Study On The Difference Of Strength And Toughening Mechanisms Between Cu-containing Aged Steel And NiCrMoV Steel

Based on the traditional NiCrMoV steel,Cu-containing aged steel was developed by reducing carbon content and adding a small amount of Cu.In this paper,both test steels are quenched at 900? for 1 hour and tempered for 2 hours at 630?.The difference of strengthening and toughening mechanism between Cu-containing aged steel and NiCrMoV steel was studied.In this paper,the microstructures of two kinds of test steels were studied by means of SEM,EBSD and TEM.In particular,the structures near the tensile fracture were compared and analyzed.The following conclusions are drawn:1)After quenching,the structure of Cu-containing aged steel is mainly lath martensite and partially granular bainite.The structure of NiCrMoV steel after quenching is martensite with obvious lath characteristics and good hardenability.After tempering treatment,the martensite of Cu-containing aged steel is obviously decomposed,the matrix is mainly granular bainite,and a small amount of martensite still exists in some areas.The martensite of NiCrMoV steel is also decomposed,but the martensite character is more obvious than that of Cu-containing aged steel.The main reason for this phenomenon is that the carbon content of aging steel containing steel is relatively low.TEM results show that the martensitic lath width of Cu-containing aged steel is relatively large.2)The results of EBSD analysis show that,The matrix structure of quenched Cu-containing aged steel and NiCrMoV steel is mainly lath martensite.The coexistence of grain boundaries with large and small angles illustrates the multi-scale structure of lath martensite.The distribution of grain boundary orientation difference shows bimodal characteristics,which is mainly due to the special orientation relationship between bainite or martensite and austenite.The martensite characteristics of Cu-containing aged steel and NiCrMoV steel after tempering are obviously weakened.However,the characteristics of lath martensite in NiCrMoV steel are relatively obvious,and the grain boundary orientation difference distribution of tempered structure also presents a bimodal feature.3)The mechanical properties at room temperature were tested.The strength of tempered aging steel is lower than that of NiCrMoV steel,but its elongation is higher.TEM analysis near the fracture surface shows that the second phase particles in NiCrMoV steel are Cr23C6 particles.There are two types of second phase particles in Cu-containing aged steel:The size of Cr23C6 particles distributed along the slab boundary is about 150 nm,and Randomly distributed Cu particles containing 10 to 20 nm in size,The interaction between Cu-containing particles and dislocation may be the reason for the better plasticity of Cu-containing aged steel.