Microstructural variations in a high-strength structural steel weld under isoheat input conditions

Bead-in-groove submerged arc welding of quenched and tempered (Q & T) HSLA steel using a suitable welding wire and an agglomerated basic flux (basicity index = 3.1) was carried out under heat input conditions varying from 1.9 to 3.7 kJ/mm. The heat input was adjusted by varying the welding current and welding speed with the machine operated in the constant voltage mode (32-33 VDC). From several welds prepared using a range of currents (400-800 A) and speeds (3-13 mm/s), nine welds were selected. These welds represented those prepared under isoheat input conditions with different current and speed combinations. It was found all the parameters (namely, prior austenitic grain size, inclusion characteristics, cooling rate) influencing the volume fraction of intragranularly precipitated acicular ferrite in the weld showed significantly different dependence at a particular heat input depending upon the welding current and speed combination used. A new cooling rate parameter (NA/C1) based on the weld nugget cross-sectional area (NA) and the fusion zone/heat-affected zone interface length (Cl) were defined. Using multiple regression analysis, a correlation between acicular ferrite content and the different influencing parameters as mentioned above was defined having -90% correlation coefficient. This correlation can be utilized in setting up the trial welding parameters for similar grades of steel substrates and consumables with an aim to maximizing the acicular ferrite content