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    • A software based GillAc basic electrochemical system was use

    2018-11-15

    A software-based GillAc basic electrochemical system was used to conduct potentiodynamic polarization tests to study the pitting corrosion behaviors of polished samples of friction surfaced coatings. All experiments were conducted in an electrolyte of 0.5M H2SO4 + 0.5M NaCl. Steady state potential was recorded 10 min after immersion of specimen into the electrolyte, and the potential was raised anodically at a scan rate of 2m· Vs−1. The potential at which the current increases abruptly after the passive region was taken as pitting potential Epit. Specimens that exhibited higher positive potential were considered to have better pitting corrosion resistance.
    Results and discussion
    Conclusions
    Acknowledgments Financial assistance from Defence Research Development Organization is gratefully acknowledged. The authors would like to thank Dr. Amol A Gokhale, Director and Outstanding scientist, Defence Metallurgical Research Laboratory, Hyderabad, India for his continued encouragement and permission to publish this work.
    Introduction High strength low alloy (HSLA) steels were primarily developed to replace low-carbon steels for the automotive industry in order to improve the strength-to-weight ratio and meet the need for higher-strength construction grade materials. When high strength steel is welded, non-uniform heating and cooling in weld metal and oligomycin metal generate harder heat affected zone (HAZ), cold crack susceptibility and residual stress in weldments. HSLA steels demonstrate unique properties, such as high strength, excellent ductility, and good weldability, and also exhibit outstanding low temperature impact toughness superior to that of high yield strength (HY) steels. HSLA steels have much improved weldability compared to HY steels [1]. Now-a-days, the micro-alloyed or HSLA steels become an indispensable class for different applications like construction of large ships, oil and gas transmission lines, offshore oil drilling platforms, pressure vessels, building construction, bridges, storage tanks. DMR-249A is a low carbon micro-alloyed high strength low alloy (HSLA) steel, which is far superior grade compared to the numerous grades which have been in use for naval applications like construction of warships. Obviously, DMR249A demands weld metal with superior properties compatible with its own, i.e., a combination of high strength and high toughness. This is due to its composition that consists of 0.001–0.1wt% of alloying elements such as V or Ti [2]. An acicular ferrite in weld metals and wrought steels has predominant one owing to its combination of high strength and high toughness [2,4,5], thus this steel has designed to have a ferrite microstructure with small amount of pearlite less than 10% by volume [2,3]. In these grade steels, the heat affected zone (HAZ) is prone to failure due to the possibility of hydrogen induced cracking and only way to weld such steels is to use low hydrogen ferritic steel filler wire [6]. Charpy impact fracture of HSLA steel was improved by intercritical heat treatment which enhances the microstructure through the formation of ferrite microstructure with various morphologies, irregular martensite and 75% of microstructure with high angle grain boundaries [7]. The resistance to hydrogen-induced cracking and stress corrosion cracking was improved by coarse grain heat affected zone which consists of martensite-austenite constituents, thus showing the importance of reduction in carbon content of these steels [8]. Friction stir welding (FSW) is a novel solid state joining technique that is presently attracting significant attention on welding of hard metals such as steel and titanium [9–11]. FSW has appeared as an easy, ecological and promising productive welding method that reduces material waste and avoids radiation and harmful gas emissions, usually associated with the fusion welding processes. Mechanical action in the form of frictional stirring on the base material has modified the microstructure from the coarse grains to very fine grains due to plastic deformation and fast cooling rate [12–14]. Welding of steels is affected by both the temperature and composition which extensively affects the microstructure evolution. Friction stir welding enables us to control these factors and produce superior joint strength [1]. Much of the tool degradation may be attributed to the high heat (temperature around 1200 °C) and the stresses generated during friction stir welding of the high strength materials. However, the development of the wear resistant tool materials has benefited the FSW process and paved way for the rapid implementation of this process in the fabrication of high strength steel structures [15,16]. The present investigation is to study the feasibility of friction stir welding of naval grade HSLA steel and compare the mechanical properties and metallurgical characteristics of FSW joints with the fusion welded (SMA and GMA welded) joints.