In-situ Research On Microstructure Evolution Of Copper-bearing Steel During Aging

Cu precipitation strengthened steel is a low-alloy structural steel which takes into account good strength,toughness matching and excellent welding performance.Through solution quenching+aging heat treatment process,it is widely used in large machinery,offshore drilling platform,shipbuilding and other fields.In this paper,the Cu-containing precipitation-strengthened steel produced by a domestic steel mill was used as the research object,and the dynamic structure transformation of its austenite continuous cooling process,solution quenching process and aging process was studied by high temperature-confocal laser scanning microscope(HT-CLSM).Combining diffrent heat treatment processes,the microstructure and properties of Cu-containing precipitation strengthened steels after quenching and aging were studied.Using metallographic microscope(OM).scanning electron microscope(SEM),transmission electron microscope(TEM)and other instruments to analyze and characterize the structure of the test steel.So that the law of the evolution of the structure and the changes in properties under different cooling rates,different Ni contents,different aging times,and different aging temperatures were studied.The conclusions reached are as follows:(1)Taking the formation of parallel bunched martensite when the overall martensite proportion is about 5%as the criterion,the Ms point of the copper-containing steel with 3%Ni content measured by the high temperature-confocal laser scanning microscope at 20℃/s is 447.6℃.In the field of HT-CLSM view,it is observed that martensite and bainite are preferentially formed at the grain boundaries of prior austenite,the relief produced by martensite is deeper with explosive and phased trends,and most of them gather and distribute in parallel bundles,the whole is relatively regular;the relief produced by bainite is relatively shallow,showing a tendency to spread and grow,and at the same time there may be "interlocking" phenomenon,and the whole is relatively messy.As the cooling rate increases from 5℃/s to 20℃/s,the microstructure evolves from bainite to mart ensite,and the samples’ microstructure increases from 263.4 HV to 301 HV.(2)Regardless of the quenched state or the aging state,the hardness of Cu-containing precipitation strengthened steel shows an increasing trend with the increase of Ni content.With the increase of Ni content from 1%to 3%,the microhardness of the quenched state increased from 309.0 HV to 333.6 HV,and the microhar dness of the aging state increased from 240.2 HV to 251.8 HV;the increase of Ni content reduces the initial phase transformation point in the quenching process of Cu precipitation-strengthened steel,improves the hardenability of Cu-containing precipitation-strengthened steel,and reduces the size of martensite lath.(3)For the Cu-containing precipitation hardened steel with 3%Ni content,after aging for 2 h at different aging temperatures,the results show that,as the aging temperature increases,the microhardness of the material first decreases and then increases at 475℃ to reach a peak of 337.7 HV,and then the hardness value gradually decreases,reaching a.minimum value of 246 HV at 650℃,and there is a small range recovery to 256.4 HV at 700℃.The martensite embossment in the quenched state is very obvious.During the aging process,the degree of surface embossment is inversely related to the aging temperature.After aging at different temperatures for 2 h,the lath martensite gradually decomposes and tranforms into polygonal ferrite.At 700℃,granular bainite and M-A components are produced in the structure.There is a positive correlation between the size of copper-rich precipitates and the aging temperature.As the aging temperature increases,the copper-rich precipitates gradually become coarser,and their morphology changes from dots to small spheres and short rods.(4)For the Cu-containing precipitation hardened steel with 3%Ni content,after aging at 650℃ for different times,the results show that,the microhardness of the material increases first and then decreases with the extension of the aging time,reaching the peak hardness of 268.3 HV when the aging time is 1h.During the aging process,the martensite matrix recovers and evolves to polygonal ferrite The average width of the lath increases from 290 nm to 386 nm,the surface relief becomes weaker and weaker,and the original austenite grain boundaries become blurred.Before the aging peak(t=1h),the copper-rich precipitates in the matrix are dispersed and fine,after passing the aging peak,the copper-rich precipitates grow up and coarser,and the morphology gradually changes from point to small spherical and short rod-shaped.