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In submerged arc welding (SAW), selecting the right wire and equipment are critical factors to success. Just as important is choosing the proper flux.
A wide range of fluxes are available for the SAW process. The basic job of flux is to shield the weld pool from contamination. In addition, some fluxes are specially formulated to bring specific properties and characteristics to the finished weld.
With many types of flux available, how do you know which option is the best choice for your application? Review these key considerations for help in making the choice, so you can unlock the full potential of the SAW process.
The role of flux
If you want best-in-class toughness or high travel speeds, the right flux can help achieve those goals. Choosing the optimal flux for your welding application is key to maximizing performance.
When selecting SAW flux, be sure to consider all variables in the welding operation, such as joint design, mechanical property requirements and productivity expectations. Then, compare these factors with the unique characteristics of each flux.
Among flux options, understanding key terminology used in product literature can help narrow down the right choice for the job. These include active flux versus neutral flux and high-basicity flux versus low-basicity flux.
Flux neutrality describes how much a flux can influence the chemical composition of the weld deposit. As a general rule, consider neutral fluxes for general purpose applications and active fluxes for fast one- or two-pass welds.Active or neutral flux
As the name suggests, neutral fluxes remain neutral; they do not significantly influence the chemistry of the weld deposit. This is beneficial when making tough, crack-free multi-pass welds. Neutral fluxes have no limitations on material thickness, are not very parameter sensitive and have minimal effect on the weld&#;s impact properties. Flux neutrality describes how much a flux can influence the chemical composition of the weld deposit. A flux is either active or neutral.
Active fluxes can provide more unique welding characteristics because they actively influence weld chemistry. The additional manganese and silicon in active fluxes alloy the weld to compensate for high-dilution welds. Active fluxes are best suited for single- or two-pass fillet and groove welds and are typically not recommended for large multi-pass welds.
Often, these fluxes improve weld cleaning, helping operators make quality welds on lightly rusted or scaled materials. The formulas of many active fluxes also help to maintain good bead contour at high travel speeds. Keep in mind that active fluxes are very parameter sensitive, so it&#;s important to follow the manufacturer&#;s recommendations for operating parameters and to always maintain good control over the welding procedure. Also, be aware that some welding codes limit or prohibit the use of active fluxes in certain applications.
As a general rule, consider neutral fluxes for general purpose applications and active fluxes for fast one- or two-pass welds.
High or low basicity
Travel speed and productivity aren&#;t always the most important considerations in every welding application. One example is offshore structure fabrication, where welds need to resist harsh waves and storms and very low seawater temperatures, while also helping to prevent environmental disaster. Weld toughness is critical in these applications.
Toughness is the ability of a weld to absorb rapidly applied energy. When this is a primary design concern, consider a flux&#;s basicity. Basicity is the ratio of chemically basic to acidic compounds that make up the welding flux. Basicity is often expressed in terms of a Basicity index (Bi), which is a calculated value based on flux composition.
In most cases, fluxes with higher basicity indexes offer improved toughness, while fluxes with lower basicity indexes typically offer more appealing welding characteristics such as good weld bead appearance and easy slag removal. It&#;s recommended to select fluxes with a high-basicity index for critical applications and demanding service conditions. But be aware that high-basicity fluxes weld much differently than low-basicity fluxes &#; typically producing a rougher bead appearance, less weld pool wetting action and slag release that is somewhat more difficult.
While fluxes with similar basicity index values tend to weld similarly, minute variations in flux composition can alter the way they weld. Selecting the best flux for your application is often a balancing act between mechanical properties, such as toughness, and operating characteristics, such as slag release. In most situations, a good rule of thumb is to select a flux with the lowest basicity that will still provide sufficient mechanical properties for the in-service weld.
Proper use and storage
In addition to choosing the right flux, proper use and storage of SAW flux also helps promote success.
Any moisture in the flux contributes additional diffusible hydrogen into the weld metal, which can increase the risk of hydrogen-induced cracking. This risk is even greater when welding thick, restrained or high-strength materials.
Look for flux that is formulated and packaged with low moisture content and take care to store it properly to reduce the chance of moisture absorption and, therefore, hydrogen-induced cracking. Most Hobart® fluxes are heat-sealed in durable bags constructed to protect flux from moisture absorption, so the contents can be used directly from unopened packaging stored for up to three years without reconditioning.
Once opened, fluxes should be stored in a heated drying cabinet or hopper at 255 to 345 degrees Fahrenheit. If flux has been exposed to a moisture source, such as being outside of a heated hopper for a considerable amount of time, it&#;s necessary to recondition the flux by heating it in a drying cabinet at 570 to 660 degrees Fahrenheit for at least two hours. It&#;s important that the entire volume of flux (including the flux in the middle of a container) reaches this temperature, otherwise the average moisture level may still be higher than recommended.
To maintain good welding characteristics, reconditioning should not be performed on SAW flux more than twice.
A critical component
Submerged arc welding flux does much more than simply protect the weld. The right flux can help give you the best balance of properties and performance for optimized results.
Always consider the wire and flux combination for any application &#; no matter how simple or complex. A filler metal manufacturer can also provide suggestions and help you choose and implement the best solutions.
Flux is a mixture of various minerals, chemicals, and alloying materials that primarily protect the molten weld metal from contamination by the oxygen and nitrogen and other contaminants in the atmosphere. The addition of certain chemicals and alloys also help to control arc stability and mechanical properties.
Flux is used in the following arc welding processes Shielded Metal Arc Welding (SMAW), Flux Cored Arc Welding (FCAW) and Submerged Arc Welding (SAW). Let&#;s look at these welding processes and how flux is added and used in shielding the welding zone and in alloying.
Shielded Metal Arc Welding (SMAW) uses a solid core wire for the electrode material. To add the flux to the bare electrode we mix all the ingredients such as sodium and potassium silicate to the dry mix of materials and alloys to create a binder that is then added to the bare electrode by extrusion to make the SMAW coated electrode as shown in Figure 1
Figure 1, Image of extruding the flux on the bare electrode
As stated earlier, the main function of the flux coating is to protect the molten metal from contamination from the atmosphere by forming a shielding gas and slag to cover the molten metal as seen in Figure 2
Figure 2, A covered electrode SMAW
Other functions of the flux are to;
Ease of arc striking assist, arc stability and ionization
Control of bead shape
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Influence Penetration
Add alloying elements to the weld
Control the hydrogen to reduce the chance of induced cold cracking
Aid in slag removal
Control mechanical properties of the weld
In principle there are three differing categories of flux coatings used with SMAW electrodes:
cellulosic based fluxes (e.g. E)
rutile-based fluxes (e.g. E) and
basic fluxes (e.g. E).
Of the above, only the basic flux types, (E XX18, EXX28, E XX16) are classified as low hydrogen.
Flux Cored Arc Welding (FCAW) uses an outer metal sheath and a core containing a flux and alloying components. The process of adding the flux to the wire is by using a metal strip and passing it through a set or rollers which form a U shape. The flux compound is then added into the strip and it is closed by another U shape roll to make the final flux cored wires as seen inFigure 3
Figure 3, Image of manufacturing flux cored wire
There are three basic manufacturing types of fluxed wires, the butt, folded or overlap methods Figure 4
Figure 4, Typical cross section of flux cored wires
As with shielded metal arc welding electrodes the flux in the flux cored wire is made up of materials and alloys to protect the molten metal from contamination from the atmosphere by creating a shielding gas and a molten slag to cover and protect the weld Figure 5
Figure 5, Flux cored arc welding process
The difference with flux cored welding wires is the are two classes, those that require an external shielding gas (FCAW-GS) to help with protecting the molten metal and, those that do not and are classed as self-shielding flux cored wires (FCAW-SS). These self-shielding wires contain more complex elements to help shield the molten metal.
The fluxes for the gas shielded wires are made up of two main types, 1) rutile or titania types and 2) lime or basic types. In principle the basic fluxes are selected when improved weld metal properties are required.
Submerged Arc Welding (SAW) uses two separate consumables, the solid wire electrode, and the flux. The wire is fed from a coil through the feeder into the weld and covered by the flux which is fed on top of the joint to be welded through a hopper. The arc is created under the granular flux hence the name submerged arc welding. Some of the flux is melted to create the slag which covers the weld pool and protects the molten metal from the contamination from the atmosphere while the remainder of unmelted flux can be recovered and reused. The submerged arc welding process can be seen in Figure 6
Figure 6, Submerged arc welding process
The solid welding wire electrodes are classed by base metal compositions and the fluxes are made by dry mixing carefully proportioned quantities of materials such as silica sand, metal oxides and amounts of halide salts. These materials are melted together at 1,500 to 1,700 degrees Celsius and then the molten material is chilled to cool. The product, when cooled, is ground and screened to certain particle sizes that forms the granular flux for welding.
Basic fluxes for SAW are made from elements such as Calcium, magnesium, sodium, potassium and manganese oxides, calcium carbonate and calcium fluoride whilst silica, and alumina are the constituents of acid-based fluxes.
Submerged arc fluxes can be measured by their basicity index which is commonly used to describe the metallurgical behavior of a welding flux. The basicity index is a ratio between basic and acid compounds (oxides and fluorides) of which the flux is composed.
Welding fluxes can be divided into three groups:
Acid fluxes with a basicity index of <0.9
Neutral fluxes with a basicity index of 0.9-1.2
Basic fluxes with a basicity index of > 1.2
Basicity has significant influence on weld metal properties, particularly on toughness. Increasing basicity brings down the oxygen content and hence the inclusion level in the weld metal and thus increases the toughness.
Fluxes and their behaviour are a complex science and the above is meant to provide an introduction only to the types of flux and how they are categorized between the welding processes that use them.
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