Investigation On Welding Usability Of A New Medium Alloy MAG Welding Wire Used For Railway Freight Cars

According to the “Twelfth Five-Year” Plan of railwayfreight cars, Three technologyplatforms,88-100t special freight cars,75-80t general freight cars and160km/h rapid freightcars will be constructed. The manufacturing technology of railway freight cars willreach anew stage of30t axle weight load and160km/h speed, and reaches the top level in the world.To realize the goal, high weather resistance and high strength structure steel to decrease theweight and to improve the performance of railway freight car is becoming prominent andurgent. At recent years some domestic steel plants have successively developed a new highcorrosion resistant weathering steel S450EW with650Mpa strength and a new MAG weldingwire （TH650EW-Ⅱ）based on the crresponding weathering steel. The recommended shieldinggas is80%Ar+20%CO₂. However, there are some problems in practical welding, for example,large spatters, instable arc, lower toughness of welded joint.In order to compressively master the welding usability of TH650EW-â…¡wire, this paperinvestigatedthe effectof carbon dioxide （CO₂） content in argon rich shielding gas on weldingprocess technology and mechincal properties by adopting means of arc analyzer, high speedCCD and SEM, etc. such as arc characteristic and shape, droplet transfer, weld formation,mechanical properties of deposited metal and welded joint. The conclusions are as follows:As the increasing of carbon dioxide content in the shielding gas, the arc shape changesfrom stable umbrella-shape into the instable bell jar shape, and the droplet transfer changesfrom projected transfer with small spherical droplets and high transition frequency into largegranular transfer with low transition frequency. Moreover, instable short circuit transfer withlarge spatters appears and welding usability become poorer. The weld reinforcement doesn’tchange and the penetration and weld width decreases somewhat. But the low temperaturetoughness of all-deposited metal and weld metal shows an evident decrease.The lateralexpansion and sheared section rate of impact fracture specimen also have a significantdownward trend. Chemical compositions analysis shows that there is a linear increase ofoxygen content in the deposited metal with CO₂content proportion. The dimples of impactfracture become larger and shallower, and the diameter of inclusions （oxide of silicon andmanganese） also has a relative increase.With80%Ar+20%CO₂shielding gas, the welding arc shows instable bell jar shapedandthe arc voltage and welding current is dispersed and instable. Moreover, the droplet is largewith instable shape and size and it is difficult to transfer the large granular droplets with largespatters. The impact absorbtion energy of deposited metal and weld metal is only48J and41J respectively, therefore, the margin of impact toughness is very small. However, with95%Ar+5%CO₂shielding gas, arc shows stable umbrella-shaped and the arc voltage andwelding current is very stable. Furthermore, the droplet presents a projected transfer of hightransition frequency with steady spherical shape and small size.The weld bead has a stablewettability and nearly no spatter. The impact absorbtion energy of deposited metal and weldmetal reaches to140J and138J respectively.The test of comprehensive mechanical properties and microstructures on welded jointbased on the new high corrosion resistant weathering steel S450EW and new welding processshows the welded joint has a good mechanical properties such as low temperature impacttoughness, tensile properties, bending properties, hardness, etc..