Mass Transfer and Weld Appearance of 316L Stainless Steel Covered Electrode During Shielded Metal Arc Welding
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INTRODUCTION
STAINLESS steels were widely used in chemical equipment and power generation as structural supports, containers, pipes, etc. because of their good corrosion resistance. In stainless steel catalogue, austenitic stainless steels comparatively have both good mechanical properties, particularly ductility and toughness, and acceptable workability, formability, and weldability; thus, they were extensively used in welded structure or weld assembly.[1] Because of the relatively low heat conductivity and high thermal expansion, the austenitic stainless steels require special welding precautions and appropriate welding materials as filler metals so as to prevent welding distortion, residual stresses, and weld defects in either the weld bead or the heat-affected zone (HAZ).[1,2] Apart from weld forming, the chemical composition of the weld metal (depending on the filler metal and the welding process) is extremely important for maintaining adequate mechanical properties and high corrosion resistance. For shielded metal arc welding (SMAW), the metal mass transfer from the covered electrode to the weld pool depends crucially on the flux ingredient coated on the electrode and the welding parameters.[3,4] This definitively determines the final chemical composition of the weld. Although one can empirically estimate the mass transfer during SMAW, the relatively accurate transfer coefficients of the key elements (i.e., Ni, Cr, and Mo) in the stainless steel covered electrodes, especially for the newly developed ones with faintly acidic flux coatings that bring the good weld quality and easy welding operation together, are practically useful for welding engineering design and welding material development. ZHAOLING DUAN, Master Student, RENYAO QIN, Doctoral Student, and GUO HE, Professor, are with the State Key Lab of Metal Matrix Composites, School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, P.R. China. Contact e-mail: [email protected] Manuscript submitted February 1, 2013. Article published online October 30, 2013 METALLURGICAL AND MATERIALS TRANSACTIONS A
Traditionally, the flux coating on the electrodes is either ‘‘rutile type’’ or ‘‘basic type.’’[3] The former is mainly composed of TiO2, Al2O3, SiO2, and carbonates with which a very stable welding process can be achieved; the latter principally consists of CaF2, MgO, carbonates, and silicates. With such a coating ingredient, the crack sensitivity will be reduced because the hydrogen in the weld can be controlled in a very low level.[3] In recent years, the development of the flux coating ingredient has tended to take advantage of both the rutile type and basic type, forming a new type composed of TiO2, SiO2, Al2O3, and CaF2. With the help of this new flux coating, we developed some stainless steel covered electrodes that have both a stable welding process and a high weld quality (especially the impact resistance performance). However, the mass transfer of the key elements in such new flux coating is unclear. What factors contribute to the
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