Research On Microstructure And Mechanical Properties Of High Strength Steel Weld Metal

In this paper, the reason of low values and variation of the impact toughness of weld metal was investigated, which was welded by a flux-cored wire used in X70 pipeline; then the effect of Si and Cu on the microstructure and mechanical properties of X120 pipeline steel weld metal was studied, which was welded by metal-cored wire used in X120 pipeline.The work in this paper was divided into two groups:(1) Study on the microstructure, the tensile properties and the impact properties of the weld metal of flux-cord wire used for X70 pipeline steel. The first group is composed of three multipass welds with three different kinds of welding position: flat(PC2048), vertical down(PC2049) and vertical up(PC2224) were conducted from one tensile sample and 10 impact samples respectively which were adjacent each other to in the middle of the welded joint. Tensile tests at room temperature and Charpy V impact tests at-40℃, respectively.(2) Analysis on the microstructure and the mechnical properties of the X120 pipeline steel weld metal. The second group is composed of four kinds of multipass welds, which were designed on the basis of the composition of a new set of welding wire used for X120 pipeline steel, the content of Si and Cu were modified in order to study the effect of Si and Cu on microstructure and properties of weld metal, when the content of Cu is 0.05 wt.%, the content of Si were 0.4wt.%, 1.2wt.%, 2.0wt.% respectively; when the content of Si is 0.4wt.%, the content of Cu is 0.05wt%, 0.2wt.% respectively. The tensile tests were conducted at 20℃,-50℃,-80℃,-110℃,-196℃ and the Charpy V impact tests were conducted at 20℃,-50℃,-80℃,-90℃,-196℃. Observation of the fracture surfaces of tensile and impact specimens(OM and SEM) and metallographic examination of the weld microstructures were analyed. The software of Image J was used to measure the impact fracture’s microscopic parameters.The first part of research shows some reasons related to the the variation and low values of impact toughness:(1) there was coarse grain in weld metals. Cleavage crack were initiated in coarse grain zone, the larger grain size is, the lower the toughness is.(2) Welding defects such as inclusions were the cleavage fracture initiation origins. When the weld defects, such as inclusions, just on or near the position of impact specimen notch root, once these defects were inducted at the path of the crack propagation, the value of impact toughness will decrease sharply.(3) When the position of these defects is near to the notch, at lower applied load, the tensile stress reaches the critical values, thus the smaller impact energy is obtained. In conclusion, the main reason for the impact toughness variation and low values is that the defect or the brittle second phase particle is located in the large grain region and the formed weak region is close to the notch root and sampled by its high stress field, and the nearer coincidence position to the notch root the lower impact energy.The second part of research shows that:(1) In the case of low carbon content(0.06 wt.%), Si also can promote the formation of lath bainite. When the content of Si is increased from 0.4wt.% to 2.0wt.%, the microstructure of the weld metal changes from blocky ferrite + a large amount of granular bainite into lath bainite + a small amount of martensite structure; the overall trend of the mechanical properties is that the strength of weld metal is increased, the plastic and impact toughness were decreased. When the content of Si is 2.0wt.%, tensile properties of the weld metal becomes lower, there are some hydrogen cracks in these weld metals. The reason is that hydrogen concentrated in weld metal, and non-tempered martensite induces high sensitivity to hydrogen induced cracks.(2) When the content of Cu increased from 0.045 wt.% to 0.188 wt.%, Cu suppresses the formation of the granular bainite and the bainite segregation at columnar grain boundary. With the increasing of Cu, the microstructure of weld metal changes from the blocky ferrite + granular bainite + a small amount of lath bainite segregation at columnar grain boundary to bulky blocky ferrite + granular bainite. The fraction of granular bainite was decreased, the fraction of ferrite increased, and the overall microhardness decreased. The overall trend of weld metal mechanical properties is that the strength, the plastic were decreased,and the impact toughness improved a bit.