| Low carbon bainitic high strength steel JG950belongs to1000MPa grade engineering mechanical welding steel featured by ultra low carbon, high purity, super fine grain, high strength and toughness, which is the upgraded product of low alloy steel as well as the alternative to plain carbon steel plate. This kind of steel is widely used in heavy-duty industries, e.g. mining machinery, bridges, railways, cranes, etc., and it is internationally recognized as a new generation steel of21st century. The weldability of this steel is an urgent issue to be solved because it possesses high strength and complex alloying system and poor weldability. Some new problems are generally arised from the welding in the heat affected zone and how to deal with these problems becomes significant for the engineering application. In this thesis, welding thermal simulation and welding technology are used to study weldability of JG950low carbon bainitc high strength steel. The main work is as follows:(1) By measuring the SH-CCT diagram of tested steel with welding thermal simulation, the influence of cooling rate on microstructure and hardness of CGHAZ was obtained to reveal the phase transition of HAZ at different cooling rates. The results indicate that intermediate temperature phase transformation to bainite can occur in a wide range of cooling rates. The microstructure of HAZ is mainly martensite at fast cooling rate (≥40℃/s). The hardness decreases gradually with the increase of t8/5and the maximum hardness is425HV0.1.(2) The welding thermal simulation, electron microscopy analysis and mechanical property test were adopted to study the effect of peak temperature and cooling rate on the microstructure and properties of heat affected zone. The results show that CGHAZ is the brittle zone in single-pass welding thermal cycle and has the highest hardness value. Microstructural analysis indicates that the main reason for the deterioration of toughness is the grain coarsening of original austenite. All reveal that the tested steel is not suitable for single pass welding. The microstructure of CGHAZ contains two main types of bainites, i.e. granular and lath. When t8/5is in the range of5-45s (corresponding to E of7.52-22.56kJ/cm), the shape, distribution and amount of M-A constituent change with t8/5and it has a significant impact on the toughness of CGHAZ, at the same time the effective austenite grain size in CGHAZ increases with the increasing of t8/5. Therefore, the impact energy increases at first and then decreases with the increasing of t8/5. When t8/5is30s, the toughness of coarse grained region is the best. Besides, the hardness of the heat affected zone is lower than the base metal as t8/5exceeds30s so that CGHAZ will be softened. So, the important measures to ensure the strength of welding joints are to control the heat input and to reduce the heat affected zone softening.(3) In multi-pass welding, low-temperature toughness can be effectively improved by tempering CGHAZ by post welding-pass. When the peak temperature is in SCR CGHAZ, the low-temperature toughness reaches the best due to the recrystallization of coarse grains. Meanwhile, toughness of UA CGHAZ can be improved to some extent because carbon and alloy elements can sufficiently diffuse and impurity elements get partially diffused and remelted, the microstructure become relatively uniform as well. Local brittle phenomenon appears in the experimental steel when the peak temperature is below AC3, and this phenomenon is caused by the inhomogeneous grain size and coarse M-A constituents. The low-temperature toughness of CGHAZ can be effectively improved by the secondary thermal cycle.(4) The weldability of the steel was theoretically analysed and the cold crack sensitivity was evaluated by oblique Y-groove cracking and HAZ maximum hardness tests. It is shown that this steel has low quenching hardenability and cold or hot crack sensitivity under the experimental conditions. Increasing preheating temperature or heat input can reduce the maximum hardness of HAZ so as to obtain low hardenability. The ratio of cracks decreases with the increasing of preheating temperature. Even if in severe constraint conditions, preheating temperature of100℃is sufficient to avoid cold crack.(5) The experiments of mixed gas metal arc welding under different welding processing parameters were carried out. Mechanical property tests such as tensile, bending and impact test were utilized to evaluate the mechanical behavior of welding joint. Moreover, microstructure and fracture behavier of welding seam, fusion-line and HAZ were investigated by means of optical microscope and scanning electron microscope (SEM). The effects of heat input and inter-bead temperature on microstructure and mechanical property of welding joint were systematically analyzed. The impact of microstructure on strength and toughness of welding joint was studied to explore the adaptability of the steel to the welding technology. The results indicate that lower heat input is suitable for the steel and excellent comprehensive mechanical properties can be obtained by using80%Ar+20%CO2gas metal arc welding and GM120welding wire as the filler and10.38-15.26kJ/cm as the heat input energy and100℃or so as preheating temperature,150℃or so as inter-bead temperature. |
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