Xiaofei Guo

Hydrogen embrittlement (HE) behavior of 31CrMoNiNbV steel subjected to austempering below the M_s temperature was investigated. The material exhibited a martensitic/bainitic dual-phase microstructure, with uniformly dispersed V- and Nb-rich MC carbides as well as Cr-rich M₃C carbides precipitated in the matrix. It achieved an ultimate tensile strength of 1688 MPa and an elongation to fracture of 10.7%. Slow strain rate tensile tests revealed that the HE susceptibility index reached 13.5%. Thermal desorption spectroscopy and hydrogen permeation tests showed that the martensite/bainite phase boundaries and the dispersed MC/M₃C carbides acted as effective hydrogen-trapping sites, lowering the hydrogen diffusivity and alleviating local hydrogen accumulation in stress-concentrated regions. Microstructural analysis further indicated that the dual-phase structure may simultaneously mitigate the hydrogen-enhanced localized plasticity and hydrogen-enhanced decohesion mechanisms, through combined stress relaxation at martensite/bainite interfaces and hydrogen immobilization by carbide traps.

The temperature-dependent carbide precipitation behavior and mechanical properties in 31CrMoNiNbV secondary hardening martensitic steel were investigated over a tempering temperature range of 200–650 °C. Sequential carbide precipitation behavior was characterized using scanning electron microscopy, electron backscatter diffraction and transmission electron microscopy, which was further correlated with tensile properties, hardness and low-temperature toughness. At the high tempering temperature of 580 °C, the investigated material achieved a well-balanced combination of tensile strength of 1633 MPa and total elongation of 15.1%, wherein tempering-induced M₂C precipitation contributed to strength retention, while dislocation recovery and the formation of high-angle grain boundaries led to concurrent improvement in ductility and toughness. A predictive framework linking tempering temperature to carbides evolution and the resultant mechanical properties has been established for the investigated material, guiding the microstructural and heat treatment parameter design of ultra-high strength secondary hardening martensitic steels.

The hydrogen embrittlement (HE) behavior of an Fe–18Mn–8Al–1C–5Ni lightweight steel containing a fine and uniformly distributed B2 phase and κ-carbide was investigated by slow strain rate tensile testing with in situ hydrogen charging. Hydrogen charging reduces the elongation from 28.2% to 11.2%, while preserving an ultimate tensile strength above 1100 MPa and yielding an HE index of 60.2%. A thermal desorption analysis reveals a multi-peak desorption curve corresponding to diffusible hydrogen, hydrogen reversibly trapped at κ-carbides, and hydrogen strongly bound at the B2/γ interfaces, revealing a hierarchical hydrogen trapping behavior. Electron backscatter diffraction and electron channeling contrast imaging analyses near the fracture head region further reveal that localized hydrogen enrichment at the B2/γ boundaries induces severe stress concentration and interfacial weakening, shifting the fracture mode from ductile micro-void coalescence in air to hydrogen assisted intergranular and interphase cracking. This study clarifies the distinct roles of coherent κ-carbide and B2/γ interfaces in hydrogen trapping and crack initiation, offering a microstructure-based perspective for designing high-strength, HE resistant lightweight steels.

Hydrogen embrittlement (HE) critically limits the application of ultra-high-strength press-hardened steels (PHS) in hydrogen-containing environments. This study investigated the effect of Nb microalloying on HE resistance of 1500 MPa-grade PHS. Even with higher hydrogen contents, steel 1500Nb exhibits better HE resistance than steel 1500. The results show that Nb addition plays effective role in grain refinement, mitigating stress concentration, and effectively postponing the initiation of intergranular cracks under hydrogen-charged conditions. Additional, hydrogen diffusivity in 1500Nb steel is lower than 1500 steel, attributed to both grain refinement effect and solute drag effect of Nb.

Austenitic Fe-Mn-Al-C lightweight steels have attracted considerable interest for automotive applications due to their exceptional specific strength, where κ-carbides precipitation critically influences mechanical properties. This review systematically examines the crystal structure, classification, and precipitation kinetics of κ-carbides, emphasizing their spatial distribution-dependent effects: coarse κ-carbides at austenite grain boundaries induce embrittlement and degrade toughness, while nanoscale κ’-carbides within grains enhance strength and ductility through dislocation interactions (e.g., Orowan bypassing and shearing), activating deformation mechanisms such as Dynamic Slip Band Refinement (DSBR), Shear Band-Induced Plasticity (SIP), and Microband-Induced Plasticity (MBIP). Thermodynamic calculations guide alloy design to ensure a single-phase austenite structure at the typical hot-rolling finishing temperature (~900 °C), avoiding harmful phases while promoting beneficial precipitates. Mn suppresses κ-carbide formation, whereas Al and C act as promoters, with intragranular κ’-carbides favoring higher Al/C concentrations (e.g., >6.2% Al and >1.0% C). Heat treatment parameters critically influence κ-carbide distribution, where rapid cooling (e.g., water quenching) suppresses κ-carbides, and subsequent aging (500–700 °C) enables homogeneous precipitation of κ’-carbides. Pre-deformation prior to annealing further accelerates κ-carbide nucleation by introducing crystal defects. Optimal performance requires integrated composition-processing-microstructure optimization to achieve a nnanoscaleκ’-carbide-strengthened austenite matrix through controlled composition and thermo-mechanical processing to achieve an optimal strength-ductility balance.

This study compares the hydrogen embrittlement susceptibility of a Fe-30Mn-8Al-1.2C austenitic low-density steel aged at 600 °C for 0 (RX), 1 min (A1) and 60 min (A60), each exhibiting varying sizes and distributions of nano-sized κ-carbides. Slow strain rate tests were conducted to assess hydrogen embrittlement susceptibility, while thermal desorption analysis was applied to investigate hydrogen trapping behaviors. Fracture surface analysis was employed to discuss the associated failure mechanisms. The results suggest that nano-sized κ-carbides with sizes ranging from 2–4 nm play a crucial role in mitigating hydrogen embrittlement, contrasting with the exacerbating effect of coarse grain boundary κ-carbides. This highlights the significance of controlling the sizes and morphology of precipitates in designing hydrogen-resistant materials.