Research Of Crack Tip Stress Field In Hydrogen Corrosion Environment

This paper is supported by National Nature Science Foundtion of China. With the development of the modern energy industry system, oil gas and other related petrochemical energy industry become the nation's major lifeline. However, metal materials used in the petrochemical equipment produce a variety of structural discontinuity phenomena inevitably. The discontinuous of material or structure can become hydrogen trap, which make local hydrogen concentration increases, thus produce crack and other irreversible hydrogen damage. Therefore, the development of defect evaluation and life prediction method of the existing macroscopic defects in actual working condition is the key and the pressing needs to the development of the energy industry healthily and rapidly.The paper takes common 45 steel as an example, in order to simulate the actual state of defective part, pre-cracked specimens are used and stresses of crack tips are measured. Considering the influence of hydrogen on the stress state, hardness of crack tips are tested and hydrogen induced stresses are calculated, then the stresses distribution of crack tips are obtained and the stress field model of crack tip under the hydrogen and working load coupled condition are built. The correctness of the model is verified by fracture toughness tests.The main research achievements are as following:(1) In air environment, the corresponding relationship between hardness and biaxial surface stress field is studied by instrument hardness tests and the expressions of hardness and tensile/compression stresses are analyzed. By considering the machining stresses and the plastic zone, the experimental curve coincides well with the theoretical one. Hardness can be used as a calibration method of surface stress field.(2) On the basis of relation between the hydrogen content and hydrogen induced stress, the theory of hydrogen permeation and stress-induced hydrogen diffusion, under in-plane stress condition, the expression of hydrogen induced stress at crack tip were analyzed. Hardness tests were used to measure the stress field distribution of crack tip under the hydrogen corrosion and working load coupled condition. Through comparing the experimental stress with the theoretical stress, the crack tip stress field is not a simple superposition of applied stress and hydrogen induced stress, but the coupling relationship. The method of iterative calculation should be adopted when simulate stress field of crack tip. After iterative calculation, experimental stress curve coincides well with theoretical curve.(3) The inner part of the crack tip is the most dangerous position. Crack tip stress state is a typical plane strain state in this area. However, the actual stress value of this dangerous area cannot be measured, on the basis of coupling relationship of hydrogen induced stress and working load, hydrogen induced stress expression under plane strain condition is obtained. Finally, the correctness of the model is verified by the experimental results of fracture toughness in the air or the corrosion environment.