Research On Fatigue Performance Of Super Duplex Stainless Steel S32760

Super duplex stainless steel S32760 is an engineering steel with high strength and excellent corrosion resistance.It has good development prospects as a material for offshore oil mining equipment.Alternating loads in the ocean caused by typhoons,tides,ocean currents,etc.act on the working equipment,and after a certain accumulation,the materials cause fatigue damage;on the other hand,seawater is rich in various salts,and some onshore performance The good material has a significant decrease in mechanical properties due to the strong corrosion of seawater,especially the presence of high concentrations of chloride in seawater,which leads to corrosion fatigue of the material.The participation of corrosive media further accelerates the rate of fatigue crack growth.Based on this,the fatigue performance of super duplex stainless steel S32760 in two different environments in air and 3.5%Na Cl solution was studied by rotating bending fatigue test,the fatigue limit and stress life curve were measured,and the fatigue failure mechanism was studied as S32760 stainless steel structural parts provide a reliable basis for anti-fatigue design,fatigue life prediction and improved technology to improve their anti-fatigue performance.The research contents and conclusions are as follows:?1?The fatigue limit of super duplex stainless steel S32760 in air was556.667MPa and the fatigue ratio was 0.54 measured by the kid-like lifting method;the fatigue limit in 3.5%Na Cl was 490MPa and the fatigue ratio was 0.47.Compared with the fatigue limit in air,it decreased by 66.667MPa,and the decrease was about12%.?2?The S-N curve of S32760 is measured by the group method.The power function expression for fitting the S-N curve in the air in Origin is S=1055.4285N^-0.04136,and in 3.5%Na Cl it is S=832.12404N^-0.03367,two The S-N curves under various working conditions are linearly distributed on single logarithmic coordinates.?3?Scanning electron microscopy analysis of the air break shows that the inclusion of slag and accompanying pores in the steel is more likely to cause crack initiation than inclusions,and the fatigue performance of the material is weakened more than the inclusions.After the fatigue crack enters the first stage of propagation,the austenite phase and part of the ferrite phase slip fracture and the secondary phase?phase and part of the ferrite phase cleavage fracture.After the cracks propagate from the austenite phase to the ferrite phase through the secondary cracks,the decrease in the growth rate is not entirely caused by the increase in strength.The slowing of the crack growth rate caused by the secondary cracks is also one of the reasons.In fatigue crack growth,diversified crack growth mechanisms are beneficial to reduce the growth rate.The fatigue fracture surface of S32760 is mostly characterized by ductile fractures such as slippage,tearing corrugation and dimples.Brittle cleavage fractures are relatively rare and no intercrystalline fractures occur.?4?In the 3.5%Na Cl solution,the fatigue source of the super duplex stainless steel S32760 corrosion fatigue fracture comes from the impurities remaining in the metallurgical process.Among them,the impurities composed of silicate,Ca S and Al2O3 are caused by the initiation of cracks due to the alternating load.The residual The deoxidizer Al caused crack initiation by electrochemical corrosion.Na Cl solution promotes the transformation of ferrite in duplex stainless steel from slip fracture to cleavage fracture,which increases the brittle fracture of duplex stainless steel.In the second stage of fatigue crack growth,the growth rate of fatigue cracks in austenite is greater than that in ferrite.The propagation of cracks is dominated by transgranular fracture.The common intergranular fracture in corrosion fatigue occurs only at the secondary crack parallel to the rolling surface.Super duplex stainless steel S32760 is in a passive state in 3.5%Na Cl solution,which will not produce stable electrochemical corrosion,but will accelerate the rate of fatigue crack propagation inside the metal matrix.