Study On Microstructure And Fatigue Resistance Of Ship Plate Steel EH36

With the development of the world economy,maritime trade becomes more and more frequent,and ships are also developing in the direction of large size and deep sea.Large ocean-going ships suffer from wind and waves for a long time,and under the action of periodic loads,it is very easy to have microcrack fatigue failure and cause serious damage accidents.So it is of great significance to study the fatigue resistance of ship plate steel.In this thesis,the controlled rolling,controlled cooling and normalizing EH36 ship plate steel was studied by using light mirror,scanning electron microscope,transmission electron microscope,electron backscattering analyzer and other analysis and testing technologies.The microstructure and fatigue resistance of EH36 ship plate steel were studied through fatigue crack growth rate experiment,fatigue fracture toughness experiment and numerical simulation,and the main research results were obtained as follows:（1）The microstructure of the controlled-rolling and controlled-cooling and normalizing EH36 ship plate steel consists of ferrite,pearlite and carbide.The large Angle grain boundary occupies a larger proportion under controlled rolling and controlled cooling process,and the normalizing process has a stronger texture and a stronger{110}and{111}plastic texture.Precipitation strengthening and fine grain strengthening have great contribution to yield strength of controlled rolling and controlled cooling and normalizing processes respectively.The microstructure of precipitates of EH36 ship plate steel under both processes are regular quadrilateral,and their composition is（Nb,Ti）（C,N）.（2）The fatigue crack propagation life prediction formulas of controlled-rolled,controlled-cooled and normalizing EH36 ship plate steel under specific load ratio are calculated as da/d N=1.68×10^-9（?K）^3.32and da/d N=1.07×10^-9（?K）^3.49,respectively.The fatigue fracture toughness value K_{J0.2BL（30）}was calculated by J-integral method to be 464 MPa·m^1/2and 387 MPa·m^1/2,respectively.The dynamic evolution of fatigue crack growth process is analyzed from macroscopic and microscopic perspectives.There are fringe,secondary crack and tear ridge at the fracture of fatigue crack growth.The fatigue fracture toughness fracture shows that EH36 ship plate steel has a mixed fracture mechanism of cleavage transgranular fracture and micropore growth polymerization fracture.（3）With the increase of stress ratio 0.03,0.1 and 0.3,the fatigue crack propagation life of EH36 ship plate steel decreases under the same crack length.Under the same stress intensity factor amplitude,the fatigue crack propagation rate increases.The threshold value of fatigue crack propagation tends to decrease.It is suggested that the fatigue resistance of materials can be improved by increasing the yield strength,increasing the large Angle grain boundary and reducing the average stress.（4）The numerical simulation results of 3D fatigue crack front SIF amplitude and fatigue crack growth life are in agreement with the experimental results,and the average error is small（SIF amplitude 2.358%,residual fatigue life 3.465%）,which can effectively evaluate and predict the fatigue crack growth behavior of EH36 ship plate steel.The results show that the amplitude of SIF increases with the increase of the maximum external load under the same fatigue crack length under the same stress ratio.When the maximum external load is constant,the SIF amplitude decreases with the increase of stress ratio under the same fatigue crack length.When the fatigue load is constant,the amplitude of the fatigue stress intensity factor decreases with the increase of the thickness of the model.