Microstructure And Hydrogen Embrittlement Fracture Mechanism Of AISI430 Ferritic Stainless Steel

As a new type of green energy,hydrogen has gradually replaced traditional coal mine energy.Due to the increase in the transportation volume and transportation scope of hydrogen pipelines,the requirements for the mechanical properties of pipeline materials have been greatly improved.However,high-strength steel is more susceptible to hydrogen embrittlement.Hydrogen penetration of metal significantly deteriorates the mechanical properties of the material,especially the reduction in plasticity,which limits the application and development of high-strength steels.Many existing research works have identified that hydrogen embrittlement（HE）is due to the interaction between hydrogen and the internal microstructure of the material.Hydrogen embrittlement（HE）involves the study of hydrogen trapping sites inside the material and the influence of hydrogen on the change of the fracture modes of the material.However,the characterization and observation of the precise location of hydrogen atoms in metallic materials limits our understanding and research progress to a certain extent.Although the coexistence of different hydrogen embrittlement mechanisms can be observed in different fracture morphologies of metal materials,the interrelationship and effect of different mechanisms（which mechanism is dominant）is still a challenging and critical issue.AISI430 ferritic stainless steel is widely used in electrical appliances,marine environment materials and pipeline transportation due to its good corrosion resistance,excellent mechanical properties and low price.As the basic structure of high-strength steel,the hydrogen embrittlement mechanism of ferrite is still in the research stage.Therefore,in this study,the effects of electrochemical hydrogen charging time and current density on the mechanical properties of AISI430 ferritic stainless steel were studied through tensile tests.Using scanning electron microscope（SEM）,electron backscatter diffraction（EBSD）technology system,time-of-flight secondary ion mass spectrometer（TOF-SIMS）and thermal desorption spectroscopy（TDS）to study the microstructure of AISI430 ferritic stainless steel on its hydrogen embrittlement sensitivity;The influence of hydrogen on the mechanical properties of AISI430 ferritic stainless steel and the effect of the change of its fracture mode are mainly discussed,and different fracture models are proposed according to different fracture morphologies.The final research results show that:under the premise of constant sample size,constant hydrogen charging current density and constant hydrogen charging time,,the increase of current density and time leads to the increase of the hydrogen embrittlement sensitivity index of AISI430 ferritic stainless steel;The hydrogen charging conditions were adopted with a current density of 50 m A/cm²,when the hydrogen charging time reaches 4 h,the saturation of the internal hydrogen content of the material causes that the increase of hydrogen charging time no longer affects its hydrogen embrittlement sensitivity index.Using time-of-flight secondary ion mass spectrometer（TOF-SIMS）to characterize the microstructure of AISI430ferritic stainless steel for hydrogen capture,it was found that carbide(M₂₃C₆)acts as a strong hydrogen trap and the trapping effect of hydrogen atoms are greater than that of ferrite grain boundaries.The strong trapping effect causes hydrogen atoms to accumulate at the carbide(M₂₃C₆).Through the analysis of the fracture morphology and thermal desorption spectrum（TDS）of single-phase AISI430 ferritic stainless steel with microstructure containing carbides(M₂₃C₆)before and after hydrogen charging,it is shown that under low diffusible hydrogen concentration,hydrogen-induced fracture is mainly based on carbide(M₂₃C₆)treatment cleavage fracture and fine ductile microvoid coalescence（F-MVC）fracture guided by the Hydrogen Enhanced Decohesion（HEDE）+Hydrogen Enhanced Localized Plasticity（HELP）+Hydrogen Enhanced Strain-Induced Vacancies（HESIV）mechanism.Under high diffusible hydrogen concentration,the hydrogen-induced fracture modes are transgranular（TG）dominated by cleavage（C）fracture,which guided by the HEDE+HELP mechanism.The research results of hydrogen embrittlement susceptibility of AISI430 ferritic stainless steel after single-phase（α）hot rolling and dual-phase（α+γ）annealing process show that the test steel has the highest hydrogen embrittlement sensitivity after hot rolling.The susceptibility to hydrogen embrittlement is reduced after annealing at 940°C;The analysis results of the fracture morphology show that the hot-rolled specimens are mainly intergranular fractures（IG）accompanied by cleavage fractures;The fracture modes of the annealed specimens are cleavage fracture and quasi-cleavage fracture（QC）;Comparing the effects of the microstructure at different annealing temperatures on the hydrogen embrittlement susceptibility and fracture mode of AISI430 ferritic stainless steel,it is found that the presence of intergranular carbides(M₂₃C₆)leads to a significant increase in the strength of the material after hydrogen charged.The single-phase（α）annealed specimens have lower hydrogen embrittlement sensitivity than the dual-phase（α+γ）annealed specimens.This is due to the strong trapping effect of intergranular carbides(M₂₃C₆)on hydrogen and the existence of a high-strength martensite phase with high hydrogen embrittlement sensitivity.