Influence of Gas Mixtures in GMAW of Modified 409M Ferritic Stainless Steel Up to 10% CO2 with argon may be commercially utilized in the gas mixture for fabricating Introduction SUPPLEMENT TO THE WELDING JOURNAL, APRIL 2015 Sponsored by the American Welding Society and the Welding Research Council welded joints of 409M ferritic stainless steel BY M. MUKHERJEE, J. SAHA, P. KANJILAL, T. K. PAL, AND S. SISODIA The gas metal arc welding (GMAW) process is commonly used for fabricating various components of ferritic stainless steel. One of the unique characteristics of the GMAW process is the way molten metal is transferred across the arc. Metal transfer is controlled by several parameters, including current, voltage, polarity, electrode extension, shielding gas composition, and electrode diameter. Previous work (Refs. 1, 2) reported that the microstructural constituents such as grain size, martensite content, and precipitation in ferritic stainless steel weldments are strongly dependent upon the variation in modes of metal transfer and heat input. The spray mode of metal transfer (S-mode) produces a greater WELDING RESEARCH amount of grain boundary austenite along with lath martensite, which inhibits ferrite grain growth in the weld metal and induces higher strength and toughness (Ref. 1). However, the effect of martensite on mechanical properties is controversial and may promote hydrogen-induced cracking (Ref. 3). The variation in shielding gas mixtures in the GMAW process has a direct impact on welding costs, and it also affects the weld quality through its influence on metal transfer. A mixture of carbon dioxide (CO2) and argon (Ar) is widely used as a shielding gas for arc welding processes (Ref. 4). Carbon dioxide is more plentiful, widely available, and two to three times less expensive than argon. Therefore, substantial savings are possible if welds are made with the addition of more CO2in an Ar-CO2 mixture. However, weld bead quality and deposition rates often decrease with the increase of CO2 in a binary Ar-CO2 mixture (Ref. 5). Using uncoated steel electrodes and nonpulsed power supplies with direct current electrode positive (DCEP), Smith (Ref. 6) reported stable, axial type of free-flight transfer when the CO2 concentrations in Ar-CO2 mixture are less than 25%. Above 25% CO2, the operating characteristics of the process changed to repelled transfer during free-flight mode. However, quality welds can be made with 100% CO2 at decreased deposition rates using short circuiting transfer. Also, the addition of CO2 in the shielding gas increases the transition current APRIL 2015 / WELDING JOURNAL 101-s ABSTRACT The present study describes in detail the effect of shielding gas mixtures on the bead geometry, microstructure, and mechanical properties of gas metal arc welded modified ferritic stainless steel (409M) sheets (as received) of 4 mm thickness. The welded joints were prepared under spray (S) mode of metal transfer at same heat input using 308L austenitic filler metal and four different shielding gas mixtures, i.e., pure Ar, Ar + 5% CO2, Ar + 10% CO2, and Ar + 20% CO2. The welded joints were evaluated by means of microstructural changes, hardness, tensile strength, and toughness. The dependence of weld metal microstructure on shielding gas mixtures has been determined by bead geometry, Creq/Nieq ratio, Ms, Ms, optical microscopy (OM), transmission electron microscopy (TEM), and electron probe microanalyzer (EPMA). It was observed that the variation in shielding gas mixture effectively manipulates the solidstate phase transformation and precipitation behavior of the welded joints. Variations in microstructure ultimately affect the mechanical properties of the weld metal as well as coarse-grained HAZ (CGHAZ). The present study concluded that up to 10% CO2 may be commercially utilized in the shielding gas mixture for fabricating welded joints of 409M using 308L filler metal without deteriorating microstructural and mechanical properties. KEYWORDS • Modified Ferritic Stainless Steel • Shielding Gas Mixtures • GMAW • Microstructure • Mechanical Properties M. MUKHERJEE is a senior research fellow, J. SAHA is a graduate student, and T. K. PAL (tkpal.ju@gmail.com) is a professor, Metallurgical and Material Engineering Department, Jadavpur University, Kolkata, India. P. KANJILAL is a scientist SD (Mechanical), National Test House, Saltlake, Kolkata, India. S. SISODIA is general manager (Quality), Salem Steel Plant, Steel Authority of India Ltd., Salem, Tamil Nadu, India.
Welding Journal | April 2015
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