13th International Conference on Fracture June 16–21, 2013, Beijing, China 1 Microstructure and properties of welding joint of 8%Ni high strength steel R. Cao1, J. H. Chen1 , Y. Peng2 1.State Key Laboratory of Gansu Advanced Non-ferrous Metallic Materials and Key Laboratory of Non-ferrous Metal Alloys of the Ministry of Education, Lanzhou University of Technology，Lanzhou 730050, China 2. Central Iron and Steel Research Institute, Beijing 100081, China * Corresponding author: zchen@lut.cn(J.H.Chen), caorui@lut.cn(R.Cao) Abstract: This paper analyzes the mechanism of unmoral phenomena that in the HAZ of 8% Ni high strength steel, the charpy V toughness at -50℃ in the coarse grain HAZ is higher than that of fine grain HAZ. It is caused of different fracture. The former fracture is induced by void growing until impingement-resulting coalescence in the CG-specimen, and the latter fracture is caused by forming shear sheets of the secondary voids to connect the neighboring primary voids before their impingement. This paper reveals that the critical event for cleavage fracture in this high strength steel and weld metals is the propagation of a bainite packetsized crack across the packet boundary into contiguous packets and the bainitic packet sizes control the impact toughness. The high-angle misorientation boundaries detected in a bainite packet by EBSD form fine tear ridges on fracture surfaces. However, they are not the decisive factors controlling the cleavage fracture. The effects of Ni content are essential factors for improving the toughness. The extra large cleavage facets seriously deteriorate the toughness, which are formed on the interfaces of large columnar crystals growing in welding pools with high heat input. Keywords 980MPa 8%Ni high strength steel, weld metal, impact toughness, fracture, high-angle misorientation boundaries 1. Introduction Due to its coarse grains and high hardenability, the CGHAZ(CG), which is overheated to a high temperature (up to 1350℃) shows a lowest ductility and toughness and limits the performance of welding structure [1]. On the contrary the FGHAZ (FG), which is heated to the normalization temperature (850-930℃) and has much finer grains, always shows the mechanical properties (including strength, ductitlity and toughness) superior than other regions within the HAZ. In the our paper, the abnormal phenomena that the Charpy V toughness in simulated CG are higher than those in the simulated FG are investigated with analyzing the different ductile fracture micromechanisms in associated microstructures formed by heating to 1320℃ and 900℃ [2]. With sustained improvement of the properties of high strength steels, the impact toughness has reached around 200J at lower temperatures of -50oC for 980MPa grade steels[2]. However the toughness of the weld metals with matching strength is appreciably lower than this value. How to increase the toughness of weld metal through the modification of the microstructure has been attracting a great attention of welding metallurgists. The microstructural parameter, which controls the toughness of weld metal may be the grain size, martensite or bainite packet size, the size of area with misorientation angle larger than 15o and the brittle second phase particle size[3-8]. In this work, we use instrumented Charpy V tester, OM, SEM, TEM, EBSD and FEM calculation to identify which one in above microstructural parameters really controls the fracture and the toughness of impact Charpy V specimens. In addition, the effects of nickel addition are investigated. 2. Materials and procedures Compositions of an 8% Ni 980MPa grade steel and various weld metals in Table 1 were used. Specimens cut in the rolling direction were heated by a heat simulating machine Thermorestor W to preset temperatures: FG (900℃), CG (1320℃), 730℃ and 600℃. The simulated heating procedures were designed as those shown in Table 2.Double heating simulations were carried out for temperatures (1320℃+1320℃, 1320℃+900℃, 1320℃+730℃, 900℃+1320℃, 900℃+900℃, and 900℃+730℃). Welding parameters of various welded joints are presented in Table 3. The microstructures were observed by optical microscope (OP), scanning electron microscope (SEM 6700F) and transmission electron microscope (TEM JEOL2001). The grain sizes were measured by linear section method. ZEISS ULTRA 55 Field-Emission Scanning Electron Microscope was used to collect EBSD grain boundary maps and to define the orientations of microstructural features. Retained austenite films were
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