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<p>The strength of welded joints is closely related to the hardening behaviour in the heat affected zone (HAZ). To investigate the intrinsic relationship between microstructure transformation and hardening behaviour of HAZ for 1.0 GPa-grade ultra-high strength seamless steel tube, the evolution of microstructure and hardness with different thermal cycles were analyzed. The gas metal arc welding (GMAW) method was used to prepare welded joints with heat inputs of 14 and 34&#xa0;kJ/cm. The microstructures of typical lath martensite (LM) and granular bainite (GB) were observed with different thermal cycles of CGHAZ. As the heat input increases, the grain size in coarse-grained heat-affected zone (CGHAZ) increased from 48.84 to 110.84&#xa0;μm, and that of inter-critical reheated coarse-grained heat-affected zone (ICCGHAZ) increased from 54.92 to 120.90&#xa0;μm. The area fraction of GB increased with the increase of heat input, and are accompanied by an increase in the area fraction of M-A constituents. By using the Vickers hardness test and nanoindentation test, the&#xa0;hardness distribution and hardness mechanism of HAZ were analyzed. The results show that the CGHAZ hardness is always higher than the base metal (BM) hardness, and the ICCGHAZ hardness is always similar to BM. The hardening behaviour of CGHAZ weakens as the heat input increases from 14 to 34&#xa0;kJ/cm. The microstructure type generated by heat input is the intrinsic factor of the CGHAZ hardening behaviour. The weakening of the hardening behaviour of CGHAZ is closely related to the increase of GB area fraction and the decrease of LM micro-hardness. The decrease of ICCGHAZ hardening behaviour is determined by the extremely uneven structural characteristics generated by the secondary thermal cycle. Some dislocations are reduced by slip or other forms of rearrangement after the secondary thermal cycle. The micro-hardness and yield strength of LM and GB decrease and are close to each other.</p>

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Effect of Welding Thermal Cycles on Microstructure Transformation and Hardening Behaviour of HAZ for 1.0 GPa-grade Ultra-high Strength Seamless Steel Tube

  • 期刊论文

  • 发表日期：

    2025年5月2日

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  • Junya Han,
  • Chengning Li,
  • Xiaocong Yang,
  • Yuke Yang,
  • Ying Wang,
  • Yanqing Zhao,
  • Xinjie Di

摘要

The strength of welded joints is closely related to the hardening behaviour in the heat affected zone (HAZ). To investigate the intrinsic relationship between microstructure transformation and hardening behaviour of HAZ for 1.0 GPa-grade ultra-high strength seamless steel tube, the evolution of microstructure and hardness with different thermal cycles were analyzed. The gas metal arc welding (GMAW) method was used to prepare welded joints with heat inputs of 14 and 34 kJ/cm. The microstructures of typical lath martensite (LM) and granular bainite (GB) were observed with different thermal cycles of CGHAZ. As the heat input increases, the grain size in coarse-grained heat-affected zone (CGHAZ) increased from 48.84 to 110.84 μm, and that of inter-critical reheated coarse-grained heat-affected zone (ICCGHAZ) increased from 54.92 to 120.90 μm. The area fraction of GB increased with the increase of heat input, and are accompanied by an increase in the area fraction of M-A constituents. By using the Vickers hardness test and nanoindentation test, the hardness distribution and hardness mechanism of HAZ were analyzed. The results show that the CGHAZ hardness is always higher than the base metal (BM) hardness, and the ICCGHAZ hardness is always similar to BM. The hardening behaviour of CGHAZ weakens as the heat input increases from 14 to 34 kJ/cm. The microstructure type generated by heat input is the intrinsic factor of the CGHAZ hardening behaviour. The weakening of the hardening behaviour of CGHAZ is closely related to the increase of GB area fraction and the decrease of LM micro-hardness. The decrease of ICCGHAZ hardening behaviour is determined by the extremely uneven structural characteristics generated by the secondary thermal cycle. Some dislocations are reduced by slip or other forms of rearrangement after the secondary thermal cycle. The micro-hardness and yield strength of LM and GB decrease and are close to each other.