Abstract
An investigation has been made of the effects of heat input and shielding gas composition on the properties and microstructure of welds which was produced on 950 MPa class high strength steel by gas metal arc welding. The microstructure of the weld obtained, consisting principally of martensite and bainite, was altered from martenstic to bainitic, as the CO_2 content in the shielding gas of Ar was increased from 0 to 25%. The effect of the increase in the heat input from 2.5 to 4.5 kJ/mm on the microstructure exhibited a similar tendency, but the observed difference was less significant. The weld metal produced by shielding gas of less than 10% CO_2 contents at heat inputs less than 4 kJ/mm had superior mechanical properties. It was also found that the M-A constituent, embrittlement microstructure, formed in the granular bainitic area, whose volume fraction was increased with the CO_2 content and heat input.
