Study On Flux-cored Wire For Offshore Engineering And It's Welded Joints

At present,most domestic flux-cored wires used for marine engineering over 440 MPa have problems,such as low temperature toughness and unstable welding processability.In this thesis,five kinds of flux-cored wires were trial-produced,and their strength and toughness mechanisms were analyzed.Then the influence of heat input on the microstructure and properties of flux-cored wire welded joints was studied.Finally,the fracture toughness of the welded joints under a heat input was investigated.Through the system research above,we provide theoretical basis and technical support for the engineering application of flux-cored wire for marine engineering.The strengthening and toughening mechanisms of five flux-cored wires were studied.It can be seen that solid solution strengthening is the main strengthening method for the five flux cored metal deposits.The increase of Ni(0.06% to 1.0%)is not significant for the solid solution strengthening,the increase Cu and Ti(Cu content increases from 0.025% to 0.24%,Ti content increased from 0.004% to 0.025%)will enhance the effect of precipitation strengthening;Ni and Cu elements improve toughness by increasing the content of acicular ferrite and grain refinement;Ti element improves the toughness by increasing the oxide inclusions of Ti and the nucleation core of acicular ferrite.In summary,the comprehensive mechanical properties of G5 flux-cored wire deposited metal(Ni content 1.0%,Cu content 0.24%,Ti content 0.025%)are optimal.The microstructure and properties of the welded joints under three kinds of heat input(8 kJ/cm,14 kJ/cm,20 kJ/cm)were studied.It is known that the microstructure of the final weld is pre-eutectoid ferrite,side-slat ferrite,acicular ferrite,and a small amount of retained austenite.With the increase of heat input,the acicular ferrite content decreased from 75% to 58%,the average ferrite size increased from 0.4?m to 1.1?m,and the retained austenite between the laths changed from film to bulk.Weld inclusions are mainly composed of high melting point oxides and MnS composite inclusions.With the increase of heat input,the average size of inclusions increases from 1.04 ?m to 1.49 ?m,and below 1.25 ?m,the proportion of inclusions that promote acicular ferrite nucleation decreases from 59% to 48%.The distribution ratio of the sector has dropped from 83.3% to 73.7%.The above micro-texture changes lead to a decrease in weld strength and low temperature toughness.The microstructure of the coarse-grained zone in the heat-affected zone under the three kinds of heat input is mainly lath bainite.With the increase of heat input,the prior austenite grain boundaries and the slat bainite size in the grain increased,and the parallel slat beam increased,resulting in the impact work at-40°C dropping from 129 J to 90 J.Considering the actual engineering welding cost and offshore platform index requirements,the heat input of 14 kJ/cm is most suitable.In order to find out the law of the fracture characteristics of engineering welded joints,a series of temperature impact tests were carried out.The FATT of the weld seam at 14 kJ/cm heat input is-40 °C,and the FATT of fusion line is-55 °C.Through 7 sets of CTOD experiments,the delta value of the weld seam is 30.08%,and the single value is greater than 0.15 mm.This satisfies the requirements of the crack resistance of marine engineering.