa, then it has a tendency to decrease a little over 1 MPa.
5) Content of hydrogen in molten metal by D4313 electrode, C₀, is seemed to increase with pressure up to 0.3 MPa and then it tends to decrease. The value of C₀^* is thought to be larger than the critical concentration of hydrogen C_cr^* under the pressure range from 0.13 MPa to 0.65 MPa, therefore pores occur in weld metal in this pressure range.