| Abstract Scope |
Pipeline failure due to corrosion is a common problem in the oil and gas and petrochemical industry. The application of corrosion-resistant weld overlays (WOLs) is a cost-effective way to prevent such failures. Hot wire gas tungsten arc welding (HW-GTAW) is a popular WOL process, however, the process’s high heat input increases dilution resulting in multiple clad layers needed to meet minimum iron requirements. Research pertaining to the nuclear industry indicates the low heat input cold metal transfer (CMT) process can produce WOLs which corrode ten times slower than overlays produced with CW-GTAW, with up to four times higher deposition rates.
A design of experiments approach was used to create overlays of nickel-base alloys 625, 686 and 825 onto low alloy steel X65 with CMT. Bead geometry, heat input, deposition rate, and the presence of lack of penetration defects from each sample were measured and categorized to identify parameter sets that produced welds adhering to certain criteria such as low dilution, moderate toe angles, and lack of defects. CMT overlays were then compared to HW-GTAW with respect to presence of defects, heat affected zone and weld metal hardness, microstructural characteristics such as dilution and arm spacing, compositional gradients, and corrosion resistance.
Analysis of CMT sample microstructure revealed smaller planar growth regions, less swirls as well as smaller swirls, and smaller secondary dendrite arm spacing as compared with HW-GTAW. These characteristics respectively contribute to a lower potential for solidification cracking, a lower potential for hydrogen assisted cracking (HAC), and increased corrosion resistance. Microhardness maps revealed CMT produced samples with lower heat affected zone (HAZ) hardness values along with higher weld metal (WM) hardness values. Higher WM hardness increases erosion resistance and lower HAZ hardness decreases the need for PWHT. Overlays of all three filler metals were constructed using both processes. Specimens extracted from the CMT overlays passed ASME Section IX bend tests. The bend tests were validated using digital image correlation (DIC) as lack of strain concentration along fusion boundaries indicated proper fusion had occurred. EDS traverses across fusion boundaries showed a transition from base metal to minimally diluted filler metal within tens of micrometers for CMT as compared with millimeters for HW-GTAW. Compositional comparisons from EDS traverses parallel to fusion boundaries and top surfaces showed nearly identical compositions for CMT at both locations but not for HW-GTAW. Corrosion testing following ASTM G48 found critical pitting temperatures (CPT) of 60°C, 85°C, and 20°C, respectively, for alloy 625, 686, and 825 single-layer overlay samples created using CMT. Double-layer HW-GTAW samples tested at the same temperatures as their CMT counterpart had deeper maximum pitting as well as greater overall weight loss.
CMT and HW-GTAW weld overlays created using optimized weld parameters were evaluated with respect to presence of defects, heat affected zone and weld metal hardness, microstructural characteristics such as dilution and arm spacing, compositional gradients, and corrosion resistance. Through comparative analyses, CMT was determined a superior WOL production process to HW-GTAW as CMT increases structural integrity and decreases cost with its ability to produce defect-free, corrosion-, erosion-, and cracking-resistant WOLs at higher deposition rates and with less filler material. |