Study On Microstructure And Fatigue Behavior Of Heat-affected Zone In 690MPa Grade Extra-thick Rack Steel

The large-section extra-thick rack steel with a yield strength of 690 MPa and the halfchord plate of the same strength are welded to form a jack-up offshore platform lifting leg structure,which is fixed in the pile boots of the seabed and supports the lifting weight of the entire platform,used as the main bearing parts of cyclic loads such as nonlinear dynamic waves under complex sea conditions have anti-fatigue fracture performance and service life,which play an important role in the overall platform safety.The pile leg rack is composed of extrathick steel plate in quenching +tempering state.Flame cutting and welding are the main processing methods for manufacturing the pile legs.The upper and lower surfaces and the semicircular chord are welded together along the length to form a pile leg.The pile legs are butt welded to the end of the rack to increase the length,so as to assemble a pile leg of 100 to 200 meters.The flame cutting and welding process forms a heat-affected zone(HAZ)of a certain depth in the tooth surface of the rack steel,the joint between the rack steel surface and the halfchord plate,and the joint portion of the rack steel end.According to the survey,after the jackup offshore platform has been in service for a period of time,these HAZs are prone to fatigue cracks,especially in the splash zone where the corrosion enviroriment is severe.Therefore,it is important to study the fatigue performance of the HAZ of rack steel.This thesis mainly studies the microstructure and high cycle fatigue performance of the flame-cutted HAZ and the welded HAZ of 690 MPa grade rack steel.The main research results are as follows:(1)Microstructure difference:In the HAZ,the coarse-grained zone consists of coarse martensite,lath bainite and granular bainite;the fine-grained zone consists of lath bainite and granular bainite.A small amount of ferrite is formed;the matrix structure is composed of tempered martensite and bainite;the mixed crystal region is composed of martensite and bainitic ferrite structure.(2)Difference in tensile strength at room temperature:The yield strength of flame cutted HAZ is 845 MPa,and the tensile strength of that is 990 MPa,the yield strength of the matrix is 700 MPa,and the tensile strength is 850 MPa.The yield strength of welded HAZ is 745 MPa,and tensile strength of that is 875 MPa,the yield strength of the matrix is 710 MPa,and the tensile strength is 845 MPa.(3)Hardness difference:It is reflected from the hardness test that there is a very thin martensitic hardened layer on the flame cutted surface,the thickness is about 0.1 mm,the hardness is 360 HV;the hardness of the coarse-grained zone is 290 HV,and the width is about 1.2 mm3 accounting for 1 7%of the HAZ;the hardness of the fine grain zone is 330 HV,the width is about 2.8 mm,accounting for 40%of the HAZ;the hardness of the mixed crystal zone is uneven,falling along the heat input direction,and the width is about 3 mm,accounting for 43%of the HAZ;the heat affected zone of the welded rack steel is about 5 mm,the hardness of the coarse grain zone is 320 HV,the width is about 2 mm,which accounts for 40%of the HAZ;the hardness of the fine grain zone is 380 HV,and the width is about 2 mm,accounting for 40%of the HAZ;the hardness of the mixed crystal zone is also unevenly distributed,and the hardness decreases along the heat input direction,accounting for 20%of the HAZ.(4)Fatigue performance difference:High-cycle fatigue experiments were carried out on the heat-affected zone of the flame-cutted and welded rack steel and the corresponding matrix respectively,and the S-N curve and fatigue limit of the four sets of samples were obtained.The condition fatigue limit of flame-cut rack steel is 540 MPa,the fatigue limit of flame cutted rack steel matrix is 560 MPa,the fatigue limit of welded rack steel HAZ is 650 MPa,and the fatigue limit of welded rack steel matrix is 675 MPa.The fatigue limits of the HAZ are reduced compared to the matrix.(5)Effect of seawater corrosion on fatigue performance:The effect of corrosion on fatigue performance was studied by simulating seawater corrosion treatment of samples in HAZ.After observing the fatigue fracture morphology by SEM,it is found that the corrosion pits appear mostly at the original austenite grain boundary,indicating that the original austenite grain boundary is easily corroded to form a crack source.