The specimens for transverse tensile

The specimens for transverse tensile test and SCC tests were extracted from the parent metal and weld joints using wire-cut electric discharge machining. The tensile properties of as-received parent metal and weld joints are listed in Table 2. In order to reveal the susceptibility of Super 304H parent metal and weld joint to intergranular corrosion (IGC), the specimens were subjected to oxalic abscisic acid etch test as per ASTM A262 practice A. The specimens were probed under light microscope to reveal the level of Super 304H\'s susceptibility to IGC before and after welding. The SCC test was carried out using the smooth tensile specimen (shown in Fig. 1) in a custom-built constant load setup with maximum loading capacity of 10 kN. The applied loads are measured using a load cell with an accuracy of ±10 N. The strain measurements were done using an LVDT with measurable range of ±5 mm and an accuracy of <1 μm. The schematic representation of the SCC constant load setup is shown in Fig. 2. The environment for SCC testing of Super 304H was chosen as 45% MgCl2 boiling at 155 °C, and the tests were conducted in accordance with ASTM G36. The constant load SCC tests were conducted for parent metal and weld joints of Super 304H at stress levels of 100%, 80%, 60% and 40% of the parent metal\'s yield strength. The specimens for microstructure analysis were polished and etched using Glyceregia. The fracture surfaces of the SCC specimens were ultrasonically cleaned and analyzed using scanning electron microscopy (SEM) to reveal the modes of failure. Energy dispersive spectroscopy (EDS) attached with SEM was used to reveal the elemental composition of interested spots in fracture surface.
Results
Discussion The SCC was transgranular at all the stress levels i.e. SCC-dominated and stress-dominated regions, in parent metal and weld joint, which is characteristic of the higher steady elongation observed [20]. Out of the numerous mechanisms proposed to explain SCC behaviour the anodic path cracking mechanism (APC) belongs to the cracking of austenitic stainless steel in chloride environment [21]. The mechanism proposed by Nishimura for transgranular SCC is also valid for this work, where the entire cracking is based on a cyclic event of passive film formation and rupture, dealt elsewhere in detail [22,4]. The anodic metal dissolution and passive film formation at the crack tip result in dislocation pile-up at the crack tip, resulting in an increase in local stresses higher than the applied local stress at the vicinity of the crack tip. The film ruptures when the local stress exceeds a critical value and a crack propagates by exposing fresh metal to repeat the process. Such event of crack propagation was recorded as noise peaks in corrosion elongation curves until tss, resulting in steady state elongation (refer Fig. 13). The downward arrows indicate the critical stress at which the film ruptures and the upward arrows indicate the applied mean stress. In weld joints, HAZ is the most susceptible region to SCC due to the metallurgical changes caused by weld thermal cycles. The oxalic acid etch test reveals a dual structure in the HAZ, confirming the deterioration in the corrosion resistance of the zone. In the worst case, the welding residual stress can be superimposed and may be as high as yield strength of the material [15,4]. The microstructure examination of the SCC weld joints reveals that the failure occurred between the weld interface and HAZ, indicating the susceptibility of the zone to SCC. At lower applied stress (0.4 × YS), the cracks are found to initiate in the HAZ and propagate in to the weld metal. The crack initiation sites are not observed in the weld metal at lower stress (refer Fig. 11(f)), which confirms the enhanced pitting resistance of weld metal by Mo addition [15]. The cracks initiated in the weld metal at higher stress (refer Fig. 11(d)) are attributed to the local strain field around the interface of matrix and precipitates, caused by the misfit between matrix and precipitates.