Study On The Welding Material With High Strength And Super Corrosion Resistance And The Microstructure And Properties Of The Welded Joint

With the rapid development of railway industry, the corrosion resistance and strength of weathering steels used for railway vehicle is improved continually. In order to obtain welded structure with strength, toughness and corrosion resistance comparable to the steel, the studies of welding materials and related welding technology are very important. The development of welding materials with high strength and good corrosion resistance have theoretical and practical significance.In this paper, a gas metal arc welding wire with high strength, excellent corrosion resistance and good low temperature toughness is successfully developed. Experiment results indicate that the strength and hardness of deposited metal increase with higher carbon equivalent. The microstructure of the deposited metal is mainly composed of acicular ferrite (AF), granular bainite (GB) and small amount of lath bainite (LB), which are the transformation products of super-cooled austenite in the middle temperature region. TEM analysis demonstrates that both acicular ferrite and bainite of deposited metal have high dislocation density. More content of Mo in deposited metal induces decrease of phase transformation temperature, enlargement of the bainite transformation region, and rightward shift and region reduction of ferrite and pearlite transformation region.With more Cr and Mo contents, M-A constituent and granular bainite increase while the acicular ferrite decreases. Twin morphology of M-A constituent in deposited metal was observed with TEM. These microstructural variation results in larger mean grain size and less large angle boundary, which may induce more crack initiation sites and lower crack propagation resistance, and result in lower toughness. Large angle boundary is a key factor which influences the toughness with a positive linear proportion relationship.The influence of alloy elements in deposited metal on corrosion resistance in industrial environment was studied by means of cyclic immersion corrosion test and electrochemical test. Experiment results show that Mo and Cr have a significant effect on the corrosion resistance, while the influence of Ni, Mn and Cu is not obvious. Electrochemical test indicates that Mo、Cr、Ni、Cu and Mn can not significantly decrease the thermodynamics tendency of corrosion for bare deposited metal. With the increase of Mo and Cr contents, the dissolution rate of bare deposited metal decrease, and the anodic dissolution reaction of deposited metal with rust is hindered. Cr and Mo enrich in the rust layer as Cr2O3, FexCr3-xO4, MnFexCr2-xO4, Mn1-xFexCr2O4, NiFexCr2-xO4and MoO3, which may impede the anode dissolution and enhance the protect performance of rust, resulting in improvement of corrosion resistance. Under the corrosion condition in this paper, Mo shows better improvement effect on corrosion resistance than Cr. Granular bainite has better corrosion resistance than acicular ferrite in the initial corrosion period, but microstructure variation has no obvious effect on long-term corrosion behaviour.The effect of welding heat input on the microstructure and mechanical properties of welded joints of the high strength, super corrosion resistance weathering steel was tested with the best corrosion resistance welding wire. It is shown that the microstructure of both weld metal and coarse grain heat affected zoon (CGHAZ) is composed of LB and GB. With the increase of heat input, the width of lath bainite and the amount of M-A constituent increase. The original austenite grain size in CGHAZ is slightly increased as the heat input increases from8kJ to12kJ. When welded with16kJ heat input, the original austenite grain size is increased remarkably. With the increase of heat input, the yield strength increases at first and then decreases, while the tensile strength and impact toughness decrease. The corrosion rate in different region of welded joint is almost the same.