28-10-2016, 11:49 AM
1461987472-AReviewontheEffectofFluxCoredArcWeldingonProcessParameters.docx (Size: 1.72 MB / Downloads: 7)
Abstract
Flux Cored Arc Welding (FCAW) is a type of welding processes which uses continuous flux-cored filler wire. Here, the flux is used as a protection of welding from the atmospheric conditions. This study is about the effect of FCAW on various welding parameters with the variables in welding current, speed, arc voltage and the heat input. The effects will be in welding penetration, microstructure and hardness of the welded metal. They considered mild steel and duplex stainless steel as the base metal. This process leads to deposit good welds on high-integrity applications, even it gives a significant economy due to its higher deposition. Likewise, as in the case of cladding duplex stainless steel on the low carbon structural steel plates, it is being considered that four factor five level central design on rotational basis with full replications. Welding voltage and travel speed of arc were adjusted to keep a constant arc length and the area of deposition of weld. Thus, the effect of input process parameters such as arc voltage and welding speed will influence mainly, the weld penetration using Flux Cored Arc Welding process.
Key words: Depth of penetration, hardness, microstructure, welding speed, deposition, cladding
1.Introduction
Many engineering applications need both high strength and corrosion resistant materials for high reliability and good performance. Often the good strength can be achieved by the using steels which do not have the needed corrosion resistance. A possible solution regarding materials to provide the structural components which blends the attributes of high strength and corrosion resistance is mainly to make cladding of the steel with a metallurgically suitable corrosion resistant alloy. The desirable properties in that kind of cladding alloy are appropriate strength, weldability, general and localised corrosion resistance, and good fatigue properties.
Duplex stainless steel is the best material for cladding which is having good corrosion resistance and weldability. These possess chloride stress corrosion cracking resistance and strength significantly greater than that of the austenitic stainless steels. In recent years, weld cladding processes has been increased rapidly and are now applied in enormous industries like chemical industries, fertilizer industries, nuclear and steam power stations, food processing industries and petrochemical industries, etc., The greatdifference between welding process and cladding process is the percentage dilution. The composition and other properties of cladding are highly influenced by the obtained dilution. Controlling the dilution is important factor in cladding, where low dilution is highly desirable.
In a welding aspect, Flux-Cored Arc Welding (FCAW) process is being commonly used in various industries to blend the metals and alloys. It has very few benefits such as high deposition rates, most permissive of rust and mill scale than GMAW, simplest and more adaptable when comparing with SAW, less operator skill is required than GMAW, Very high productivity than SMAW and good surface finish. For repairing works in industries, they are using manual metal arc welding (MMAW), even though the flux cored arc welding (FCAW) process is of more benefits and have been appreciated by many industries. The welding process parameters for FCAW must be well recognized and categorized to automate and include robotization in arc welding. The selection of welding process should be specific to assure the good quality in weld bead. To obtain the required quality in welds, it is very important to have total control over the appropriate process parameters to obtain geometry of bead and relationship in shape of a weldment dependent. The results from this study on process parameters, geometry of weld bead and properties in low carbon steel joints by FCAW-G has been shown that the increase in welding arc voltage or welding current in FCAW-G decreases the ultimate tensile strength and yield strength of the weld. Whenever the welding speed increases, the weld strength also increases.Therefore, from this study we can also come to know that the effects of various FCAW process parameters on the qualityparameters of cladding in duplex stainless steel cladding of low carbon structural steel plates.
2.Experimental Work
The mild steel plate of 100mm x 100mm x 6mm were used as the base metal for this experiment. The spectrometric analysis of the base metal has been made by using Optical Emission Spectrometer Machine to get the chemical composition of the metal was mentioned in the Table 1. FCAW process was performed by the OTC Almega AII-B4 series articulated robot welding. Electrode wire (K-71T AWS A5.20) with diameter of 1.2mm were used, 100% of carbon dioxide was used as a shielding gas, nozzle to work distance is 12mm, the torch angle is 5° and only one pass weld was made on the plate.
The variables those have been chosen in this study are arc voltage, welding current and welding speed. The arc voltages and the welding currents have been chosen as 22V, 26V and 30 V and 90A, 150A and 210 A approximately. The welding speed has been chosen as 20, 40 and 60 cm/min. After the welding process, the specimens will be made into cut perpendicular to the welding direction by using a cut-off machine. Then the specimens will be grounded and polished. Then, the specimens will be etched using 2% or 10% Nital to clearly see the boundary of grains in the metal zone of welding. The depth of penetration will be measured and microstructure will be observed on the specimens using optical microscopy which is of 10x magnification. Finally, for the values of Vickers Hardness for HAZ, 1kg of load will be applied up to 20 seconds of time on the specimens [1]. The composition of the base metal used here is shown in the table
The experiments were done by using UNIMACRO 501C programmable welding machine by using DC electrode positive (DCEP) polarity. Specimens of size 200 mm × 150 mm × 20 mm has been cut from low carbon structural steel (IS: 2062) plate and its surfaces were grounded to remove oxide scale formed and dirt before cladding. Flux cored duplex stainless steel welding wire (E2209T1-4/1) of 1.2 mm diameter was used to deposit in the weld beads. The Chemical composition of the base metal and welding wire is given in Table 2. CO2 gas at a constant flow rate of 18 L/min was used for shielding to protect the weld from the atmosphere. The experimental setup used here consists of a travelling carriage with a table to support the specimens.
The carriage speed can be continuously adjustable from 6 cm/min to 72 cm/min. The welding torch was being held in the same position in a frame mounted above the work table, and it was provided with an attachment for upward, downward and angular movement for
setting the needed nozzle-to-plate distance and welding torch angle respectively. The experiments were conducted by laying three passes of beads using stringer bead technique with a constant overlapping range of 40%. The inter-pass temperature was maintained at 150 ◦C during all the cladding experiments
The clad quality parameters such as the Weld bead depth, Depth of penetration, Height of reinforcement and Percentage dilution will be influenced by the welding current, welding speed, Nozzle-to-plate distance and Welding torch angle. All the above mentioned parameters will be increased with the increase in welding current. This is because of the increase in welding current density and the weight of the filler wire fuse per unit time. Also, the arc becomes stiffer and much hotter that penetrates most deeply and melts more base metal. The increase in welding speed leads to the decrease in bead width and height of reinforcement. Whereas, the depth of penetration and percentage dilution increases. The decrease in height of reinforcement and bead width can be directly combined to the reduced heat input per unit length and lesser filler metal applied per unit length of the weldment. The increase in percentage dilution is due to the weight of deposited metal per unit length decreases with the cross sectional area of the bead which decreases lesser. With the low welding speed, the arc is almost vertical in position and in this instance the weld pool provides the effect of the arc made and restricts the deeper penetration.
Results and Discussions
The FCAW process on the base metals with the variables in the process parameters such as welding speed, arc voltage and welding current were done and the observation of the microstructure, depth of penetration and hardness has been measured for all cases. As a result, the process parameters will be affecting the depth of penetration, microstructure of the weld and hardness of metal [1]. The effect of various parameters on the welding process are as explained as follows:
3.1 Effect of the welding current on depth of penetration
The effect of welding current on depth of penetration has beenrepresented in figure 4. The entire graph is showing the depth of penetration versus welding current on three different arc voltages with the constant welding speeds 20cm/min, 40cm/min and 60cm/min. In figure 4, arc voltage was kept as constant as 30V and the value of depth of penetration has been increased by increasing the value of the various welding speeds. The higher depth of penetration is 2.732 mm at the welding speed of 40cm/min, welding current of 210A and arc voltage of 30V. The lower depth of penetration is 0.26mm at the welding speed of 20cm/min, welding current of 90 A and arc voltage of 22V. From the figure 4, it is cleared that the increasing the welding current from 90A to 210A will be influencing and increasing the depth of penetration at all the arc voltages. The following graph shows that when the weldingspeeds and the welding current increases the penetration increases
Observation of Microstructure
The Changes in the various process parameters influences the properties of microstructure of the weld metal. The increase in the value of process parameters will affect the grain size of the microstructure of the weld.
The Figure 5 shows the variation of microstructure of the weld present in the different phases of grain boundaries at arc voltage of 30V, welding current of 210A and with the varying welding speeds of 20cm/min, 40cm/min and 60cm/min. Comparatively, the grain boundaries are large in size of 25.4µ at 20cm/min, smaller in size of 19.4µ at 40cm/min and smallest in size of 16.9µ at 60cm/min which forms more martensite than others
3.3 Vicker’s Hardness Test
The specimens will be made into cut perpendicular to the direction of welding process by using abrasive cutter, grinder and polisher with different grades of grit. Before the experiment, the specimen will be etched using 2% Nitalto clearly see the metal zone of
welding. For getting Vickers Hardness values for HAZ, the 1kg of load will be applied up to 20 seconds of time on the surface of the specimens. The hardness values were determined and performed and as shown in the figure 6. Generally, the losses of originality strength for the material is caused by the strain hardening effect in the fused zone during the solidification. For the low carbon steel, the point of the fusion zone which leads to the formation of bainite or martensite and the hardness of the material has been increased. The effect of welding current to the hardness of the weld metal is as shown in figure 6. The entire graph represents the hardness of the metal versus welding current on three different arc voltages with the various welding speeds of 20cm/min, 40cm/min and 60cm/min. The hardness of the metal decreases, as the welding current and arc voltage increases.
The clad quality parameters such as the Weld bead depth, Depth of penetration, Height of reinforcement and Percentage dilution will be influenced by the welding current, welding speed, Nozzle-to-plate distance and Welding torch angle. All the above mentioned parameters will be increased with the increase in welding current. This is because of the increase in welding current density and the weight of the filler wire fuse per unit time. Also, the arc becomes stiffer and much hotter that penetrates most deeply and melts more base metal. The increase in welding speed leads to the decrease in bead width and height of reinforcement. Whereas, the depth of penetration and percentage dilution increases. It is as shown in the figure 8.
The decrease in height of reinforcement and bead width can be directly combined to the reduced heat input per unit length and lesser filler metal applied per unit length of the weldment. The increase in percentage dilution is due to the weight of deposited metal per unit length decreases with the cross sectional area of the bead which decreases lesser. With the low welding speed, the arc is almost vertical in position and in this instance the weld pool provides the effect of the arc made and restricts the deeper penetration. It is as shown in the figure 9.
The increase in the nozzle-to-plate distance lead to the slight decrease in depth of penetration and percentage dilution. Whereas, the bead width and height of reinforcement increases. This consequently increases the circuit resistance that reduces the welding current. This reduced current is the reason for the depth of penetration and dilution. But, with an increase in nozzle-to-plate the arc length will be increased. Therefore, the bead width will be increased due to the wider arc area at the weld surface and this increases the height of reinforcement that is due to the addition of same volume of the filler material. It is as shown in the figure 10.
The increase in welding torch angle leads to the decrease in height of reinforcement, depth of penetration and percentage dilution. Whereas, the bead width increases. The main reason is that when the angle got increased, the arc force pushes the weld metal in forward i.e. the direction is towards the cold metal that reduces the penetration. reinforcement and percentage dilution as the width of the weld is getting increased.
Conclusions
Since, the Robot welding system is highly appropriate in increasing the production rate, quality and to reduce production time and cost for a desired product, because it has attained a great deal of attention. Hence, the conclusions from this study are:
1) The increase in value of depth of penetration by increasing the value of welding current from 90, 150 and 210 A. Welding current is a factor which affects the depth of penetration. The Arc voltage and the welding speed will also be a factor which can affect the depth of penetration. From the graph represented at the figure 6, the best value of the depth of penetration from three different welding speeds is 30V. It has been plotted with the highest value of depth of penetration and comparing with 22V and 26 V.
2) The sizes of grainboundary changes in microstructure from bigger to smaller size when the welding speeds increases.
3) The value of hardness for the weld metal will be higher than hardness in HAZ in case of FCAW if the base metal is mild steel. The other parameters will affect the values of hardness for the welded portion in metals and HAZ. It is shown that the various parameters have an HAZ value that is higher than welded portion of metals. The hardness will be decreasing as the arc voltage and welding current increases that makes a change in the sizes of grain boundary of microstructure [1].
Thus, The Percentage Dilution value increases with the increase in welding current and the welding speed and decreases with the increase in the nozzle-to-plate distance and the welding torch angle. The height of Reinforcement increases with the increase in the welding current and the nozzle-to-plate distance and decreases with the increase in the welding speed and the welding torch angle. The Weld bead width increases with the increase in the welding current, the nozzle-to-plate distance and the welding torch angle and decreases with the increase in the welding speed. The depth of Penetration increases with the increase in the welding current and the welding speed and decreases with the increase in the nozzle-to-plate distance and the welding torch angle.
The Bead width increases with the increase in welding current at each and every level of the welding speed. But the rate of increase in the bead width with the increase in the welding current which decreases significantly with the increase in the welding speed. The Increase in the welding torch angle decreases the depth of penetration when welding speed is high, but the depth of penetration slightly increases with the increase in the welding torch angle when welding speed is low. The Percentage dilution decreases with the increase in the nozzle-to-plate distance at each and every level of the welding current. But, the rate of decrease in percentage dilution with the increase in the nozzle-to-plate distance which decreases significantly with the decrease in the welding current