Korean Journal of Metals and Materials, Feb 5, 2023
In this study, the liquation cracking temperature range (LCTR) of over-aged 247LC superalloy repa... more In this study, the liquation cracking temperature range (LCTR) of over-aged 247LC superalloy repair welds was quantitatively evaluated using spot-Varestraint testing, especially for application in the repair weld heat-affected zone (HAZ). The metallurgical mechanism of the evaluated liquation cracking susceptibility and variation of the LCTRs of the over-aged 247LC superalloys was clarified by analyzing the microstructural characteristics at the liquation cracking surface, and using thermodynamic calculations. From the spot-Varestraint tests of the over-aged materials, the LCTR was quantitatively evaluated to be 410 and 620 K for the 500 and 1000 h over-aged specimens, respectively. These results demonstrate that the overaged 247LC superalloys face significant and serious liquation cracking susceptibility in repair weld HAZ. Based on local element distributions at the liquation cracking surface, analyzed by transmission electron microscopy and the Thermo-Calc calculations, the underlying metallurgical mechanism of high LCTRs of overaged materials could be attributed to the precipitation of fine MC and M 6 C carbides during the over-aging treatment. The carbides promote local liquation at the interface between the gamma phase at temperatures lower than the equilibrium solidus (1530 K) of the 247LC. The estimated liquation initiation temperatures at the MC/γ and γ/M 6 C interfaces were 1125 and 1420 K, respectively.
Korean Journal of Metals and Materials, Aug 5, 2018
Martensitic 10Cr1MoW steels are widely used for high temperature components in turbine engines be... more Martensitic 10Cr1MoW steels are widely used for high temperature components in turbine engines because they have good mechanical properties and corrosion resistance. Delta-ferrite can form locally in large ingots and the ferrite phase degrades the metal's high-temperature properties. To decrease the fraction of delta-ferrite the solidification process should be controlled. In this paper, directional solidification experiments were carried out with martensitic 10Cr1MoW steel at various growth velocities, 5 μm/s-50 μm/s. The fraction and size of the delta ferrite declined with increasing growth velocity, and also declined with increasing distance from the solid/liquid interface of the directionally solidified sample. These results are discussed in relation to the dendrite spacing of the delta primary phase, and precipitation of the gamma phase with solidification rates. As the solidification velocity decreased and the distance from the solid/liquid interface increased, the fraction of delta ferrite increased.
This article is an open access article distributed under the terms and conditions of the Creative... more This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY
Korean Journal of Metals and Materials, Dec 5, 2020
In this study, the weldability of the as-cast CM247LC superalloy for turbine blade applications w... more 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.
This article is an open access article distributed under the terms and conditions of the Creative... more This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY
In this study, the liquation cracking temperature range (LCTR) of over-aged 247LC superalloy repa... more In this study, the liquation cracking temperature range (LCTR) of over-aged 247LC superalloy repair welds was quantitatively evaluated using spot-Varestraint testing, especially for application in the repair weld heat-affected zone (HAZ). The metallurgical mechanism of the evaluated liquation cracking susceptibility and variation of the LCTRs of the over-aged 247LC superalloys was clarified by analyzing the microstructural characteristics at the liquation cracking surface, and using thermodynamic calculations. From the spot-Varestraint tests of the over-aged materials, the LCTR was quantitatively evaluated to be 410 and 620 K for the 500 and 1000 h over-aged specimens, respectively. These results demonstrate that the over-aged 247LC superalloys face significant and serious liquation cracking susceptibility in repair weld HAZ. Based on local element distributions at the liquation cracking surface, analyzed by transmission electron microscopy and the Thermo-Calc calculations, the unde...
The metallurgical aspects of weld cracking in Ni-based superalloys remain relatively unexplored i... more The metallurgical aspects of weld cracking in Ni-based superalloys remain relatively unexplored in existing research. The present study performed comprehensive metallurgical and manufactural investigations into the weldability of an Ni-based superalloy, CM247LC, from the viewpoint of the liquation cracking behavior and its susceptibility. Metallurgical solutions to suppress the liquation-cracking susceptibility were derived via the visualization-based Varestraint test, and the possibility of liquation crack-free welding was explored by employing pre-weld heat treatments and laser beam welding. The alloy that was subjected to aging treatment exhibited the lowest liquation-cracking susceptibility (liquation cracking temperature range: 66 K), while the as-cast alloy specimen exhibited the highest liquation-cracking susceptibility (liquation cracking temperature range: 620 K). The metallurgical mechanisms of the liquation cracking susceptibility of as-cast CM247LC weld were elucidated v...
The Korean Institute Of Metals And Meterials, Aug 1, 2020
The effect of aging heat treatment conditions on the mechanical properties and microstructure of ... more The effect of aging heat treatment conditions on the mechanical properties and microstructure of the base and weld metal of Alloy 282 superalloy was investigated. The aging heat treatment conditions employed in this study were as follows: two-steps (1010 °C for 2 hours plus 788 °C for 8 hours) and one-step at 788 °C, 738 °C, and 688 °C for 4 hours. The base metal with the one-step aged condition exhibited lower hardness but longer creep life than that treated with the two-step aged condition. The base metal subjected to the two-step aging exhibited the highest hardness and the shortest creep life, mainly due to the precipitation of Mo-rich M6C-type carbides and coarse γ'' (Ni3(Al,Ti)) phase. For the weld metal, regardless of aging heat treatment conditions, creep elongation and life decreased significantly compared to the base metal, due to the coarse effective grain size and inhomogeneous distribution of γ'' precipitates. (Received May 27, 2020; Accepted July 01, 2020)
Abstract The high-temperature oxidation resistances of Ni-based superalloys with the compositions... more Abstract The high-temperature oxidation resistances of Ni-based superalloys with the compositions of Ni–(0–15)Co–(8–15)Cr–(0–5)Mo–(0–10)W–(3–8)Al–(0–5)Ti–(0–10)Ta–0.1C–0.01B were analyzed using an artificial neural network (ANN). The oxidation resistances of the alloys were evaluated based on the weight change measured during cyclic oxidation tests. An ANN was constructed with the contents of the alloying elements as input and mass gains as output. The ANN provided highly accurate regression results. The present results were compared with those obtained using response surface methodology (RSM) in a previous study. The regression model of the ANN could effectively detect the effect of an additional alloying element explicitly. The main and interaction effects of the alloying elements were plotted based on the results with random compositions. The optimum composition of superalloys with the highest oxidation resistance at high temperatures was determined using the trained ANN.
The effects of thermal gradient and solidification rate on the dendrite arm spacing and carbide m... more The effects of thermal gradient and solidification rate on the dendrite arm spacing and carbide morphology were investigated in directionally solidified Ni-base superalloy, CM 247LC. Thermal gradient was controlled by changing the position of the cold chamber and the furnace set temperature. The interface morphology changed from the planar to dendritic as increasing solidification rate. It was found that the dendrite spacing decreased as increasing the thermal gradient as well as the solidification rate. Also, as increasing solidification rate, carbide morphology changed from blocky shape to script and spotty shapes.
Korean Journal of Metals and Materials, Feb 5, 2023
In this study, the liquation cracking temperature range (LCTR) of over-aged 247LC superalloy repa... more In this study, the liquation cracking temperature range (LCTR) of over-aged 247LC superalloy repair welds was quantitatively evaluated using spot-Varestraint testing, especially for application in the repair weld heat-affected zone (HAZ). The metallurgical mechanism of the evaluated liquation cracking susceptibility and variation of the LCTRs of the over-aged 247LC superalloys was clarified by analyzing the microstructural characteristics at the liquation cracking surface, and using thermodynamic calculations. From the spot-Varestraint tests of the over-aged materials, the LCTR was quantitatively evaluated to be 410 and 620 K for the 500 and 1000 h over-aged specimens, respectively. These results demonstrate that the overaged 247LC superalloys face significant and serious liquation cracking susceptibility in repair weld HAZ. Based on local element distributions at the liquation cracking surface, analyzed by transmission electron microscopy and the Thermo-Calc calculations, the underlying metallurgical mechanism of high LCTRs of overaged materials could be attributed to the precipitation of fine MC and M 6 C carbides during the over-aging treatment. The carbides promote local liquation at the interface between the gamma phase at temperatures lower than the equilibrium solidus (1530 K) of the 247LC. The estimated liquation initiation temperatures at the MC/γ and γ/M 6 C interfaces were 1125 and 1420 K, respectively.
Korean Journal of Metals and Materials, Aug 5, 2018
Martensitic 10Cr1MoW steels are widely used for high temperature components in turbine engines be... more Martensitic 10Cr1MoW steels are widely used for high temperature components in turbine engines because they have good mechanical properties and corrosion resistance. Delta-ferrite can form locally in large ingots and the ferrite phase degrades the metal's high-temperature properties. To decrease the fraction of delta-ferrite the solidification process should be controlled. In this paper, directional solidification experiments were carried out with martensitic 10Cr1MoW steel at various growth velocities, 5 μm/s-50 μm/s. The fraction and size of the delta ferrite declined with increasing growth velocity, and also declined with increasing distance from the solid/liquid interface of the directionally solidified sample. These results are discussed in relation to the dendrite spacing of the delta primary phase, and precipitation of the gamma phase with solidification rates. As the solidification velocity decreased and the distance from the solid/liquid interface increased, the fraction of delta ferrite increased.
This article is an open access article distributed under the terms and conditions of the Creative... more This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY
Korean Journal of Metals and Materials, Dec 5, 2020
In this study, the weldability of the as-cast CM247LC superalloy for turbine blade applications w... more 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.
This article is an open access article distributed under the terms and conditions of the Creative... more This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY
In this study, the liquation cracking temperature range (LCTR) of over-aged 247LC superalloy repa... more In this study, the liquation cracking temperature range (LCTR) of over-aged 247LC superalloy repair welds was quantitatively evaluated using spot-Varestraint testing, especially for application in the repair weld heat-affected zone (HAZ). The metallurgical mechanism of the evaluated liquation cracking susceptibility and variation of the LCTRs of the over-aged 247LC superalloys was clarified by analyzing the microstructural characteristics at the liquation cracking surface, and using thermodynamic calculations. From the spot-Varestraint tests of the over-aged materials, the LCTR was quantitatively evaluated to be 410 and 620 K for the 500 and 1000 h over-aged specimens, respectively. These results demonstrate that the over-aged 247LC superalloys face significant and serious liquation cracking susceptibility in repair weld HAZ. Based on local element distributions at the liquation cracking surface, analyzed by transmission electron microscopy and the Thermo-Calc calculations, the unde...
The metallurgical aspects of weld cracking in Ni-based superalloys remain relatively unexplored i... more The metallurgical aspects of weld cracking in Ni-based superalloys remain relatively unexplored in existing research. The present study performed comprehensive metallurgical and manufactural investigations into the weldability of an Ni-based superalloy, CM247LC, from the viewpoint of the liquation cracking behavior and its susceptibility. Metallurgical solutions to suppress the liquation-cracking susceptibility were derived via the visualization-based Varestraint test, and the possibility of liquation crack-free welding was explored by employing pre-weld heat treatments and laser beam welding. The alloy that was subjected to aging treatment exhibited the lowest liquation-cracking susceptibility (liquation cracking temperature range: 66 K), while the as-cast alloy specimen exhibited the highest liquation-cracking susceptibility (liquation cracking temperature range: 620 K). The metallurgical mechanisms of the liquation cracking susceptibility of as-cast CM247LC weld were elucidated v...
The Korean Institute Of Metals And Meterials, Aug 1, 2020
The effect of aging heat treatment conditions on the mechanical properties and microstructure of ... more The effect of aging heat treatment conditions on the mechanical properties and microstructure of the base and weld metal of Alloy 282 superalloy was investigated. The aging heat treatment conditions employed in this study were as follows: two-steps (1010 °C for 2 hours plus 788 °C for 8 hours) and one-step at 788 °C, 738 °C, and 688 °C for 4 hours. The base metal with the one-step aged condition exhibited lower hardness but longer creep life than that treated with the two-step aged condition. The base metal subjected to the two-step aging exhibited the highest hardness and the shortest creep life, mainly due to the precipitation of Mo-rich M6C-type carbides and coarse γ'' (Ni3(Al,Ti)) phase. For the weld metal, regardless of aging heat treatment conditions, creep elongation and life decreased significantly compared to the base metal, due to the coarse effective grain size and inhomogeneous distribution of γ'' precipitates. (Received May 27, 2020; Accepted July 01, 2020)
Abstract The high-temperature oxidation resistances of Ni-based superalloys with the compositions... more Abstract The high-temperature oxidation resistances of Ni-based superalloys with the compositions of Ni–(0–15)Co–(8–15)Cr–(0–5)Mo–(0–10)W–(3–8)Al–(0–5)Ti–(0–10)Ta–0.1C–0.01B were analyzed using an artificial neural network (ANN). The oxidation resistances of the alloys were evaluated based on the weight change measured during cyclic oxidation tests. An ANN was constructed with the contents of the alloying elements as input and mass gains as output. The ANN provided highly accurate regression results. The present results were compared with those obtained using response surface methodology (RSM) in a previous study. The regression model of the ANN could effectively detect the effect of an additional alloying element explicitly. The main and interaction effects of the alloying elements were plotted based on the results with random compositions. The optimum composition of superalloys with the highest oxidation resistance at high temperatures was determined using the trained ANN.
The effects of thermal gradient and solidification rate on the dendrite arm spacing and carbide m... more The effects of thermal gradient and solidification rate on the dendrite arm spacing and carbide morphology were investigated in directionally solidified Ni-base superalloy, CM 247LC. Thermal gradient was controlled by changing the position of the cold chamber and the furnace set temperature. The interface morphology changed from the planar to dendritic as increasing solidification rate. It was found that the dendrite spacing decreased as increasing the thermal gradient as well as the solidification rate. Also, as increasing solidification rate, carbide morphology changed from blocky shape to script and spotty shapes.
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