Study On Corrosion Fatigue Crack Propagation Of Maraging Steel

Under the action of Marine corrosive media,the occurrence of pore corrosion,intergranular corrosion and denudation will cause local stress concentration and promote the initiation of fatigue cracks.Stress corrosion at the crack tip and deep sea high pressure load will accelerate the crack growth rate,resulting in failure and corrosion fatigue of the structure.Maraging steel 18Ni（250）has high strength,hardness,excellent toughness and good plasticity which meets the requirements of the deep-sea pressure spherical hull and other deep sea equipments.Therefore,it is of great significance to study the corrosion fatigue properties of maraging steel 18Ni（250）and establish the corrosion fatigue crack propagation model for the practical application of Marine equipment.The main research contents and conclusions of this thesis are as follows:（1）The corrosion fatigue crack growth characteristics of 18Ni（250）three-point bending specimens of maraging steel 18Ni（250）under different stress ratios were studied,and the corrosion fatigue crack growth characteristics were compared with those in air environment,and the mechanism of corrosion fatigue crack growth was analyzed.The results showed that the corrosion fatigue crack growth rate when the stress ratio was 0.1 was larger than that of the stress ratio was 0.3.And the crack growth rate under corrosion environment was about 2.1 times that under air environment.The micropore aggregated fracture was a typical quasi-cleavage morphology in the crack propagation mode under corrosive environment.（2）The stress corrosion process of 18Ni（250）was analyzed from the perspective of energy conservation.The stress corrosion crack growth rate was calculated from the theoretical analytical perspective.The process of stress corrosion crack propagation was simulated based on FRANC3D software.The results showed that the theoretical analytical values were consistent with the software simulation values in a certain range.And the results showed that the FRANC3D simulation method was effective.（3）Fatigue crack propagation and corrosion fatigue crack propagation were numerically simulated by FRANC3D software.Compared the simulation results and the experimental results,the error between the numerical simulation value and the experimental data was less than 10%.The results showed that the numerical simulation method was reliable.The influence of loading frequency on the crack growth rate of corrosion fatigue was discussed by numerical simulation.The results showed that the loading frequency had little effect on the corrosion fatigue crack growth rate in a certain range.The crack growth rate at lower frequencies was higher than that at higher frequencies.（4）The model of corrosion fatigue crack growth rate was modified according to the cumulative model by introducing environmental factor C_env and stress ratio factorβ.The model of corrosion fatigue crack growth rate of maraging steel 18Ni（250）was established.The parameter value was solved by using numerical simulation results.The model was verified by the experimental results.And the rules were similarity.