Experimental Research On Hydrogen Damage Of Metals Corroded By H₂S And Damage Mechanism Analysis Of Crack Growth Rate Of Metals Corroded By H₂S

Many industrial disasters are caused by hydrogen damage to metal,so research on hydrogen damage to metal in the hydrogen environment is necessary.Hydrogen is a ubiquitous element that enters metal materials from many different sources.Once the hydrogen enters the metal,it causes irreversible damage and affects the mechanical properties of metal materials,such as fracture toughness,tensile strength,and ductility.It can decrease the toughness and plasticity of metal,which easily causes cracking and brittle fractures in metals under low-stress conditions.Once metals are exposed to hydrogen,they can interact with it,resulting in various kinds of structural damage,e.g.,hydrogen-environment-assisted cracking,delayed hydride cracking(DHC),and hydrogen embrittlement.There is still a unified understanding of the basic laws underlying the changes in the mechanical properties of metals caused by hydrogen damage.However,descriptions of the influence of hydrogen damage on mechanical properties are still based on experiments.Most of the hydrogen damage models are data-fitting models,and it has been difficult to ascertain quantitative relationships using theoretical methods.In this paper,we systematically characterize the mechanical properties of materials after hydrogen enters the metal by means of experimental testing and theoretical analysis,and we developed a quantitative evaluation method of hydrogen damage,revealing the nature and regularity of hydrogen damage affecting the properties of materials.The research into this subject has important reference value for the control and prevention of material hydrogen damage in practical engineering.The research described in this paper mainly includes the following:(1)A method for measuring hydrogen damage distribution in metals is proposed.The method uses a microhardness tester to measure the distribution of hydrogen damage in metals under different conditions.The distribution behavior of hydrogen in metal under different concentrations of corrosives,corrosion time,and applied stress conditions was systematically studied by using the layer-by-layer microhardness method of cylindrical specimens and the microhardness test method of non-uniform stress field at the tops of cracks in bolt-loaded WOL specimens.(2)An experimental study of crack growth rate of metal materials before and after corrosion was carried out,and the curves of crack growth rate in different environments,different corrosion times,and different stress ratios were obtained.The nature of change that hydrogen damage causes in the mechanical properties of materials was discussed through analysis of the variations in the characteristics of crack growth rate parameters under different conditions.(3)The tensile properties and fracture toughness of metal materials before and after corrosion were experimentally studied.The effects of hydrogen damage on metal strength,elasticity,plasticity,crack growth resistance curves,and fracture toughness were analyzed.(4)The hydrogen damage behavior in the plastic zone at the tips of cracks tip was investigated by means of elastic-plastic fracture mechanics.Then,the mechanism underlying hydrogen damage in the plastic zone of the crack tip was analyzed by considering the changes in mechanical properties of materials under monotonic loading,one-off loading,and cyclic loading and unloading before and after corrosion.The main research results of this paper include:(1)The distribution trend of hydrogen damage in unsaturated metal was determined by analyzing the incremental change of microhardness of metal caused by hydrogen damage along the direction of depth under different corrosion conditions.There is a saturated state of hydrogen damage in metals,which is consistent with the solid solubility of hydrogen in metals.When hydrogen enters the metal,a hydrogensaturated layer develops in the metal along the direction of depth,and the thickness of the hydrogen-saturated layer increases as corrosion time continues,which indicates that the hydrogen damage saturation state increases as corrosion time continues.After the hydrogen-saturated layer forms,the hydrogen content in the metal decreases gradually along the direction of depth until it reaches zero,and the trend is consistent with the theoretical distribution curve.The thickness of the hydrogen-saturated layer is not affected by corrosion concentration but only by corrosion time.(2)Based on the trend of hydrogen damage distribution along depth under different corrosion conditions,a model of hydrogen damage distribution in unsaturated metals is obtained.For the same material,there is an exponential function relationship between the thickness of hydrogen-saturated layer and corrosion time.We here established a new damage distribution model of hydrogen in metals.(3)The distribution trend of hydrogen damage under two-dimensional equal stress field was here found using the incremental changes in microhardness of hydrogeninduced metals in non-uniform stress field.The theoretical analysis and experimental results show there to be a simple superposition relationship between the incremental change in microhardness affected by two-dimensional equal stress field and the incremental change in microhardness affected by hydrogen damage corresponding to the stress field.(4)A mathematical expression of hydrogen damage in two-dimensional equal stress field is established using the trend of hydrogen damage distribution in stress environments.(5)Based on the study of hydrogen damage distribution,the incremental changes in microhardness can serve as the main parameter to characterize hydrogen damage behavior in local areas of metals.The theoretical analysis and experimental results show that not only can the layer-by-layer hardness method measure the hydrogen damage distribution in metals accurately and quantitatively,but the hydrogen damage distribution of materials can also be measured by microhardness test in known twodimensional equivalent engineering stress environments.(6)There was no significant difference in crack growth rate of metals under different stress ratios before and after corrosion,which indicates that the changes in the crack growth rate of metals after hydrogen damage is due to changes in the mechanical properties of those materials due to hydrogen damage.(7)Based on the differences in tensile properties of metals before and after corrosion,it was further proved that hydrogen damage mainly affects the plastic properties of metals.(8)A stress distribution analysis method based on the assumption of linear strain distribution in the plastic zone at the crack tip was constructed,and the stress distribution and strain distribution in the plastic zone at the crack tip were found.(9)The plastic damage mechanism by which hydrogen corrosion damage changes the crack growth rate was qualitatively analyzed based on the linear strain distribution method in the plastic zone at the tip of the crack.According to the mode of crack opening,assuming that the strain in the plastic zone at the crack tip would increase linearly as radius decreased,the plastic properties of metals would be affected by hydrogen corrosion,and the maximum plastic strain required for crack propagation of materials would also be decreased,resulting in a significant reduction of crack initiation toughness at the tip of the crack,easier crack propagation,and more hydrogen damage under one-time loading.The fracture toughness of metals would decrease and the crack growth rate would increase after hydrogen damage under cyclic loading and unloading.The main innovations are as follows:(1)Given different corrosion solution concentrations and different corrosion soaking time,hydrogen-saturated layers appeared in the metal along the direction of depth,and the thickness of said hydrogen-saturated layer would increase with as corrosion time continued.After the hydrogen saturated layer,the hydrogen content in the metal decreased gradually along the direction of depth until it reached zero.The thickness of hydrogen saturated layer was not affected by corrosion concentration but only by corrosion time.There is a simple superposition relationship between the incremental changes in microhardness affected by two-dimensional equal stress field and the incremental changes in microhardness affected by hydrogen damage corresponding to the stress field.This proves that measurement of microhardness can be used to assess the distribution of hydrogen damage in a two-dimensional equivalent engineering stress environment.(2)There is no significant difference in the crack growth rate of metal under different stress ratios after hydrogen sulfide corrosion,which indicates that the changes in the crack growth rate of metal materials after hydrogen corrosion damage is due to changes in the mechanical properties of materials due to hydrogen damage.(3)A stress distribution analysis method based on the assumption of linear strain distribution in the plastic zone at the tips of cracks is constructed.Based on this method,the mechanism underlying hydrogen corrosion damage to the plastic properties of metals and changes in crack the growth rate is qualitatively analyzed.