| 300 series austenitic stainless steel(304,316,316L)widely used as the lining of hydrogenation reactor,hydrogen storage vessel,high-pressure hydrogen compressor,pressure gauge,high-pressure valve,piping and pipeline joint materials in high-pressure hydrogen system due to its excellent hydrogen embrittlement resistance.However,the strength of austenitic stainless steel is generally low,except for a few precipitation strengthening grades(such as A-286),its yield strengthσ_s is 200-300mpa.For the high-pressure hydrogen system,this kind of material is difficult to meet the lightweight design requirements,many parts use strain strengthened austenitic stainless steel.Although austenitic stainless steel show almost no hydrogen embrittlement,the problem of hydrogen embrittlement after strain strengthening cannot be ignored.The coupling effect of hydrogen and alternating load in high pressure environment and the residual effect of strain strengthened structure are the main causes of the above accidents.Moreover,the fatigue damage of strain strengthened austenitic stainless steel in high pressure hydrogen environment remains to be studied.With the influence of alternating load,high pressure hydrogen and strain hardening residual,the fatigue damage of strain strengthened austenitic stainless steel bearing parts directly affects the service safety of high pressure hydrogen system.In this paper,based on the difference of nucleation dynamics of deformation structure,the microstructure of deformation twins,dislocation structure,predeformation and dynamic martensite can be controlled by changing the deformation temperature and deformation amount,and the subsequent mechanism is discussed.By separating the different deformation microstructures and their residual effects effectively,the coupling effect between them and environment hydrogen is discussed respectively.Finally,the correlation mechanism of forming and manufacturing microstructure hydrogen transfer and distribution hydrogen induced cracking of austenitic stainless steel is obtained.Three new mechanisms of hydrogen induced cracking were established.In this study,304 and 316 austenitic stainless steels were rolled and deformed.The mechanical properties under high pressure hydrogen environment,HRTEM,EBSD and in-situ mechanical properties of hydrogen were used to study the effects of strain strengthening and localized deformation of crack tip on hydrogen transport behavior,hydrogen distribution characteristics and hydrogen induced fatigue damage The results show that the evolution mechanism of strain strengthened austenitic stainless steel from micro to macro multi-scale fatigue damage under the coupling action of high pressure hydrogen and alternating load.The main conclusions are as follows:(1)For cold rolled samples:in the region of lowΔK value,a large number of initial twins make it difficult for dislocations to cross the twin boundary,resulting in cross slip,displacement,and fault accumulation,which makes the cracks accelerate passivation,thus showing a lower fatigue crack growth rate;in the region ofΔK value,the stress value at the crack tip increases gradually,and dislocations begin to cross the twin boundary to form shear slip bands,and then the dynamicα’martensite slips The transition zone nucleates at the junction of dislocation.Due to the great difference in hydrogen solubility and hydrogen diffusion coefficient betweenα’martensite and austenite matrix,hydrogen tends to accumulate at the interface between them,which makes the material preferentially crack at the shear slip band/Twin interface,and finally leads to high fatigue crack growth rate.(2)For warm rolled samples:since there is no twinning and dislocation distribution is more uniform during warm rolling,martensitic transformation is difficult to occur in the region of low or highΔK value,so the crack growth mechanism is relatively simple.Under the condition of lowδK value,the plastic deformation takes place in the plastic zone at the crack tip,which fully evolves into dislocation wall structure,which divides the grains into multi oriented sub crystal structure,and finally leads to the multi-directional crack development path.When the value ofδK is high,there are many fine secondary twin structures in the sub crystal due to the large in-situ stress amplitude.The fine twin structure eventually leads to the appearance of more fine and less undulating fracture surface.(3)The larger the rolling deformation and the lower the rolling temperature in the forming process,the larger the proportion of the initial twin deformation in the pre deformation process.These initial twins play an important role in the nucleation and growth of the dynamicα’martensite in the subsequent crack growth process.There are great differences in hydrogen diffusion coefficient and hydrogen solubility between the dynamicα’martensite and the original austenite matrix,which have a great influence on the diffusion and segregation of hydrogen in the material,and then on the hydrogen embrittlement resistance of the material.(4)Twin andα’martensite increase the hydrogen embrittlement sensitivity of austenite materials through two mechanisms:hydrogen segregation at the twin boundary/slip band,resulting in the rapid development of cracks along the slip band/twin boundary,forming step like fracture micro morphology or small plane morphology along the twin boundary;hydrogen tends to accumulate in the massiveα’martensite at the crack tip,resulting in the cracks at{100}The cleavage surface cracks preferentially,forming a large number of microcracks and micropores.These micro pores and microcracks eventually merge with the main crack,accelerate the crack propagation forward,and finally form the micro fracture morphology of quasi cleavage plane. |
