Study On Fatigue And Corrosion Fatigue Characteristics Of Maraging Steel

Maraging steel 18Ni?250?has extremely high strength,excellent formability,simple heat treatment process,and minimal aging deformation,as an early rocket engine shell material which meets the design requirements of large-ocean-depth pressure hull and deep-ocean manned submersible.Studying the fatigue characteristics of 18Ni?250?in air and solution and exploring the factors that affect the characteristics of corrosion fatigue are of great significance for the application of 18Ni?250?in offshore structures.The main contents and conclusions of this study are as follow:?1?The fatigue tests of 18Ni?250?were conducted,then fatigue analysis of 18Ni?250?was performed with Fe-safe,the load parameters of fatigue analysis were consistent with the fatigue tests.The results showed that the distribution of fatigue life in the working section was not uniform and the positional fatigue life fluctuated obviously,but it would become uniform as the load increased.Under large load,the results of fatigue tests with constant load amplitude were basically consistent with the analysis of Fe-safe.Under small load,due to the effect of defects and test errors,fatigue life analyzed by Fe-safe was higher than the fatigue life of tests.?2?The fatigue tests of 18Ni?250?with constant load amplitude were conducted,then S-N curves were fitted to the corrosion fatigue life,and the best fitting equations were selected.Combined with S-N curves,the effects of load amplitude,frequency f and stress ratio R on corrosion fatigue life of 18Ni?250?were studied.The results showed that the change of load amplitude would lead to the change of corrosion damage.The larger the load was,the smaller the corrosion damage N_Dwas.Obviously,under small load,the effect of corrosion was great.Increasing load frequency f or stress ratio R both could improve the corrosion fatigue life of 18Ni?250?.However,the effect of frequency f was very unstable and the difference under large load and small load was great;while the effect of increasing stress ratio R was stable and controllable,and there were no large fluctuations due to changes in load amplitude.For structures made of 18Ni?250?,it was better to increase stress ratio R to improve the corrosion fatigue life.?3?In air,fatigue crack of 18Ni?250?propagated in a quasi-cleavage way;in 3.5%Na Cl solution,intergranular cracking and plastic deformation coexisted,and the more severe the intergranular cracking,the shorter the fatigue life.Decreasing frequency f and stress ratio R would exacerbate intergranular cracking and affect the initiation of corrosion pits and cracks.A decrease in frequency f would promote the initiation of corrosion pits.A decrease in stress ratio R would not only exacerbate corrosion but also lead to formation of large-cracks.The propagation mode of corrosion fatigue crack under low load was mainly intergranular,as the loading amplitude increased,the propagation mode gradually evolves to intergranular cracking+cleavage and intergranular cracking+quasi-cleavage.Anodic corrosion occurred on the surface of 18Ni?250?in 3.5%Na Cl solution,and Fe₂O₃was the final precipitate of anodic corrosion.?4?Corrosion fatigue life with constant load amplitude of 18Ni?250?was used to predict corrosion fatigue life with variable load amplitudes,and the accuracy of prediction was verified by corrosion fatigue tests with variable load amplitudes.The results showed that loads staggered,and large load with small cycle,small load with large cycle,in which mode the results of tests were basically consistent with the prediction.Miner criterion was effective for predicting the corrosion fatigue life with variable load amplitudes,which was accurate and reliable,and the error was small.