Weld metal/ferritic steel interface in laser welded austenitic/ferritic dissimilar steel joints

Abstract

Austenitic/ferritic dissimilar steel joints are widely used in power generation systems. Such joints are normally produced using conventional welding processes such as tungsten inert gas welding and manual arc welding. Research and application experiences have proved that nickel based filler is preferable for producing an austenitic/ferritic joint when using conventional welding processes [1-5]. Laser beam welding has, in recent years, attracted more attention due to its special features: a small heat-affected zone (HAZ) and narrow weld bead due to the low heat input; welding at high speed; welding can be carried out in areas of difficult access; contactless energy transfer; welding in an exact and reproducible manner; possibilities for automation and robotization, and welding performed in various atmospheres [6]. Industrial application potentials of laser welding are being actively investigated worldwide. Laser welding of dissimilar metals has also been a topic of interest recently. The possibility of using a laser to weld austenitic/ferritic dissimilar metal joints and the effects of processing parameters have been reported [7, 8]. Weld metal/ferritic steel interfacial microstructure and properties have been a critical issue fo r austenitic/ferritic dissimilar steel joints. Since it is the weak point of the joints, considerable efforts have been devoted to characterize it and to understand its influence on the quality of the joints made by conventional welding processes [9-13]. Data for such interfaces of laser welding joints is, however, limited, but still very important. In the present study, a high power CO2 laser was used to join an austenitic stainless steel to a low alloy ferritic steel. The joints were produced in two modes: autogenous welding (without filler) and welding with nickel based filler wire. The interfacial microstructure between the weld metal and ferritic steel was examined in both the as-welded and post-weld heat treated conditions. The objective of this letter is to report some results concerning the interface made by laser welding in order to provide useful knowledge for assessing the process. Base materials were AISI 347 austenitic and 13CrMo44 low alloy steel tubes with 43.5 mm outside diameter, 4.5 mm wall thickness and length of 23 mm. The chemical compositions of the base materials are listed in Table I. The filler wire used in the present study was commercial nickel based ENiCrMo-3 of diameter 1.2 mm. Laser welding was performed using a continuous wave CO2 laser. Plasma control and shielding gases (helium) at flow rates of 20 and 32 lmin -1, respectively, were used. The welding nozzle, wire feed and workpiece arrangement is shown schematically in Fig. 1. Welding parameters are given in Table II. The tube/tube joints were cut after welding, and some of them were heat treated at 650 °C for 1 h. The metallographic samples were prepared using standard procedures including grinding, polishing and etching. Optical and electron microscopy were used, with energy dispersive X-ray spectroscopy (EDS) and electron probe microanalysis (EPMA) for analysing major alloying elements.