Research On Fatigue Property And Dynamic Fracture Toughness Of 1Mn18Cr18N Retaining Ring Steel

1Mn18Cr18N is nonmagnetic austenitic stainless steel,which has high strength and toughness,suitable thermal expansion rate,low magnetic permeability and high corrosion resistance.It has been widely used as a main material in turbo generator retaining rings.The retaining ring is one of the key components of the turbogenerator set.It is the component with the highest stress load in the turbo generator set,and the running environment is bad,which is in the strong magnetic field and corrosive atmosphere for a long time.In order to ensure the safety and reliability of the retaining ring in the long-term operation of the turbo generator set,it is very important to study the fatigue property of the 1Mn18Cr18N retaining ring steel material.Since 1Mn18Cr18N stainless steel material only has austenitic organization,good mechanical properties such as high ductility and toughness of retaining ring can not be achieved by heat treatment,only through the cold deformation strengthening or increase the nitrogen content of the formation of supersaturated solid solution to cause lattice distortion mode.This article analyzed 600MW turbogenerator retaining ring,test results of chemical composition,tensile,impact,microstructure,magnetic properties and stress corrosion show that the domestic600MW turbine generator retaining ring has excellent comprehensive performance.In the direction of section thickness,the strength index of the inner ring to outer ring shows a decreasing trend,and the index of plasticity and toughness increases.Mechanical properties of the retaining ring varied in different parts is due to the non-uniform deformation of retaining ring in hot forging and cold hardening.The differences of strength,ductility and toughness are caused by the different condition of recrystallization and hardening after nonuniform deformation.Under normal operating conditions,the retaining ring bears the function of complex stress,such as centrifugal force,electromagnetic force,assembly stress and so on.It needs to have high fatigue strength.Aiming at the difference of mechanical properties of different parts of retaining ring and the stress state during the operation of retaining ring,the high cycle fatigue test of 1Mn18Cr18N retaining ring steel was carried out.The fatigue limit and S-N curve of the retaining ring steel were accurately tested by lifting method and least square method.High cycle fatigue tests were carried out at different stress levels,the fatigue life of each stress level was obtained.The fatigue life curve of the retaining ring steel under different loads are obtained by fitting curve in double logarithmic coordinates,stress amplitude and fatigue life showed a linear relationship.1Mn18Cr18N retaining ring steel material 10⁷ fatigue limit is about 655MPa,the relationship between fatigue limit and tensile strength approximately?_-1?0.61R_m.In view of the different temperatures during the installation,disassembly and operation of the retaining ring,influences of different heating temperatures on the performance of the 1Mn18Cr18N ring steel were studied.The results show that the grain boundary precipitates will appear at the heating temperature above 500°C and have different effects on the properties of the materials.During the frequent startup or shutdown of a turbogenerator set or the peak load,the stress level of the retaining ring will be higher than the normal working condition,and the low cycle fatigue of strain will occur in long term operation.For this reason,the low cycle fatigue characteristics of materials treated at different temperatures were studied,and the stress-strain curves and strain-life relationship curves of 1Mn18Cr18N retaining ring steel were tested.The test results show that the 1Mn18Cr18N ring steel has the characteristic of cyclic softening,and the degree of cyclic softening increases with the increase of the stress amplitude.In abnormal conditions,the generator may bear transient high load impact.This paper test the dynamic fracture toughness of 1Mn18Cr18N after heated by heat set temperature by the combined method of experiment and numerical simulation.JId is the characteristics of dynamic fracture toughness of 1Mn18Cr18N steel.It can be used for evaluating the resistance of cracking induced by high load impact.