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<p>This study systematically compared WC–Co and polycrystalline cubic boron nitride (PCBN) tools for friction stir welding (FSW) of 1700&#xa0;MPa martensitic ultra-high-strength steel (UHSs). The microstructural analysis revealed complete martensite transformation in the stir zone (SZ), while the thermomechanically affected zone (TMAZ) showed a martensite–ferrite mixture with decreasing ferrite content toward SZ. The heat-affected zone (HAZ) exhibited tempered martensite with grain boundary carbides. The increase in heat input led to larger prior-austenite grains and coarser martensitic microstructure during cooling after FSW, which reduced the hardness of the SZ. The TMAZ width expanded as the heat input increased, whereas the tempering degree of martensite in the HAZ remained relatively unchanged due to the far distance from the SZ center. The tensile tests indicated that at a low rotation speed of 300 rpm, the use of PCBN tool effectively suppressed the softening in the HAZ due to its low heat input, achieving a welding efficiency of 70%. At a high rotating speed of 500 rpm, the formation of defects was effectively prevented owing to the excellent high-temperature stability of the PCBN tool. The results establish PCBN as the preferred tool material for high-strength martensitic steel FSW, offering both low heat input and high-temperature stability.</p>

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A comparative study on the microstructure and mechanical properties of friction stir welded martensitic ultra-high-strength steel joints using PCBN and WC–Co tools

  • 期刊论文

  • 发表日期：

    2025年6月5日

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  • Xiangxiang Zhu,
  • Wenyuan Lv,
  • Shiquan Cui,
  • Yufeng Sun,
  • Wenbo Cao,
  • Shaokang Guan

摘要

This study systematically compared WC–Co and polycrystalline cubic boron nitride (PCBN) tools for friction stir welding (FSW) of 1700 MPa martensitic ultra-high-strength steel (UHSs). The microstructural analysis revealed complete martensite transformation in the stir zone (SZ), while the thermomechanically affected zone (TMAZ) showed a martensite–ferrite mixture with decreasing ferrite content toward SZ. The heat-affected zone (HAZ) exhibited tempered martensite with grain boundary carbides. The increase in heat input led to larger prior-austenite grains and coarser martensitic microstructure during cooling after FSW, which reduced the hardness of the SZ. The TMAZ width expanded as the heat input increased, whereas the tempering degree of martensite in the HAZ remained relatively unchanged due to the far distance from the SZ center. The tensile tests indicated that at a low rotation speed of 300 rpm, the use of PCBN tool effectively suppressed the softening in the HAZ due to its low heat input, achieving a welding efficiency of 70%. At a high rotating speed of 500 rpm, the formation of defects was effectively prevented owing to the excellent high-temperature stability of the PCBN tool. The results establish PCBN as the preferred tool material for high-strength martensitic steel FSW, offering both low heat input and high-temperature stability.