STUDIES ON HOT CORROSION BEHAVIOUR OF DISSIMILAR WELDMENTS

Abstract

Dissimilar-metal joints are used widely in various industrial applications due to both technical and economic reasons. The adoption of dissimilar metal combinations provides possibilities for the flexible design of the product by using each material efficiently, i.e. benefiting from the specific properties of each material in a functional way. Fusion welding is far the most important process used in the fabrication of modern boilers. Components are joined by the formation of molten pool of metal between them. The production of high quality welds with a high degree of consistency is readily achievable. I-Iowever, defects are more likely to occur in the welds than in wrought material. In all are welding process, the intense heat source produced by the are and the associated local heating and cooling results in a number of consequences in the material corrosion behaviour and several metallurgical phase changes occur in different zones of the weldments. The materials used for high temperature applications are subjected to hot corrosion and high temperature wear. Hot corrosion can be defined as deposits modified, gas induced degradation of materials at high temperatures. For example, during the combustion stage in heat engines, particularly in gas turbines, sodium and sulphate impurities present either in fuel or in combustion air, react to form sodium sulphate. The present study is an attempt to evaluate the behaviour of un-welded and dissimilar weldments of AISI 304 (stainless steel) and 20MnMoNi55 (ferritic low alloy steel) made by TIG welding at temperature 900°C samples when exposed to high temperature oxidation in air and molten 75%wt Na2SO4 + 25%wt K2SO4 salt under cyclic condition. Compare the oxidation and hot corrosion performance of the as un-welded steel with dissimilar weldments sample and evaluate its suitability for using in boiler applications. The experiments are conduct in laboratory under cyclic condition and after high temperature oxidation, hot corrosion studies were characterized by the techniques such as XRD, SEM/EDAX and X-ray mapping to know an insight corrosion mechanism based on morphology of the corroded products formed on the un-welded and dissimilar weldments sample. The result shows that corrosion resistance is in order of AISI 304 —►DSW --►20MnMoNi55 in both air as well as in molten salt environment