Study On Plasma Spray Welding And Overlaying In Repairing The Surface Defects Of Cast Iron Parts

Due to the simple production process and low cost, gray cast iron is widely usedin machinery manufacturing, metallurgy, mining, petrochemical and other industrialsectors. However, porosities, slag inclusion and other defects in casting process or forthe sake of a series of wrong operations such as overproof in the machining processand defects like wear and scratches in using process inevitably exist in gray cast iron.Therefore, surface defects repair of cast iron has significant economic and socialbenefits. Nowadays, many techniques are applied to repair of cast iron surface defects.But plasma arc spray welding is widely used for its high degree of automation, simpleoperation and high efficiency.The plasma spray welding and overlaying in repairing the surface defects of thecast iron is investigated in this paper. By the means of Ni25powders plasma spraywelding, the surface defects of cast iron are repaired and cast iron is welded by meansof arc cold welding using both Z116and Z308welding electrodes. Opticalmicroscopy, EDS (Energy Dispersive Spectrometer) and XRD (X-ray diffraction) areused to analyze the microstructures and chemical compositions of the alloy layers,and the tests of microhardness, tensile property and crack resistance are used tomeasure the quality of the alloy layers. Conclusions are as follows:(1) Plasma spray welding process parameters are optimized by the means oforthogonal experiment. The optimal parameters of plasma spray welding techniqueare: transferred arc current is55A, plasma gas flow rate is1.5L/min, powder gasflow rate is2.0L/min, transferred arc voltage is25V, shielding gas flow rate is12L/min, the distance between nozzle and workpiece is10mm.(2) By means of EDS, the mutual diffusion of alloy elements between the spraywelding layer and parent metal indicates that the combination of spray welding layerand the parent metal belongs to metallurgical joint. In the cooling and solidificationprocess of molten pool, spray welding layer grains have the characteristics ofdendrites due to the large composition supercooling and the microstructure is mainlycomposed of austenite combined with a small amount of other compounds. (3) The microstructure prepared by Z116electrode is mainly composed offerrite, distributed with Fe3Si、Fe2SiV、FeV in ferrite matrix. The microstructureprepared by Z308electrode is mainly composed of austenite, distributed with graphitephase in the matrix.(4) There is no white iron microstructures in semi-fusion zone of Ni25spraywelding layer, but different width of the white iron microstructures exist in thesemi-fusion zone of surfacing layer of Z308and Z116, this indicates that plasma arcspray welding is more suitable in repairing the surface defects of the cast iron,compared with arc surface welding.(5) The average microhardness of Ni25spray welding layer is close to basematerial hardness, so repairing HT250is the most appropriate; The average tensilestrength of the specimen after being repaired by plasma spray welding is198.23MPa;the anti-cracking properties of Ni25spray welding layer and Z308surfacing layer arebetter than that of Z116surfacing layer.