An algorithm for heat transfer prediction of in-service welding operations in a forced flow of fluid is presented. The algorithm presented is derived from Rosenthal’s 3D heat flow equation and boundary layer approximations. This was possible by the introduction of an apparent thermal conductivity , which is a function of the boundary layer’s heat transfer coefficient and the base material’s thickness . This implies that a weld cooling time in a forced flow of fluid can now be calculated by an ordinary engineering calculator and thus enabling suitable welding parameters to be determined. The magnitude of was established by regression analysis of results from a parametric finite element analysis series of a total number of 112 numerical simulations. Furthermore, the result of the regression analysis was validated and verified by a welding experiment series accomplished on an in-house designed and constructed in-service welding rig. The principle design of the welding rig as well as its instrumentation, a PC based Data Acquisition system, is described. In addition, a method to measure the weld metals cooling time by means of thermocouple elements is described. Finally, the algorithm presented in this study proved feasible for industrial in-service welding operations of fine-grained Carbon and Carbon–Manganese steels with a maximum Carbon Equivalent (IIW) (CE) of 0.32.