Production and Weld Joint Performance Evaluation of Arc Welding Electrodes From Dana Rolling Mill Scales
Aim and Objectives
This research work is aimed at utilizing mill scales from Dana steel rolling mill for the production of arc welding electrodes and to evaluate and compare the performance of welded joints using the produced electrodes and a foreign electrode.
The specific objectives are:
- To obtain mill scales from Dana steel rolling mill, prepare and analyze the mill scales, to determine the constituent
- To formulate a new flux composition for the electrodes to be produced
- To produce various types of Iron oxide electrodes such as E6020, E6027, E6024, and E6030 by varying the composition of the constituent
- To determine the welded joints performance of the electrodes produced and compare them with the foreign electrodes obtained in the
- To carry out the microstructural analysis of the weldments.
- LITERATURE REVIEW
- Electrode Overview
Electrode is a specially prepared rod or wire that not only conducts electric current and sustain the arc, but also melts and supply the filler metal to the joint; as in the case of a consumable electrode (Hwaiyu, 2004). In arc welding, an electrode is used to conduct current through a work piece to fuse two pieces of materials together. Depending upon the process, an electrode is either consumable as in the case of shielded metal arc welding or non-consumable such as gas tungsten welding (Lincoln Electric, 2006).
Electrode is a metal in rod or wire form with baked minerals around it, which can also be referred to as filler wire used in electric arc welding to maintain the arc and at the same time supply molten metal. Electrode used in arc welding is basically made of steel core wire, and the covering(coating). Electrode can be bare, fluxed and can be heavy coated. Bare electrodes have limited applications as during welding operations, they are exposed to oxygen or nitrogen of the surrounding air which form non-metallic constituents and they are trapped in the rapidly solidifying weld metal, thereby decreasing the strength, and ductility of the weld metal. When bare electrodes are used, the weld appearance is poor, and there is difficulty of maintaining a stable arc. Bare electrodes are generally used for welding wrought iron. Improved weld may be obtained by applying light coating of flux on the rod using a dusting or washing process. The flux coatingon an electrode assists both in eliminating undesirable oxides and preventing their formation. The heavy coated electrodes are by far the most important and the most widely used (Jain, 2008).
Arc welding electrodes are identified using the American welding society numbering system. The E stands for electrode, next is a four digit number, the first two number of the four digits indicates the maximum tensile strength in thousands of pounds per square inch (psi) of the weld that the rod will produce, the third digits indicates the position in which the electrode can be used, and the last two digits together indicates the type of coatings on the electrode and the welding current the electrode can be used with. The positions in which electrodes can be used are: 1- Flat, vertical and horizontal position, 2-Horizontal and flat, while 3-Flat position. The types of coatings are: Iron oxide coating, cellulosic coating, and rutile coating.
Fluxes are chemical compound which are composed of different minerals such as oxides, carbonates and fluorides used to prevent oxidation or the formation of oxides and other unwanted chemical reactions. Fluxes help to make welding easier and ensure making of a good and sound weld (Davies, 1972). Fluxes and their slags provide a blanket to protect the weld metal from the action of extraneous gases, flux can also perform cleaning, alloying actions and also produces shielding gas that prevents molten weld metal from oxidation (Jackson, 1973). Fluxes protect, prevents atmospheric oxidation and clean up welded joint chemically and reduces impurities in the metal joining processes (Parma, 2005).
The type of flux coating depends on the weld metal composition. Electrode coating facilitate striking the arc and also provide a stable arc. Coating on an electrode also provide gaseous shield, and prevents the oxidation of molten weld metal, a good flux covered electrode will produce a weld that has an excellent physical and chemical properties.
Commercially electrodes are available in 1.5-9.5mm diameter and 35-45mm in length, the diameter of an electrode is selected depending mainly on the thickness of the material to be joined, and the welding current. Diameter of an electrode controls penetration, the thicker the material to be welded the higher the current needed and the larger the electrode needed (Jain, 2011). The size of an electrode called the gauge is the diameter of the core wire and does not include the flux coatings. Electrodes are available in the following different type of gauges, 16G, 14G, 12G, 10G, 8G, 6G and 4G, the smallest number refers to the largest diameter.
Types of Arc Welding Electrodes
There are two main types of arc welding electrodes. These are consumable electrodes(used in shielded manual metal arc welding) and non-consumable electrodes (used in plasma arc welding and tungsten inert gas welding).
These types of electrodes slowly burns away with usage and are replaced when they become too short (50mm) for further use.
MATERIALS AND METHODS
This chapter outlines the various materials and equipment used for this research work as well as the production process adopted.
- Mill scales
- Sodium silicate
- Calcium carbonate
- Manganese dioxide
- Calcium fluoride
- Mild steel wire
- Wooden mould.
Mill scales: The mill scales used for this research was obtained from Dana steel rolling mill. Mill scales are available in abundance as an industrial waste in Dana steel rolling mill. The mill scales was prepared and analyzed using the modern Oxford 800 X Supreme XRF machine to determine the percentage by weight of all the elements that constitute the mill scales. Primary function is to act as the slag former and also stabilizes the arc.
Sodium silicate: Sodium silicate acts as the binder that is, it binds the flux materials together and also to the core wire. Binders are needed to hold the materials together, so that the flux won’t flake off the core wire. A measured quantity of sodium silicate (water glass) on a weighing balance (Plate II).
This chapter presents the results of the mechanical tests conducted on the welded joints of on all the samples as well as the microstructural analysis of the weldments and heat affected zone using the produced electrodes and a foreign electrode.
DISCUSSION OF RESULTS
Elemental Analysis of Mill Scales
The elemental composition of mill scales, using the modern Oxford 800 X Supreme XRF machine is presented in Table 4.1 and it can be observed that Si02, Al203 and Fe203 were found to be the major constituent elements, with Fe203 constituting more than 80% of the total constituent element and Fe203 being an important constituent in electrode covering, therefore this suggests that Mill scales can be used in electrode production. Other elements such as Mn203, P205, Mg0 were found to be present in traces.
Performance Evaluation of the Welded Joints
Table 4.2 shows the results of the tensile test conducted on some welded specimen using the produced electrodes and the foreign electrode under the same welding condition(Input voltage and Current setting). It can be observed that all the produced electrodes with the exception of electrode type E6030 compete well with the foreign electrode. The welded specimen using electrode type E6020 produced the highest tensile strength as the welded joint shows that it can withstand a tensile strength of up to 453.0 N per mm2. The produced electrodes did not only give a clean and sound weld, the welded joints produced by these electrodes are also defects free. The little difference in the tensile strength of the weld produced by the foreign electrode and the produced electrodes can be attributed to the production process adopted, meanwhile the difference in the tensile strength of the four produced electrodes must have been due to variation in the process parameters
Figure 4.1 is the variation of load against extension of the tensile test of the weld joint using electrode type E6020, it can be observed that with a gradual increase in load, there is a corresponding increase in extension of the specimen, that is the extension produced is directly proportional to load, this continues until the maximum load is reached. At this point of maximum load, which is 8.2KN a reduction in area of the specimen occurs and small increase in extension also take place and a neck is formed. The reduced area was not able to sustain the load being applied, hence the specimen finally fractured at this new point which is called the breaking load point. For the specimen welded with electrode type E6020, it can be observed that the maximum load is different from the breaking load. The reason for this is that, the specimen did not fractured at the welded point, because the welded joint is stronger than other part of the specimen, hence fracture occurs outside the welded zone. The type of fracture that occurs here is referred to as ductile fracture. This type of fracture is aided by plastic deformation of the specimen and a visual observation of the fractured surface, revealed a fibrous surface, indicating that plastic deformation occurred before the specimen fractured.
Figures 4.2, 4.3, 4.4 present the variation of load against extension of the tensile strength tests of the weld joints using electrode type E6024, E6027 and E6030. It can be observed that there is little elongation and the specimen fractured at the point of maximum load. Here the maximum load is the same as the breaking load, and specimen welded with electrode type E6024, E6027 and E6030 fractured at the welded point. Therefore it can be deduced that the material is stronger than the welded joint. A close examination of the fractured surfaces of each of the specimen shows a crystalline surface, indicating that the specimen was brittle and fracture of the specimen occurred with little plastic deformation.
Figure 4.5, is the variation of the tensile test of the specimen welded with the foreign electrode, from the graph it can be observed that as the applied load increases, extension also increases. Just like what was obtained in Fig 4.1, the specimen did not fractured at the point of maximum load. At maximum load there is increase in extension(elongation of the specimen) and reduction in the area, and with the load concentrated at the reduced area, the specimen finally fractured at this new point called the breaking load point. It can also be said here that the welded joint is stronger than other part ofthe specimen, and thisexplain the reason why the specimen fractured outside the welded zone, therefore the maximum load is also different from the breaking load. Here also we have ductility fracture, the initiation and initial growth of cracks observed occurred due to plastic deformation.
Figure 4.6 shows the variation of load against extension of the tensile tests conducted on all the specimen welded with the produced electrodes and the foreign electrode super-imposed in each other. It can be observed that the sample welded with the foreign electrode gave the highest yield strength with the highest extension, then followed by the specimen welded with electrode type E6020. The specimen welded with electrode type E6030 produced the lowest yield strength with the lowest extension, this shows that the specimen welded with electrode type E6030 produced the joint that is least ductile when compared to joints produced using the foreign electrode and the other three types of electrodes produced.
Figure 4.7 shows the bar chart of percentage elongation against electrode type, the foreign electrode produced the joint with the highest percentage elongation while the welded joint produced electrode type E6030 gave the lowest percentage elongation as evident in the chart.
From Table 4.3 it can be observed that the foreign electrode (commercial) produced the highest average value of hardness test and of the four different electrodes produced, electrode type E6020 produced the highest average value of hardness test. A similar trend was observed in the result of the hardness test conducted in relation to tensile test result earlier reported. This corroborate what was reported by Ebelebe (2014): that there is a relationship between the hardness value of a material and the tensile strength of a material, that is hardness of a material varies directly with strength of a material.
Table 4.4 shows the results of the impact test conducted on some welded specimen using the produced electrodes and the foreign electrode. The test was carried out using the Izod impact testing machine, it can be observed that the specimen welded with electrode type E6030, produced the highest impact energy while the foreign electrode gave the lowest impact energy (measure of toughness) as a result of this it can be deduced from this results that as the hardness properties of the specimen welded with the produced electrodes and the foreign electrode increases, the impact energy (measure of toughness) decreases. In other words it could be said that hardness properties varies inversely proportional to impact energy, the harder the joint the less tough it is.
Micrographs of Weldment and Heat Affected Zone
Plate XVII shows the microstructure of mild steel in the as- received condition, revealing an even distribution of the pearlite (black matrix) in ferrite (white matrix). Etchant; 2% Nital, Mag; x200.
Plate XVIII shows the microstructure of the weldment of electrode type E6024 revealing a coarse structure of pearlite in ferrite matrix, while Plate XIX revealed the heat affected zone having an intermediate grain size. Etchant; 2% Nital, Mag; x200.
Plates XX and XXI are the microstructures of the weldment and heat affected zone of electrode type E6027.The weldment revealed a coarse pearlite and an elongated ferrite matrix, while the heat affected zone shows a fine grain size of both the pearlite and the ferrite grain structure. Etchant; 2% Nital, Mag; x200.
Plate XXII revealed the microstructure of the weldment of electrode type E6030 showing a coarse and dominant pearlite matrix in ferrite matrix, while Plate XXIII shows the heat affected zone with a fine pearlite matrix in ferrite matrix. Etchant; 2% Nital, Mag; x200.
Plate XXIV shows the microstructure of the weld metal using electrode type E6020 revealing coarse pearlite grain matrix in ferrite matrix, while Plate XXV shows the heat affected zone of the weld metal of electrode type E6020 showing a fine pearlite in ferrite matrix. Etchant; 2% Nital, Mag:x200.
Plate XXVI shows the microstructure of the weldment using the foreign electrode, revealing an intermediate pearlite in ferrite matrix, while Plate; XXVII shows the heat affected zone with fine pearlite and ferrite matrix. In all cases it can be observed that the microstructure of the weldment revealed that it has bigger grain size due to the reason of high heat input at the weld during welding operation and a relatively lower cooling rate. The slow rate of cooling which probably might have been caused by the shielding gas, which insulate the weldment from the atmospheric air and thus reducing the rate of cooling and hence permitting greater grain growth. The heat affected zone has a fine grain size, because of lower heat input and consequently higher cooling rate which resulted in grain refinement.
CONCLUSION AND RECOMMENDATIONS
In this study Iron oxide based arc welding electrodes were produced using mill scales (an industrial waste). The produced electrodes and a foreign electrode were used in welding, and the mechanical properties of the welded joints were obtained and compared. The metallographic examination of the welded joints was also determined and the following conclusions were drawn from this research.
- The analyzed mill scales shows the presence of predominantly Iron oxide, and Iron oxide is one of the important constituents of covering on mild steel arc welding electrodes, therefore it can be concluded that mill scales can be used in the production of Iron oxide electrodes such as E6020, E6024, E6027 and
- It is an established fact that the mechanical properties and performance of an electrode depends on the coating formulation, as a result of this a new flux, composition was generated by the application of Hadamard multivariate chemical composition model, and it was observed that the newly formulated flux produced a good fusion and it can also be concluded that the newly formulated flux did not only perform well, it also did not produce any weld defect, such as gas porosity and slag inclusion etc.
- With an appropriate variation in the flux constituent elements different types of electrodes can be
- The foreign electrode was able to yield the best performance in terms of strength and visual appearance of the welded specimens this is because these electrodes were produced under a more ideal condition.
- The microstructure analysis of the weldments revealed an acceptable grain
It is recommended that future research on the production of arc welding electrodes should focus on the following area
- Extrusion production process should be adopted (use of extruder) so as to have electrodes with improved
- Potassium silicate binder should be used, as the binding agent, because research has proved that it is a more efficient binder than Sodium
- The assessment of the hardness of the weld, heat affected zone and parent metal.
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