Study On The Welding Material And The Microstructure And Properties Of The Welded Joint In A 800MPa Grade Structural Steel

Ultra low carbon bainitic (ULCB) structure possesses high strength, high toughness and good cracking-resistance. In addition, its strength, ductility and low temperature toughness are insensitive to the heat input and cooling-rate, therefore the development of ULCB welding materials is the key solution to weld high strength steels. In this paper, the welding wire with superior strength and excellent low temperature toughness is successfully developed on the basis of ULCB designing idea. The results of further study indicate that with the increase of carbon equivalent, the strength and hardness of ULCB as-deposited metal increase, but the toughness and ductility fall. The microstructure of the ULCB as-deposited metal is mainly composed of carbide-free lath bainite, granular bainite and acicular ferrite (AF), which are the transformation products of super-cooling austenite in the middle temperature region. TEM analysis reveals that the as-deposited metal mainly consists of lath-like bainitic ferrite with high dislocation density. With the change of t 8/5, the transformation temperatures of the ULCB as-deposited metal maintain stable. This is beneficial to get stable structure and properties in a wide range of welding heat input. The thermodynamic transformation driving force of ULCB in the weld metal is calculated using the KRC model. The results show that the transformation driving force ofULCB is minus, which conforms to the thermodynamic theory that transformation makes the free energy of the system depress. With the decrease of the transformation temperature, the driving forces of ULCB transformation, nucleation and growth increase. With the increase of the carbon content in the weld metal, the driving forces of ULCB transformation, nucleation and growth decrease. When the carbon content is extremely low, ULCB transformation tends to occur in the pre-eutectoid mechanism. By applying the technology of computer-aided auto-modeling to study the ULCB welding material, the qualitative model, geometrical model and quantitative II model are established. Using the artificial neural network model to forecast the strength and ductility of the as-deposited metal, satisfied results can be obtained. Using ULCB wire, the welded joint of the800MPa grade ultra lowcarbon micro-alloyed steel obtains high low temperature toughness and high tensile strength that is over 90% of the base metal tensile strength under different heat inputs. With the heat input increases, the tensile strength of the welded joint decreases gradually, but the low temperature impact toughness initially increases and then decreases. The different microstructure is corresponding to the different heat input. At lower heat input, the fine and dense bath bainite is the main microstructure, and the AF and granular bainite are the less. When the heat input increases gradually, the amount of AF increases, but the amount of the lath bainite decreases greatly. The width of the dendrite also increases with the increase of the heat input. The HAZ of the 800MPa grade ultra low carbon micro-alloyed steel can be classified into three zones, i.e.