HAZ hardness in laser beam welds

The thermal profiles that result from a numerical model of laser beam welding are used to calculate the hardness in the heat-affected zone of two 10-mm-thick BS4360 50D steel welds. The numerical model assumes a cylindrical keyhole with the same radius as that of the incident laser beam. The temperature at the keyhole/liquid interface is assumed to be the vaporization temperature of the steel. The numerical model adopts liquid/solid boundary data as constraints on a system matrix representation of the equation of heat conduction. The influence of convection, the temperature dependence of material properties and any incomplete information concerning material properties, including the coefficient of surface tension of the liquid metal, is taken into account by the numerical model by means of constrained optimization of the system matrix elements. Thermal profiles were calculated across the fusion and heat-affected zones. The laser beam welds were full penetration welds and were made at two different laser powers at the test piece (4.5 and 9 kW) and two different welding speeds (5 and 20 mm/s), respectively. Hardness traverse were made across the fusion and heat-affected zones. The experimental hardness values were compared to calculated values. The calculated values were based on the chemistry of the steel and the calculated thermal profiles. The agreement between the experimental and calculated values is good.