Self-shielded Flux Cored Arc Welding Research Articles

Flux-cored arc welding of 316L austenitic stainless steel: the effect of different shielding gas mixtures on microstructural features and weld performance

ABSTRACT Double-pass flux cored arc welding (FCAW) of 316 L austenitic stainless steel was performed using different shielding gas mixtures. Microstructural features and weld performance were evaluated. Based on the Creq/Nieq ratio of fusion zone and cooling rate, different morphologies of ferrite (skeletal, lathy, and dendritic) and austenite with various volume fractions were produced. An increase in Creq/Nieq ratio can increase the strength level because the presence of ferrite in the weld metal acts as a second-phase strengthening agent. On the other hand, ferrite is a suitable place for sigma phase precipitation. Highest joint efficiency resulted in the specimen welded via GTAW. The ductility of welds was ranked as CO2 +Ar shielded FCAW > GTAW > CO2 shielded FCAW > self-shielded FCAW. At room temperature, variation of delta-ferrite content (Creq/Nieq ratio) does not affect toughness. However, an increased amount of CO2 in shielding gas will result in decreased toughness values because of the enhanced formation of oxide inclusions. Increasing CO2 content of shielding gas from 0% to 18% and finally to 100% will result in decreasing toughness values from the highest value of 76J to 40 J and finally the lowest value of 38J.

Underwater wet welding of high-strength low-alloy steel using self-shielded flux-cored wire with highly exothermic Al/CuO mixture

Underwater wet welding processes with minimal energy consumption and high efficiency is the future goal for researchers in the field. Al/CuO exothermic mixtures in consumables for land welding process can improve welding process stability and decrease the electrical dependence of the welding process. To confirm the beneficial effects of Al/CuO thermite and gain a better understanding of metallurgical properties of the thermite welds in underwater environment, this study investigated underwater wet self-shielded flux-cored arc welding (FCAW-S) of high-strength low-alloy steel with highly exothermic Al/CuO mixture. The effects of Al/CuO thermite on arc stability, weld metallurgical behavior and melting characteristics of flux-cored wire were clarified. The ratio of chemical heat in the flux to the heat required to melt welding wire increased from 4.7% to 38.4% with the addition of Al/CuO, potentially accelerating the melting of flux-cored wires. The arc stability with thermite addition was superior to that without the addition of thermite. The presence of copper as a result of metallurgical reactions had a substantial impact on metallurgical behavior of underwater wet welds. The ferrite weld was a non-equilibrium Cu supersaturated solid solution, which increased the microhardness and tensile strength of wet welds. The measured cooling time proved that the incorporation of Al/CuO had the capacity to reduce cooling rate. The incorporation of 50 wt% thermite yielded a 7.8% increase in welding process efficiency compared to the base formulation. Current work provided an important direction for the development of a chemically self-heating flux-cored wire.

Strain ageing embrittlement behaviour of X80 self-shielded flux-cored girth weld metal

ABSTRACT Strain ageing embrittlement behaviour of X80 self-shielded flux-cored arc welding (FCAW) girth weld metal at different strain ageing conditions (1% prestrain, and aged at 150°C/100°C for 1 –6 h, and 80°C for 1–30 days) was studied. Microstructure of FCAW girth weld generally consisted of coarse granular bainite and lath bainite. After strain ageing, toughness notably decreased, and higher ageing temperature or longer ageing induced lower toughness, until ageing is fully yielded. At initial stage of strain ageing, more dimples were found accompanying with cleavage facet. With the progress of strain ageing, the embrittlement was more obvious, while larger cleavage facet and less dimples were found. Meanwhile, the critical size for cleavage fracture initiation site became smaller. Martensite–austenite constituent (1–2 μm) was found to initiate cleavage fracture when partially strain aged, while submicron inclusions could initiate cleavage facet when fully strain aged.

Effect of Exothermic Addition (CuO - Al) on the Structure, Mechanical Properties and Abrasive Wear Resistance of the Deposited Metal During Self-Shielded Flux-Cored Arc Welding

Effect of Exothermic Addition (CuO - Al) on the Structure, Mechanical Properties and Abrasive Wear Resistance of the Deposited Metal During Self-Shielded Flux-Cored Arc Welding

Application of Taguchi method and ANOVA analysis for optimization of process parameters and exothermic addition (CuO-Al) introduction in the core filler during self-shielded flux-cored arc welding

The main factors affecting the mechanical properties of the deposited metal in arc hardfacing are primarily the chemical composition and the cooling rate. The latter depends on the filler materials composition, hardfacing condition (mode), and hardfacing technology, determining the geometric parameters and parameters of the welded bead shape, the amount of deposited metal dilution with the base metal, and the amount of the introduced heat. The goal of this work is to analyze the effect of exothermic addition introduction to the core filler and the effect of welding condition on weld bead morphology, thermal indicators, and mechanical properties of the deposited metal. The deposited metal of the Fe-C-Cr-Cu-Ti-V-Al alloying system was used. The Taguchi technique was adopted as the experimental plan design, as follows: the orthogonal array L9 (3 ^ 4) and signal-to-noise ratio (S/N). The analysis of variance (ANOVA) was also applied for determination of the variables contribution to the dependent parameters. Application of the Taguchi method is simpler, more efficient, and quicker. It requires fewer experiments to determine the optimal values of the studied variables. Microstructural studies using an optical microscope were additionally performed for individual samples of the weld metal with the best mechanical properties, for each of the experimental flux-cored wires. We determined that introduction of CuO-Al exothermic addition of to the core filler had a significant effect on such indicators of the weld bead morphology as the weld reinforcement height and the weld reinforcement form factor (WRFF). Introduction of exothermic addition (CuO-Al) to the core filler had a significant effect on the welding current (I), heat input rate (HIR), and microhardness. The arc voltage (Р(Ua) = 45.8 %) had the greatest influence on the microhardness value, while the percentage of exothermic addition in the core filler (P (EM) = 26.9%) and the wire feed rate (P (WFS) = 21.5%) had a smaller effect. It was shown that it is important to take into account complex parameters, such as dilution variation and weld reinforcement form factor (WRFF) for the geometric characteristics of the weld bead and heat input rate (HIR) for heat indicators in order to optimize hardfacing conditions and composition of the core filler (introduction of exothermic addition). According to the results of the article, the value is the most correlated with the average values of microhardness.

Microstructural and wear investigation of high chromium white cast iron hardfacing alloys deposited on carbon steel

The investigation of the microstructural development during solidification as well as wear performance of Nb and Mo added hypereutectic WCI hardface deposits with W and V as minor additives was aimed during the study. This would enable the development of the hardface deposits with superior abrasive performance for severe wear applications. The hardfacing alloys were deposited on carbon steel using self-shielded flux cored arc welding (FCAW) technique. Phase formation, microstructural and mechanical properties of hardface deposits were investigated in the as-deposited condition. The microstructural examination showed that the addition of alloying elements to the high-Cr WCI resulted in grain refinement of the primary proeutectic M7C3 (M = Cr and Fe) carbide phase. EDX analysis revealed that the addition of Nb and Mo resulted in the formation of MC (M = Mo and Nb) carbides which resulted in grain refinement and higher wear resistance. Phase investigation of the deposits using XRD analysis shows the existence of M7C3 for all alloys and MC carbides for alloy B, C and D along with the ferrite phase. The main wear mechanism observed for the base high-Cr WCI is abrasive wear, while hardface alloys with alloying addition showed surface delamination which was attributed to the presence of soft phase adjacent to the M7C3 carbides in the microstructure. Among all the alloys, hardface deposits prepared using a filler with 5.5 -Mo, 6 - Nb, 2 - W and 1 - V (wt%) addition showed highest wear resistance due to grain refinement, MC carbide formation and solid solution strengthening by W and V.

Effects of electrode polarity on the droplet transfer mode in self-shielded flux-cored arc welding

Abstract Self-shielded flux-cored wires (AWS E71 T FCW) are widely applied in the fields of construction. In this study, the effect of the electrode polarity on the mode of droplet transfer and arc stability in self-shielded flux-cored arc welding was investigated by the high-speed video camera system and electric signal. The results showed that the change of electrode polarity determined different mode of droplet transfer. In direct current electrode positive (DCEP), droplet transfer mode presented the globular repelled transfer. And the area of the arc root decreased at the bottom of the droplet because of the metallurgical reaction effect of the flux core, it resulted in the relatively lager electromagnetic force acting on the droplet, which hindered the droplet transfer. Moreover, when the droplet was separated from the welding wire, the droplet was kept rotation state under the action of the resultant force couple. In the direct current electrode negative (DCEN), the electromagnetic force promoted the droplet transfer and droplet transfer mode shown spray transfer mode. Due to the particularity of the composition and distribution of the molten slag, the conductive path was generated easily at the connection between the droplet and the wire, it made the arc root climb up and wrap the droplet, the necking phenomenon appeared obviously. In DCEN or DCEP, arc voltage and arc current have little effect on the droplet transfer mode. At the same time, compared with the welding process in DCEP, the welding process in DCEN had the advantages of higher arc stability, fewer welding spatter.

Influence of the core filler composition on the recovery of alloying elements during the self-shielded flux-cored arc welding

Introduction of the exothermic mixture into the flux-cored electrode core filler and its’ influence to the deposited metal composition and recovery factors of alloying elements were investigated in the article. An influence of FCAW-S core filler composition with exothermal mixture implemented for recovery hardfacing by wear-resistant alloys was investigated in this article. An influence of oxidizing agent to reducing agent ratio (CuO/Al) in the composition of exothermic mixture, on the recovery factors of the alloying system components was considered. As well as the ratio of exothermic mixture oxidizing agent to graphite (CuO/C) and the amount of exothermic mixture (TM) in the flux-cored wire filler composition. Some mathematical models of the recovery factors dependence on the studied factors were received as a result of the investigation.An analysis of the results of the study showed that the ratio of the oxidizing agent to the reducing agent in the exothermic mixture (CuO/Al) had the greatest influence on the recovery factors of alloying elements. Received mathematical dependences of the alloying elements recovery rates allowed to optimize the calculation of composition of the FCAW-S core filler mixture with exothermic mixture and the deposited metal composition. To receive the high values of the elements recovery factors, the parameters subject to investigation should have the following values:CuO/C=3.75 … 5.25, CuO/Al=3.75 … 5.25, CuO+Al=33 … 45wt.%.

Diffusible hydrogen management in underwater wet self-shielded flux cored arc welding

This article reports the effect of underwater wet welding parameters and conditions on the diffusible hydrogen content in deposited metal for welding with a self-shielded flux cored wire. The diffusible hydrogen content in deposited metal was determined using the glycerin method according to the Plackett-Burman design determining the significance of the effect of the stick out length, welding current, arc voltage, travel speed and water salinity. The results of the measurements of the diffusible hydrogen content in deposited metal ranged from 25.85 to 44.12 ml/100 g. The effect of all the tested factors is statistically significant. An equation was also developed, the analysis of which showed, that the hydrogen content cannot be reduced by technological methods below 21 ml/100 g.

Microstructure and Abrasion Resistance of Fe-Cr-C and Fe-Cr-C-Nb Hardfacing Alloys Deposited by S-FCAW and Cold Solid Wires

This work assesses the wear resistance behavior of different high Cr and Nb alloyed hardfacing deposits carried out by Self-shielded Flux Cored Arc Welding, considering the resulting microstructures as base for the analysis. To obtain non-commercial alloys, an “in-situ” technique was employed with addition of cold wire into the weld pool. Six combinations of two commercial FeCrC and FeCrNbC tubular wires and three different solid wires (AWS ER 70S-6, ER308LSi and ER 430) were used to obtain white cast iron hardfacing deposits on plain carbon steel subtracts. The welding parameters were identical for all combinations. Each deposit was characterized taking into account the resulting microstructure, hardness and abrasion wear resistance. The addition of a ferritic stainless solid wire in the weld pool resulted in the best wear resistance, which was boosted if Nb is present in the S-FCAW wire. The reason for these results was microstructure refinement and the presence of NbC and M7C3, the latter being present in all combinations.

Microstructure and Properties of Fe-Cr-C Hardfacing Alloys Reinforced with TiC-NbC

AbstractThe hypereutectic Fe-Cr-C hardfacing alloys with different contents of TiB2 and Nb were prepared by self-shielded flux cored arc welding. The microstructure of a series of hypereutectic Fe-Cr-C hardfacing alloys added with various TiB2 and Nb contents was investigated by using optical microscopy (OM), scanning electron microscopy (SEM) and X-ray diffraction (XRD). In addition, their Rockwell hardness, microhardness and resistance to abrasive wear were tested. The results showed that the microstructure of a series of hypereutectic Fe-Cr-C hardfacing alloys consisted mainly of martensite, austenite, primary M7C3 carbides and eutectic M7C3 carbides. With the addition of TiB2, a new hard-phase TiC was produced in the hardfacing alloys. And in the alloys added with TiB2 and Nb, a new hard composite phase TiC-NbC was formed. The microhardness of the matrix was improved by adding TiB2 and Nb, but the effect on the Rockwell hardness of Fe-Cr-C hardfacing alloys was insignificant. The addition of TiB2 and Nb can also decrease the size of the primary M7C3 carbides and make the primary M7C3 homogeneous. As a result, the reinforced matrix, the more homogeneous primary M7C3 carbides, and the new hard-phase TiC-NbC all improved the wear resistance of Fe-Cr-C hardfacing alloys.

Characteristics of short-circuit behaviour and its influencing factors in self-shielded flux-cored arc welding

The characteristics of short-circuit behaviour during the process of self-shielded flux-cored arc welding and its corresponding welding electric information are investigated by a synchronous acquisition system of high-speed images and welding electric information. The research results show that when the welding arc between the wire tip and the weld pool extinguishes, a short-circuit event occurs and its corresponding arc voltage is below ∼10 V, which can be used to judge the occurrence of the short-circuit procedure. Meanwhile, the influence of welding process setting parameters on the short-circuit behaviour is explored in detail. The existence of liquid slag increases the difficulty of arc re-ignition, which makes it easy to produce weld defects. Therefore, the short-circuit procedure in self-shielded flux-cored arc welding should be avoided as much as possible.

Low Heat Input Welding to Improve Impact Toughness of Multipass FCAW-S Weld Metal

Multipass self-shielded flux cored arc welding with different heat inputs (1.3-2.0 kJ/㎜) was conducted to determine the effects of the heat input on the proportion of the reheated region, impact toughness, and diffusible hydrogen content in the weld metal. The reheated region showed twice the impact toughness of the as-deposited region because of its fine grained ferritic-pearlitic microstructure. With decreasing heat input, the proportion of the reheated region in the weld metal became higher, even if the depth of the region became shallower. Accordingly, the greatest impact toughness, 69 J at -40℃, was obtained for the lowest heat input welding, 1.3 kJ/㎜. Irrespective of the heat input, little difference was observed in the hardness and diffusible hydrogen content in the weld metal. This result implies that low heat input welding with 1.3 kJ/㎜ can be performed to obtain a higher proportion of reheated region and thus greater impact toughness for the weld metal without the concern of hydrogen cracking.

셀프실드아크 용접금속의 확산성수소량에 미치는 용접변수의 영향

The effects of the welding parameters, contact tip-to-workpiece distance (CTWD), current, and voltage on the diffusible hydrogen content in weld metal deposited by self-shielded flux cored arc welding were investigated and rationalized by comparing the amount of heat generated in the extension length of the wire. This showed that as CTWD increased from 15mm to 25mm, the amount of heat generated was increased from 71.1J to 174.8J, and the hydrogen content was decreased from 11.3mL/100g to 5.9mL/100 g. Even if little difference was observed in the amount of heat generated, the hydrogen content was increased with an increase in voltage because of the longer arc length. A regression analysis showed that the regression coefficient of voltage in self-shielded flux cored arc welding is greater than that in arc welding. This implies that voltage control is more important in self-shielded flux cored arc welding than in arc welding.

Microstructural and Mechanical Properties Investigation of TiC Reinforced Hardface Alloy Deposited on Mild Steel Substrate

Wear is one of the major problems faced in industry that reduces the life of industrial components and increases the operating costs. Therefore, hardfacing is widely employed by engineers to minimize components’ wear, in which single or multiple layers of hard material that mainly consists of carbides deposited on the base metal. However, these carbide-based materials suffer from solidification cracking due to lack of ductility. In this study, titanium carbide (TiC) reinforced alloy steel deposited using self-shielded flux cored arc welding technique. The microstructure and phase analysis in the hardfaced deposit by scanning electron microscope, energy dispersive x-ray and X-ray diffractometer revealed that the microstructure consists of finely distributed TiC within matrix of martensite and some retained austenite. Microhardness test and pin-on-disk wear test had shown that the clad deposit posses better hardness and wear resistance. Retained austenite formation and TiC precipitation were discussed in detail and correlated with the mechanical properties.

Effect of cerium on abrasive wear behaviour of hardfacing alloy

Hardfacing alloys with different amounts of ceria were prepared by self-shielded flux cored arc welding. The abrasion tests were carried out using the dry sand-rubber wheel machine according to JB/T 7705-1995 standard. The hardness of hardfacing deposits was measured by means of HR-150AL Rockwell hardness test and the fracture toughness was measured by the indentation method. Microstructure characterization and surface analysis were made using optical microscopy, scanning electron microscopy (SEM) and energy spectrum analysis. The results showed that the wear resistance was determined by the size and distribution of the carbides, as well as by the matrix microstructure. The main wear mechanisms observed at the surfaces included micro-cutting and micro-ploughing of the matrix. The addition of ceria improved the hardness and fracture toughness of hardfacing deposits, which would increase the resistance to plastic deformation and scratch, thus the wear resistance of hardfacing alloys was improved.