TRENDS IN WELDING PROCESSES FOR STRUCTURAL FABRICATION
S Sankaran, Welding Consultant, Bangalore
Introduction
A variety of welding processes are in use to fabricate and erect steel structures. The criteria for selection of the welding process are the need for high quality and economical fabrication. In order to achieve these objectives, it is important that all people involved should have proper understanding of the welding process fundamentals, their capabilities and limitations. In this paper an attempt is made to describe the commonly used welding processes for fabrication and erection of steel structures, and the latest trends.
SMAW-Shielded Metal Arc Welding
Shielded metal arc welding (SMAW) is also commonly known as manual arc welding. The process is characterised by its versatility, simplicity and flexibility.
However, there is a practical limitation with this process. The electrodes are limited to typically 350 to 450 mm in length. When higher current is used, electrical resistance heating within the length of electrode will become so high that the coating ingredients get overheated and result in weld quality degradation. The restriction on the length of electrodes and maximum current, limits the deposition rates that can be achieved with SMAW. Frequent interruptions in welding to change electrode and remove the slag cover, limits the productivity and affect the quality of welding.
A wide range of electrode flux coatings are available for application in structural fabrication. The Bureau of Indian Standard classification is IS-814, which is similar to ISO standards. But the most widely used classification is as per American Welding Society AWS A5.1.
The E6013 electrodes are rutile (TiO2) based. They are suitable for use with both AC and DC. They produce very smooth arc and are welder friendly to produce welds with good visual appearance. The slag detaches easily. But they are not hydrogen controlled and toughness requirement is not specified. So their application is restricted to light structural fabrication of mild steels below 16mm base material thickness.
The E7016/E7018 type electrodes are Lime Carbonate (CaCO3) based. They produce basic slag. The slag is very sticky and difficult to remove. They are used preferably with DCEP polarity. The diffusible hydrogen content in the weld is controlled to very low levels and the electrode is suitable for welding higher thickness C-Mn steels and high strength low alloy steels. The weld will meet the toughness requirement at room temperature and at low temperatures up to minus 50o C. The low hydrogen electrodes need to be stored in humidity and temperature controlled store rooms to prevent them from absorbing atmospheric moisture during storage. Further they have to be conditioned in electrode baking ovens and held in heated holding ovens till they are used.
E7024 and E7028 type electrodes contain good amount of iron powder in the coating and improve the deposition rate considerably. However their usage is restricted to down hand welds and horizontal fillet joints.
For welding weather resisting steels, E7018-W type electrodes are available with good atmospheric corrosion resistance.
The shelf life of all types of SMAW electrodes will depend on the type of packing and storage condition.
Recent Trends in SMAW
In the recent years, like in all other industrial operations, the main driving force behind new developments is the environmental issues related to the welding operations. Among the various environmental issues, atmospheric pollution is the one causing major concern. One of the main causes for atmospheric pollution is energy production by burning coal, oil or gas. Even Hydal and Nuclear power generation have their own environmental and safety issues. So conservation of energy in welding and related operations is now getting greater attention. In construction activities the share of energy used in welding and related activities is around 19 % of total energy consumed.Some years back the American Welding Society along with the US Government has brought out a document titled “Vision for Welding Industry”. One among the many strategic goals (or Performance Targets) identified in that document is to “Reduce Energy use in welding and related operations by 50% from current level by the year 2020”.
It is a common requirement that all SMAW welding electrodes are to be stored in temperature and humidity controlled stores. This is to prevent the electrode coating absorbing atmospheric moisture during storage. All high class fabricators maintain a hot room for storage of welding consumables. The hot room is equipped with de-humidifier and heaters to control the temperature and humidity. Power consumption for maintaining the hot room is quite significant. Further the electrodes have to be baked in an oven at temperature ranging from 150o C to 300o C depending on the type of electrode. Then they have to be held in holding ovens at 80-100o C till they are issued to the welder in portable holding ovens. This is a cumbersome process, particularly in construction sites to manage the procedures for storage control, conditioning of electrodes and issue control.
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| Vacuum Sealed Aluminium Foil packet |
In the recent years with the availability Aluminium foils, the electrode manufacturers’ offer vacuum sealed aluminium foil packets of 1kg to 2 kg electrodes. As long as the vacuum sealing is intact the electrode packets can be stored in ordinary covered store rooms without any limitations on their shelf life. Further the electrodes from the vacuum sealed packets can be directly issued to the welders in their portable holding ovens. There is no need to dry the electrodes in the baking ovens and keep them in holding ovens. This will simplify the cumbersome procedures for electrode storage control and conditioning them before issue. By changing over to procurement of low hydrogen electrodes in vacuum-sealed packets, the need for hot room storage can be reduced and use of baking and holding ovens can be brought down.
The power rating of baking oven is about 4 to 5 KW and the holding oven is 2 KW. Considering that the baking oven is used for one baking cycle per day and the holding oven is operated for 24 hrs the energy saved would be around 50KWH per day which can more than offset the extra price to be paid for procuring the electrodes in vacuum sealed pack.
In construction sites, traditionally Transformer or Rectifier type welding power sources are employed. It is important to consider the power factor of these welding power sources. Power factor is defined as the ratio of real power to apparent power. In a purely resistive circuit, voltage and current waveforms are in phase, and the power factor is almost equal to 1.
In circuits with inductors and capacitors, the current and voltage waveforms are out of phase and not all of the power is available to do useful work. Power charged by electricity distributers will be for the apparent power, and not on the real power utilized. Constant current power sources used in SMAW typically have a PF of about 0.6. Whereas modern inverter based power sources offer power factors up to 0.95. Further, Inverters have conversion efficiency of around 85-95%, when compared to rectifier and transformer at around 70%. For the same output welding current inverter draws about 25 - 30% less primary current than a rectifier.
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| 6800 kwh/year |
8500 kwh/year |
| 20 kg |
250 kg |
| Inverter |
Rectifier |
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| 12500 kwh/year |
| 350 kg |
| Motor generator |
Energy consumption per year for SMAW welding power sources
The energy consumption per year for various types of SMAW power sources at 30% arcing time and it brings out the energy saving potential by using Inverter type power sources. Further, the inverter provides ripple free output as the switching takes place at high frequency. So the welders find it easy to produce good uniform welds with Inverters. The size of the inverters is very compact and light weight for the same power output and they are highly portable which is a significant advantage in construction sites.
Submerged Arc Welding (SAW)
In Submerged arc welding the arc is struck between the work and a bare continuous wire electrode, the tip of wire is submerged in the loose granular flux added separately. Since the arc is completely covered by the flux, it is not visible and the weld is made without spatter, arc radiation and very little fumes. The process can be fully mechanised, the semiautomatic version is becoming more or less obsolete.
High currents can be used in submerged arc welding resulting in extremely high current densities. This allows for high deposition rates and deep penetration. Welds made under the protective layer of flux are excellent in appearance and are free from spatter. The easy adaptability of the process to full mechanization makes it popular for the manufacture of plate girders and fabricated columns. The major disadvantage of the process is that its application is limited to mostly indoors and down-hand /horizontal welding positions.
In SAW the weld metal composition and properties are dependent on both wire and flux selected. Most popular classification of SAW wire is AWS A5.17 which indicates the percentage of C, Mn and Si in the wire. Fluxes are of different types such as Acidic or Basic on the basis of the nature of slag, Agglomerated or Fused on the basis of method of production, Neutral or Alloying based on the addition of alloying elements to the weld metal.
Agglomerated fluxes are more tolerant to some rust and mill scale on the surface, slag is easy to remove. But, they are hygroscopic and absorb moisture. When unused flux is recovered the fine particles can be lost and consistency of composition will be affected. Whereas fused fluxes are chemically homogenous and loss of fine particles during recovery of unused flux do not alter the composition. They are resistant to moisture pick up. But, it is difficult to add de-oxidisers to fused flux and hence they are not suitable for welding over rusted surfaces. Neutral fluxes do not add any alloying elements to weld metal except for compensation of Mn and Si. They are more suitable for multi-pass applications. Alloying fluxes are primarily intended for single pass welds. They should not be used where input voltage fluctuations exist. Acidic fluxes give very good operating characteristics and smooth bead appearance with self lifting slag, but the mechanical properties are not the best. Basic fluxed give the best mechanical properties, particularly toughness. Bur the slag is very sticky and difficult to remove and bead appearance is not good.
Recent Trends in SAW
In the recent years many equipment manufacturers have come out with special purpose welding systems with multiple welding heads for I beams, H columns, + Columns, Box Columns etc.These machines not only reduce the cycle time for welding, they help to control the distortion due to balanced welding and minimize the time and cost of distortion correction. They also bring down the time spent on setup of the joints and enable accurate dimensional control.
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| Tandem-arc and Multi-head SAW systems for welding structural members |
The use of cored wires for submerged arc welding was prevalent for many years, but it made very little impact until recently. The deposition rate in submerged arc welding can increase by 20-25% when cored wires are substituted for a solid wire at the same welding parameters. Another interesting option is the possibility of using relatively acidic type submerged arc flux in combination with a basic type cored wire.
Acidic fluxes give a good bead appearance and slag detachability, and are less susceptible to moisture pickup than basic fluxes, but do not generally produce welds with good weld-metal toughness. However, when used with a basic flux-cored wire, all the good attributes of the flux are retained, while the small amount of basic components, delivered directly to the arc cavity, lower the weld oxygen level and allow good toughness to be achieved. The combination of acidic and fused fluxes with basic cored wires is finding application in offshore structures and has many potential applications in other sectors.
Flux Cored Arc Welding (FCAW)
The Flux Cored Arc Welding (FCAW) is a continuous wire welding process. The wire is tubular and the core of the wire is filled with Flux, De-oxidisers or Metal Powders which provide protection and composition control to the weld metal. Additional shielding can be provided from an externally supplied gas like CO2 or Argon-CO2 gas mixtures.
In the early days flux cored wire developers took the SMAW coating formulations and applied them to flux cored wires. Thus they failed to capitalise the biggest advantage that the flux cored wire offer to eliminate binders and extrusion aids which are the main source of hydrogen pick up. The early flux cored wires had the bad reputation for high hydrogen content and porosity. Subsequent developments in cored wires have overcome all the initial deficiencies and enhanced their usage in wide range of applications in structural fabrication.
Recent Trends in FCAW wires
Basic Flux Cored Wires
Basic slag systems produce weld metals low in oxygen, which is good for ductility and toughness. But, to ensure a stable, fine droplet transfer to take place, low surface tension of the transferring droplet surface is necessary. Since this does not happen with low oxygen levels in basic wires, the metal transfer tends to be quite globular. Fluorspar inhibits hydrogen pickup by the droplets, so basic flux-cored wires produce weld metals much lower in hydrogen. The developers of basic flux cored wires dispensed with the use of alkali metal-based arc stabilisers, thus producing relatively non-hygroscopic wires.
The low oxygen basic weld metal gives good toughness at much higher strength levels. With the increasing use of steels with high yield strengths, basic wires are the best way to achieve satisfactory mechanical properties. Due to inherently fluid lime–fluorspar slag, the basic wires are difficult to manage in positional welding, while the globular transfer leads to spatter.
Rutile Flux cored wires
The development of flux cored arc welding is in pursuit of higher "efficiency and speed” to achieve the high deposition rate, high-speed welding and robotisation. Less-spatter, less-porosity, smooth wire feedability are the requirements for higher efficiency and higher speed. Less fume generation is a new requirement from the environmental considerations. A large variety of rutile flux cored wires have been developed meeting the above requirements. The main characteristic of these wires is the ability to give extremely smooth, spray type metal transfer over a wide range of currents.
The main factor which controls the size of the droplet transfer is the oxygen level. While it is quite possible to formulate a rutile wire which will give a lower oxygen weld metal, this would lead to unacceptable welding characteristic similar to the basic types. By adding titanium and boron as micro-alloying elements, wires have been developed that matched the levels of toughness required for offshore structures.
Another characteristic contributing to the versatility of rutile wires is their ability to develop a wide range of melting points and viscosities. Rutile melts between 1700 and 1800o C. With the addition of suitable fluxing agents it is easy to make slag with melting point around 1200o C and to fine tune these to suit the application. Thus, for vertical up welding where the slag has to support the weld metal and mould it to a flat contour, rutile wires are the optimum choice. With such wires, currents up to 300A can be used for vertical-up welding without losing control of the weld pool.
For high current down-hand fillet welding, a slower freezing, but more viscous slag may give the best results and this too can be readily formulated on a rutile base. Many such wires are developed for use with CO2 shielding, since this allows cooler running of the torch and is more comfortable for the operator.
The rutile flux-cored wires of early days had high weld metal hydrogen content up to 30ml/100g. These wires contained significant amounts of hygroscopic synthetic titanates and were produced by a drawing process using solid soap traces of which was left on the wire surface. Today wires are using better formulations and a soap-free production route, are giving less then 5ml/100g of hydrogen.
Rutile flux-cored wires are easy to use and are available for many types of steel, from mild steel to high strength steels. It is therefore not surprising that they have overtaken basic types in popularity in the last 20 years, and growth in their use is expected to continue.
Self Shielded Flux Cored Wires
The self-shielded tubular wires came up in the late 1950s. The driving force was the need for a process that would be faster to use than SMAW, but which would be independent of a supply of shielding gas. It was important for welding in remote areas where the infrastructure for gas supply does not exist. Even in welding on tall structures, the need for carrying heavy gas cylinder was undesirable.
In the absence of an external shielding gas, the lime–fluorspar system produced a gas shield if the wire stick-out is kept long enough for the resistance heat generated to break down the lime into CO2 and CaO, while the fluorspar vaporized in the arc. Addition of metallic aluminium, as a de-oxidant and nitride former, allowed more than normal stick-outs to be used and a range of wires were produced which could be used without external shielding gas.
Self-shielded wires are not remarkable for their mechanical properties, mainly because the atmospheric nitrogen was trapped as aluminium nitride in the weld metal. The excess aluminium remained in solution in the weld metal. Aluminium is a strong ferrite former and if present in sufficient amount tends to reduce austenite formation. Thus the beneficial austenite transformation products that give steel its combination of strength and toughness may not be fully formed. In the recent years, other shielding mechanisms, involving, for example, lithium compounds which produce metallic lithium in the arc, were developed so that aluminium levels could be reduced and the toughness improved. Nowadays such self-shielded wires are being used to weld thick section offshore platforms meeting stringent charpy and CTOD requirements.
Another major advantage of self-shielded wires is in the welding of high-rise buildings due to their relative immunity to winds and draughts. This is because the metal vapour shielding is not easily blown away and the weld is rich in nitride formers and deoxidants.
Over the years, self-shielded wires were developed for many different applications, from high deposition rate welding of heavy plate to the welding of thin sheet at low currents and voltages. Special wires have been made for welding of pipeline girth welds, and for welding galvanised steel etc. They are convenient to use, particularly suitable for outdoor applications and torches are light weight. Nevertheless, for several environmental and occupational health reasons, self shielded wires are not the first choice where other options are available.
Metal Cored wires
Metal-cored wires were patented in 1957 mainly to overcome shortage of solid welding wire. The addition of small amounts of non-metallic material to the core together with the metal powders, have subsequently made metal-cored wires so useful in their own right. High deposition rate per ampere of welding current may be one of the deciding factors in metal cored wire selection. They also give very good under-bead profile on argon-rich gases, which reduces occurrence of lack of fusion defects. This makes them particularly suited to fillet welding. Slag levels are low, so it is possible to make three or more runs without de-slaging. The lack of slag also makes welding over primers easier.
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Like solid wires, metal-cored wires must be used in the dip transfer mode when welding uphill, which limits deposition rates and they may not offer much advantage over solid wire. However, where vertical down welding is permitted, very high deposition rates can be achieved. Another approach to positional welding is the use of a pulsed arc. This allows a high deposition rate in the uphill direction and is very suitable for robotic applications, where guidance is by through-arc sensing.
Mechanisation allows metal-cored wires to be used at high productivity and the recent commercial development of tandem pulsed arc welding takes this a step further. In this system, two wires are fed through contact tips which are insulated from each other but share a gas shield. If the pulses on the wires are synchronized , the arcs do not interfere with each other.
Selection of right process for structural fabrication
Submerged arc welding has the potential to deliver the highest deposition rates. Multiple electrode applications of submerged arc extend this capability even further. For structural joints requiring high volume of weld metal deposition, submerged arc is ideal processes to contribute to low cost welding. Deep penetration is offered by the submerged arc welding process.
When the specific conditions are not suitable for SAW, but reasonably high deposition rates are still required, flux cored arc welding is the next best alternate. When FCAW is the choice, gas shielded flux cored welding offer deep penetration.
Out-of-position welding capability is strongest for the flux cored and shielded metal arc welding processes. The slag coatings that are generated by these processes can be instrumental in retaining molten weld metal in the vertical and overhead positions. Submerged arc is not suitable for out of position welding.
Fabricators often make the mistake of selecting a process and procedure capable of extremely high deposition rates, but with limited travel speeds. Oversized welds can result from the inability to achieve the matching high travel speeds. A more economical approach would be to optimize the procedure according to the desired travel speed. This may result in a lower deposition rate but a lower overall cost because over welding has been eliminated.
Self shielded flux cored welding is ideal for outdoor conditions. Quality deposits can be obtained without the erection of special wind shields and protection from air drafts. Shielded metal arc welding is also suitable for these conditions, but productivity is considerably slower.
The self shielded FCAW is the first choice for field erectors and construction sites at remote areas. The main advantage of the self shielded FCAW process is its ability to deposit quality weld metal under field conditions, which usually involve wind. Self shielded FCAW is immune to this problem. The codes limit wind velocity in the vicinity of a weld to a maximum 2.3 m/s. To employ gas shielded processes under these conditions, it is necessary to erect wind shields. Under conditions of severe shielding loss, weld porosity can result. A variety of other gas-related issues are also eliminated, such as ensuring availability of gas, safety issues in handling of high pressure cylinders, protection of gas distribution hose under field conditions, and the cost of shielding gas.
Weld Quality and Process-Specific Influences
Some welding processes are more sensitive to the generation of certain types of weld defects; some welding processes are nearly immune from certain types of weld defects.
SMAW — The limitations of shielded metal arc welding falls into three categories:
- Arc length related defects,
- Start-stop related defects, and
- Coating moisture related problems.
In SMAW, the welder controls arc length. Excessively short arc lengths can lead to arc outages, where the electrode becomes stuck to the work. When the electrode is mechanically broken from the joint, the area where the shorting has occurred needs to be carefully cleaned, usually ground, to ensure conditions that will be conducive to good fusion by subsequent welding. The electrode is usually discarded since a portion of the coating breaks off of the electrode when it is removed from the work. Excessively long arc lengths will generate porosity, undercut, and excessive spatter. Because of the finite length of the SMAW electrodes, an increased number of starts and stops is necessitated. During arc initiation with SMAW, starting porosity may result during the short time after the arc is initiated and before adequate shielding is established. Where the arc is terminated, under-filled weld craters can lead to crater cracking. The coatings of SMAW electrodes are sensitive to moisture pick-up. It is necessary to ensure that the electrodes remain dry in order to be assured of low hydrogen welding conditions. Improper care of low hydrogen SMAW electrodes can lead to hydrogen assisted cracking.
Gas shielded FCAW — In gas shielded FCAW, as with all gas shielded processes, it is important to protect the gas shielding around the weld deposit. If gas shields are disturbed by winds, fans, or smoke exhaust equipment, porosity can result. The deep penetrating characteristics of FCAW are generally advantageous, but excessive penetration can lead to centreline cracking because of a poor aspect ratio (width-to-depth ratio) in the weld bead cross section.
| Self shielded FCAW — Excessively high arc voltages, or inappropriately short electrode extension can lead to porosity with self shielded FCAW. When excessive voltages are used, the demand for shielding increases, but since the amount of shielding available is relatively fixed, porosity can result. When the electrical stick out distance is too short, there may be inadequate time for the various ingredients contained within the electrode core to chemically perform their function before they are introduced into the arc. This too can lead to porosity. Because of the extremely high deposition rate capability of some of the self shielded FCAW wires, it is possible to deposit quantities of weld metal that may result in excessively large weld beads. |
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GMAW — When solid electrodes are used, and particularly when welding in out-of-position, the short arc transfer mode is frequently used. This can directly lead to cold-lap, a condition where complete fusion is not obtained between the weld metal and base material. This is a major shortcoming of the GMAW process and is one of the reasons why its application is restricted by structural welding codes. GMAW is sensitive to the loss of gas shielding.
Conclusion
A thorough understanding of the capabilities and limitations of welding processes will help the structural fabricators to select right process for their specific applications. Having an insight into the recent trends in these welding processes will help them to maximize productivity and quality and at the same time cut down the welding cycle time and cost of fabrication
References
1) Fabricators’ and Erectors’ Guide to Welded Steel construction, by James F. Lincoln Arc Welding Foundation
2) Tubular wire welding by David Widgery
3) Self shielded arc welding by T Boniszewski
4) Flux Cored Arc Welding – Past, Present & Future by S. Sankaran & R. D. Pennathur
5) Vision for welding industry -2020 by American Welding Society
Acknowledgement: This article was presented by Mr. Mr. S Sankaran, Welding Consultant, at the National Seminar on Steel Structures- Steelcon, held at Mysore on 26th and 27th of April, 2013. Mr. Sankaran can be contacted on- shreyas45@hotmail.com