Effect of Different Chromium Additions on the Microstructure and Mechanical Properties of Multipass Weld Joint of Duplex Stainless Steel

The correlation between the mechanical properties and ferrite volume fraction (approximately 40, 50, and 60 Ferrite Number [FN]) in duplex stainless steel weld metals were investigated by changing the Cr content in filler wires with a flux-cored arc-welding (FCAW) process. The interpass temperature was thoroughly maintained under a maximum of 423 K (150 A degrees C), and the heat input was also sustained at a level under 15 KJ/cm in order to minimize defects. The microstructure examination demonstrated that the delta-ferrite volume fraction in the deposited metals increased as the Cr/Ni equivalent ratio increased, and consequently, chromium nitride (Cr2N) precipitation was prone to occur in the ferrite domains due to low solubility of nitrogen in this phase. Thus, more dislocations are pinned by the precipitates, thereby lowering the mobility of the dislocations. Not only can this lead to the strength improvement, but also it can accentuate embrittlement of the weld metal at subzero temperature. Additionally, the solid-solution strengthening by an increase of Cr and Mo content in austenite phase depending on the reduction of austenite proportion also made an impact on the increase of the tensile and yield strength. On the other hand, the impact test (at 293 K, 223 K, and 173 K [20 A degrees C, -50 A degrees C, and -100 A degrees C]) showed that the specimen containing about 40 to 50 FN had the best result. The absorbed energy of about 40 to 50 J sufficiently satisfied the requirements for industrial applications at 223 K (-50 A degrees C), while the ductile-to-brittle transition behavior exhibited in weldment containing 60 FN. As the test temperature decreased under 223 K (-50 A degrees C), a narrow and deep dimple was transformed into a wide and shallow dimple, and a significant portion of the fracture surface was occupied by a flat cleavage facet with river patterns.