Study On Microstructure Properties And Precipitation Behavior Of High-Heat Input Welded Joint Of Copper-Bearing Marine Steel

In this thesis,Cu-containing steel is taken as the research object.By comparing the differences in microstructure and mechanical properties of the modulated base metal with different components,the influence of chemical composition on the microstructure and properties of the base metal is determined.The simulation welding thermal cycle experiment is carried out,and the microstructure and mechanical properties of the welding heat affected zone of experimental steel under different heat input conditions are characterized by experimental means.The evolution mechanism was analyzed by coupling the relation between microstructure and mechanical properties.The welding of 0.98Cu1 Ni steel,1.3Cu1 Ni steel,1.62Cu1 Ni steel and1.3Cu0 Ni steel was carried out on site.Then the microstructure and mechanical properties of the heat affected zone were analyzed to observe the macroscopic distribution and microstructure of the weld.The results are as follows:(1)By comparing the base material 0.98Cu1 Ni steel,1.3Cu1 Ni steel and1.62Cu1 Ni steel,it is found that with the increase of Cu content,the hardness value gradually decreases,the yield strength and tensile strength are gradually increased,and the morphology and quantity of bainite in steel have changed obviously.The Cu content in the white granular Cu-rich phase also increased.(2)The comparison between 1.3Cu1 Ni steel and 1.3Cu0 Ni steel shows that,when Cu content is constant,the hardness without Ni is higher,the tensile strength is lower and the toughness is poorer than that with Ni.The bainite in Ni-bearing steel mostly exists in the form of island,and the residual austenite is more.The granular bainite in 1.3Cu0 Ni steel is mostly black granular.This difference in microstructure indicates that Ni can effectively stabilize austenite,improve the enrichment and precipitation of Cu,and improve the strength and toughness of the test steel base metal.(3)In the welding thermal simulation experiment,the microstructure of0.98Cu1 Ni steel and 1.3Cu0 Ni steel under different heat input was observed.It was found that with the increase of heat input,the speed of phase transition slowed down,which was conducive to the formation of multi-ferrite,the increase of residual austenite,the growth of bainite,the precipitation time of Cu became longer,and the content of Cu in the precipitated substance increased.It shows that large input is conducive to the enrichment and precipitation of Cu in experimental steel.However,when the speed is too slow,pearlite will be formed in the steel at the heat input of200 k J/cm.(4)In the actual welding experiment,it is found that with the increase of Cu content,the ferritic size of 0.98Cu1 Ni steel,1.3Cu1 Ni steel and 1.62Cu1 Ni steel is smaller,and the hardness of the coarse crystal zone is gradually increased,indicating that the increase of Cu content improves the performance of the heat affected zone.At the same time,the microstructure of the fine crystal zone of 1.3Cu1 Ni steel and1.3Cu0 Ni steel is very different.The fine crystal zone of 1.3Cu1 Ni steel contains a lot of pearlite and ferrite,while that of 1.3Cu0 Ni steel does not contain Ni element,and C will precipitate together with Cu-rich phase in the form of carbide,which reduces the nucleation rate of pearlite.The result shows that Ni can not only improve the strength of base metal,but also improve the strength of steel after welding.