EFFECT OF LONG DURATION PWHT ON THE MECHANICAL AND MICROSTRUCTURAL PROPERTIES OF SMA WELDED Cr-Mo ALLOY STEEL AND HOT CORROSION BEHAVIOUR OF Cr-Mo BOILER STEEL IN AIR AND SALT Na2SO4.60%V2O5 ENVIRONMENTS

Abstract

Welding process involves melting and subsequent cooling and the result of this thermal cycle causes residual stress and degradation of mechanical properties in weldments, When welding thicker sections. In thicker sections like pressure vessels, boilers which are used in petrochemical, chemical, oil & gas industries the existence of residual stress and undesirable mechanical properties are unacceptable as per criticality of these structures. In order to impart high mechanical properties to thicker sections for sophisticated applications PWHT is carried out. Extensive research has been done by many authors on PWHT to investigate the effect of PWHT on mechanical properties and residual stresses and the problems occurred during PWHT like reheat cracking. In the present work the effect of LONG-TIME POST WELD HEAT TREATMENT on the Mechanical and micro structural properties of Chromium-Molybdenum alloy steel of ASTM A 387 Gr-22, 2.25 % Cr-1%Mo alloy steel is determined ,which is commonly used in boilers, pressure vessels, heat exchangers, turbine parts etc. The regions in the weldment like base metal, HAZ and weld metal were studied, through the results obtained from the Tensile test, Hardness measurement, Charpy-V-Notch impact test. Microstructural characterization using SEM and Optical microscope and chemical analysis using E-DAX, Spectroscopy. It is concluded that the degradation of mechanical properties of steel is due to the changes that occurred in the microstructure: Material degradation at high temperature is a serious problem in power plants, which encounter severe corrosion problems resulting in the substantial loss. The problem becomes more prominent as the power plants are getting older. The boiler tubes used for super heaters and re-heaters in the steam generating systems are subjected to corrosion, resulting in tube wall thinning and premature failure. Hot corrosion has been identified as a serious problem in high temperature applications such as boilers, gas turbines, diesel engines, coal gasification plants and chemical plants. The present study has been performed to evaluate the hot corrosion behaviour of chromium-molybdenum alloy steel when exposed to high temperature oxidation in air and molten salt (Na2SO4-60%V205) environment, under isothermal conditions at a temperature of 900°C, in actual degrading conditions prevailing in a coal fired boiler of a thermal plant. The work is focused on Un-welded material, weld metal and base metal welded by SMAW process. The specimens were characterized by SEM, E-DAX, X-Ray mapping, EPMA, XRD analysis. Isothermal treatments resulted in parabolic oxidation kinetics and thus resulted in weight gain.