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Evaluation of Welding Metal Zones in the Case Welded with 0.5 Mo Filler Metal to Forged Steel for Piston Crown Material

Since the oil price has been significantly jumped for recent some years, the diesel engine of the merchant ship has been mainly used the heavy oil of low quality. Thus, it has been often exposed to severely corrosive environment more and more because temperature of the exhaust gas of a combustion chamber is getting higher and higher with increasing of using the heavy oil of low quality. As a result, wear and corrosion of the most parts surrounded with combustion chamber is more serious compared to the other parts of the engine. Therefore, an optimum repair welding for these parts is very important to prolong their lifetime in a economical point of view. In this study, 0.5Mo filler metal was welded with SMAW method in the forged steel which would be generally used with piston crown material. And the corrosion properties of weld metal zone, heat affected zone and base metal zone were investigated using electrochemical methods such as measurement of corrosion potential, anodic polarization curves, cyclic voltammogram and impedance etc. in 35% H2SO4 solution. The heat affected and base metal zones exhibited the highest and lowest values of hardness respectively. And, the corrosion current density of the heat affected zone indicated the lowest value, having the highest value of hardness. It appeared that the corrosive products with red color was wholly covered on the surface of the base metal zone, while its products was not observed in the heat affected zone. The microstructure of the pearlite with black color was more or less observed in the base metal zone with patterns such as crystal and needle, in particular, the crystal pattern of ferrite microstructure with white color was considerably included in the base metal and heat affected zone, and the ferrite microstructure was significantly observed in the weld metal zone. In particular, the polarization characteristics such as impedance, polarization curve associated with corrosion resistance property were well in good agreement with each other. Keywords: Forged steel, Microstructure, 0.5Mo filler metal, SMAW, Weld metal zone, Electrochemical method, Corrosion current density, Hardness.