Study On Structure And Property Of Haz Of JB800 Bainite Steel

JB800 bainite steel is Mn-alloy bainite steel developed by Jinan Iron & Steel Group Company and Tsinghua University. Its carbon equivalent is 0.63%, so it has high hardenability and cold Cracking sensitivity. For this reason, we should study on the structure and property of heat affected zone (HAZ) of JB800 bainite steel thoroughly, which is significant for designing scheme of welding procedure, controlling the structure and property of welding joint and engineering application of JB800 bainite steel. Meanwhile, it has important factor in enriching the theoretics of welding of bainite steel.First of all, by testing the SH-CCT chart of JB800 bainite steel with welding thermal simulation, the paper obtains the influence of cooling rate on structure and hardness of CGHAZ. And it analyses the structure of fractography and impact fracture appearance of CGHAZ thoroughly when t8/5 is 20 seconds. The results indicate that the hardness decreases with the increase of t8/5, maximum of which is 390. In the condition of single weld thermal cycle and when t8/5 is 20 seconds, the low-carbon martensite grain is refined because lower bainite separates austenite grain; the grain which is composed of low-carbon martensite and lower bainite has large-angle grain boundary which prevents crack extending, improves the impact toughness of JB800 bainite steel. When experiment temperature is 20℃ and t8/5 20 seconds, the radical zone of impact fracture of JB800 bainite steel obtains measurable dimples, besides quasi-cleavage fracture, so HAZ has fine impact toughness.Secondly, the paper establishes analyse model of welding heat transfer of JB800 bainite steel based on finite element software SYSWELD; it studies the influence of welding procedure, such as joint geometry and welding parameters, on GMAW temperature field and thermal cycle of JB800 bainite steel with numerical simulation.After the welding experiments for JB800 bainite steel, experimental validation for the established model has been finished through the weld cross-section figures and the welding thermal cycles on the weldment. A better agreement has been achieved between prediction and measurement, which validates the reliability of the established model. Through the combination of established model and the SH-CCT chart for JB800 bainite steel, the paper successively realizes the prediction of HAZ structure and hardness.Thirdly, Through Y-slit type cracking test and HAZ maximum hardness test the paper assesses the cold crack sensitivity of JB800 bainite steel. The results indicate that in the case of severe constraint, to prevent 20mm thickness JB800 bainite steel from welding cold crack, preheat temperature should not low 100℃ for welding wire of GFM-60; the preheat temperature should not low 150℃ for welding wire of GFM-70. The permitted maximal hardness of welding joint is 375 and the established model can exactly predict HAZ maximal hardness in different welding procedure. The paper also studies the influence of welding heat input on mechanical property of JB800 bainite steel, which indicate that welding joint with better mechanical property can be obtained between heat input 13 kJ/cm and 15 kJ/cm.