Abstract
The aim of this study concerns the effect of multi-pass shielded metal arc welding (SMAW) on the corrosion behavior of CW6MC cast nickel alloy. Using optical and SEM techniques the welded joint is analyzed. Vickers microhardness mapping and potentiodynamic polarization in NaCl and H2SO4 solutions are also evaluated. Both the Laves phase and NbC-type carbides are identified in the base metal (BM) and weld metal (WM) regions. The main microstructural difference observed between these regions is the morphology aspect and fineness of the dendritic arrays. The welding process promotes the finer columnar grains formation with refined intermetallic particles in the WM than equiaxed grains of the BM, which in turn results in higher microhardness values in the former region. However, no substantial changes were observed in the corrosion behavior between the BM and WM regions, considering both acid and saline media. Nevertheless, during the multi-pass SMAW process, some non-metallic micrometric inclusions (Mo and S-rich regions) can be constituted to occur in the WM region. This is associated with a significant drop in the corrosion performance of this region when the electrochemical tests are evaluated.
Highlights
Nickel-base superalloys are engineering materials with outstanding high-temperature stability, a good combination of mechanical properties and high pitting corrosion resistance [1,2,3]
(2) Since the microsegregation of alloying elements that take place during the solidification of both the base metal (BM) and weld metal (WM) promote the formation of the Laves phase and NbC-type carbides in the examined regions
The main microstructural difference observed between these two welding regions lies in the morphology and fineness of the dendritic grains, i.e., the directional heat extraction of the multi-pass Shielded Metal Arc Welding (SMAW) process promotes the formation of both fine columnar and dendritic grains in the WM
Summary
Nickel-base superalloys are engineering materials with outstanding high-temperature stability, a good combination of mechanical properties and high pitting corrosion resistance [1,2,3]. The aforementioned stainless steel may lack the required corrosion resistance in these environments, being considered economically unviable in some cases In such applications, most of the components have complex geometries and are obtained from casting processes (e.g., pumps and valves). The majority of defects found in the casting are designated as inclusions (particles of impurities) and porosities (small holes) These may be corrected by Shielded Metal Arc Welding (SMAW) process. SMAW is the most used processing in the industry due to the low complexity of its equipment, high versatility besides the low cost of consumables (i.e., filler metal) Another issue in nickel cast alloys is the occurrence of cracking during the welding process. These cracks are commonly associated with the chemical composition evolution during the welding, which favors the formation of eutectic with a low melting point, segregating the alloying elements to the grain boundaries or to the interdendritic regions that act as preferred locations for the crack formation [3,9]
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