PERFORMANCE INTEGRITY INVESTIGATION OF GAS TUNGSTEN ARC WELDED 304L AUSTENITIC STAINLESS STEEL SUGAR PLANT BOILER PIPE

Abstract

Frequent leakage of steam from failed portions of butt joint gas tungsten arc welded 304L austenitic stainless steel (ASS) boiler pipe of plants in processing industries has been generating a lot of concerns to the operators. The conventional metallurgical approaches for weld joint integrity enhancement, such as induced harmonic vibration, alloying of electrode and reduction in residual stress gradients are expensive and not locally available. Despite that post weld heat treatment is relatively less expensive and locally available, in situ post weld heat treatment is intricate and laborious, and therefore, it is almost impossibly used. Consequently, low cost and effective metallurgical route was sought to enhance performance integrity of the sugar plant boiler pipe in this work. To this end, sixty (60) samples with dimensions (50 x 30 x 4.5)mm3 thickness were cut from the as- received experimental 304L ASS boiler pipe with a simple hacksaw, and made into butt joint samples. The weld configuration was comprised of a single - V geometry with angle 60° within its edges, and root opening of 2.5 mm. Weldment samples were produced at varied ranges of GTAW speed, current and voltage in accordance with ASTM A778 / A778M (2016), and characterised before corrosion and after corrosion in demineralised water at ambient (25oC) and elevated (55oC) temperatures using X-ray diffraction (XRD), optical microscopy, scanning electron microscopy (SEM) and energy dispersive Spectroscopy (EDS) in accordance with ASTM standards. Tensile and microhardness tests were carried out before corrosion and after corrosion at ambient (25oC) and elevated (55oC) temperatures using tensile testing machine (INSTRON 3369) in accordance with ASTM E 8-04 (2008) standard and Vickers micro hardness testing machine (LECO 700AT) in accordance with ASTM 384-11 (2011) standard. The weldment was subjected to electrochemical studies using AUTOLAB PGSTAT 204N instrument, piloted by NOVA software. The experiments were performed according to ASTM G3-14 (2014). Fractured tensile specimens were obtained and analysed using scanning electron microscope (SEM). MINITAB statistical software package with coefficient of determination (R2) was used to predict the welding heat input at any range of GTAW speed, current and voltage. From the results,x microstructure of the boiler pipe sample is comprised of austenite matrix with equiaxed grains and small amount of δ-ferrite along grain boundaries. Microstructures of the weldment samples are heterogeneous, comprising of varied volume fraction of interspersed ferrite in austenite matrix. Dendrites of different sizes, precipitates, δ-ferrite and inclusions were found to exist within the austenite matrix. Microstructures of the weldment samples after corrosion at 25oC and 55oC are characterised by pits and cracks respectively. Mechanical properties (tensile and microhardness) were improved at the ranges of GTAW speed, current and voltage that favour low heat input relative to the ranges of GTAW speed, current and voltage that favour high heat input. Optimum tensile and micro-hardness properties before corrosion were achieved at GTAW speed of 7.2 mm/s, current of 120A and voltage of 20V. Least tensile and micro hardness properties before corrosion were achieved at GTAW speed of 1.7 mm/s, current of 200A and voltage of 40V. Ecorr was found to shift to more negative direction at the ranges of GTAW speed (1.7-4.6 mm/s), current (160 - 200A) and voltage (30 - 40V), while converse was true for the ranges of GTAW speed (5.4 - 7.2 mm/s), current (120 - 140A) and voltage (20-25V). Generally, corrosion rate was increased at 55oC relative to 25oC. More fibrous structures, dimples and voids were seen on fractured surfaces of the weldment samples at the ranges of GTAW speed (1.7-4.6 mm/s), current (160 - 200A) and voltage (30 - 40V) as compared to GTAW speed (5.4-7.2 mm/s), current (120 - 140A) and voltage (20 - 25V). More corrosion products were observed on the fractured surfaces after corrosion at 55°C relative to 25oC. Using MINITAB statistical software and coefficient of determination (R2) criterion, accurate prediction of heat input values at any range of GTAW speed, current and voltage can be made. A combination of GTAW speed of 5.4 mm/s, current of 140A and voltage of 25V was found most suited for maintaining boiler pipe of the sugar plant.