Effect of Shielding Gases on Austenitic Stainless Steel Overlay by FCAW Process on Low Alloy Steel

Pressure vessels and high pressure heat exchangers used for chemical, petrochemical and refineries are normally fabricated from creep resistant low alloy steel material. The internal surfaces of these equipments are given a surface treatment (cladding) using austenitic stainless steel in order to prevent the low alloy steel from aggressive corrosion effects of the process fluids. Small diameter process nozzles and other components are normally weld overlaid using flux cored arc welding process, pure CO2 is used as a shielding gas. Nowadays, stringent requirements are being enforced by many customers/process licensors on restricting the use of 100 % CO2 as shielding gas for FCAW process. This necessitates development of stainless steel FCAW weld overlay procedures employing various shielding gases/mixtures. The current study involves the use of flux cored arc welding (FCAW) process to deposit austenitic stainless steel (Type 347) overlay on low alloy steel (1 Cr Mo). Cladding was performed by depositing a barrier layer of type 309L stainless steel and one layer of type 347 stainless steel over the barrier layer. In the first phase of experiment two different shielding gases were employed pure Ar, and pure CO2. Samples from various overlay coupons were subjected to macroscopic examination for studying bead characteristics and microscopic examination for studying ferrite phase distribution and inclusions. Chemical analysis using spectroscopic method was also carried out with intent to understand, in detail, the effect on carbon content due to nature of reactions it undergoes with the shielding gas and flux of the cored-wire. Ferrite survey, using Feritescope and bend ductility tests were also carried out.