Submerged Arc Welding (SAW)
The submerged arc welding process or commonly called SAW welding is an automatic and high productivity welding process. Compared to the shielded metal arc welding process, the flux to provide shielding is laid in granular form on the unwelded seam ahead of the bare metal electrode. The electrode is fed continuously from a coil, thus avoiding the interruptions inherent in the SMAW process to change electrodes. The flux is quite effective in preventing the atmosphere from contaminating the molten weld metal and no external shielding gas is required.
Principles of SAW Operation
The arc is struck beneath the flux between the bare electrode and the workpiece, which melts a small amount of the flux. Although a non-conductor when cold, the flux becomes highly conductive when molten (about 1300°C) providing a current path to sustain the arc between the continuously fed metal electrode and the workpiece. The heat generated by the arc melts the end of the electrode, the flux, and part of the base metal at the weld seam. The arc transfers the molten metal from the tip of the melting electrode to the workpiece, where it becomes the deposited metal. As the molten flux
combines with the molten metal, certain chemical reactions occur that remove some impurities and/or adjust the chemical composition of the weld metal.
While still molten, the flux, which is lighter than the weld metal, rises to the surface of the weld pool and protects it from oxidation and contamination. On further cooling, the weld metal solidifies at the trailing edge of the moving weld pool, and the weld bead usually has a smooth surface due to the presence of the molten glass-like slag (molten flux resulting from all the chemical reactions) above it.
The slag freezes next and continues to protect the weld metal as it cools. Frozen or solidified slag is readily removable, sometimes popping off the bead spontaneously. Excess, unmelted flux can be recovered and reused after proper processing.
Equipment and current types in SAW Welding
The equipment set-up for single wire submerged arc welding is shown in below figure. In addition to the power supply, a submerged arc welding system requires a wire feeder to maintain a continuous feed of the electrode wire through the torch. For single wire submerged arc welding, direct current electrode positive (DCEP) is used for most applications as it provides better control of bead shape, ease of arc initiation, and deeper penetration welds with greater resistance to porosity.
Direct current electrode negative (DCEN) polarity is also occasionally used to provide a greater deposition rate. However, penetration is reduced and there is some increased risk of lack of fusion- type flaws. From a practical point of view, a change from DCEP to DCEN may necessitate an increase in voltage of about 2 to 3 V if a similar bead shape is to be maintained.
Both constant voltage and constant current (drooping voltage characteristics) power sources can be used. With constant potential power sources, used in conjunction with constant speed wire feeders, the arc length self-adjusts to a nearly constant value depending on the voltage, as in GMAW.
Power sources are available that can deliver up to 1500 A. However, direct current is usually kept below 1000 A since there can be excessive arc blow. Alternating current can be used to reduce arc blow in high current applications and other situations prone to arc blow, e.g., multiwire and narrow gap welding. Alternating current power sources are usually constant current type with a nearly square wave output voltage to assist in arc ignition at each polarity reversal. Square wave constant potential power sources have also become available that provide both voltage and current in square wave form and therefore have less difficulty in arc re-ignition at polarity reversals. The weld bead penetration obtained with alternating current is in between that for DCEP and DCEN.
Advantages and Applications of Submerged Arc Welding
- high quality weld with good finish.
- high welding speed and weld metal deposition rate.
- smooth, uniform finished weld with no spatter.
- little or no smoke.
- no arc flash, thus minimal need for protective clothing.
- high utilization of electrode wire.
- ease for automation for high-operator factor.
- Not much involvement of manipulative skills of the welder/ operator.
Most submerged arc welding applications are for carbon and low alloy steels. The process is also used for joining stainless steel and nickel based alloys. However, the fluxes are proprietary in nature and flux manufacturers must be consulted for optimum flux selection.
Because of the mechanized nature of the process, it is most effectively used when numerous similar welds are to be made (splicing of plates and panels in shipyards, fabricated structural shapes, welding longitudinal or spiral seams of large diameter oil and natural gas pipelines and when the thickness to be welded is large (circumferential and longitudinal seams in thick wall pressure vessels). Other applications of submerged arc welding include overlaying (stainless steel overlay on
chromium-molybdenum steels for high temperature, high pressure hydrogen applications) and rebuilding and hard surfacing.
Classification of Submerged arc Welding (SAW) Electrodes
In AWS A5.17 wires are split into three groups of
2. medium and
3. high manganese.
The first digit, ‘E’, identifies the consumable as a bare wire electrode. If supplemented by ‘C’ the wire is a composite (cored) electrode. The composition of the solid wire is obtained from an analysis of the wire. However, since the composition of a cored wire may be different from that of its weld deposit the composition must be determined from a low dilution weld deposit made using a specific, named flux.
The next letter, ‘L’, ‘M’ or ‘H’ indicates a low (0.6% max), medium (1.4% max) or high (2.2% max) manganese content.
This is followed by one or two digits that give the specific composition. An optional letter ‘K’ indicates a silicon killed steel.
There are a final two or three optional digits identifying the diffusible hydrogen in ml/100g weld metal, H16, H8, H4 or H2.
A full designation for a carbon steel flux/wire combination could therefore be F6P5-EM12K-H8.
This identifies this as being a solid wire with a nominal 0.12% carbon, 1% manganese and 0.1 to 0.35% silicon capable of achieving an ultimate tensile strength of 60 k.p.i. (415MPa), Charpy-V impact strength of 27J at -50°F (-46°C) in the post-weld heat-treated condition.
Another example F43A2-EM12K is a complete designation for a flux-electrode combination. It refers to a flux that will produce weld metal which, in the as-welded condition, will have a tensile strength of 430 to 560 MPa and Charpy V-notch impact
strength of at least 27 J at -20 °C when produced with an EM12K electrode under the conditions called for in this speciﬁcation. The absence of an “S” in the second position indicates that the flux being classified is a virgin flux.
F48P6-ECI is a complete designation for a flux-composite electrode combination when the trade name of the electrode used in the classification is indicated as well. It refers to a virgin flux that will produce weld metal with that electrode which, in the postweld heat treated condition, will have a tensile strength of 480 to 660 MPa and Charpy V-notch energy of at least 27 J at -60 °C under the conditions called for in this specification.
Variants of Submerged arc Welding (SAW)
One of the great advantages of the submerged arc welding process is the ability to use multiple electrodes fed into the same weld pool thus considerably increasing the deposition rate. Some configurations for multiple wire submerged arc welding are:
1. Parallel Electrode Welding : Also called twin wire welding, two electrode wires are connected in parallel to the same power source. Both electrodes are fed by means of a single wire feeder and through the same welding head. Welding current is the sum of currents for each electrode and a single deep penetrating weld pool is obtained.
2. Multiple Arc Welding (Tandem SAW): Also called tandem welding, two (or more) electrodes can be connected to individual power supplies and fed by separate drive rolls through separate contact tips. The lead electrode in such cases is connected to a DC power source and the trailing electrode to an AC source to reduce interaction between the magnetic fields of the two arcs. It is important to ensure that the spacing between the arcs is not too large. The trailing arc is usually positioned close enough to the leading arc that the slag cover does not solidify between deposits. The total current in multiple wire welding can be as high as 2000 A, although in most applications it does not exceed 1200 A.
3. Series Arc Welding: Two electrodes, fed through separate guide tubes, are connected in series. Separate sets of drive rolls and contact tips, insulated from each other, need to be employed. The current path is from one electrode to another, through the weld pool. The weld bead has relatively shallow penetration, making this arrangement useful for overlay welding.
Factors of dilution rate in SAW Welding
In SAW, joint configuration is the main factor that affect the dilution rate along with welding current. In general, below figure shows the effects of the welding joint type of the weld dilution.
types of flux in SAW Welding
Fluxes for submerged arc welding can be categorized by method of manufacture or effects on weld metal composition. There are two types of fluxes:
1. fused flux and
2. bonded flux.
3. Active flux
4. Neutral Flux
The manufacture of fused fluxes involves melting together various ingredients to provide a homogeneous mixture, which is then allowed to solidify by pouring it onto a large chilling block. The glass-like, solidified particles are crushed, screened for sizing and then packaged for use. The main advantages of fused fluxes are their chemical uniformity (irrespective of the flux particle size), resistance to moisture absorption and
easy recycling without changes in particle size or composition. The disadvantage of fused fluxes is that it is difficult to add deoxidizers and ferroalloys because these compounds tend to oxidize during the melting process.
In comparison, bonded fluxes are made by finely grinding the individual components of the flux, mixing them in appropriate proportions and then adding a binder, typically potassium and/or sodium silicate. The wet mixture is then baked at a relatively low temperature and ground to size for packaging. The main advantage of bonded fluxes is that it is easier to add deoxidizers and ferroalloys.
On the negative side, such fluxes are prone to moisture pick up, and to local changes in composition due to segregation or removal of fine mesh particles.
Fluxes that significantly influence the composition of the weld metal through slag/metal reactions are termed active fluxes. Typically, these fluxes add manganese, silicon and chromium to the weld metal.
The extent of this addition increases with arc voltage, since higher arc voltage leads to increased flux consumption. Very active fluxes may be used to deposit single or two pass welds only, since the increase in the Si and Mn content of subsequent passes may be sufficiently large to impair the weld metal ductility and also make it more prone to hydrogen cracking.
Neutral fluxes also participate in slag-metal reactions but the changes in silicon and manganese are smaller and not dependent on arc voltage. There is little build up of elements and such fluxes are therefore well suited for multipass welds.
chemically basic, neutral or acidic flux Classification
Fluxes are also referred to as chemically basic, neutral or acidic. Chemically basic fluxes have Calcium Oxide (CaO) and Magnesium Oxide (MgO) as the major ingredients. Chemically acidic fluxes have Silicon Oxide (SiO2) as the main ingredient.
When the ratio of basic oxides to acidic oxides present is greater than 1, the flux is chemically basic and
when it is less than 1, it is chemically acidic. Ratios near 1 imply a chemically neutral flux. Basic fluxes transfer smaller amounts of Si, Mn and oxygen to the weld metal, and therefore are preferred for critical applications.