Bainite Transformation Characteristics And Fracture Micromechanism Of Q690CF Steels Under The Welding Thermal Cycle

Since welding technique is widely used in the manufacturing industry of steel structure, the microstructural evolution and related performance of the welded joint have always been some hot issues in the welding metallurgical research field of the steel materials. The welding thermal cycle (mainly fusion welding) has its own unique features, e.g., rapid heating, inhomogeneous cooling rate, extremely high peak temperature, and negligible hold time at the peak temperature, which produces the continuous changes of microstructure and unstable properties at the heat affected zone (HAZ). The catastrophic damage of structural components easily occurs in service from the welded joints with imperfection. Especially for high strength steel, its practical application is always limited due to the deteriorations in toughness and the resistance to hydrogen cracking of the welded joint. Therefore, it is an important guiding significance for steel welding system design to analyze deeply the phase transformation behavior during the welding thermal cycle and the fracture microscopic features of the HAZ. In this work, the research focuses on the low carbon multi-microalloyed high strength bainitic steels (also named as crack free steels) after subjected to different welding thermal cycle processes.First, the isothermal bainite transformation of the experimental steel was studied to reveal some new phenomena of the incomplete bainite transformation behavior. Two different phase transformation kinetic curves were observed distinctly in the range of isothermal bainite transformation temperature. According to Johnson-Mehl-Avrami overall transformation kinetics phenomenon model, the results of phase transformation overall activation energy as well as a suite of microstructure characterization techniques, the bainite transformation mechanism is completely interpreted:the transformation kinetics changes from interface migration controlled mechanism to carbon diffusion controlled mechanism with the progress of phase transformation. The incomplete bainite transformation behavior can form under the following two different conditions:one is that the coupled solute drag-like effect between carbon and solute elements hinders transitorily the moving of interface when the transformation driving force is relatively small; the other is the Gibbs free energy of retained austenite equals to that of the product phase due to the carbon enrichment in the retained austenite, which makes the bainite shear transformation impossible.The effect of austenite grain size on the continuous cooling phase transformation was analyzed quantificationally. Under the same cooling schedule condition, the fine austenite grain size reduces the transformation temperature range and improves the average transformation rate, which can infer the sequence of phase transformation at the sub-zones of the HAZ. The crystallographic feature of the partially transformed microstructure is studied by electron backscatter diffraction (EBSD) technique. The primary bainite lath not only has nearly a classic Kurdjumov-Sachs (K-S) orientation relationship with its parent phase, but also holds an approximate K-S relationship with the neighbor austenite grain. This variant selection reduces the bainite transformation resistance force. The cooling rate can affect the variant selection mechanism of bainite transformation. The variants of the bainitic laths with low misorientation angles are formed from the same Bain zone at large welding cooling time process, which enlarges the effective grain size. Therefore, variant selection mechanism is an important crystallographic feature for the formation of coarse bainite microstructure. And a new kind of micro variant selection, the habit plane direction of primary bainite having a small inclined angle with grain boundary, is proposed according to the orientation features of the bainite laths at the nucleation stage.The toughness of the simulated HAZ microstructure was examined using an instrumented Charpy impact tester to reveal the fracture micromechanism. According to Griffith fracture theory and the experimental fracture stress, the relationship between the size of microcrack and the width of brittle martensite/austenite constituents is established, which proves that the change in the width size of the M/A constituent with the welding cooling time (t8/5) is consistent with the calculated size of microcrack. The interface debonding mechanism of the massive M/A constituent stimulates the nucleation of cleavage microcrack. Several microcracks can be formed simultaneously in the same Bain zone for the coarse bainite microstructure and these microcracks propagate and interconnect with each other to lead to complete cleavage fracture. The cleavage fracture modes in two different matrix micro structures are proposed based on the double barriers model.Welding cold cracking test showed that the cold cracks appear at the HAZ of the experimental steel when the welding heat input is about 0.92 kJ/mm or less (such as the section cracking ratio can reach 98.5% according to the results of Y groove cracking test). The welding cold cracks always distribute along the prior austenite grain boundaries with random high misorientation angles (20～40°). By contrast, the low misorientation grain boundaries and CSL grain boundaries, e.g. low Σ twinned boundary, are relatively hard to form the cold cracks. Meanwhile, the fine grained HAZ has higher cracking ratio compared with the coarse grained HAZ. In combination with hydrogen diffusion kinetics and HAZ phase transformation features, it can be concluded that the sequence of phase transformation at the subzones of the HAZ may be an intrinsic factor to influence the distribution and enrichment of diffusion hydrogen during the stage of phase transformation.The submerged arc welding test showed that the reasonable welding heat input range of the experimental steel should be controlled about 1.4-2.5 kJ/mm to obtain relatively high HAZ toughness. As regards the weld metal, the good toughness is obtained at all experimental conditions because the weld metal has predominantly acicular ferrite. According to the hydrogen permeation test, the welded joint with higher heat input process improves the apparent hydrogen diffusivity and steady-state hydrogen permeation flux. Quite a number of precipitates form in the HAZ microstructure after the welded joint is subjected to high temperature tempering. These precipitates can act as the effective hydrogen traps to reduce the apparent hydrogen diffusivity and to improve the reversible hydrogen solubility. Therefore, the tempering process may enhance the trend of environment hydrogen embrittlement of weld microstructure.