Investigation Of Susceptibility To Local Embrittlement In Heat-affected Zone Induced Of Q500qENH Steel

The middle section of the national key project Sichuan-Tibet Railway(Ya’an-Linzhi)crosses the Dadu River,Jinsha River and other river areas,and several super-large span weather-resistant steel bridges are planned to be built.Complex geological environmental conditions require materials with excellent toughness,weldability and weather resistance and other comprehensive performance.Q500qENH weathering resistant steel is the preferred material.In this study,the microstructure,mechanical properties and local embrittlement tendency of Q500qENH weathering steel in the reheated coarse grain heat affected zone by secondary thermal cycling during multi-layer multi-pass welding process were studied,and optimized the welding process.In this paper,Gleeble3500 thermal simulation tester was used to simulate different conditions of welding thermal cycling process,the use of metallurgical microscopy,scanning electron microscopy,electron back scattering diffraction and transmission electron microscopy and other means of characterization of the reheated coarse crystal heat affected zone tissue morphology to observe,based on the charpy impact test results,the microstructure low temperature impact toughness change patterns of reheated coarse grain heat affected zone of Q500qENH steel were studied.The results are listed below point by point:The microstructure change of Q500qENH steel under continuous cooling condition was studied by a thermal cycle test,and the SHCCT curve was established,with the cooling rate decreases,lath bainite(LB),granular bainite(GB)and acicular ferrite(AF)phase transition regions appear successively,the hardness decreases,and the phase transition start and end points gradually increase,and the phase transition start and end points gradually increase.The changes of microstructure and mechanical properties of different reheated coarse grained heat affected zone were studied by simulating different peak temperatures of secondary thermal cycles.In which the microstructure and impact toughness of the intercritically reheated coarse grained heat affected zone(ICCGHAZ)were the worst when the peak temperature was between 750 ℃ and 800 ℃.At this stage,a large number of chainlike M-A constituents were formed at the Prior Austenite Grain Boundaries(PAGB).The M-A constituents’ formations in this zone were defined and quantified,and the surface energy of different M-A constituents was calculated by dihedral angle,and the microscopic mechanism of fracture caused by M-A constituents was established.The stress is concentrated on the interface between M-A constituents and ferrite,causing elongated stringer M-A more prone to debonding from the matrix,while the blocky M-A more likely to crack.When the peak temperature was outside the two-phase zone,the percentage,size and aspect ratio of M-A constituents decreased and the low temperature impact toughness increased.The microstructure and property changes of ICCGHAZ were studied by simulating different heat inputs of secondary thermal cycles.With the increase of heat input,the microstructure in ICCGHAZ of Q500qENH steel became coarser gradually,chainlike M-A constituents at the PGAB became coarser and change from stringer like to block like,its percentage and size increased gradually,so that the low temperature impact toughness decreased.The formation of M-A constituents at the CGHAZ can be inhibited by reducing the heat input,thus reducing the susceptibility of local embrittlement of ICCGHAZ.