Modeling macro-and microstructures of gas-metal-arc welded HSLA-100 steel

Fluid flow and heat transfer during gas-metal-are welding (GMAW) of HSLA-10 0 steel were studied using a transient, three-dimensional, turbulent heat t ransfer and fluid flow model. The temperature and velocity fields, cooling rates, and shape and size of the fusion and heat-affected zones (HAZs) were calculated. A continuous-cooling-transformation (CCT) diagram was computed to aid in the understanding of the observed weld metal microstructure. The computed results demonstrate that the dissipation of heat and momentum in the weld pool is significantly aided by turbulence, thus suggesting that pr evious modeling results based on laminar flow need to be re-examined. A com parison of the calculated fusion and HAZ geometries with their correspondin g measured values showed good agreement. Furthermore, "finger" penetration, a unique geometric characteristic of gas-metal-are weld pools, could be sa tisfactorily predicted from the model. The ability to predict these geometr ic variables and the agreement between the calculated and the measured cool ing rates indicate the appropriateness of using a turbulence model for accu rate calculations. The microstructure of the weld metal consisted mainly of acicular ferrite with small amounts of bainite. At high heat inputs, small amounts of allotriomorphic and Widmanstatten ferrite were also observed. T he observed microstructures are consistent with those expected from the com puted CCT diagram and the cooling rates. The results presented here demonst rate significant promise for understanding both macro-and microstructures o f steel welds from the combination of the fundamental principles from both transport phenomena and phase transformation theory.