PQ600 Low Alloy High Strength Steel Weld HAZ Microstructure And Properties

Combined with "high-strength low-alloy steel for mechanical engineering development" project of Panzhihua Iron and Steel Group, this paper researched PQ600 steel's welding ability. By the methods of thermal simulation, samples of HAZ which have different peak temperature and cooling rate in the first thermal cycling and the second thermal cycling was obtained. Study the organizational changes in the law through the observation in microscope. Acquired its mechanical properties by tensile and impact test, and the fracture of mechanism were studied through the observation of the sample surface in scanning electron microscope.The research show that, the microstructure of CGHAZ are martensite and granular bainite when t8/5=10s; When t8/5=35,60s, the microstructure of CGHAZ are granular bainite. The degrees of CGHAZ is between 3 to 4.Recrystallization phase transition zone is small and uniform ferrite and pearlite due to the recrystallization. The microstructure of incomplete recrystallization zone is pearlite,ferrite, and little of M-A million. The strength,toughness and hardness of HAZ reduced as t8/5 increase. The mechanical properties is the worst when t8/5=60s, the yield strength of CGHAZ is 555MPa, the impact energy is 20J,and the hardness is HV220. The yield strength of recrystallization phase transition zone is 570MPa, the impact energy is 50J, and the hardness is HV201. The yield strength of incomplete recrystallization zone is 500MPa, the impact energy is 40J, and the hardness is HV180.When the second peak temperature is up to 1050℃, part of the organization recrystallized, when t8/5=10,35,60 s, the impact energy increased by 45.8%,57%,60%. When the second peak temperature is up to 850℃, the microstructure of CGHAZ did not become smaller, the performance is the genetic of microstructure. M-A million in the zone contains martensite which has high carbon content, and lead to its decline in toughness. When t8/5=10,35,60 s, the impact energy decreased by 15%,23.5%,25% compared to CGHAZ. When the second peak temperature is up to 1200,600℃, the impact energy of reheat CGHAZ is larger than the impact energy of CGHAZ, the toughness of CGHAZ is improved.Through the test of welded joints'mechanical properties, we can see that:welded joints of PQ600 hot-rolled steel plates have good tensile strength, large wealth of impact toughness, and qualified plastic when t8/5=10s.