Study On Relation Between Microstructure And Mechanical Behavior Of A 2.5 GPa Maraging Steel

When the tensile strength of maraging steel reaches to 2.5 GPa or above,its fracture toughness will decrease sharply which limits its industrial applications.Hence,the issue of how to improve the fracture toughness without sacrificing its strength has become an important research direction for maraging steel with high strength and toughness.In general,the high strength and good toughness of maraging steel are realized by the combination of solid solution strengthening,phase transformation strengthening and precipitation strengthening,and the precipitation strengthening induced by nano-sized precipitates which formed in the aging process contribute the most to strength and toughness.Therefore,study on the evolution of precipitates during aging process and its relationship with strength and toughness will be helpful for understanding the strengthening and toughing imechanism.In addition,it will also provide relevant ideas to avoid the trade-off dilemma between strength and toughness.In this thesis,the heat treatment process of the 2.5 GPa grade maraging steel has been optimized to the following:solution treatment at 820? for 1 h +cryogenic treatment for 1 h+aging treatment at 480 ? for different time.The tests on mechanical properties show that with increasing aging time,the Lultimate tensile strength of the 2.5 GPa grade maraging steel increases from 800 MPa at cryogenic condition to 2560 MPa at peak aging condition.On the contrary,the fracture toughness reduced from 106 MPa·m1/2 to 20 MPa·m1/2.After peak aging treatment,the tensile strength of the maraging steel begins to decrease,and the fracture toughness gradually increased.Besides,the fatigue crack growth rate of the maraging steel shows a same tendency with ultimate tensile strength during aging process.Observation on the microstructure indicates that the matrix is composed of lath martensite with high density dislocations and some volume fraction of austenite after cryogenic treat,ment,no clustering and precipitation have been detected by atom probe tomography.It is found that at the initial stage of aging treatment,Ni and Ti atoms segregated rapidly and Ni-Ti clusters can be found in the matrix,when the aging time is 4 h,the Ni-Ti clusters transform into Ni3Ti,and the Mo-rich phase appears at the Ni3Ti/matrix interface.With the elongation of aging time,especially when the aging times are 48 h and 96 h.it is found that N i3Ti phase starts to coarsen and the Mo-rich phase transform to Ni3Mo.It should be noted that the volume fraction of austenite in the matrix always keep increasing tendency during the whole aging process.Finally,based on the results about the evolution of precipitates during aging process,the effect of the morphology of the precipitates on both the strength and toughness is discussed.