Investigation Of Stress Corrosion Cracking Of Austenitic Stainless Steel In High Temperature And Pressure H₂S Environment

The high susceptibility of stress corrosion cracking of austenitic stainless steel in H₂S-containing environment makes it challenging to replace nickel-based alloys in the development of sour oil and gas fields.The key to solving this problem is to clarify the stress corrosion cracking mechanism of materials in H₂S-containing environment.In this paper,the law and mechanism of stress corrosion cracking of austenitic stainless steel in H₂S-containing environment were studied by slow strain rate tensile test;the relationship between stress concentration caused by dislocation motion and crack propagation path was revealed by microscopic characterization.The relationship model between stacking faults energy and dislocation motion was established,and the dislocation motion mechanism of stress corrosion cracking was proposed.Finally,the influence mechanism of Ni content,grain size,and aging treatment on stress corrosion cracking of austenitic stainless steel in H₂S-containing environment was explored.The main conclusions are as followed.As the ambient temperature of H₂S-containing environment increases from 303K to563K,the stress corrosion cracking sensitivity of 316L austenitic stainless steel first increases and then decreases,and the crack propagation path undergoes multiple transformations of"transgranular-intergranular-transgranular".When the temperature is at 303-473K,the failure mechanism is hydrogen-induced cracking stress corrosion;while at 473-563K,the failure mechanism gradually changes to anodic dissolution stress corrosion with the increase in temperature.In H₂S-containing environment at 303K,the microstructures such as stacking faults and deformed martensites are generated in the grains of 316L austenitic stainless steel during plastic deformation,which hinders dislocation slip,and then forms stress concentration inside the grains,resulting in transgranular fractures.In H₂S-containing environment at 473K,it is difficult to form the above obstacles inside the grains during plastic deformation,and dislocations are easy to move along the slip plane to the grain boundaries and pile up forming the stress concentration,causing the crack to propagate along the grain boundary to form intergranular fracture;when the ambient temperature rises to 563K,dislocations are prone to cross-slip,thus forming dislocation entangles inside the grain,and the stress concentration position shifts to internal again,leading to transgranular fracture.The transition of the dislocation movement mode from plane slip to cross slip is due to the increase of the stacking fault energy caused by temperature increase.The influence of temperature in H₂S-containing environment on the stacking fault energy of 316L austenitic stainless steel was quantitatively calculated.The model of stacking fault energy and dislocation movement mode was established.When the stacking fault energy is less than 50 m J/m²,the perfect dislocations decompose to form partial dislocations during plastic deformation,and the plane slip of partial dislocations generates stacking faults,and the overlapping of stacking faults will form deformation martensite or deformation twins.When the stacking fault energy is between 50 and 70 m J/m²,perfect dislocations move to the grain boundary through coplanar slipping because of difficulty in decomposing.When the stacking fault energy is higher than 70 m J/m²,the dislocations mainly move in a cross-slip manner,forming dislocation entanglements inside the grains.The research results of the effects of Ni content,grain size,and aging treatment on stress corrosion cracking of austenitic stainless steel in H₂S-containing environment show that when the Ni content reaches 20 wt.%,the stress corrosion cracking of austenitic stainless steel in 303K H₂S environment can be prevented;when the Ni content reaches30 wt.%,it can prevent stress corrosion cracking in H₂S-containing environment at 473K.Grain refinement of 316L austenitic stainless steel to 2μm by"Cold rolling-Recrystallization annealing"can also prevent stress corrosion cracking in H₂S-containing environment at 303K,but due to the existence of residual dislocations,it deteriorates its higher temperature stress corrosion cracking resistance.After 973K-120h aging treatment,the stress corrosion susceptibility of 316L austenitic stainless steel in H₂S-containing environment at 303K can be reduced to 5%,and 1173K-24h aging treatment can reduce its stress corrosion susceptibility in H₂S-containing environment at 473K to 70%.Microscopic characterization and electrochemical measurement of hydrogen permeation experiments found that increasing Ni content,grain refinement,and aging treatment can not only affect the dislocation movement mode during plastic deformation,and reduce stress concentration;it can also reduce the diffusion rate in the matrix or solid solubility in lattice interstitials of hydrogen atoms.Therefore,the resistance to hydrogen-induced cracking stress corrosion of austenitic stainless steel is improved.