Investigation of Weldability in High-Cr Ni-base Filler Metals
High-Cr Ni-base alloys have been used in the nuclear industry for weld overlay repair of dissimilar metal welds which have been affected by primary water stress corrosion cracking. During welding, defects may result from super-solidus and sub-solidus cracking phenomena. The ability to evaluate and quantify the degree to which a given alloy is susceptible to each cracking mechanism is of great importance in alloy selection, alloy development and weld design. The first aim of this project was to improve the reliability of the cast pin tear test (CPTT) as a tool for evaluation of solidification cracking susceptibility. The second aim was to evaluate the weldability of a new heat of Inconel alloy 52MSS (ERNiCrFe-13) with reduced iron content as well as a single heat of Kobelco 690NB (ERNiCrFe-7A) comparing these materials with other available alloys of the same type.The incorporation of levitation melting has enabled precise control of the casting process in the CPTT. Active temperature monitoring during melting enables control of the casting temperature with a standard deviation of 8.8 C. Gradual reduction in the power applied to the coil has enabled focused transfer of the charge into the mold. Thermocouple measurements indicated that the cooling rate during solidification in the CPTT was approximately twice that which is experienced in GTAW welding using parameters typical for use in weld overlay repairs. Finite element simulations indicated that a significant reduction in cooling rate of the casting could be obtained by altering the mold material, mold outer diameter or cast pin diameter. Prototype mold screening tests enabled design of a mold that provides a cooling rate of 160 C/s based on FEA modeling, compared with the previous cooling rate of 245 C/s, while maintaining feasibility of the CPTT. A longer pin length is required to cause cracking with the redesigned molds when compared with the previous molds. Also, significant crack healing with Nb-rich eutectic-like constituents was observed in pins cast from alloy 52MSS in the new molds. A decrease in strain rate most likely accompanies the decrease in cooling rate which would allow increased time for the flow of fluid to enable crack healing in alloys which exhibit a sufficient fraction eutectic. The solidification cracking susceptibility in the heat of alloy 52MSS with reduced iron content (52MSS-E) was evaluated and compared with alloy 690NB. 52MSS-E was determined to have a wider solidification temperature range based on thermodynamic modeling and single-sensor differential thermal analysis. This indicates that 52MSS-E may be more susceptible to solidification cracking than alloy 690NB. Transvarestraint testing showed that the maximum crack distance (MCD) was greater in 52MSS-E at 3%-7% augmented strain. CPTT also indicated that alloy 52MSS-E is more susceptible to solidification cracking than alloy 690NB. All tests, therefore, indicate that alloy 52MSS-E is more susceptible to solidification cracking than alloy 690NB. Alloys 690NB, 52MSS-E and 52i-B were evaluated based on their susceptibility to ductility dip cracking (DDC) using the strain to fracture test at 950 C. Alloys 690NB and 52MSS-E were found to have an equivalent threshold strain of 8% for DDC to occur, indicating similar resistance to DDC. Alloy 52i-B was found to have a threshold strain of 14% indicating superior resistance to DDC compared with 690NB and 52MSS-E.
Materials Science, Metallurgy, weld overlay, nickel, dissimilar metal weld, inconel, weld cracking, solidification cracking, ductility dip cracking, cast pin tear test, weldability testing
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