As the demands for energy continue to increase worldwide, fabricators serving this industry will be challenged to remain competitive by changing from the stick welding process to a gas-shielded flux-cored welding process. Before making this sort of switch, they must carefully assess their quality needs and available labor or training ability.
Most fabricators regularly look for ways to increase their productivity and reduce costs. As the demands for energy continue to increase worldwide, companies serving this industry will be especially challenged to meet those goals if they are to remain competitive. One option to help may be to change from the stick welding (SMAW) process widely accepted in this industry to a gas-shielded flux-cored (FCAW) welding process.
Even though stick electrodes provide the chemical and mechanical properties necessary to weld the chrome-moly steel tubing and pipe that is prevalent in many energy applications (e.g., pressure vessels, boilers, etc.), the stick welding process is notoriously slow. Advancements in gas-shielded flux-cored wires, however, are now making these filler metals a more viable option for gaining the weld quality necessary on such critical jobs – and completing them much faster.
There are two main types of gas-shielded flux-cored wires for chrome-moly applications: T-1 and T-5. As a general rule, T-1 wires (those with a rutile or acidic slag system) have an easy-to-remove slag and generate low amounts of spatter, making them a good option for welding on multi-pass applications. They do not require as much time to clean after passes, so more time can be spent welding instead.
They contain 1.25 to 10.5 percent chrome and 0.5 to 1 percent molybdenum and are divided into multiple product classes (B2, B3, B6, B8 and B9) according to the levels of each. Many T-1 flux-cored wires can weld out of position, which is helpful for welding tube and pipe applications within an existing piping system. Typical applications that can benefit from these wires include high temperature, corrosion-resistant steam piping and heat exchangers.
T-1 flux-cored wires are known for being very welder-friendly because of their stable arc and smooth welding characteristics. They are also relatively simple to train welding operators to use, so they can be a good choice for companies who are adding new employees to their staff to meet increasing production demands. They are also now available in low hydrogen versions (as little as 4 ml of diffusible hydrogen per 100 g of weld metal), which helps reduce instances of hydrogen cracking that can lead to downtime for rework. Adding to that benefit, these wires also offer high deposition rates and efficiency compared to stick electrodes to help improve productivity.
Another flux-cored wire option companies may want to consider for their chrome-moly applications are those with a basic slag system, also called T-5 wires. These wires create welds with good toughness in high heat applications, making them a good choice for welding P21 and P22 chrome-moly pipe.
Historically, many welding operators have found T-5 wires for chrome-moly to be somewhat difficult to use and, until now, available versions have been limited to welding in flat and horizontal positions. In recent years, however, new versions with improved operating characteristics have become available. American Welding Society (AWS) E81T5-B2M H4 and E91T5-B3M H4 flux-cored wires are now options for welding out of position on chrome-moly, particularly vertical up and vertical down, and they are significantly more welder-friendly and easier to train with than T-5 wires of the past. Still, the wires do create a unique weld puddle that may take a bit of practice to master.
Instead of using straight CO2 like other T-5 wires, these newer wires operate with a mixture of Argon and CO2 to create a stable arc similar to that of a T-1 flux-cored wire. They offer much better slag coverage than older versions of T-5 wire, too, which helps ensure good quality welds. Also, that slag is much easier to remove than older versions. These newer T-5 wires require a power source capable of welding with a DC current on negative polarity (DCEN). The wires provide reliable low diffusible hydrogen weld deposits that are well shaped, uniform and smooth, so quality and aesthetic concerns are at a minimum.
Standard (older versions) of T-5 wires for welding chrome-moly are also still available and can similarly provide advantages over stick electrodes in terms of increasing productivity. These wires, however, are limited to welding in the flat position (due to their very fluid weld puddle) and they do tend to generate more spatter and have slag that is more difficult to remove than that created by newer versions of this wire or by T-1 wires. Still, because of the excellent toughness properties these wires provide, companies may consider using them as an alternative to stick welding, particularly for tube and pipe applications within a shop environment.
MAKING AN INFORMED DECISION
As with any type of filler metal, there are some limitations to using flux-cored wires for welding chrome-moly steel and companies should take the necessary precautions to protect against any issues. For example, T-1 wires tend to create welds with slightly higher oxygen content than other filler metals, so they do not have as great of toughness properties, particularly when exposed to cooler temperatures. For that reason, they should not be used in the cold start of a power plant or in applications subject to very low temperatures.
Similarly, if a company chooses to use either the older or newer versions of T-5 wires, it may take slightly more training to make welding operators proficient with them because of their unique weld puddle characteristics.
Companies should carefully assess their quality needs and available labor (or ability to train welding operators) before making the switch to a new welding process. Gas-shielded flux-cored wires for chrome-moly may be the answer to obtaining good quality welds and establishing a higher level of productivity compared to the stick welding process – but the process change needs to be made after careful assessment.
Also, because this process change will necessarily increase the volume of welding, companies need to assess the current ventilation in their welding areas to make sure they can safely accommodate the additional activity.
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