| The national "carbon neutrality and emission peak" strategic goal drives the new development of hydrogen energy,but it also puts forward new and higher requirements on material microstructure,mechanical properties and hydrogen embrittlement susceptibility of hydrogen equipment.The application of rare earth(RE)in steel is generally focused on purification,modifying inclusions and micro-alloying to collaboratively improve the mechanical properties of materials,but actually,RE modified oxygen sulfides,and RE elements as point defects forming hydrogen traps etc.,also play a beneficial role in delaying and regulating the hydrogen enrichment,thus effectively improving the hydrogen embrittlement(HE)phenomenon in steel.In this work,a new design concept of RE yttrium micro-alloying forming hydrogen traps was adopted in AISI 321 austenitic stainless steel,and the effects of RE yttrium and solid solution temperatures on the microstructure,mechanical properties and HE susceptibility of AISI 321 austenitic stainless steel were systematically and deeply investigated with the help of various characterization methods,and the mechanism of RE yttrium on hydrogen trapping of carbides was deeply revealed.The main research findings are as follows:(1)The addition of appropriate amount of RE yttrium helps to refine austenite grains,improve microstructure uniformity and significantly affect the number,size and distribution of carbonitrides.With the increase of RE yttrium content,the large particles of Ti N gradually spheroidized,the number of small size Ti C increased and gradually refined,from 185.5 nm 0Y specimen to 57.7 nm in 0.045 Y specimen.As the solid solution temperatures increased,the tensile strength of AISI 321 austenitic stainless steel showed a tendency to rise and then decrease,and the microstructure was relatively more uniform and the strength was the highest at 1100 ℃ for 30 minutes.When the solid solution temperature was 1100 ℃,with the increasing content of RE yttrium,the strength showed a trend of rising and then decreasing,and the highest strength was 636 MPa in the 0.035 Y specimen.The fracture observation results indicated that the carbides or nitrides at the dimples were important factors affecting the strength and toughness of the material.(2)The addition of appropriate amount of RE yttrium helps to reduce the HE susceptibility of AISI 321 austenitic stainless steel.The 0.015 Y specimen had the lowest HE susceptibility of16.15% after solid solution at 1050°C for 30 minutes and its HE susceptibility gradually increased with the increase of RE yttrium content,but even in the 0.045 Y specimen,the HE susceptibility was still less than that of the 0Y specimen.The contribution of RE yttrium to the reduction of HE susceptibility is mainly attributed to the positive effect of RE yttrium on the distribution,number and size of carbides,which is a more critical factor affecting the HE of AISI 321 austenitic stainless steel than grain size and martensitic phase transformation.(3)The positive effect of t appropriate amount of RE yttrium on carbides or nitrides,which reduces the HE susceptibility of AISI 321 austenitic stainless steel comes from two deep mechanisms: the first is that the modified large particle Ti N,under the multi-field coupling effect of stress-corrosion-hydrogen,alleviates the corrosion tendency of its surrounding area,reduces the channel for hydrogen atoms to diffuse into the material interior,and effectively buffers the emergence of hydrogen-cracking.In addition,the spheroidized nitride significantly improves its stress concentration distribution and reduces the nucleation of hydrogen cracks around the hard phase.Secondly,the RE yttrium micro-alloying AISI 321 austenitic stainless steel shows irreversible hydrogen trapping with a binding energy of 44.23 k J/mol,which mainly comes from the transformation of the interface relationship between the refined Ti C particles and the austenitic matrix and the increase of the mismatch value,which increases a hydrogen grasping effect of the Ti C particles and finally improves the HE resistance of AISI 321 austenitic stainless steel. |
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