Effect of welding processes on microstructural characteristics mechanical properties and stress corrosion cracking behavior of 316ln austenitic stainless steel joints

Abstract

The high molybdenum grades of austenitic stainless steels such as 316, 316L, 317 and 316LN are used for the fabrications of chemical storage tanks, pressure vessels and structural components of nuclear reactors. The presence of nitrogen in the 316LN grade provides good mechanical properties at high temperatures (gt 427 O C), hence it is successfully used in the development of the fast breeder reactors. Also, the presence of a very low weight percentage of carbon (0.026 %) content is capable to eliminate sensitization related issues during the welding process and gives resistance to the Intergranular Corrosion (IGC) during its services. This research work explores the effect of different welding processes namely Gas Tungsten Arc Welding (GTAW), Activated flux Gas Tungsten Arc Welding (AGTAW), Laser Beam Welding (LBW) and Friction Stir Welding (FSW) on the weld thermal cycles, microstructures, mechanical properties and stress corrosion cracking behavior on 316LN welded joints. The weld joints of 316LN base plates with the dimension of 75 mm x 300 mm x 3 mm were fabricated using the above mentioned four welding techniques. The major parameters namely current(65 A), voltage (12 V) and welding speed (60 mm/min) were used to fabricate GTAW joints. The major parameters namely current (120 A), voltage (12.6 V) and welding speed (85 mm/min) were used to fabricate AGTAW joints. The major parameters namely power (2.5 kW) and welding speed (1500 mm/min) were used to fabricate LBW joints. The major parameters namely tool rotational speed (600 rpm), vertical thrust force (14 kN) and traverse speed (50 min/mm) were used to fabricate FSW joints. The macrostructure study revealed that the absence of cracks, voids and porosities inside the Fusion Zone (FZ) and Heat Affected Zone (HAZ) of GTAW, AGTAW, LBW joints as well as stir zone and Thermo-Mechanically Affected Zone (TMAZ) of the FSW joint. The microstructural characterization of the base metal and different welded joints was done using Optical Microscopy (OM), Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM). The major microstructural morphologies namely cellular structure (17.18 ± 3.5 and#956;m), equiaxed grains (19.30 ± 1.5 and#956;m) with large dendrites, equiaxed dendrites (7.30 ± 1.5 and#956;m) with columnar structure and fine equiaxed grains (5 and#956;m) were revealed at the weld zones of GTAW, AGTAW, LBW and FSW joints respectively. Grain boundary feature study using TEM at 500 nm scale revealed that there were no precipitates formed in the weld zones of different welded joints newline