Effect of Beam Oscillation on Microstructure and Mechanical Properties of AISI 316L Electron Beam Welds
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INTRODUCTION
AISI 316L grade austenitic stainless steel (SS) shows better mechanical properties at elevated temperature, superior oxidation resistance, and better resistance to corrosion. This grade finds wide application in the field of shipbuilding industries, oil and gas industries, nuclear sector, cryogenic sector, refineries, chemical and petrochemical plants, architectural applications and medical implants, etc. During its application, welding is regularly used as a fabrication tool and failure in such engineering components often initiates from the weldment region. During its joining by conventional arc welding processes, one of the major concerns is the formation of hot cracks in the fusion zone (FZ) and/or heat-affected zone (HAZ).[1,2] It is reported that retention of 3 to 9 pct delta ferrite in the FZ avoids hot cracking tendency,[3] but higher ferrite content is also a concern as it reduces the creep life and corrosion resistance of the joints.[4,5] Coarse grain structure in the FZ, inadequate cleanliness, inability to join thicker plates in a single pass, and wider FZ and HAZ are some of the added problems which hamper the service life of the engineering components fabricated by conventional arc welding processes. Welding methods based on high energy density heat sources such as electron beam
JYOTIRMAYA KAR, SANAT KUMAR ROY, and GOUR GOPAL ROY are with the Department of Metallurgical and Materials Engineering, Indian Institute of Technology Kharagpur, Kharagpur, West Bengal 721302, India. Contact e-mail: jyotirmaya007@gmail. com Manuscript submitted July 11, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS A
welding (EBW) and laser beam welding (LBW) have an edge over conventional arc welding processes. In such processes, high energy density and lower heat input produce welds with larger penetration in a single pass resulting in narrower FZ and HAZ. Such processes also reduce the chance of defect formation such as microcracks, porosity, and undercut.[6–8] Kumar et al.[9] prepared butt joints of 316LN SS by CO2 laser at two different powers (3 and 3.5 kW) and reported that both joints possessed similar creep and mechanical properties equivalent to the base metal (BM). Joseph et al.[10] prepared butt joints of 316LN SS by EBW and reported that faster cooling produced fine grain structure in the FZ that improved the Charpy impact strength. Tjong et al.[11] prepared butt joints of 316L SS by both EBW and LBW and reported the presence of central parting line segregation in the FZ, which reduced the creep life of the joints. In EBW process, the beam can be made to oscillate in various predetermined patterns such as elliptical, circular, and sinusoidal. Unlike in a linear beam, oscillating beam is supposed to produce a churning action of the liquid in the weld pool, which improves mixing, restricts segregation, and renders the weldment region more uniform in composition and structure.[12] Babu et al.[13] and Fu et al.[14] studied the effect of the beam oscillation on Ti-6Al-4V alloy joints and reported improved me
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