Improvement of heat affected zone in multi-pass narrow gap submerged arc welding

The study emphasizes the effects of multi-pass narrow gap (NG) Submerged-Arc welding (SAW) on the Heat Affected Zone (HAZ). The differences between the HAZ of single-pass welds and multi-pass welds are radical and are caused by the subsequent thermal cycles which only exist in multi-pass welding.;Theoretical and experimental analyses indicate that the HAZ's in multi-pass welds can be improved significantly by the welding process itself. However, multi-pass welding does not always improve its HAZ. To realize the improvement, some special criteria must be met. The most important parameters are heat input, welding speed, deposit thickness and inter-pass temperature. This study establishes the relations between these parameters and the effects of HAZ refinement. There are some domains of the parameters in which HAZ refinement can be realized. The research reveals that in multi-pass NG welding, a set of welding parameters can always be found to fulfill the conditions for HAZ improvement.;Real welding processes, with welding parameters optimized according to the analyses, were performed and a totally refined HAZ was achieved under laboratory conditions. It is suggested that the method can be applied to in situ welding situations.;High heat input does not necessarily lead to inferior microstructural and mechanical properties in multi-pass NG welding. As long as the conditions for HAZ refinement are satisfied, an improved HAZ will be obtained no matter how high the heat input is. This also leads to the conclusion that the HAZ in multi-pass NG welding is less sensitive to heat input than that in single pass welding.;A series of experiments were carried out to establish the relationship between the welding thermal cycles and the mechanical and microstructural properties of a HAZ. Both real and Gleeble simulated HAZ's were investigated.;The tempering parameter, which has been used to evaluate the tempering effects at constant temperature, is successfully introduced into welding (non-isothermal) conditions. The effects of precipitates in 2.25Cr-1Mo steel are theoretically investigated.