FROM 5G TO 6G: IMPROVING PRODUCTIVITY ON OUT-OF-POSITION PIPE WELDING

Making the change to a tubular wire for out-of-position pipe welding applications can yield significant productivity gains. It can also help ensure that the chosen wires best match the base material for the application, and prevent an already difficult task from becoming more so.

Welding out of position is rarely easy – and rarely the most efficient method. But when welding on pipe, there is often no other option. The fabrication of smaller diameter pipe systems, such as those found in power plants or oil refineries, as well as their maintenance and repair in later years, frequently require welding in the 5G and 6G positions – fixed-horizontal and 45 deg fixed positions, specifically.

The same holds true in the construction of larger diameter onshore transmission pipelines, which are generally welded in the 5G (horizontal) position and require welding both vertical-up and vertical-down depending on the chosen filler metal. Both applications require skilled, certified welding operators to manage the task, as well as the right filler metal for the job.

In past years, many pipe fabrication and repair applications, regardless of the diameter of the material, relied on the use of SMAW (shielded metal arc welding) or stick electrodes. Not only do these filler metals provide the necessary chemical and mechanical properties for many pipe applications (including good toughness for high and low temperatures, and corrosion resistance), but they are also widely accepted and specified for these jobs. Despite that, stick welding is a notoriously slow process due to the frequent stick electrode changeovers it requires. Welding operators can typically weld only 10 in to 12 in with a stick electrode before needing to replace it.

In recent years, however, advancements in filler metal technology have brought forth tubular wire options (flux-cored and metal-cored) that provide excellent mechanical and chemical properties – particularly good resistance to cracking – and they also can improve productivity on the out-of-position welding required on many pipe applications.

WELDING IN 5G AND 6G POSITIONS ON SMALLER DIAMETER PIPE
Smaller diameter pipe – 10 in or smaller – is commonly found in applications ranging from HVAC and high-pressure steam to process piping, including pipe spools. This pipe is most often thin (Schedule 40, for example) and can be composed of materials such as carbon steel, low alloy steel, chrome-moly or stainless steel.

During the fabrication process and for repairs as well, welding operators often must weld the pipe in place, in either the 5G or 6G position. In the 5G position, the pipe is fixed at one or both ends and the welding operator must travel in one of two directions, either vertical-up or vertical-down. Depending on the location of the pipe, the application may also require welding either overhead or in a flat position. Welding small diameter pipe in the 6G position is even more difficult. In this position, which is typically found in sub-assembly applications, the pipe is fixed at a 45 deg angle.

Both the 5G and 6G positions pose definite challenges to the welding operator. In addition to requiring specific training and certification, welding defects such as lack of fusion, slag entrapment and lack of penetration are always a possibility. Poor weld bead appearance can also occur when welding in these difficult positions. Selecting a metal-cored and/or gas-shielded flux-cored wire that provides good out-of-position welding capabilities can increase productivity compared to welding with stick electrodes and simplify training associated with welding in the 5G and 6G positions.

For root passes in both positions, metal-cored wires combined with a modified short circuit process (such as RMD^® — Regulated Metal Deposition from Miller Electric Mfg. Co., Appleton, WI) are a good option. American Welding Society (AWS; Miami, FL) E70C-6M, E80C-Ni1, E90C-K3, and E100C-K3 metal-cored wires work well depending on the material (check with a trusted welding distributor for information on matching a specific wire to a given base material).

Metal-cored wires offer faster travel speeds than other types of wires, as well as higher deposition rates, making them a good alternative for increasing productivity on out-of-position pipe applications. These wires also bridge gaps that are common in pipe applications to help ensure high-quality welds. When combined with a process like RMD, metal-cored wires can often be used for the root, fill and cap passes on small diameter pipe applications, which can further increase productivity by eliminating the need to change filler metals.

Other benefits include eliminating lack of fusion and/or lack of penetration by providing thicker root passes; reducing spatter and subsequent post-weld cleaning; and simplified training. Because processes like RMD maintain a consistent arc length even at varying wire stick-out, welding operators who may not be as skilled at maintaining wire stick-out can still have good control of the process. This feature is especially beneficial since some novice welding operators are prone to increasing wire stick-out when welding out of position.

On fill and cap passes, .035 in or .045 in diameter gas-shielded flux-cored wires with an AWS E71T-1 classification are appropriate for welding smaller diameter pipe. These wires not only offer reliable out-of-position welding capabilities, including good weld puddle control, but they also provide excellent bead appearance and slag release. Wires such as AWS E71T-1/T-9/T-12M/J, E81T1-B2C H4/-B2M H4 and E91T1-B3C H4/-B3M H4 help improve productivity compared to stick electrodes.

WELDING IN 5G ON LARGER DIAMETER PIPE
Similar advantages can be found with tubular wires when welding on larger diameter pipe, such as that used for onshore transmission pipelines. This pipe averages 24 in or more in diameter, with some distribution lines falling in the 12 in to 16 in diameter range. These pipes typically require welding in the 5G position (since the pipe is already put in place in the field), and in many cases today, the material is high strength – 80 ksi or greater. For that reason, it requires filler metals that can provide low hydrogen weld deposits capable of mitigating weld cracking.

As with tubular wires for smaller diameter pipe, wires for pipeline projects need to have good all-position welding capabilities. They also need to have good toughness properties, since much of the material is often subject to temperature extremes.

Gas-shielded flux-cored wires for larger diameter pipe have proven to be an effective alternative to stick welding for gaining productivity on pipeline applications. These wires are often used in .045 in diameters on automated and semi-automatic pipe applications and feature a rutile (or T-1) slag system that results in an easy-to-remove slag and low spatter levels.

These features make the wires well-suited for welding on multi-pass applications; they require minimal cleanup between passes or after welding to increase productivity further. Many of them feature much lower hydrogen levels than many stick electrodes – as low as 4 ml per 100 g of weld metal (compared to 16 ml per 100 g or more for EXX10 cellulosic stick electrodes) – to help reduce cracking and downtime for rework.

Good gas-shielded flux-cored wires (for standard and high-strength pipe) are those classified as AWS E71T-1/T-9/12J H4, E101T1-GM and E111T1-GM H4. These wires produce a good bead appearance and provide reliable weld penetration to prevent issues like lack of fusion or under-bead cracking. As with wires for smaller diameter pipe, they can be used for fill and cap passes when combined with a modified short circuit welding process (as described previously) and a metal-cored wire for the root pass. The result is good gap bridging and better tolerance to high-low misalignment often found in pipeline applications. The combination of a modified short circuit process and metal-cored wire root pass also eliminates the need for a “hot pass” required with stick electrodes, thereby speeding welding time.

Self-shielded flux-cored wires are also an option for increasing productivity on out-of-position pipe welding. These wires are formulated specifically to weld vertical down and feature low hydrogen levels (around 8 ml per 100 g of weld metal). These wires offer high impact strengths at low temperature and resist cracking, helping to minimize rework time and maximize productivity. Because they require no shielding gas, they are also easier and quicker to set up on the jobsite.

Good options for out-of-position pipe welding include AWS E71T8-Ni1J H8 and E81T8-Ni2J H8. Both wires are typically used in 1/16 in or 5/64 in diameters when welding on larger diameter pipe and offer an easy-to-remove slag that lessens interpass cleaning.

FINAL CONSIDERATIONS
Making the change to a tubular wire for out-of-position pipe welding applications can yield significant productivity gains. As with any modification in the welding operation, however, it is important to evaluate every factor before proceeding. Working with a knowledgeable welding distributor is a good first step in the process. It can help ensure that the chosen wires best match the base material for the application and that an already difficult task doesn’t become more so.

Keith Packard is the segment manager for the Power Generation and Refineries Group and Derick Railling is a product manager for the Global Onshore Pipeline Group, Hobart Brothers Company, 101 Trade Square East, Troy, OH 45373, 937-332-4000, [email protected], www.hobartbrothers.com.