CHAPTER 3 DEVELOPMENT OF AGGLOMERATED FLUXES

Transcription

1 CHAPTER 3 DEVELOPMENT OF AGGLOMERATED FLUXES 3.1 Introduction There is a need to have more insight into the designing of flux such that indigenously manufactured flux results in high weld metal integrity and is cost effective. For analyzing and predicting the weld metal mechanical properties, it is essential to estimate the weld metal composition with from the wire, flux and parent metal combination. The welding parameters also affect the weld metal composition. Therefore, the effect of welding parameters should also be considered while developing flux formulation to estimate the weld metal composition with reasonable accuracy. With the development of new alloys and the need for better weldment characteristics, science of welding consumable is still evolving. It would be cost effective to use a sound scientific approach (observation, characterization, correlation, modelling, etc.) in establishing a fundamental basis for the development of welding consumables. American Welding Society also defines the flux as A material used to prevent, dissolve or facilitate removal of oxides and other undesirable substances (Butler and Jackson, 1967). Submerged Arc welding flux performs several functions. The main functions are arc stabilization, protection from the atmospheric contamination, de-oxidation and alloying of the weld metal. A flux should help in attaining the appropriate weld metal composition and exhibit satisfactory welding behaviour. Welding characteristics of the welded joint largely depend upon the characteristics of the welding flux. A flux 54

2 performs effectively if it has the following characteristics within the optimum range (Butler, 1967; Nippes, 1993). (i) It should provide arc stability. (ii) The flux should have good slag detachability. (iii) Flux should be ductile at high temperature to prevent oxidation of the weld metal and brittle at room temperature to facilitate slag removable. (iv) The melting temperature of the flux must be lower than that of the molten metal so that no gases are trapped between the slag and the weld metal and complete fluxing action can take place. Therefore, the upper limit of the melting temperature of fluxes used for joining steel is nearly C. (v) The solidification range of the slag and its change of viscosity over this range have to match to that of the solidifying weld metal to ensure uniform solidification. (vi) The viscosity of a welding flux must be high enough to give it impermeability to atmospheric gases and to prevent it from running away from the molten metal and flowing in front of arc. (vii) Flux must be fluid enough in the welding operation to permit rapid separation of non-metallic particles such as oxides and evolution of gases from the molten metal. (viii) The flux should be cost effective. (ix) The flux should have a good rate of reaction, so that the various reactions among solid, liquid and gas phases are completed within a very short time before the weld deposit solidifies. (x) The weld deposit should be free from welding defects like cracks, porosity etc. 55

3 (xi) The weld deposit should give the required chemical composition and also the desired mechanical and metallurgical properties. Flux constitutes half of the total welding cost in submerged arc welding. Due to transportation and handling, approximately 10-15% of the flux gets converted into very fine particles termed as flux dust before and after welding. If welding is performed without removing these very fine particles from the flux, the gases generated during welding are not able to escape, thus it may result into welding defects like surface pitting (pocking) and even porosity. If the flux is too fine, it will pack and not feed properly. If a fine flux or a flux with small amounts of fine particles is recovered by the vacuum system, the fine particles may be trapped by the system. Only coarser particles will be returned to the feeding system for reuse, which may cause welding problems. On the other hand, if these fine particles are removed by sieving, the cost of welding will be increased significantly. If this flux dust is dumped, it will create the pollution. The present study has been conducted to investigate the feasibility of developing one acidic and basic agglomerated flux by utilizing wasted flux dust of the parent commercial available fluxes. The chemical composition and mechanical properties viz. tensile strength and toughness of the all weld metal joint using developed fluxes as well as parent commercial fluxes of the same type were compared. The radiographic examinations of all the welded joints were conducted to check weld metal integrity. It was found that the chemical composition and mechanical properties of the all weld metal prepared by using the developed fluxes are in the same range as that of parent fluxes. The welded joints were also found to be radiographically sound. 56

4 Therefore the developed fluxes prepared from the waste flux dust can be used without any compromise in mechanical properties and quality of the welded joint, thereby reducing the cost of welding. 3.2 Types of Fluxes Fluxes can be classified into the following different ways: 1. Methods of manufacture 2. Chemical nature 3. Chemical compositions Methods of manufacture Based on the choice of several manufacturing methods, the different types of fluxes are: (a) Fused fluxes (b) Bonded or Agglomerated fluxes (c) Mechanical mixed fluxes (d) Sintered fluxes Fused fluxes To manufacture a fused flux, the basic raw materials are silica, calcite, dolomite, fluorspar, rutile, ferro-alloys etc. are dry mixed and melted in electric furnace. After melting and final conditions, the furnace charge is poured and cooled. Cooling may be accomplished by shooting the melt through a stream of water or by 57

5 pouring it onto large chill blocks. The result is a product with a glassy appearance which is then crushed, screened for size and packaged. Fused fluxes have the following advantages: (i) Good chemical homogeneity (ii) Easy control of the fines without affecting the flux composition (iii) Normally not hygroscopic, which simplifies handling, storage and welding problem (iv) Readily recycled through feeding and recovery system without significant changes in particle size or composition Their main disadvantage is the difficulty of adding deoxidizers and ferro-alloys to them during manufacture without segregation or extremely high losses. The high temperature (approximately C) needed to melt the raw ingredients limit the range of flux composition. Fused fluxes are more expensive to manufacture because of the greater energy requirements to melt all of the ingredients or sophisticated equipment to withstand the higher temperatures and the additional step of cooling and crushing the liquid mass. Additionally, the fused fluxes are more expensive to use because a greater amount melts during the welding. Bonded or Agglomerated fluxes The raw materials to produce a bonded flux are powdered, dry mixed and bonded with either potassium silicate or sodium silicate or a mixture of two. Chemical bonding takes place between particles due to formation of electrovalent or covalent 58

6 unsaturated bonds. After bonding, the wet mix is passed through a 10 mesh screen to form small pallets which are subsequently baked at 380 to C and then crushed to desired grain size (Chew, 1976). Belton et al. (1963) reported different method for the formation of pallets. As per this method, binder is added in sufficient quantity to wet appropriately the dry mixed homogeneous powder. The temperature involved in preparing these fluxes is lower than that used for producing fused fluxes. The agglomeration permits the use of deoxidizers and alloying elements that provides the manufacturer to obtain the stringent weld quality requirements and better mechanical properties of the welded joint. These fluxes have lower bulk density and hence under identical welding parameters less flux is melted for a given amount of weld deposit as compared with fused flux. The advantages of the bonded fluxes include the following: (i) Easy addition of deoxidizers and alloying elements which are added as ferro-alloys or as elements to produce alloys not readily available as electrodes or to adjust weld metal compositions (ii) Usage with thicker layer of flux when welding (iii) Colour identification But it has the followings disadvantages: (i) Tendency for some fluxes to absorb moisture in a manner similar to coating on some shielded metal arc electrodes (ii) Possible gas evolution from the molten slag 59

7 (iii) Possible change in flux composition due to segregation or removal of fine mesh particles Mechanically mixed fluxes To produce a mechanically mixed flux, two or more fused or bonded fluxes are mixed in any ratio necessary to obtain the desired results. The advantages of mechanically mixed fluxes are that several commercial fluxes may be mixed for highly critical or proprietary welding operations. The following are the disadvantages of mechanically mixed fluxes: (i) Segregation of the combined fluxes during shipment, storage and handling (ii) Segregation occurring in the feeding and recovery systems during the welding operation (iii) Inconsistency in the combined flux from mix to mix Chemical nature Fluxes are also identified as chemically basic, chemically acidic, or chemically neutral. The basicity or acidity of a flux is related to the ease with which the component oxides of the flux ingredients dissociate into cation and an oxygen anion (Ward, 1965). Chemically basic fluxes are normally high in MgO or CaO, while chemically acidic fluxes are normally high in SiO 2. The basicity or acidity of a flux is referred to as the ratio of CaO or MgO to SiO 2. Fluxes having ratios more than one are 60

8 called chemically basic. Ratios near unity are called one chemically neutral and those less than one are chemically acidic. Generally, the chemically basic flux is manufactured by agglomeration technique and has lower oxygen content, which contributes to better mechanical properties due to less density of inclusions and gases. The flux has lower density and viscosity, which contribute to lower current carrying capacity rate and penetration as compared to acidic flux. The flux is hygroscopic and therefore, less tolerant to rust and scaling. Basic fluxes have recently become the prime fluxes for welding critical applications where close control on weld deposit and chemistry are required. Basic fluxes are available to suit any weldable material by submerged arc welding. The chemically acidic flux has higher density and viscosity, which contribute to higher current carrying capacity rate and penetration. It is non-hygroscopic and therefore can tolerate higher degree of rust and scale. Generally the flux is manufactured by fusion technique and has higher oxygen content, which contributes to inferior mechanical properties due to higher density of inclusions and gases. The characteristics of chemically neutral flux are between the characteristics of acidic and basic fluxes. A neutral flux adds little or no alloying elements to the deposited weld metal. The main ingredients of the flux are alumina, titanium oxide, manganese oxide etc. Notch toughness is higher due to presence of aluminum and titanium which promote the formation of acicular ferrite in the weld metal. 61

9 3.2.3 Chemical compositions On the basis of chemical composition the fluxes can be classified as: (i) Calcium Silicate (high silica) (ii) Calcium Silicate (medium silica) (iii) Calcium Silicate (low silica) (iv) Alumina Rutile (v) Fully Basic Flux (vi) Manganese Silicate (vii) Alumina Basic (viii) Rutile 3.3 Properties of Flux The flux properties affect the quality, bead geometry and mechanical properties of the weld. Therefore the relation between the properties of a flux and its welding behavior should be known. The performance of the weld metal depends upon the properties of the flux, which are further depended upon the chemical composition of the flux. The properties of the flux undergo a change with the change of flux composition in order to obtain the required weld chemistry. The various studies have been conducted to analyze the effects of the flux properties on the welding behaviour. Flux consumption rate, weld penetration, viscosity, slag detachability etc. are determined by the properties of the flux. The important properties of flux, which affect 62

10 the weld metal chemistry and welding behaviour, are classified into the three categories, namely physical, metallurgical and technological properties Physical properties Physical properties of a flux include grain size, melting point, heat capacity, thermal conductivity, electrical conductivity, surface tension, viscosity, grain size etc. These properties influence the welding parameters, bead geometry, arc stability, bead quality, heat affected zone and inclusion rating of the weld metal etc Particle size and distribution Flux particle sizes and their uniform distribution within the bulk flux are important because that influences feeding, recovery, amperage level, and weld bead smoothness and shape. As amperage increases, the average particle size for fused fluxes should be decreased and the percentage of small particle should be increased. If the amperage is too high with a given particle size, the arc may be unstable and leave ragged, uneven bead edges. Finer fluxes can be used at higher current density. Generally, fluxes for current higher than 800 ampere are used with finer grain (Vishvanath, 1982). Patchet (1983) observed that the particle size influenced the dimensional instability of the weld bead especially at higher current. When rusty steel is welded, coarse particle fluxes are preferable, because they allow gases to escape more easily. 63

11 Flux grain size and its distribution influence the flux consumption rate, weld bead shape, surface quality and arc stability. The finer the flux, the more is the flux density, which contributes towards high flux consumption rate and less tolerance to rusting. It also affects the operating range of the flux, particularly with respect to welding current Electrical conductivity Cold flux is nonconductor of electricity but once it melts due to heat of the arc, it becomes highly conductive and hence the current flow is maintained between the electrode and the job through the molten flux. Electrical conductivity of the flux influences the arc stability and slag current, which heat the slag and therefore improves the transition region between weld and parent metal (Wanka, 1980). The electrical conductivity of the flux depends upon basicity index, flux composition and temperature. It increases with the increase in basicity index and temperature. Flux consumption rate increases with the increase in electrical conductivity (Bennett, 1970; Renwick and Patchet, 1976; Gupta and Gupta, 1988). Kaushal et al. (1988) reported that the electrical conductivity influenced the heat affected zone Arc stability Arc stability can be defined as the fluctuation of voltage from the average arc voltage during welding. The larger the magnitude of voltage fluctuation, the greater is the arc instability. A stable arc will produce good weld bead and a defect free weld 64

12 nugget. Defects commonly introduced by unstable arc are slag entrapment, porosity, blow holes and lack of proper fusion. Arc stability also affects the initiation and maintenance of welding arc and even the weld bead morphology (Patchett, 1974). The arc should initiate easily and be able to maintain itself under a varying arc length (Schwemmer and Williamson, 1979). If due to certain reasons arc length decreases, arc voltage will decrease, arc current and therefore burn off rate will increase thereby causing the arc to lengthen. The reverse occurs if the arc length increases than the normal. Arc stability is also affected by non symmetrical flow of plasma jet. The flux ingredients play an important role in welding by providing easily ionized atoms, which helps in improving the arc stability. Lithium oxide, potassium oxide and sodium oxide produce vapours that are easily ionized and therefore increase the arc stability. However, potassium oxide is more effective than sodium oxide. Farias et al. (1997) reported that the addition of magnesium improved the arc stability. Witting (1980) observed that the addition of fluorides of calcium and magnesium caused arc instability. Addition of compounds such as aluminum oxide and chromium oxide decreases the arc stability and addition of potassium oxalate, potassium silicate and lithium carbonate to a flux increases the arc stability (Nadkarni, 1988) Interfacial tension The surface tension at the interface of molten flux and weld metal is known as the interfacial tension. The interfacial tension depends upon the flux and weld metal 65

13 composition (Hazlett, 1957; Yakobashvili, 1970). Interfacial tension of a flux influences the protection of weld metal from the atmospheric gases and thus welds bead surface quality. It also influences the spreading tendency of weld pool and thus nature of the heat transfer during welding. A high interfacial tension between molten flux and weld metal gives rise to undercut, whereas low interfacial tension cause easy separation of slag from the weld metal. An increase in interfacial tension between flux and molten weld causes an increase in penetration. Komapov et al.(1983) observed that low value of interfacial surface tension promoted better profile of the deposited weld metal. Researchers (Goloshubov, 1972; Wanka, 1980) reported that the range of the surface tension of the fused fluxes should be between dynes/cm depending on welding application Viscosity Viscosity is a measure of resistance to flow. Viscosity of the molten flux depends upon flux ingredients and temperature. Fluxes having more silica content, exhibit high viscosity due to network formation tendency of silicon ion (Ward, 1965). The viscosity can be reduced by the addition of calcium fluoride, calcium oxide, manganese oxide etc. Also, viscosity decreases with increase in temperature. Tarlinsku (1980) reported that the preferred range of viscosity for the satisfactory welds was between poise at the temperature range of C. Viscosity of the molten flux affects the inclusion rating of the weld, internal gas porosity, pocking, weld bead appearance, welding position and heat transfer during 66

14 welding (Butler and Jackson 1967). A flux with a high viscosity tends to confine the weld pool, thus increasing the heat input for a given area and resulting in deeper penetration. The optimum range of slag viscosity improves the formation of the weld by restricting the disturbing factors and suppressing oscillations of the weld pool as they move in viscous slag towards the solidifying boundaries of the molten metal (Kuzmenko, 1985). A flux must remove undesirable elements and gases away from the weld metal by first absorbing atoms and molecules into the molten slag at the liquid metal-slag interface followed by diffusion of these species away from the interface. Viscosity influences the velocity of separation of the liquid metal from the liquid slag. Hence, a flux with low viscosity has more bulk diffusion rate and, therefore results in faster reaction rate at the metal-slag interface. This phenomena contributes to cleaner weld metal and low non-metallic inclusions such as oxides, sulfides, etc. while, a higher viscosity may result in entrapment of gases, which contributes to the surface defects like pocking. However, the viscosity of a welding flux must be high enough to give it impermeability to atmospheric gases and to prevent it from running away from the molten metal and flowing in front of the arc (Butler and Jackson, 1967). Selection of proper viscosity also depends on the position and speed of the welding. A more viscous flux is desirable for vertical and overhead welding to assist in reinforcing and protecting the weld pool and low viscous flux is preferred for high welding speed. 67

15 Melting point Melting point of the flux influences the weld metal protection characteristic, flux viscosity, current carrying capacity and flux consumption rate. The flux should be in molten stage after the weld metal has solidified to protect the weld metal and to obtain the good weld surface appearance. Therefore, the upper limit of the melting point of the flux, used for steel, is generally considered to be about C (Nippes, 1993). The flux consumption rate increases where as the current carrying capacity and viscosity decreases with the decrease in melting point. The melting point depends upon the ingredients and their ratios. In general, it decreases with the addition of calcium fluoride and manganese oxide Metallurgical properties Metallurgical properties influence the microstructure of the weldment, which in turn affect the overall mechanical properties of the weldment. The oxygen level and chemistry of weldment are also influenced by metallurgical properties. These include chemical characteristics, basicity index and oxidation power of flux. The chemical characteristics of a flux depend upon composition and chemical characteristics of the flux ingredients. It influences the slag-metal-gas reactions and the weld metal composition, which further affect the mechanical soundness of the weldment Basicity index Basicity index is a measure of basicity of the flux / slag oxide system and it is used as a parameter by which the chemical behaviour of the flux or slag and 68

16 mechanical properties of the weld metal can be correlated (Potapov, 1978). The basicity of a flux is related to the ease with which the component oxides of the flux ingredients dissociate into a metallic cation and oxygen ion. Chemical behaviour of the flux and slag influences the slag-metal-gas reactions in the weld pool, which in turn further affects the elements transfer behaviour. It exerts a strong effect on the process of flux hydration and oxidation-reduction process taking place at the heterogeneous boundary of slag and metal. Basicity index influences the oxygen content in the weld pool, which further affects the porosity, inclusions level, and oxidation of alloy and mechanical properties of the weld metal (Lancaster, 1993). In general, oxygen level in the weld metal decreases with increase in basicity index. Eager (1978) also observed that weld metal oxygen content reduced from 900 ppm to 250 ppm for a basicity index change from 0.50 to 1.5 and then remained constant at 250 ppm level with further increase in basicity index. Basicity index also affects the dephosphorization and desulphurization process in the weld pool (Tarng and Chang, 2002) Oxidation power Oxidation power of the flux influences the oxidation-reduction reaction taking place during welding. It depends upon the stability of the various oxides present in the flux and increases with the increase in amount of oxides which have lower thermal stability such as silica, manganese oxide etc. It also depends upon the basicity of the flux. In general, high basicity index results in low oxidation power of the slag. 69

17 3.3.3 Technological properties The important technological properties which influence the economics and the productivity of the welding include detachability, flux usage and penetration Detachability Detachability can be defined as the ease with which the slag is removed from the weld metal after solidification. Time and effort required to remove slag lowers productivity. Incomplete removal of slag can lead to weld defects like slag inclusions, lower the corrosion resistance of the weld and impair the appearance of the weldment and can compromise weldment integrity. Therefore, the ability to remove slag with relative ease is an important factor for higher production rate, better mechanical properties of the weld metal. The composition of the flux plays an important role in the risk of slag inclusions through its effect on the weld bead shape and the ease with which the slag can be removed. Melting point of the slag has an effect on the amount of inclusions in the weld metal and the ability to weld out of position. A weld pool with low oxygen content will have a high surface tension producing a convex weld bead with poor parent metal wetting. Thus, an oxidizing flux, containing for example iron oxide, produces a low surface tension weld pool with a more concave weld bead profile, and promotes wetting into the parent metal. High silicate flux produces a glass-like slag, often self-detaching. Fluxes with lime content produce an adherent slag which is difficult to remove. 70

18 Detachability also varies depending upon the type of flux used. For rutile or acidic fluxes, i.e. large amounts of titanium oxide (rutile) with some silicates, the oxygen level of the weld pool is high enough to produce a flat or slightly convex weld bead. The fluidity of the slag is determined by the calcium fluoride content. Fluoride-free coatings designed for welding in the flat position produce smooth bead profiles and an easily removed slag. The more fluid fluoride slag designed for positional welding is less easily removed. For basic fluxes, the high proportion of calcium carbonate (limestone) and calcium fluoride (fluorspar) in the flux reduces the oxygen content of the weld pool and therefore its surface tension. The slag is more fluid than that produced with the rutile coating. Fast freezing also assists welding in the vertical and overhead positions but the slag coating is more difficult to remove. Consequently, the risk of slag inclusions is significantly greater with basic fluxes due to the inherent convex weld bead profile and the difficulty in removing the slag from the weld toes especially in multi-pass welds. Addition of corundum, zirconia, rutile and alumina in a flux improves detachability (Bennett, 1970) Flux usage Besides economic consideration, flux consumed in SAW influences the pick up or reduction of alloying elements and therefore affects the mechanical soundness and metallurgical properties of the weld metal. The flux consumption depends on physical properties of the flux such as melting point, density, thermal properties, chemical composition, basicity index and welding parameters viz. wire feed rate, open circuit 71

19 voltage, welding speed etc. The flux consumption increases with the decrease in melting point and decreases with the decrease in the density of the flux and thermal conductivity of the base material. The flux consumption initially increases with the current, reaches maximum and then decreases. The flux consumption increases with the increase in welding voltage (Pandey and Mohan, 2003). In applications where low hydrogen considerations are important, fluxes may be kept dry. Fused fluxes do not contain chemically bonded H 2 O, but particles hold surface moisture. Bonded fluxes contain chemically bonded H 2 O, and may hold surface moisture as well. Bonded fluxes need to be protected in the same manner as lowhydrogen shielded metal arc electrodes. The user should follow the directions of the flux manufacturers for specific baking procedures. When alloy-bearing fluxes are used, it is necessary to maintain a fixed ratio between the quantities of flux and electrode melted, to obtain consistent weld metal composition. There ratio is actually determined by the variables of the welding procedure. For example, deviation from an established volt-ampere relation will change the alloy content of the weld metal by changing the flux-electrode melting ratio Penetration Penetration is the distance from base plate top surface to the maximum extent of the weld nugget. Penetration determines the load carrying capacity of a welded structure. It affects the weld strength. Flux ingredients and welding parameters influence the penetration. Indocochea and Olson (1983) reported that the penetration 72

20 increased with the increase in slag viscosity and interfacial tension, and represented the same by the following equation: P = k ( l - s) (V- V) (3.4) Where, η, γ l, γ s, and `k are the viscosity, interfacial tension of liquid, interfacial tension of solid and constant respectively. The flux ingredients which increase the viscosity and oxygen activity of the slag like silica also increase the penetration. 3.4 Ingredients of Fluxes and their Functions Fluxes used in SAW are granular fusible minerals containing oxides of manganese, silicon, titanium, aluminum, calcium, zirconium, magnesium and other compounds such as calcium fluoride. The flux is specially formulated to be compatible with a given electrode wire type so that the combination of flux and wire yields desired mechanical properties. All fluxes react with the weld pool to produce the weld metal chemical composition and mechanical properties. The ingredients of the fluxes have varying degrees of influence on the physical properties. The functions of the main ingredients are as follows: Manganese oxide Manganese oxide is chemically basic in nature. According to Ferrera, and Olson (1975), its addition improves the arc stability, reduces the viscosity and thereby favours high welding speed. It also reduces the harmful effect of sulphur. 73

21 3.4.2 Silica Silica is considered as acidic in nature. Its addition improves good weld appearance and slag detachability. It also increases the viscosity and current carrying capacity. Higher silica content gives higher content of O 2 ions that react with carbon and cause transfer of silicon and oxygen into the weld metal (Tuliani et al. 1972). Volobuev (1982) reported that the silica content of the flux controls not only the welding and technological properties but also the metallurgical properties of the fluxes and has a strong effect, especially on the susceptibility to hot cracking of the weld deposit Halides Borikov et al. (1983) observed that halides affects the oxidizing and reducing reaction taking place at the slag metal boundary and therefore transfer of alloying elements. Potapov et al. (1981) reported that the addition of halides of alkali metals improves the metallurgical and mechanical properties by providing uniform distribution of non-metallic inclusions. The addition of potassium fluoride and manganese fluoride reduces the amount of diffusible hydrogen in the weld metal Calcium fluoride Calcium fluoride (CaF 2 ) occurs naturally as the mineral fluorite and is basic in nature. It is also called as fluorspar and is the least expensive of the fluorides. It is used in some amounts to provide protection from the atmosphere and lower the melting range of the slag. Too much fluoride adversely affects the arc stability and can increase 74

22 the tendency for undercutting at the edges of the weld bead. Weymueller (1981) reported that calcium fluoride increased the fluidity of the flux, which in turn resulted in better weld coverage and helped to escape gases from the weld pool. The addition of calcium fluoride reduces the dissolved hydrogen and silicon content of weld metal Rutile Rutile is the most common natural form of TiO 2 and is a chemically neutral oxide.it is effective in reducing the viscosity, especially when aluminum oxide or silica is to be replaced and is added for bead appearance. It promotes the formation of acicular ferrite and refines the grains that results in increase of ductility and toughness of the weld metal. Rutile is employed to provide for good slag removal after the weld has solidified and to reduce the oxygen content of the weld metal (Kohno et al. 1982) Calcium oxide Calcium oxide is normally present as a stable complex compound with SiO 2 and/or Al 2 O 3 and is chemically basic. It is quite hygroscopic in nature and increases the basicity index and hence, decrease the sulphur and phosphorous content in the weld metal. It also improves the arc stability and decrease the viscosity of the slag (Rissone, 2002). It is added to the flux to maintain the desired fluidity and oxygen content in the weld metal. 75

23 3.4.7 Sodium oxide The sodium oxide is present as a silicate which functions as a binder for the fluxes. Sodium oxide can also be present as some form of mineral. It is highly basic in nature and enhances the arc stability Potassium oxide Potassium silicate is formed when potassium oxide combines with silica. The potassium oxide gives improved arc stability and is also basic in nature Aluminum oxide Aluminum oxide is a mild acidic oxide. It is added for maintaining good weld bead appearance and easy detachability of slag. Its addition promotes acicular ferrite and refines the grain, thereby improves the mechanical properties of the weld metal. It also improves the slag detachability. 3.5 Weldability The weldability or joinability of a material refers to its ability to be welded. It indicates the ability of a material to respond to the welding process under given fabrication conditions in order to enable successful fabrication of a well designed structure which, in turn, should successfully render the intended service when put to use (Wiseman, 1976). Thus, weldability is a measure of how easy it is to: (i) Obtain defect free welds (ii) Achieve adequate mechanical properties 76

24 (iii) Produce welds resistant to service degradation Weldability is not a fixed parameter for a given material, but will depend on a number of factors including chemistry, surface finish, heat-treating tendencies, joint details, service requirements, and welding processes and facilities available. 3.6 Experimental Procedure The entire experimental plan is presented in the form of flow chart in Fig.3.1 Development of Fluxes Conducting trials to check behaviour of developed fluxes Chemical composition F of weld metal laid by developed and parent fluxes Preparation of welded joints using developed and parent flux Radiographic examination of the welded joints Machining of the joints and testing for mechanical properties Fig. 3.1 Flow chart of experimental plan 77

25 3.6.1 Development of fluxes In the present study, agglomerated cost effective fluxes were developed by using the flux dust of acidic and basic flux respectively, with addition of potassium silicate as binder and aluminum powder as deoxidizer. Numbers of welding trials were conducted to find the percentage of binder and deoxidizer to the flux dust. The weld metal was checked for smooth bead appearance free from visual defects, chemical composition and slag detachability. The trials were repeated until satisfactory results were obtained after analyzing the weld bead. It was found that the 90 ml addition of potassium silicate binder in 550 grams of flux dust, and aluminum powder (4% of the weight of the flux dust) gave an excellent bead appearance, free from any visual defects and satisfactory detachability Flux preparation The above said flux ingredients were weighed and wet mixed for 10 minutes and then passed through a 10 mesh screen to form small pallets. Potassium silicate was added as binder because of better arc stability (Bennett, 1970; Renwick and Patchet, 1976). The pellets of the flux were dried in air for 24 hours and then baked in the muffle furnace between C for nearly three hours (Gupta and Gupta, 1988). After cooling, these pallets were crushed and subsequently sieved. After sieving, fluxes were kept in air tight bags and baked again at C before welding. 78

26 3.6.3 Chemical analysis To avoid the dilution, four-layer-high weld pad was made as per AWS A standard as shown in Fig.3.2, using 4 mm diameter wire and the laboratory made agglomerated fluxes. Fig. 3.2 Weld metal pad for chemical analysis The compositions of the electrode and base metal are shown in Table 3.1. The welding parameters, as shown in Table 3.2, were kept constant. The inter pass temperature was maintained between C. The weld sample of approximately 50mm length was cut from the middle of the each weld pad. The cut weld pad was cleaned at the upper most layers. The chemical composition was determined at the clean surface of the upper most layers by spectroscope. 79

27 Table 3.1 Chemical composition of base plate and electrode wire Element (%) Base Plate Electrode Wire C Mn Si S P Ni Cr Nil Nil Table 3.2 Welding parameters Parameter Units Values Current Ampere 550 Open Circuit Voltage Volt 38 Electrode stick-out Millimeter 30 Welding speed m/hr 28 Arc Voltage Volt Preparation of welded joints The four butt weld joints were made with mild steel as base plate and backing strip. A constant voltage D.C submerged arc welding power source was used for forming the joints with base plates having the dimension 275 x 125 x 25 mm using 4 mm diameter wire electrode of grade C (AWS EH-14). DCEP polarity was used throughout the experimentation. The backing plates of 12 mm thick were tack welded to these 25 mm thick base plates. The dimensions of the groove weld laid on the weld plate are shown in Fig.3.3; 25mm length of all weld material was cut from each end of the groove weld and discarded. All the specimens were cleaned thoroughly 80

28 and their surfaces polished using 600 no. and 800 no. grit polishing paper before subjecting them to the testing. Fig.3.3 Geometry and dimensions of groove weld in mm Radiographic examination The welded plate was cleaned and thereafter backing plate and crown were removed by machining. The well cleaned weld plate was radiographed and interpreted according to standard AWS D Tensile test Three all weld metal tensile test pieces were cut from each welded plate and machined to the standard dimensions. The tensile tests were carried out on a universal testing machine (Make FIE-India) on three test specimens for each type of developed and parent fluxes. The location and dimensions of the tensile sample are shown in Figs 3.4 and 3.5 respectively. 81

29 The groove welds were laid as per AWS A and welding parameters were maintained as per Table 3.2 using DCEP polarity. The dimension of the weld plate and location of tensile and Charpy test pieces are shown in Fig Fig.3.4 Location of the tensile sample Fig. 3.5 Dimensions of the tensile sample in mm 82

30 Fig.3.6 Dimensions of weld plate in mm Impact test Charpy V notch impact test was carried out to evaluate the toughness of the welded joints at 0 0 C. The zero degree centigrade temperature was attained by keeping impact test specimens under ice cubes. Five all weld metal impact test specimens were cut from each welded joint of plates as per AWS standard A The orientations and dimensions of the sample are shown in Figs.3.7 and 3.8 respectively. The specimens were fine polished by the surface grinder. The specimens were positioned properly and load was released suddenly behind the notch and recorded on the scale of joules with 2 joules least count. Among the five values of the impact strength, 83

31 the lowest and the highest values were discarded and average of other three values was taken for the evaluation of impact strength of the groove welds. Fig.3.7 Location of the impact sample in mm Fig. 3.8 Dimensions of the impact sample in mm 84

32 3.6.8 Scanning electron micrograph (SEM) Scanning electron micrograph of the fractured surfaces of tensile test specimens were laid at 20kVand 1500 X on microscope (Make JOEL Japan, JSM-6100). 3.7 Results and Discussion The flux behaviour of the developed fluxes was found to be satisfactory. The bead surface appearance was found to be excellent and free from any visual defects and it was comparable with the parent fluxes. The slag was easily detachable form the welded joint laid by the developed fluxes. As shown in Table 3.3 the compositions of all weld metal of the developed and parent fluxes are found to be in the same range. However, manganese content of the weld metal laid by using the developed acidic and basic fluxes is slightly lower than the weld metal laid by using the parent acidic and basic fluxes. The silicon content of the weld metal laid by using both of the developed fluxes is higher than the weld metal laid by using the parent fluxes. The carbon equivalent was being computed from the following equation (Mercado et al. 2005) C equivalent = C + Mn/6 + Si/24 + Ni/40 + Cr/5 + Mo/4 + V/4 (3.1) Where C, Mn, Si, Ni, Mo and V represent the metallic content, expressed as percentage. 85

33 Table 3.3 Chemical composition of all weld metal laid by developed and parent fluxes Element (%) C Mn Si S P Ni Cr Carbon Equivalent Parent Basic Flux Developed Basic flux Parent Acidic Flux Developed Acidic flux Nil Nil Potassium silicate binder was added to the flux dust. The silicon di-oxide, associated with potassium silicate binder dissociate into oxygen and silicon due to heat during welding (Lau et al., 1980). It causes the additional amount of oxygen and silicon content in the weld pool. The additional amount of oxygen results in oxidation of manganese and hence the less manganese content in the weld metal laid by using the developed fluxes as compared to the weld metal laid by using the parent fluxes. The additional amount of silicon results in increase of silicon content and hence the higher silicon content in the weld metal laid by using the developed fluxes as compared to the weld metal laid by using the parent fluxes. The radiographs of the welded joint which were prepared using developed fluxes were found to be acceptable as per of AWS D radiographic standard of dynamic loading. The average values of tensile properties, yield strength, ultimate strength, elongation percentage, area reduction percentage and impact strength of the developed fluxes as well as parent fluxes are shown in Table 3.4 and 3.5 respectively. 86

34 Table 3.4 Tensile strength of all weld metals laid by using parent and developed fluxes Flux Parent Basic Flux Developed Basic flux Parent Acidic Flux Developed Acidic flux Yield Strength (N/mm 2 ) Tensile Strength (N/mm 2 ) Elongation (%) Area Reduction (%) Table 3.5 Impact strength of all weld metals laid by using parent and developed fluxes Flux Impact strength / Toughness (joules) Observations Parent Basic Flux Developed Basic flux Parent Acidic flux Developed Acidic Flux Average The average values of tensile strength and impact strength of all weld metal obtained by using the developed and parent fluxes are reported to be in the same range. However, the tensile strength and impact strength of all weld laid by using the parent acidic and basic fluxes are slightly higher than that of the all weld laid by using the corresponding developed fluxes. It is attributed to slightly higher carbon equivalent of 87

35 all weld metal laid by using parent fluxes than that of carbon equivalent of all weld metal using the developed fluxes. The weld metal composition and mechanical properties viz. tensile strength and impact strength of the developed fluxes were also found satisfactory and comparable with parent fluxes available in the market. The highest and lowest tensile strengths were obtained for the weld metal with highest and lowest equivalent carbon respectively. It has been reported by Lancaster (1980) that carbon equivalent higher than 0.45 had a high susceptibility to cold cracking after welding. The carbon equivalent of welds for all fluxes is lower than Carbon equivalent for acidic fluxes are more than that of basic fluxes, resulting in higher value of tensile strength of the weld laid using acidic fluxes. The higher value of impact strength of the weld prepared from basic flux can be attributed to lower oxygen content due to higher thermal stability of basic oxides. The oxygen may be present in the weld bead in the form of oxides of iron or of any of the metals contained in the welding flux, electrode or base metal. These oxides, if present in the weld bead, appears as microscopic particles which fail to float to the surface of the molten weld metal before it solidifies and thus remain interspersed throughout the weld metal along the grain boundaries resulting in potential low energy fracture regions. Figs.3.9 and 3.10 show the scanning electron micrographs of the fractured tensile test specimens of the weld laid out at same parameters using developed as well as parent fluxes. The micrographs of all specimens show the ductile mode of fracture. 88

36 (a) (b) Fig. 3.9 SEM image of tensile test fractured surface of weld laid by (a) Parent acidic flux, (b) Developed acidic flux (a) (b) Fig SEM image of tensile test fractured surface of weld laid by (a) Parent basic flux, (b) Developed basic flux 89