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<p>A novel heat treatment strategy is proposed to address microstructure heterogeneity and the strength-ductility mismatch in plasma arc additive manufactured (PAAMed) ultrahigh strength steel (UHSS) induced by complex thermal cycling. The strategy involves quenching, tempering and deep cryogenic treatment (DCT). In this study, the microstructural evolution and mechanical properties of PAAMed UHSS subjected to varying DCT times are explored. Results show that the as-deposited UHSS primarily consists of plate- and lath-shaped tempered martensite, bainite and carbide. Quenching dissolves carbides, allowing carbon atoms to diffuse into the martensitic matrix, followed by carbide precipitation during tempering. DCT promotes martensite refinement, improves microstructural uniformity and stability, and increases dislocation density. Lattice parameter analysis further reveals that DCT induces pronounced lattice shrinkage and distortion, thereby enhancing internal stress and lattice strain. Mechanical tests demonstrate a concurrent improvement in strength and ductility after the combined heat treatment, with an optimal strength-ductility balance achieved by the 12-hour DCT process. The yield strength increased by 52% and elongation improved by 78% compared to the initial as-deposited state. The enhanced strength stems from solid solution strengthening during quenching combined with martensite refinement induced by DCT. The enhanced ductility results from tempering-induced carbide precipitation, reduced solute carbon content, and residual stress relief, further supported by microstructural homogenization via DCT. This study offers a viable technical pathway for optimizing the strength and ductility of PAAMed UHSS.</p>

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Synergistic enhancement of strength and ductility for plasma arc additive manufactured ultrahigh strength steel by a novel heat treatment

  • 期刊论文

  • 发表日期：

    2026年4月8日

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  • You Li,
  • Yibo Xiong,
  • Mingjie Zhao,
  • An Lu,
  • Changmin Li,
  • Junyang Tu,
  • Lihong Jiang,
  • Zhenghua Guo

摘要

A novel heat treatment strategy is proposed to address microstructure heterogeneity and the strength-ductility mismatch in plasma arc additive manufactured (PAAMed) ultrahigh strength steel (UHSS) induced by complex thermal cycling. The strategy involves quenching, tempering and deep cryogenic treatment (DCT). In this study, the microstructural evolution and mechanical properties of PAAMed UHSS subjected to varying DCT times are explored. Results show that the as-deposited UHSS primarily consists of plate- and lath-shaped tempered martensite, bainite and carbide. Quenching dissolves carbides, allowing carbon atoms to diffuse into the martensitic matrix, followed by carbide precipitation during tempering. DCT promotes martensite refinement, improves microstructural uniformity and stability, and increases dislocation density. Lattice parameter analysis further reveals that DCT induces pronounced lattice shrinkage and distortion, thereby enhancing internal stress and lattice strain. Mechanical tests demonstrate a concurrent improvement in strength and ductility after the combined heat treatment, with an optimal strength-ductility balance achieved by the 12-hour DCT process. The yield strength increased by 52% and elongation improved by 78% compared to the initial as-deposited state. The enhanced strength stems from solid solution strengthening during quenching combined with martensite refinement induced by DCT. The enhanced ductility results from tempering-induced carbide precipitation, reduced solute carbon content, and residual stress relief, further supported by microstructural homogenization via DCT. This study offers a viable technical pathway for optimizing the strength and ductility of PAAMed UHSS.