5 Reasons Why Your Business Needs Flux Cored Wire Self-shielded？

Gas shielding prevents oxidation while arc welding, but delivering it to the weld pool requires cumbersome hoses and tanks. This also increases the risk of wind disturbing the gas flow and reducing shielding effectiveness. Flux-cored arc welding (FCAW) is an alternative solution that works well in various applications. This blog introduces FCAW and discusses when it makes sense to use flux-cored wires.

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An Overview of FCAW

In conventional MIG welding an arc is formed between the electrode and the workpiece, which melts a small region of the part being welded. The electrode is consumed in the weld where it provides filler that increases joint volume and strength.

Molten metal is very prone to oxidation and contamination, which create weld defects. Two steps are taken to prevent this:

• Shielding gas is delivered around the electrode
• A welding flux is applied to the joint region

The shielding gas helps steer and focus the arc and excludes air from the weld pool, which prevents oxidation. Flux is a compound applied to the surfaces being welded. This reacts with and cleans the metal and can also generate a gas that excludes air from the weld pool.

In FCAW the filler wire contains flux. This cleans the surfaces and may, depending on composition, generate a shielding gas. Accordingly, FCAW can be either self-shielded, where the flux provides the shielding, or it can use regular gas shielding, (which requires dual-shielded equipment). Whichever variant is adopted, the process is still arc welding, but using flux-cored wires rather than solid filler and shielding gas.

Flux-cored wires are produced in a wide range of formulations. The best results are achieved by selecting one appropriate for the metals being welded.

Comparison With Other Welding Methods

FCAW has some negative characteristics, which is why it's not a universal solution. However, it can be very useful in situations where rapid filler deposition is needed or the shielding gas is not able to perform effectively.

The downsides of FCAW are:

• Generates slag on top of the weld that requires removal
• More spatter, so more post-weld clean-up
• More smoke and fumes generated
• High cost of flux-cored wire (relative to solid filler wire)
• Puts more heat into the weld
• High cost of dual-shielded equipment (where both gas and flux-cored wire are used)

However, the beneficial attributes, and reasons for using it, are:

• Works where air movement renders gas shielding ineffective
• Slag enables welding out of position (because it holds the weld pool in place)
• Enables high weld/filler deposition rates
• Supports welds needing deep penetration
• Avoids the need for a supply of shielding gas

Applications of FCAW

The flux-cored wire process is used when:

• Metals being joined are especially dirty or contaminated
• Welding outdoors
• It's not possible to have the weld pool horizontal and the torch vertical
• The weld design calls for deep penetration

Accordingly, it's used mostly on large structural projects, like bridges, buildings, and ships. Pipelines, welds in thick steel or stainless steel, and similar heavy-duty fabrications are other applications. It is not used with material thinner than 20 Ga, (0.032').

Selection Factors

Deciding whether to use FCAW is part of developing the Weld Procedure Specification (WPS). This is a document put together by a welding engineer that tells the welder how to go about making the weld. Factors the welding engineer takes into account include:

• Where the welding will be done (indoors, outdoors, somewhere with a lot of air movement)
• Depth of weld penetration needed
• Condition of the metal being welded (if a part or structure is located outdoors it can be difficult to get it really clean)
• Weld orientation
• Type of alloy: most steels are suitable but nickel and non-ferrous alloys as well as cast iron need careful selection of flux and filler

Challenges With FCAW

Before rushing to specify FCAW it's important to understand the challenges this welding process poses for metal fabrication shops. We can discuss these under the headings of:

• Managing heat input
• Avoiding weld defects
• Clean-up

Here's a closer look at each, along with some solutions and precautions.

Managing heat input

FCAW tends to put more heat into the weld. This can increase distortion in the welded fabrication, cause weld cracking, and alter strength and hardness. Techniques for controlling heat input are:

• Raise the travel speed
• Reduce the amps and/or volts used
• Reduce the wire feed speed
• Use a discontinuous welding technique: stitch or make intermittent welds, or use the backstepping technique
• Reduce the number of passes
• Preheat the pieces being welded

Selecting the best method, or combination of methods, takes experience, and often some trial and error. It's prudent to make some test welds before finalizing the WPS.

Avoiding weld defects

Reducing thermal input goes a long way toward preventing defects, but it's also helpful to control cooling. This can reduce distortion and the risk of cracking or altering material properties. Cooling can be accelerated by quenching or using fans and slowed by covering the welded fabrication with thermal blankets.

Another major concern is porosity resulting from poor shielding, and this can be exacerbated by increasing travel speed or reducing wire feed rate, as less flux is used per unit length. Using less flux also raises the risk of weld contamination as less cleaning takes place (Note that different types of flux clean with differing levels of aggressiveness.)

Clean-up

The slag generated by the flux needs to be removed from the weld. There will also be more spatter around the weld region. When planning the welding job, and especially if estimating a time for it, allowance should be made for the additional clean-up needed.

Trust Your Welding Work to Wiley

When planning a welding job it's important to consider what could go wrong. That's especially true when the welding work will be done outdoors, when there may be problems with metal cleanliness, or when the weld requires particularly deep penetration.

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In those circumstances, the solution is often to use flux-cored arc welding. It's a process with some challenges, but if it results in higher-quality welds, they are worth taking on.

Serving the construction market is a competitive business. Between bidding against other companies and striving to meet project deadlines, contractors always are looking for ways to boost productivity. The welding operation is one key area where improvements could make a difference. For some structural steel jobs, it may be possible to convert from the stick welding or shielded metal arc welding (SMAW) process commonly used on these applications to self-shielded flux-cored welding (SS-FCAW). The conversion typically involves changing from an American Welding Society (AWS) E stick electrode to an AWS E71T-8 self-shielded flux-cored wire. The self-shielded flux-cored process can offer faster welding travel speeds, in turn increasing productivity, providing greater cost savings and giving companies an edge in the industry. The process minimizes downtime for changing out stick electrodes so that welding operators can spend more time welding.

The Self-Shielded Flux-Cored Process

Strict codes regulate the welding procedures for everything from fabricating components for structural steel erection to constructing portions of a bridge. Under the AWS D1.1 structural welding code and AWS D1.5 bridge welding code, for example, companies must follow the prescribed base metal, filler metal and power supply settings for each given application.

Converting from stick welding to a self-shielded flux-cored process, therefore, requires the requalification of welding procedures for each given project. It also involves updating welding operator certifications.

Companies that make the switch will face downtime for training welding operators the techniques of welding with an E71T-8 wire'often called a T-8 wire'and for operators to successfully complete the certification process; however, both activities are just a matter of business. Welding operators must have their certifications requalified periodically throughout the course of their careers. Transitioning to a self-shielded flux-cored welding process simply facilitates this process sooner.

Having the certifications for this process can help position companies for greater competitiveness. Being able to complete the same project faster than another contract bid gives you a clear competitive edge.

Proper Equipment

The transition also requires access to the proper welding equipment (power source). Welding successfully with a T-8 wire requires a constant voltage (CV) power source. This portion of the conversion is non-negotiable'it is critical for welding to code.

One word of caution: Adding a voltage-sensing feeder to a constant current (CC) power source used for stick welding will not suffice. This system is not acceptable for self-shielded flux-cored welding to AWS code D1.1, as it does not react quickly enough to maintain a steady voltage throughout the welding process. Those fluctuations in voltage can lead to weld defects, especially porosity, which can cause downtime and costs for rework, not to mention
potential delays and possible contract fees for missed deadlines.

Get to Know T-8 Wires

The key to using a T-8 wire is to understand its characteristics and operating requirements, its chemical and mechanical properties and how they meet structural steel application needs.

A number of T-8 wires that meet seismic requirements and offer the necessary high-impact strengths for critical code work are available on the market. A filler metal manufacturer or trusted welding distributer can help you make the best selection. Understanding the various portions of its AWS classification is a good first step in becoming familiar with a T-8 wire, beginning with its most basic arrangement, E71T-8:

• E indicates an electrode.
• 7 refers to the wire's tensile strength measured in pounds per square inch (psi). For this classification, the 7 refers to 70,000 psi.
• 1 indicates that the wire offers all-position welding capabilities.
• T denotes it is a flux-cored wire.
• 8 indicates the usability of the electrode or, in this case, DCEN single- or multi-pass welding.

Some T-8 wires also offer added designations for impact strength, seismic requirements and hydrogen content, each of which contribute to the suitability and usability of a given manufacturer's T-8 wire for a specific application.

T-8 Wire Operating Characteristics

The allowable voltage ranges for T-8 wire vary according to a given filler metal manufacturer's formula, the position in which it is used and, most important, the wire feed speed, which directly affects amperage. As a rule, T-8 wire operates at 175 amps or greater, with a voltage range from 18 to 24 V on an all-position weld regardless of wire diameter. Welding operators must weld within the recommended parameters to avoid defects that could lead to
time-consuming and costly rework.

Electrode extension, or stickout, also impacts the manner in which a T-8 wire operates and, again, varies according to the filler metal manufacturer.

Typically, most T-8 wire requires stickout in the range of ¾ to 1 ¼ inches, depending on the given diameter used. The general rule is that a smaller-diameter wire will require a shorter stickout than a larger-diameter wire.

Too short of a stickout can cause the amperage to increase disproportionately to the voltage setting, which can lead to lack of fusion. Too long of an extension causes the amperage to be too low for the given voltage setting, leading to incomplete slag coverage and other weld discontinuities.

To help minimize such problems, always check the specific stickout requirements for the T-8 wire being used. It is also important to maintain the appropriate gun angle and travel speed with a T-8 wire. Doing so helps prevent slag inclusions that could lead to poor weld quality and rework. Specific gun angles vary from wire to wire, anywhere from 10 to 45 degrees, and are determined according to the recommendations of the filler metal manufacturer.

Welding operators also should employ a drag or backhand technique when welding with T-8 wire. As a rule, the presence of a uniform slag line behind the weld puddle is a good indicator that the welding operator is using the proper gun angle and travel speed.

Finally, when using a T-8 wire, contractors must ensure that their welding operators take precautions to protect against the voltage fluctuations that commonly occur on jobsites.

Using a multi-meter or a wire feeder with digital voltage meters can help maintain proper voltage range and desirable weld results.

Maintaining good weld cable conditions and secure connections between the welding gun and the power source is also a good practice.

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