The welding electrodes used in shielded metal arc welding process are called by different names like stick electrode, covered electrode and coated electrode. Coating or cover on the electrode core wire is provided with various hydrocarbons, compound and elements around to perform specific roles. Coating on the core wire is made of hydrocarbons, low ionization potential element, binders etc. Na and K silicates are invariably used in all kinds of electrode coatings. Coating on the electrode for SMAW is provided to perform some of the following objectives:
To increase the arc stability with the help of low ionization potential elements like Na, K
To provide protective shielding gas environment to the arc zone and weld pool with the help of inactive gases (like carbon dioxide) generated by thermal decomposition of constituents present in coatings such as hydrocarbon, cellulose, charcoal, cotton, starch, wood flour
To remove impurities from the weld pool by forming Slag as constituents present in coatings such as titania, feldspar, china-clay react with impurities and oxides in present weld pool (slag being lighter than weld metal floats over the surface of weld pool which is removal after solidification of weld) For controlled alloying of the weld metal (to achieve specific properties) can
be done by incorporating required alloying elements in electrode coatings and during welding these get transferred from coating to the weld pool. However, element transfer efficiency from coating to weld pool is influenced by the welding parameter and process itself especially in respect of shielding of molten weld pool.
To deoxidize weld metal and clean the weld metal: Elements oxidized in the weld pool may act as inclusions and deteriorate the performance of the weld joint. Therefore, metal oxides and other impurities present in weld metal are removed by de-oxidation and slag formation. For this purpose, deoxidizers like Ferro-Mn, silicates of Mg and Al are frequently incorporated in the coating material.
To increase viscosity of the molten metal so as to reduce tendency of falling down of molten weld metal in horizontal, overhead and vertical welding. This is done by adding constituents like silica in coating materials which thickens the weld metal and enhances the viscosity.
Flux coating Core wire
Protective gas shield Slag
Solidified weld metal Molten weld pool
Arc
Base metal
Fig. 2 Schematic diagram showing constituents of SMAW 4.0 Welding parameters for SMAW
SMA welding normally uses constant current type of power source with welding current 50-600A and voltage 20-80V at 60% duty cycle. Welding transformer (AC welding) and generator or rectifiers (DC welding) are commonly used as welding power sources. In case of AC welding, open circuit voltage (OCV) is usually kept 10- 20% higher than that for DC welding to overcome the arc un-stability related problems due to fact that in case AC both current magnitude and direction changes in every half cycle while those remain constant in DC welding. OCV setting is primarily determined by factors like type of welding current and electrode composition which significantly affect the arc stability. Presence of low ionization potential elements (Ca, K) in coating reduces the OCV required for stable arc.
Importance of welding current
Selection of welding current required for developing a sound weld joints is primarily determined by the thickness of base metal to be welded. In general, increase in thickness of plate to be welded increases the requirement of heat input to ensure proper melting, penetration and deposition rate. This increased requirement of heat input is fulfilled using higher welding current. Thus need of high welding current dictates use of large diameter electrode. SMAW electrode are found in different sizes and generally found in a range from 1-12.5mm in steps like 1.25, 1.6, 2, 2.5, 3.15, 4, 5, 6.3, 8 and 10 mm.
Upper and lower limits of welding current for SMAW are determined by possibility of thermal decomposition of electrode coating material and arc stability respectively. Welding current (A) is generally selected in range of 40-60 times of electrode
diameter (mm). Too high current creates problem of damage to the electrode coating material due to thermal decomposition caused by electrical resistance heating of the core wire. On other hand low current setting makes the arc unstable, poor penetration and low fluidity of molten metal. All these tend to develop discontinuities in weld joints.
In shielded metal arc welding process, lower limit of current is decided on the basis of requirement for stable arc, smooth metal transfer and penetration whereas higher limit of current is decided on the basis of extent of overheating of core wire that an electrode coating can bear without any thermal damage. High current coupled with long electrode extension causes overheating of core wire of electrode due to electrical resistive heating. Excessive heating may cause the combustion/decomposition of flux much earlier than when it is required to provide inactive shielding gases for protecting the weld pool and arc. Therefore, large diameter electrodes are selected for welding of thick sections as they can work with high welding current. Large diameter electrodes allow high current setting without any adverse effect on electrode coating materials because increased cross sectional area of electrode reduces resistance to the flow of current and so the electrical resistance heating of the core wired is reduced.
Lecture 12
Shielded Metal Arc welding II Selection of type of welding current
1. Thickness of plate/sheet to be welded: DC for thin sheet for better control over heat
2. Length of cable required: AC for long cables required during welding as they cause less voltage drop i.e. loading on power source
3. Easy of arc initiation and maintenance needed even with low current: DC preferred over AC
4. Arc blow: AC helps to overcome the arc blow as it is observed with DC.
5. Odd position welding: DC is preferred over AC for odd position welding (vertical and overhead) due to better control heat input.
6. Polarity selection for controlling the melting rate, penetration and welding deposition rate: DC preferred over AC
7. AC gives the penetration and electrode melting rate somewhat in between of that offered by DCEN & DCEP.
DC offers the advantage of polarity selection (DCEN & DCEP) which helps in controlling the melting rate, penetration and required welding deposition rate (Fig. 3). DCEN results in more heat at work piece producing high melting rate and so high welding speed but with shallow penetration. DCEN polarity is generally used for welding of all types of steel except with low hydrogen ferric steel electrodes. DCEP is commonly used for welding of non-ferrous metal with low hydrogen electrodes and offers the advantage of deeper penetration. AC gives the penetration and electrode melting rate somewhat in between of that offered by DCEN & DCEP.
a) DCEN
c) AC
Fig. 3 Schematic diagram showing effect of welding current and polarity 5.0 Electrode size and coating factor
Diameter of the core wire of an electrode refers to electrode diameter (d). Diameter of electrode with coating (D) with respect to that of core wire (d) is used to characterize the coating thickness (Fig. 4). The ratio of electrode diameter and core diameter (D/d) is called coating factor. Coating factor usually ranges from 1.2 to 2.2. According to the coating factor, coated electrodes can be grouped into three categories namely light coated (1.2-1.35), medium coated (1.4-1.7) and heavy coated (1.8-2.2). Stick electrodes are generally found of length varying from 250 to 400mm. During the welding, length of the electrode is determined by welder’s convenience to strike the arc and current carrying capacity of electrode without causing excessive heating of coating materials due to electric resistive heating caused by flow of current through the core wire. Bare end of electrode is used to make electrical connection with power source with the help of suitable connectors.
D d
Flux coating
Core wire Bare end
Fig. 4 Electrode size and coating factor 6.0 Weld beads
Two types of beads are generally produced in welding namely stringer bead and weaver bead. Deposition of the weld metal in largely straight line is called stringer bead (Fig. 5 a). In case of weaver bead weld metal is deposited in different paths during the welding i.e. zigzag, irregular, curved (Fig. 5 b). Weaver bead helps to apply more heat input per unit length during welding than stringer bead. Therefore, weaver beads are commonly used to avoid problems related with welding of thin
plates and that in odd position (vertical and overhead) welding in order to avoid melt through and weld metal falling tendency.
a) b)
Fig. 5 Schematic diagram showing weld bead a) stringer bead and b) weaver bead 5.0 Metal transfer in SMAW
Metal transfer refers to the transfer of molten metal droplets from the electrode tip to the weld pool in consumable arc welding processes. Metal transfer in SMA welding is primarily affected by surface tension of molten metal at the electrode tip. Presence of impurities and foreign elements in molten metal lowers the surface tension which in turn facilitates easy detachment of molten metal drop from the electrode tip. Therefore, type and amount of coating on electrode and effectiveness of shielding of arc zone from the atmospheric gases appreciably affect the mode of metal transfer. Acidic and oxide type electrodes produce molten metal with large amount of oxygen and hydrogen at the electrode tip. Presence of these impurities in the molten weld metal lowers the surface tension and produces spray like metal transfer. Rutile electrodes are primarily composed of TiO2 due to which molten metal drop hanging
at tip of electrode is not much oxidized and therefore surface tension of the molten weed metal is not reduced appreciably. Hence, rutile electrodes produce more drop and less spray transfer. Basic electrode contains deoxidizers and at the same time moisture is completely driven off to render low hydrogen electrodes. Therefore, melt droplets at the tip of the electrode are of killed steel type having high surface tension. Since high surface tension of molten metal resists the detachment of drop from the electrode tip and hence the size of drop at tip of electrode increases to a great extent before it is detached under the effect of gravitational and electro-magnetic pinch forces. These conditions results in globular transfer with basic electrode.
In case of light coated electrodes incomplete de-oxidation (due to lack of enough flux), CO is formed which remains with single molten weld metal droplet until it grows to about half of electrode diameter. Eventually, drops with bubble of CO bursts which in turn results in metal transfer in form of fine drops and spatter. In case of basic
electrode, metal transfer occurs by short circuiting mode if molten metal drop touches the weld pool and melt is transferred to weld pool by surface tension effect.
Lecture 13
Submerged Arc Welding
1.0 Introduction
Submerged arc welding (SAW) process uses heat generated by an electric arc between bare consumable electrode wire and the work piece. Since in this process, welding arc and the weld pool are completely submerged under cover of granular and fusible flux therefore it is called so. During welding, granular flux is melted by heat generated by arc and provides protection to the weld pool contamination from the atmospheric gases. The molten flux reacts with the impurities in the molten weld metal to form slag and offers following effects on the weld joints.
Increased cleanliness of weld metal and so improved properties of weld joint
Molten flux becomes lighter than weld metal hence floats on the top of solidifying weld metal so protect the molten weld pool contamination from atmospheric gases
Shielding of the weld pool by molten flux and solidified slag and un-melted flux retards cooling rate of the weld pool and HAZ which in turns decreases the cracking tendency of hardenable steel.