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<p>Alloy 740H (Ni-25Cr-20Co, ASME Code Case 2702) is a promising candidate material for boiler tubes and headers in Indian Advanced Ultra Supercritical (A-USC) power plants operating at temperatures of 710–720&#xa0;°C and a pressure of 31&#xa0;MPa. This study investigates the changes in the microstructure and mechanical properties of Alloy 740H due to weld thermal cycles and simulated service conditions (720&#xa0;°C/1000&#xa0;h). Mock-up weld joints were fabricated from the as-received solution-annealed Alloy 740H tube (52&#xa0;mm diameter and 12&#xa0;mm thick) by automatic GTAW process and matching welding consumable ERNiCrCo-1 and qualified based on ASME Section IX. The X-ray radiography-qualified weld joints were subjected to post-weld heat treatment (PWHT) at 800&#xa0;°C for 4&#xa0;h followed by tension and bend tests for procedure qualification. The weld joints passed in side bend test with a 4&#xa0;T bend radius and met the ultimate tensile strength requirement of ASME Code Case 2702. SEM analysis revealed Ti- and Nb-rich carbides in base metal and weld in as-received/as-welded condition. Extensive precipitation of γ’ inside grains and a continuous network of M<sub>23</sub>C<sub>6</sub> along the grain/dendrite boundaries are observed after aging at 720&#xa0;°C/1000&#xa0;h. Both PWHT and aging resulted in an increase in strength and hardness with a reduction in ductility and toughness. The Digital Image Correlation performed on an as-welded cross-weld tensile specimen revealed localized straining of the weld zone compared to other regions. From the present studies, it is concluded that Alloy 740H in the solution-annealed condition possesses good weldability and the mechanical property changes correlated with the observed microstructure.</p>

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Evaluation of weldability and thermal aging behaviour on microstructure and properties of Alloy 740H

  • 期刊论文

  • 出版模式：

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  • 发表日期：

    2025年2月25日

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  • R. Kathirvelan,
  • H. C. Dey,
  • S. S. Kausal,
  • V. Karthik,
  • R. Mythili,
  • M. Vasudevan

摘要

Alloy 740H (Ni-25Cr-20Co, ASME Code Case 2702) is a promising candidate material for boiler tubes and headers in Indian Advanced Ultra Supercritical (A-USC) power plants operating at temperatures of 710–720 °C and a pressure of 31 MPa. This study investigates the changes in the microstructure and mechanical properties of Alloy 740H due to weld thermal cycles and simulated service conditions (720 °C/1000 h). Mock-up weld joints were fabricated from the as-received solution-annealed Alloy 740H tube (52 mm diameter and 12 mm thick) by automatic GTAW process and matching welding consumable ERNiCrCo-1 and qualified based on ASME Section IX. The X-ray radiography-qualified weld joints were subjected to post-weld heat treatment (PWHT) at 800 °C for 4 h followed by tension and bend tests for procedure qualification. The weld joints passed in side bend test with a 4 T bend radius and met the ultimate tensile strength requirement of ASME Code Case 2702. SEM analysis revealed Ti- and Nb-rich carbides in base metal and weld in as-received/as-welded condition. Extensive precipitation of γ’ inside grains and a continuous network of M23C6 along the grain/dendrite boundaries are observed after aging at 720 °C/1000 h. Both PWHT and aging resulted in an increase in strength and hardness with a reduction in ductility and toughness. The Digital Image Correlation performed on an as-welded cross-weld tensile specimen revealed localized straining of the weld zone compared to other regions. From the present studies, it is concluded that Alloy 740H in the solution-annealed condition possesses good weldability and the mechanical property changes correlated with the observed microstructure.