Stress Corrosion Cracking Susceptibility of 316LN Grade Stainless Steel Weld Joint in Boiling Magnesium Chloride Hexahydrate Environment

The 316LN Stainless Steel (SS) weld joints were fabricated using Tungsten Inert Gas (TIG) Welding and Activated Flux Tungsten Inert Gas (A-TIG) Welding techniques with suitable process parameters. Initially, basic mechanical properties were evaluated across weld joints. Further microstructural study of the base metal, TIG, and A-TIG joints were accomplished using Optical Microscopy (OM), Transmission Electron Microscopy (TEM), and Scanning Electron Microscopy (SEM) techniques. The microstructural characterization revealed higher grain size variations at the fusion zone of the A-TIG joint due to the slow cooling rate and reversed Marangoni convection effect. The Stress Corrosion Cracking (SCC) susceptibility of the TIG and A-TIG welded joints was assessed using five different loading/Stress conditions. The constant load boiling 45 wt% Magnesium Chloride Hexahydrate (MgCl(2 center dot)6H(2)O) solution as per ASTM G36-94 standard was used to evaluate the SCC susceptibility of the welded joints. The SCC (crack initiation and propagation) of the base metal and welded joints occurred by the anodic dissolution and Hydrogen Induced Cracking mechanisms. For the welded joints additionally, the dissolution of the delta-ferrite increased the crack growth rate. The A-TIG joint exhibited lesser SCC resistance than the TIG joint for the following major reasons: (i) Formation of the large dendrites (ii) Presence of the higher grain size variations at the fusion zone. Moreover, both welded joints showed lesser SCC resistance than the base metal due to the dissolution of the delta-ferrite and the residual stress formation. The fractographic studies for the base metal, TIG, and A-TIG joints revealed the brittle nature of transgranular SCC failure.