Effect Of Cobalt On Microstructure And Properties Of Maraging Stainless Steel

Maraging stainless steel is a new type of ultra high strength steel developed in recent years.It has excellent comprehensive properties and is widely used in the fields of aerospace and marine chemical industry.The main problem of maraging stainless steel is that when its strength increases,its plastic toughness decrease obviously.Cobalt is particular when it is used in stainless steel,its main function is to improve the strength and hardness of steel.But the influence of Co on martensitic matrix,reverse austenite and precipitate phase is rarely reported.Therefore,15-5PH stainless steel with different Co content?0,3,6,9 wt.%?was used as the research object.The microstructure of maraging stainless steel was studied by metallographic microscope?OM?,scanning electron microscope?SEM?,transmission electron microscope?TEM?and X-ray diffractometer?XRD?.Through the analysis of tensile test,hardness test,impact test and fracture toughness test,the change regulation of the mechanical properties with Co was mastered.The influence of heat treatment and Co content on the matrix structure of maraging stainless steel,the content of reverse austenite and the?-Cu phase were investigated.The main results were as follows:?1?Solution treatment from 1040 to 1100?,the original austenite grain size of the steel increases with Co content from 18.62²3.35?m to 23.63³0.35?m.The hardness of steel decreases from 283.2²69.9 HV to 274.6²65.6 HV.The solid solution conditions in this paper are determined to be at 1070?+50min+air cooling.?2?The optimization of the aging treatment parameters shows that with the increase of aging temperature,the size of lath martensite bundle and the width of lath martensite increases,the volume fraction of the austenite is increased,the amount and the size of the?-Cu phase increases.With the increase of aging temperature,the microhardness,tensile strength and yield strength decreases gradually.The elongation,the section shrinkage,the impact toughness,and the fracture toughness increases gradually with the increase of aging temperature.When the aging time increases,the hardness of steel increases,the strength increases first and then decreases and the elongation increases.The?-Cu phase is not obvious,but as the aging time prolongs,a large amount of fine?-Cu phase separates out and the volume of the inversion austenite increases.Because of the wide range of Co content of the test steel studies in this paper,the best aging system is 475⁵00?,holding time 5h+air cooling.?3?The alloying of Co results in the disappearance of high temperature ferrite and harmful phase in the test steel,and Co is dissolved in the matrix structure to improve the strength of the material.The increase of Co content will lead to the austenite grain size and martensitic plate size decreases first and then increases,but all are smaller than the size of austenite grain and martensitic lath without Co test steel.The synergistic effect of Cu-Ni will promote the formation of inversion austenite and increase the plasticity and toughness.?4?With the increase of Co content,the strong hardness of maraging stainless steel increases first and then decreases,while the elongation,section shrinkage,impact toughness and fracture toughness increases.The study shows that the best comprehensive properties of steel when the content of Co is 6 wt.%,in this condition the hardness is 434 HV,tensile strength is 1343 MPa and yield strength is 1154 MPa,the rate is 16%,the impact energy is 95 J,and the fracture toughness is 81.5 MPa?m^0.5.