T-8 wire is the wire for most structural welding.

By Charles Scharfy, Segment Manager, Structural Division Hobart Brothers Co. (www.hobartbrothers.com)

Whether fabricating components for structural steel erect ion or constructing portions of a bridge, strict guidelines dictate the procedure and the materials that can be used during the welding process.

AWS 5.20-05 states the standards to which carbon steel flux-cored welding electrodes are manufactured, specifically in regard to mechanical and chemical properties. Further, the AWS D1.1 structural welding code and the AWS D1.5 bridge welding code dictate the base metal and filler metal combination for these applications, along with the power supplies that can be used.

Under these specifications, flux-cored wire that meets the AWS E71T-8 classification is a common requirement.

Also known in various industries as T-8, this self-shielded flux-cored wire requires proper technique and training to achieve optimal results. The key to using T-8, in addition to the necessary training and practice, is a general understanding of its characteristics and operating requirements, and knowledge of how its chemical and mechanical properties are designed to meet the needs of critical construction and repair applications.

Classified for Premium Performance

Understanding the various portions of its AWS classification is a good first step in becoming familiar with a T-8 self-shielded flux-cored wire, beginning with its most basic designation: E71T-8.

In this designation

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

Due to advances in filler metal manufacturing technology, some T-8 wire also offers added designations for impact strength, seismic requirements and hydrogen content – all factors that contribute to the suitability and usability of a given manufacturer’s T-8 wire for a specific application.

For example, the addition of the letter ‘J’ to the end of the AWS classification E71T-8 – as in E71T- 8J – indicates the wire offers high ‘as welded’ impact strength properties – also referred to as Charpy VNotch values – at –40 degrees F (-40 degrees C).

A successive ‘D’ – resulting in E71T-8JD – indicates that the wire can be used for demand critical welds in special seismic applications.

To meet this designation, the wire must have adequate strength and toughness in low and high heat inputs, in the range of 30 kJ/in. to 80 kJ/in. For exact details, refer to AWS D1.8 – Structural Welding Code – Seismic Supplement under specification AWS A.520.05.

Finally, the presence of H8, or another similar such alphanumeric indicator in the end of the AWS classification, as in the example: E71T-8JD H8; specifies the amount of diffusible hydrogen in a particular T-8 wire.

In this example the H8 indicates that there is less than 8 ml. of hydrogen per 100g. of deposited weld metal. As a rule, T-8 wires available in the market place today offer diffusible hydrogen levels ranging from less than 8 ml/100 g. to as much as 16 ml/100 g., with the lower levels aiding in greater crack resistance.

Requirements that Yield Results

To achieve the best weld possible, along with the X-ray quality required for critical applications, welding operators should understand the general characteristics and operating requirements of T-8 wire.

Fluctuations in voltage are common on many job sites, and they affect the performance of T-8 wires. The length (and condition) of welding leads causes voltage drops between the power source and the work piece, and that is especially true with long welding leads.

The use of a multi-meter or a wire feeder with digital voltage meters can help to maintain proper voltage range and, with it, desirable welding results.

Also, maintaining good weld cable condition and secure connections between the welding gun and the power source helps.

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 importantly, the wire feed speed, which directly controls amperage.

Generally T-8 wire operates at 200 amps and greater, with a voltage range from 18 V to 24 V on an all-position weld regardless of wire diameter. For this reason, it is always recommended that welding operators weld within those parameters to avoid defects that could lead to time-consuming and costly rework.

In particular, excessive voltage, beyond the highest recommended for a given T-8 wire, can result in discontinuities such as porosity or worm-tracking. Conversely, voltage that is too low can prevent the wire from wetting out properly and leads to cold roll. Both of these occurrences that can lead to weld inspection failure.

Maintaining the welding parameters of a given T-8 wire additionally is dependent on electrode extension or stickout, and varies according to the filler metal manufacturer.

As a rule, most T-8 wire requires a range of ¾ in. to 1¼ in., depending on the given diameter used. The rule is that a smaller diameter wire requires a shorter stickout than a larger diameter one.

For example, a 1/16 in diameter T-8 wire could operate properly with a stickout of 7/8 in. to 1 in., but a 5/64 in. diameter wire may require stickout of 1 in. to 1¼ in.

Too short of a stickout causes the amperage to increase disproportionately to the voltage setting, which can lead to cold roll. Too long of an extension will cause the amperage to be too low for the given voltage setting leading to incomplete slag coverage, and ultimately, weld discontinuities. To minimize such problems, always check the specific stickout requirements for the T-8 wire that is being used.

Finally, T-8 wire requires an appropriate gun angle and travel speed to prevent slag inclusions.

Specific gun angles vary from wire to wire, anywhere from 10 degrees to 45 degrees, and always should be determined according to the recommendations of the given manufacturer. As a rule, the presence of a uniform slag line behind the weld puddle is a good indication that both proper gun angle and travel speed requirements have been achieved.

Remember, having a solid understanding of its characteristics and operating requirements is key to welding successfully with T-8 wire.

With the proper training for a chosen manufacturer’s wire, the advantages of T-8 can be realized.

Best of all, a bit of knowledge will ensure that the quality requirements of critical structural applications continue to be met.