Welded joint properties of Q690 weathering steel for railcar bogies
In order to ensure the high quality of Q690 high strength weathering steel welding and ensure that the welded joints have excellent toughness,the properties were studied,so as to promote its wide application in the field of railcar bogies and better realize the upgrading of products.An experimental study was conducted on Q690 weath-ering steel for railcar bogies,focusing on butt welding with two different line energies of 10 kJ/cm and 20 kJ/cm.The welding process utilized a mixed gas shield of 80%Ar and 20%CO2.The microstructure of gas shielded weld-ing joints was observed using metallographic microscope and transmission electron microscope,and the mechanical properties of each area of the welded joint were measured using impact tester and micro-hardness tester,in order to study the relationship between welding line energy,microstructure,and mechanical properties.These experimental results provide important references for understanding the internal structure and mechanical properties of welded joints,helping to optimize welding processes and improve the quality and performance of welded joints.The results indicated that preheating and post-welding heat treatment was unnecessary in this gas-shielded welding process.The weld primarily consisted of a significant amount of crisscross acicular ferrite and polygonal pro-eutectoid ferrite.At a line energy of 10 kJ/cm,the impact energy at-20℃of the weld,fusion line,1 mm outward of the fusion line,and 3 mm outward of the fusion line were measured at 75.2,56.4,69.6,and 123.4 J,respectively.The micro-hardness of the welded joint ranged from 279HV to 312HV,with local hardening zones observed in the coarse grain zone,fine grain zone,and critical zone.When the heat input of welded joint increased to 20 kJ/cm,the micro-hardness decreased compared with 10 kJ/cm,and the increase of heat input would cause the softening of welded joint to some extent.V(C,N)precipitates could promote the nucleation of acicular ferrite,so that the crisscross acicular ferrite could be better engaged,thus effectively inhibiting the propagation of cracks and improving the impact toughness of the material.A significant amount of isometric crystals were evenly dispersed throughout the fine grain zone,enhanc-ing low-temperature impact toughness through mechanisms such as fine grain strengthening and toughening of high-angle grain boundaries.The research results lay a foundation for formulating reasonable welding technology and frac-ture safety assessment of steel for railcar bogie.
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