HOT CORROSION STUDIES ON DISSIMILAR WELDED 4140 AND 304 ALLOYS UNDER SERVICE ENVIRONMENT

Abstract

Understanding the corrosion behavior of the weldment at elevated temperatures has become an object of scientific investigation recently. Investigation has been carried out on friction-welded, Tungsten inert gas welded, Electron beam welded AISI 4140(low alloy steel) and AISI 304 (Austenitic stainless steel) steels under molten salt of NaCl+Na2SO4 (50%)environment at 700°C under cyclic condition i.e lhour heating and 20 minute cooling. The resulting oxide scales in the weldment have been characterized systematically using surface analytical techniques. Scale thickness on low alloy steel side was found to be more and was prone to spalling. Weld region has been found to be more prone to degradation than base metals due to inter diffusion of element across the interface and the formation of intermetallic compound. The alloy system i.e. austenitic stainless steel (AISI 304) and low alloy steel( AISI4140) chosen for analysis has great importance in commercial application such as oil or fossil fired power plant. The AISI304 has good corrosion resistance properties even at elevated temperature and AISI 4140 is suitable for structural application. Boilers and re-heaters generally used this alloy combination. Friction welding process is a solid state process with no fusion zone present, so it offer a solution to the problem of sensitization, dilution and cracking of elements over conventional welding processes. EBW solve the problem of the large difference in melting temperature more easily than, e.g. arc welding, due to the high-energy density resulting in a high heating and cooling rate, a low total-heat input per unit length of weld of EBW can also reduce the residual stresses substantially,, as compared to arc welding. The thermal conductivity problem can be overcome by directing the beam correctly to the required location, the small weld bead size of Electron Beam welds minimizes the mixing of dissimilar metals and limits the brittle zones arising from the chemical mismatch, to some extent. Because of it EBW process is superior than the conventional welding processes when dissimilar metals are to be joined. TIG welding is commonly used high quality welding process. TIG. Welding has become a popular choice of welding processes when high quality, precision welding is required.