ESSC & DUPLEX 2019

The laser welding process is characterized by a high heating and cooling rate as well as large temperature gradients. The dwell time between heating and cooling is minimal. Especially when welding thin-walled (d ≤ 1 mm) duplex stainless steels, the austenite content in the joint zone is reduced. However, a balanced phase ratio of ferrite and austenite combines high levels of mechanical stability with outstanding resistance to corrosion. During laser welding, duplex stainless steels suffer from profound microstructural transformations, which influence the distribution and phase balance in the different areas of the weld seam and can lead to a loss of mechanical and corrosion resistance. For austenite formation in the joining zone and heat-affected zone, there are several model approaches which allow an estimate of the amount of austenite in the weld metal. The suitability of these models, especially for laser welding, is low, since the influence of the extremely high cooling rates is only insufficiently taken into account. The higher the cooling rate, the faster the conversion range is crossed and the less ferrite is converted to austenite. Furthermore, a three-dimensional heat flow is often the basis, but with thin-walled structures a two-dimensional heat flow can be assumed. This paper addresses the effect of process parameters on the cooling rate and thus directly on the phase balance between austenite and ferrite in the joining zone the grade 2205. Conclusions on the transformation behavior were obtained from the experimental welding tests and the modeling of the joining zone during the welding process. By means of metallography, scanning electron microscopy (SEM) and X-ray diffractometer (XRD), the weld seams were analyzed. The results are presented on the basis of a model, from which a direct estimate of the expected phase ratio from the process parameters is possible. By formulating boundary conditions, the model captures the extremely high cooling rates of the laser welding process. Based of the analysis the importance of the individual process parameters on the austenite transformation are shown.