Research On Strain Fatigue Characteristics Of Pressure Vessel Steel 16MnR After High Temperature Damage

In engineering practice it is of frequent occurrence that pressure vessels operate for a short time under the overtemperature accident operating conditions, which will cause short-time high temperature damage of the material, research on the effect of high temperature damage on safe operation of pressure vessel is of important significance in engineering. In this paper the low alloy steel 16MnR most extensively used in Chinese pressure vessel sector is taken as the subject investigated, a systematic and comprehensive experimental research on material property after high temperature damage is conducted based on theoretical analysis and investigation, a relatively comprehensive knowledge is obtained about mechanical property after high temperature damage and the change law of strain fatigue characteristics and research techniques for strain fatigue after high temperature damage of 16MnR material are set up. Main research contents are as follows:The effect of short-time high temperature damage on short-time tensile property and hardness of 16MnR material is inspected systematically and analysis on metallurgical structure of tensile fracture is carried out the change law of material strength and plastic indexes after high temperature damage is obtained, and the relationship between hardness and yield strength is given, thus the prediction techniques for material property with hardness as the parameter for high temperature damage is set up.Through experimental research on fracture resistance after short-time high temperature damage of 16MnR material, the effect of high temperature damage on fracture toughness, low cycle fatigue life and strain fatigue crack propagation rate is found. With the increase of damage temperature and prolongation of heating time, the crack initiation toughness Ji takes on an increasing trend with evident reduction of low cycle fatigue life, and the strain fatigue propagation rate da/Dn presents an increasing trend. An expression for crack propagation which is based on J-integral and is related to material damage temperature and time is proposed.According to the experimental results, the essential characteristics of effect of phase transition temperature on various items of mechanical property under investigation are also found.The effect of short-time high temperature damage on plastic strain energy and its response rate is taken into account, a fatigue life prediction model which takes energy of plastic deformation as fatigue propagation parameter is set up based on experiments, relationships at different damage temperatures and time are given respectively, and the change law for plastic strain energy of hysteresis loop is reflected.The crack propagation law is described using damage mechanism. Based on improvement of low cycle fatigue model, the damage model for the crack propagation law is desribed using damage mechanism is proposed, the damage temperature and the effect of holding time on strain fatigue damage and its evolutionary process are taken into account. It is also feasible in actual measurement to correctly reflect the physic characteristics of fatigue damage accumulation by the damage parameter which is defined by cumulative residual plastic strain energy. The equations for evolution of strain fatigue crack propagation damage of 16MnR material at different damage temperature and time are obtained. The expression for fatigue crack life which takes energy of plastic deformation for crack propagation as the damage variable is set up, which can better reflects the essential characteristics of accumulation of strain fatigue plastic deformation work, and the effect of high temperature damage is taken into account.Fractal geometry theory is adopted to study the relationship between the fractal dimensions of fracture surface after short-time high temperature damage of 16MnRmaterial and material property. The change law of fractal dimensions with damage is obtained. The techniques for analysis of mechanical property of the material using fractal dimensions are set up to further predict macromechanical performance of the material.The innovation points of this paper are as follows:â‘  Systematic and comprehensive experimental research is carried out on the short-time high temperature damage of 16MnR material, relatively comprehensive knowledge about the change law of mechanical property and strain fatigue characteristics after high temperature damage is acquired and the research technique for strain fatigue after high temperature damage of 16MnR material is set up.â‘¡ The effect of short-time high temperature damage on plastic strain energy and its response rate is taken into account, a fatigue life prediction model which takes energy of plastic deformation as the fatigue propagation parameter is set up, relationships at different damage temperatures and time are given respectively.â‘¢ Equations for the evolution of strain fatigue crack propagation damage of 16MnR material at different damage temperatures and time using cumulative residual plastic strain energy to define damage variable. Expression for fatigue crack life is set up which takes energy of plastic deformation for crack propagation as the damage variable.â‘£ In the fractal geometry study, after the three dimensional coordinates are obtained using image processing technology, the true fractal dimensions are calculated, the scanning pitch reaches the accuracy requirements of micrometer level for metallic material. Compared with formerly SIM method for calculation of fracture fractal dimensions, FPM method and the method for substituting height with image gray scale, this method has the advantage of undamaged fracture surface and reflection of actual profile characteristics of fracture surface and has wide application value in the research area of material science. The method for analysis of mechanical property of 16MnR material after high temperature damage using fractal dimensions with hardness as the parameter is preliminarily established.