Role of welding processes on microstructure and mechanical properties of nuclear grade stainless steel joints

Abstract

Nuclear grade 316LN austenitic stainless steel weld joints were fabricated using conventional gas tungsten arc welding (GTAW), activated flux gas tungsten arc welding (AGTAW), laser beam welding (LBW) and friction stir welding (FSW) processes. Assessment of weld beads was done by mechanical and metallurgical characterizations. Bead geometry and weld zones were studied by taking macrographs along the transverse side of the weld joints. Metallurgical features of different weld joints were carried out using optical microscopy and scanning electron microscopy. Microhardness distribution across four weld joints was recorded and hardness variations were compared. All weld zone, heat affected zone (HAZ) of GTAW and LBW, thermo-mechanically affected zone (TMAZ) of FSW processes, exhibited higher hardness values than the base metal. Reduced hardness was recorded at HAZ of AGTAW process. This was the result of a considerable grain growth. LBW joint showed the highest hardness value at the center of the fusion zone due to fine equiaxed dendrite morphology. Tensile and impact properties of different welding processes were evaluated and comparisons were made at room temperature. All weld samples displayed high yield strength (YS) and ultimate tensile strength (UTS) with a lower percentage of elongation compared to that of the base metal. FSW joint showed improved YS, UTS and impact toughness compared to other weld joints. This is attributed to the formation of strain-free fine equiaxed grains at stir zone around 5 µm in size with subgrains of 2 µm in size by severe dynamic recrystallization mechanism. Among the fusion welding techniques, AGTAW process exhibited improved toughness, besides almost equal toughness of the base metal due to low δ-Ferrite with high austenite content. Fractography studies of the base metal and different weld samples were carried out by SEM analysis and features were compared.

Probing the stress corrosion cracking resistance of laser beam welded AISI 316LN austenitic stainless steel

“…Generally, the following factors are taking the major role for the fixing of SCC behavior of austenitic stainless steel weldments: (i) amount size, the morphology of grains (ii) continuity of the δ-ferrite network (iii) residual stresses (iv) density of the metallurgical defects (v) Alloying element partitioning of γ/δ interface. 36 The high cooling rate of the LBW process (8.02° C/s) 37 preventing the microsegregation of Cr to the grain boundaries. Due to this prevention of Cr microsegregation to the grain boundary, crack initiation and propagation can be completely arrested at the center of the fusion zone between the equiaxed dendritic structure of the LBW joint.…”

Correlation between Tensile Deformation Behavior and Microstructural Morphology of Nuclear Grade Austenitic Stainless Steel Weld Joints using Infrared Thermography Technique

“…This is attributed to repeated thermal cycles and usage of filler material during the GTAW process. Both primary austenite (AF) and primary ferrite (FA) mode of solidification occurred at the FZ of the GTAW process [16]. The presence of ferrite promoting element (Cr & Mo) in the filler material and fast cooling rate are predominant factors for the FA mode of solidification.…”

Comparative Study of Friction Stir Welding and Underwater Friction Stir Welding on Magnesium ZE41 Alloy

“…The lower heat generation in FSW stands unique and avoids many defects due to higher temperature on fusion welding. Expanded details about the process of FSW were discussed in many previous publications [1][2][3][4][5].…”

“…Nowadays, there are many methods like GTAW, AGTAW, GMAW, SMAW and laser beam welding followed for welding of magnesium ZE41 alloy [1,6]. Some researchers had tried different materials with FSW in underwater condition and got better mechanical properties [7].…”