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تعداد صفحات این فایل: ۲۲ صفحه

بخشی از مقاله انگلیسیعنوان انگلیسی:Shielding gas effects on flux cored arc welding of AISI 316L (N) austenitic stainless steel joints~~en~~

Abstract

This paper deals with the flux cored arc welding (FCAW) of AISI 316L (N) austenitic stainless steel with 1.2 mm diameter of flux cored 316LT filler wire. The welding was carried out with different shielding gas mixtures like 100% Ar, 95% Ar + 05% CO2, 90% Ar + 10% CO2, 80% Ar + 20% CO2, 75% Ar + 23% CO2 + 2% O2 and 70% Ar + 25% CO2 + 5% O2 and 100% CO2. The main aim of the work is to study the effect of various shielding gas mixtures on mechanical properties and metallurgical characters. The microstructures and ferrite content of the welds were analyzed. The mechanical characteristics such as impact test, microhardness and ductility of welds were carried out. The fracture surface impact samples were analyzed through scanning electron microscope (SEM). The fracture surface revealed a ductile rupture at room temperature and ductile rupture with a few cleavages at lower temperatures occurred. The toughness and ferrite percentages of the welds were decreased for increase of the CO2 in shielding gas mixtures.

۱ Introduction

Flux cored arc welding (FCAW) has received significant attention among welders nowadays because of its advantages like easy workability and efficiency. It is suitable for mechanization and robotization and easy for applying thus providing speedy work during welding process of sheets having high thickness. It is a fusion welding process in which coalescence is produced from an arc between the work and a continuously fed filler metal electrode. The electrode core contains alloying elements that modify the molten weld metal, deoxidizers, scavenger elements that remove extra dissolved gases, stabilizers, slag forming elements and flux materials and from literatures it is evident that the use of flux wire over solid wire is more beneficial [1].

The 316L (N) austenitic stainless steel is widely used in exhaust manifolds, furnace parts, heat exchangers, jet engine parts, nuclear industry etc. The heat affected zone (HAZ) of austenitic stainless steels containing more than 0.05% C can be susceptible to a form of inter-granular corrosion called weld decay. Weld decay in austenitic stainless steels is caused by precipitation of Cr carbide at grain boundaries, which is called sensitization. Typically, the Cr carbide is Cr-enriched M23C6, in which M represents Cr and some small amount of Fe. Within the sensitization temperature range carbon atoms rapidly diffuse to grain boundaries, where they combine with Cr to form Cr carbide, so less carbon in the range of 0.03 maximum is used in 316L material [2]. Austenitic stainless steels find applications due to excellent corrosion resistance in normal atmospheres and in a wide range of corrosive media. Type 316 steels containing 2–۳% molybdenum show better general corrosion resistance as in chloride medium to pitting corrosion [3]. In 316L (N) nitrogen is added within the range of 0.06–۰۰۸%.

The shielding gas is one of the key factors which affects the microstructure and mechanical properties of the welds. A mixture of carbon dioxide and argon is widely used as the shielding gas for FCAW process [4,6]. The percentage of carbon dioxide gas has varied from the different levels with different heat input and the effects of the welds were investigated by Yilmaz and Tümer [5]. Aloraier et al. [7] has proposed that no post weld heat treatment required for FCAW process. The welding parameters that affect weld penetration were studied by Mostafa and Khajavi [8]. Erdal Karadeniz et al. [9] studied the effect of process parameters on penetration in gas metal arc welding processes. Arivazhagan et al. [10] studied the influence of shielding gas composition on toughness of flux-cored arc weld of modified 9Cr–۱Mo (P91) steel. Mohamad Ebrahimnia et al. [11] studied the effect of shielding gas composition on the mechanical weld properties of steel ST 37-2 in gas metal arc welding. In this study the influence of variation in the shielding gas composition on the weld properties of the steel ST 37-2 was investigated. Kang et al. [12] investigated the effect of alternate supply of shielding gases in austenite stainless steel GTA welding with a mixing supply of shielding gas and found that it cannot only increase the welding quality, but also reduce the energy by 20% and the emission rate of fume. To improve the efficiency of FCAW welding process the effect of carbon dioxide on the weld quality must be known.

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