br Materials and methods High nitrogen austenitic

Materials and methods High nitrogen austenitic stainless steel (HNS) plates in cold worked condition are used in the present study. Schematic diagram of joint geometry and plate dimensions are given in Fig. 1. A set of plates with single-V butt joint welded using shielded metal arc welding (SMAW) and an electrode of Cromang-N steel (17Cr–17Mn–0.36N) was chosen in present study, as shown in Fig. 2. The compositions of glucose transporter metal and electrode are given in the Table 1. The welding parameters, such as welding current and welding speed, were optimized through many welding trials. The optimized welding parameters are given in Table 2. Metallographic examination of samples was performed. The specimen were cut into pieces, which covers fusion zone, partially melted zone and heat affected zone of welds were cut, polished and etched using aqua regia reagent (HCl – 75 vol% and HNO3 – 25 vol%). Microstructures were recorded using an optical microscope and a field emission scanning electron microscopy (FESEM) was used to examine the structural morphologies. Phases were analyse using X-Ray diffraction technique. Orientation imaging microscope (OIM) studies were done to find the orientation of the grains and the amount of different phases in the various zones of weldment using Energy back-scattered diffraction (EBSD) method. The pitting corrosion resistances of base metal and welds in an electrolyte of 3.5% NaCl were tested using a software based GillAC electrochemical system. The exposure area for these experiments was 0.3 mm2.
Results and discussion Addition of Chromium and Manganese increase the solubility of nitrogen whereas nickel reduces the solubility of nitrogen. Therefore the nitrogen content in Fe–Cr–Ni alloys is much lower than that in Fe–Cr–Mn alloys with comparable concentrations of Cr. Other alloying elements like Ti, V, Nb and Zr enhance the solubility of nitrogen due to their high affinity for nitrogen, which results in nitride formation. Nitride formation has also been put forward as a possible mechanism for synergy between nitrogen and molybdenum to control the localized corrosion. Nitrogen is beneficial for pitting resistance. Like molybdenum, nitrogen also shows a strong concentration gradient in the passive film. The important requirement for welding of high nitrogen steel is the solubility of nitrogen in the weld metal. It is likely that the nitrogen content of the weld metal will decrease during welding as a result of dilution with the high nitrogen base metal. The solubility of nitrogen in binary steel at 1600°c and 1 atm. nitrogen pressure is illustrated in Fig. 3. Welded plates of high nitrogen austenitic stainless steels are shown in Fig. 2 The weldment was examined using non destructive testing to identify the surface or sub surface defects. Welds were tested visually and non-destructively using dye-penetrant testing and radiography testing. Visual examination and penetrant testing of the welds revealed no visible surface defects whereas x-ray radiographs revealed no significant defects and observed to be a sound weld as shown in Fig. 4. Fig. 5 shows the XRD results of base metal and weldment. It can be seen from the XRD pattern that there are numerous sharp peaks that correspond to the presence of austenite. Only the single phase of austenite was identified in the base metal and the presence of austenite and delta ferrite in the welds are observed which is very beneficial to avoid the cracking tendency. The optical microstructure of the high nitrogen stainless steel in cold worked condition is observed to have fine equiaxed grains of austenite and annealing twins as shown in Fig. 6. The microstructure of weld metal is fully austenitic and consists of coarse columnar austenite grains growing from the fusion boundary towards the weld centerline attributed to the high amount of chromium and manganese which helps to improve the solubility of nitrogen as shown in Fig. 7(c). At the weld interface, along the fusion boundary towards the base metal transition of coarse grains to fine grains are observed as shown in Fig. 7(b) and having maximum austenite structure due to the dilution of adjacent base metal which is having nitrogen which is completely soluble in the solid solution.