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<p>The deployment of 630&#xa0;°C ultra-supercritical (USC) units, pivotal for high-efficiency and clean coal-fired power generation, demands advanced materials with exceptional high-temperature integrity. G115 martensitic steel is a promising candidate for large-diameter piping, but current research on its welded joints predominantly focuses on individual temperature points or specific creep conditions. There is a lack of systematic study on how its mechanical properties change continuously from room temperature to 650 °C, as well as its ductile–brittle transition behavior. This study systematically investigates the high-temperature mechanical behavior and toughness transition mechanism of large-diameter G115 steel welded joints intended for 630&#xa0;°C ultra-supercritical units. The correlation between the mechanical properties and the microstructure of the joints is revealed through microstructural characterization, mechanical property testing, and fracture surface analysis. The results indicate that the weld zone exhibits optimal performance (hardness 250-270 HV) due to grain refinement strengthening and the dispersed precipitation of M<sub>23</sub>C<sub>6</sub> and Laves phases. The high-temperature tensile strength of the joint decreases exponentially with increasing temperature. At 650 °C, it remains at 298.44&#xa0;MPa, the strength retention rate is 43.7%, and the yield ratio rises to 0.9. Joint plasticity exhibits a significant decrease at 300&#xa0;°C due to dynamic strain aging, while it significantly improves at 650&#xa0;°C. The ductile-to-brittle transition temperature is determined to be 36.6&#xa0;°C, indicating a low-temperature embrittlement tendency near room temperature. Therefore, both operation and design should be carefully considered in USC units to avoid brittle fracture during low-temperature transients or start-up/shut-down cycles. This embrittlement is caused by the synergistic strengthening effect of fine lath tempered martensite and dispersed precipitates (M<sub>23</sub>C<sub>6</sub>/Laves) in the weld zone, while the coarse-grained heat-affected zone exhibits the formation of networked carbides. The findings clarify the intrinsic relationship between microstructural evolution and performance degradation, providing a theoretical basis for optimizing welding processes.</p>

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Analysis of Mechanical Properties and Ductile–Brittle Transition Behavior in Large-Diameter G115 Steel Welded Joints

  • 期刊论文

  • 发表日期：

    2026年2月10日

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  • Zhibing Lu,
  • Guozheng Xu,
  • Yalin Zhang,
  • Liu Hong,
  • Yutao Li,
  • Ziyang Zhang,
  • Guoyun Li

摘要

The deployment of 630 °C ultra-supercritical (USC) units, pivotal for high-efficiency and clean coal-fired power generation, demands advanced materials with exceptional high-temperature integrity. G115 martensitic steel is a promising candidate for large-diameter piping, but current research on its welded joints predominantly focuses on individual temperature points or specific creep conditions. There is a lack of systematic study on how its mechanical properties change continuously from room temperature to 650 °C, as well as its ductile–brittle transition behavior. This study systematically investigates the high-temperature mechanical behavior and toughness transition mechanism of large-diameter G115 steel welded joints intended for 630 °C ultra-supercritical units. The correlation between the mechanical properties and the microstructure of the joints is revealed through microstructural characterization, mechanical property testing, and fracture surface analysis. The results indicate that the weld zone exhibits optimal performance (hardness 250-270 HV) due to grain refinement strengthening and the dispersed precipitation of M23C6 and Laves phases. The high-temperature tensile strength of the joint decreases exponentially with increasing temperature. At 650 °C, it remains at 298.44 MPa, the strength retention rate is 43.7%, and the yield ratio rises to 0.9. Joint plasticity exhibits a significant decrease at 300 °C due to dynamic strain aging, while it significantly improves at 650 °C. The ductile-to-brittle transition temperature is determined to be 36.6 °C, indicating a low-temperature embrittlement tendency near room temperature. Therefore, both operation and design should be carefully considered in USC units to avoid brittle fracture during low-temperature transients or start-up/shut-down cycles. This embrittlement is caused by the synergistic strengthening effect of fine lath tempered martensite and dispersed precipitates (M23C6/Laves) in the weld zone, while the coarse-grained heat-affected zone exhibits the formation of networked carbides. The findings clarify the intrinsic relationship between microstructural evolution and performance degradation, providing a theoretical basis for optimizing welding processes.