No abstract

In this study, the weldability of the as-cast CM247LC superalloy for turbine blade applications was metallurgically evaluated in terms of its hot cracking behavior and susceptibility. For this purpose, a real blade was manufactured using a directional solidification casting process, and gas tungsten arc welding was performed at the tip and cavity of the upper blade. Hot cracking was confirmed in the heat-affected zone (HAZ) of gas tungsten arc welds, and the cracks were characterized as liquation cracks, since a cobble or dropletshaped crack surface consistent with a liquid film was clearly confirmed. Microstructural analysis of the cracking surface and thermodynamic calculations helped elucidate the metallurgical mechanisms of the liquation cracking. In other words, the cracking was attributed to liquation of the γ-γ’ eutectic colony and the constitutional liquation of the MC-type carbides: these phases existed in the as-cast microstructure. In particular, it was calculated that liquation of the γ-γ’ eutectic colony during welding occurs at least at 1488 K and that constitutional liquation of MC-type carbides begins at 1411 K, while the equilibrium solidus temperature of the CM247LC alloy is 1530 K. Finally, the liquation cracking susceptibility was quantitatively evaluated through a spot-Varestraint test, and it was confirmed for the first time that the higher susceptibility of as-cast samples can be suppressed by employing a pre-weld heat treatment such as solution treatment.

The liquation cracking temperature range (LCTR) of CM247LC was evaluated, especially in remelted and reheated weld metals produced during multi-pass repair welding. The LCTR of the remelted weld metal was 282 K, and those of the reheated weld metals were 455-805 K depending on the testing position. In the multi-bead within the reheated weld metal, the LCTR increased from 455 K far from the inter-pass boundary to 805 K near this boundary. The mechanism underlying the varying LCTR for the remelted and reheated weld metals was the amount of MC carbide at each weld position. The single-mode fibre laser welding suppressed the carbide within both areas, and lowered the liquation cracking susceptibility.

The metallurgical aspects of weld solidification cracking in Ni-based superalloys (with Ti+Al > 5 mass%) have not been widely investigated thus far. Herein, the solidification cracking susceptibility of the CM247LC superalloy and its welds with ERNiFeCr-2 filler wire was quantitatively evaluated using a novel modified Varestraint testing method, for the successful manufacturing of CM247LC superalloy gas turbine blades. It was found that the solidification brittle temperature range (BTR) of the CM247LC superalloy was 400 K. This measurement was obtained with a high-speed thermo-vision camera. The BTR increased to 486 K for the CM247LC/ERNiFeCr-2 welds (dilution ratio: 74%). Theoretical calculations (i.e., the Scheil equation, performed using Thermo-Calc software) were conducted to determine the temperature range in which both solid and liquid phases coexist, together with the microstructural characterization of the solidification cracking surfaces. The greater increase in BTR for the CM247LC/ERNiFeCr-2 welds than that for CM247LC was attributed to the enlargement of the solid–liquid coexistence temperature range. This correlated with the formation of a low-temperature Laves phase during the terminal stage of solidification, and was affected by the diluted Nb and Fe components in the ERNiFeCr-2 filler metal. Based on the experimental and theoretical results, the proposed modified Varestraint testing method for dissimilar welds is expected to be an effective testing process for solidification cracking behavior in the manufacturing of high-soundness CM247LC superalloy welds.

The metallurgical aspects of weld solidification cracking in Ni-based superalloys (with Ti+Al > 5 mass%) have not been widely investigated thus far. Herein, the solidification cracking susceptibility of the CM247LC superalloy and its welds with ERNiFeCr-2 filler wire was quantitatively evaluated using a novel modified Varestraint testing method, for the successful manufacturing of CM247LC superalloy gas turbine blades. It was found that the solidification brittle temperature range (BTR) of the CM247LC superalloy was 400 K. This measurement was obtained with a high-speed thermo-vision camera. The BTR increased to 486 K for the CM247LC/ERNiFeCr-2 welds (dilution ratio: 74%). Theoretical calculations (i.e., the Scheil equation, performed using Thermo-Calc software) were conducted to determine the temperature range in which both solid and liquid phases coexist, together with the microstructural characterization of the solidification cracking surfaces. The greater increase in BTR for the CM247LC/ERNiFeCr-2 welds than that for CM247LC was attributed to the enlargement of the solid–liquid coexistence temperature range. This correlated with the formation of a low-temperature Laves phase during the terminal stage of solidification, and was affected by the diluted Nb and Fe components in the ERNiFeCr-2 filler metal. Based on the experimental and theoretical results, the proposed modified Varestraint testing method for dissimilar welds is expected to be an effective testing process for solidification cracking behavior in the manufacturing of high-soundness CM247LC superalloy welds.