Abstract
Hydrogen embrittlement is one of the largest obstacles against the commercialisation of ultra-high strength quenching and partitioning (Q&P) steels with ultimate tensile strength over 1500 MPa, including the hot stamped steel parts that have undergone a Q&P treatment. In this work, the influence of partitioning temperature on hydrogen embrittlement of ultra-high strength Q&P steels is studied by pre-charged tensile tests with both dog-bone and notched samples. It is found that hydrogen embrittlement resistance is enhanced by the higher partitioning temperature. Then, the hydrogen embrittlement mechanism is analysed in terms of hydrogen, retained austenite, and martensite matrix. Thermal desorption analysis (TDA) shows that the hydrogen trapping properties are similar in the Q&P steels, which cannot explain the enhancement of hydrogen embrittlement resistance. On the contrary, it is found that the relatively low retained austenite stability after the higher temperature partitioning ensures more sufficient TRIP effect before hydrogen-induced fracture. Additionally, dislocation recovery and solute carbon depletion at the higher partitioning temperature can reduce the flow stress of the martensite matrix, improving its intrinsic toughness and reducing its hydrogen sensitivity, both of which result in the higher hydrogen embrittlement resistance.
Highlights
As a third-generation advanced high strength steel (AHSS), quenching and partitioning (Q&P) steels exhibit an excellent combination of strength and ductility for automotive body lightweighting [1–6]
Temperature, By tensile tests of dog-bone samples and notched samples, it is found that the higher partitioning
By tensile tests of dog-bone samples and notched samples, it is found that the higher partitioning temperature results resultsininsmaller smallerelongation elongation loss
Summary
As a third-generation advanced high strength steel (AHSS), quenching and partitioning (Q&P) steels exhibit an excellent combination of strength and ductility for automotive body lightweighting [1–6]. The production of ultimate tensile strength and total elongation has reached over. 30,000 MPa % in state-of-the-art Q&P steels [7–9]. Some Q&P steels have already been commercialized in the automotive industry, such as QP980, with ultimate tensile strength over 980 MPa and total elongation over 20% [1]. The ultra-high strength Q&P steels, with ultimate tensile strength over 1500 MPa, still face several challenges before practical application [10–12]. Hydrogen embrittlement is one of the biggest challenges for ultra-high strength Q&P steels. Hydrogen produced by galvanisation, painting, and corrosion can diffuse to regions of high residual or applied stress and degrade the fracture toughness, leading to pre-mature fracture of Q&P steel components [13,14]
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