Microstructures of gas metal arc weld metal of 950 MPa class steel

The effects of shielding gas composition on the properties and microstructure of single pass weld metals produced by GMA ( gas metal arc) groove welding of 950 MPa class steel plates have been investigated. The shielding gas employed was a mixture of argon (Ar) and carbon dioxide (CO2) ( 0 - 25%), and the weld heat input was similar to 3 kJ mm. With increasing CO2 content, the hardness of the weld metal decreased from 380 HV to 280 HV, and the absorbed energy of the Charpy impact test decreased from 130 J to 90 J. The microstructures of the weld metal, consisting primarily of low carbon martensite and carbide free bainite, became more bainitic as the CO2 content of the shielding gas was increased. It was also found that the MA constituent, embrittling microstructure, was formed in the granular bainitic area, the volume fraction of which increased with the CO2 content of the shielding gas.