Heat treatment process
Transmission pipeline welding often requires preheating of the weld zone to ensure that the joint achieves its required strength and hardness. It also helps minimize the risk of delayed hydrogen-induced cracking, which is a significant concern that impacts weld quality and integrity in transmission pipelines.
There are several heating methods available when preheating and stress relieving of parts is necessary in pipeline welding. One commonly used method is open flame. However, it presents some challenges that can negatively impact weld quality and integrity.
Another option to consider is induction heating, which offers numerous benefits for weld quality, efficiency and safety not found with other heating methods.
Induction provides greater consistency in heating and eliminates a potential hydrogen source that is a byproduct of open-flame heating. These advantages make induction a good solution to help transmission pipeline contractors meet code and quality requirements — both for new pipeline construction and the repair and maintenance of existing in-service transmission lines.
Induction on transmission pipelines
Induction heating has been successfully used in transmission pipeline applications with high-strength steels for many years.
These systems quickly heat conductive metals by inducing current into the part. Induction does not rely on a heating element or flame to transfer heat. Instead, an alternating current passes through the heating device, creating a magnetic field around it. As the magnetic field passes through the conductive workpiece, it creates localized eddy currents within the part. The resistance of the metal fights against the flow of the eddy currents, generating heat in the part. The part becomes its own heating element, heating from within. This makes induction very efficient since little heat is lost in the process.
Applications that typically require hours to heat can be completed in minutes by utilizing induction heating and the different liquid-cooled and air-cooled options. Operations can pair induction heating system with various coil configurations to induce heat, depending on the part size and geometry.
When using open-flame heating, temperatures are typically monitored manually using temperature crayons, which do not provide the accuracy of induction. In comparison, induction systems use feedback from thermocouple probes for automatic and uniform temperature control.
Eliminating a hydrogen risk
One of the main quality challenges when using open flame for preheating pipe is that a byproduct of the process introduces a hydrogen risk.
A byproduct of burning any fuel in flame heating is water vapor. The moisture in water vapor can be a source of hydrogen in the weld that could result in hydrogen-induced cracking. Reducing the risk of trapping hydrogen in the weld is critical for achieving high-quality welds in pipeline applications.
Utilizing induction heating instead of open-flame heating eliminates that hydrogen risk by keeping moisture out of the process . This therefore helps improve weld quality and integrity to meet necessary code requirements.
Quick and consistent heating
Transmission pipeline applications typically have minimum and maximum temperature requirements for preheating determined by the welding procedures for that specific alloy of pipe. Staying within the temperature window is important for weld quality and to achieve the desired properties in the finished weld.
A typical minimum preheat temperature in pipeline welding is 250 degrees Fahrenheit. Maintaining a minimum temperature helps to eliminate any moisture that can develop, since pipes are generally stored outside, where it can be cold and damp.
Open flame often results in inconsistent heating throughout the part. It’s also harder for the operator to hold a specific temperature or ensure the temperature remains within the specified window. Falling below or going over the required temperature window can adversely impact weld quality.
In contrast, induction heating provides consistent and uniform heating throughout the part. Induction systems also make it easier to hold the temperature at a specific level and to constantly monitor heat. This ensures the pipe remains within the proper temperature window throughout the weld.
Induction heating also provides a much faster time to temperature, which is important when constructing new transmission pipelines. Speed is critical in these applications because operators may be trying to weld as many joints as possible per day. These jobs typically involve numerous weld stations along the right of way. The aim is to heat pipe along the right of way, then move on quickly to heat the next weld joint ahead of the weld station.
Induction technology for the jobsite
Induction heating isn’t limited to the shop — new technology allows it to be easily used on the jobsite.
ArcReach® Heating Systems are the newest induction heating solutions from Miller. Designed for jobsite weld preheating in construction and pipe applications, this system deliver weld preheating that’s easy to use and significantly faster, safer and less expensive than open-flame or resistance methods.
With ArcReach Heating Systems, contractors can insource weld joint preheating in the field — improving productivity and profitability on the jobsite. ArcReach Heating Systems run off existing welding power sources, further enhancing convenience and lowering costs.
This technology can save several hours and hundreds of dollars per weld joint. Contractors using ArcReach Heating Systems can see a return on their investment by the 11th joint they preheat.
When welding operations hire specialized heat treat companies to perform preheating work, it can cost up to $2,000 per joint. Another option, resistance heating, requires installing expensive primary electrical power. The third solution is performing the preheating with flame heating, which consumes up to $50 per hour in propane, requires paying fire-watch personnel* and creates safety hazards. In addition, prepping and heating a weld joint using any of these methods can take hours.
By comparison, ArcReach Heating Systems allow companies to insource preheating using welding power sources already on site. The system can typically bring a joint to temperature in 20 minutes or less — saving significant time and money. An ArcReach Heating System includes an ArcReach Heater that is used with air-cooled cables or air-cooled quick wraps.
ArcReach Heaters are powered by select models of compatible ArcReach welding power sources, including all XMT® 350 FieldPro™ power sources, any Miller engine-driven welder/generator with ArcReach technology, and any Miller power source that can run a Smart Feeder. The DC power from the welding power source or engine drive converts into AC current to power the ArcReach Heater, which in turn energizes the air-cooled cables or quick wraps. Standard weld cables connect the compatible power source to the system, with a maximum distance of 250 feet.
Induction heating is faster, safer and more consistent compared to flame and resistance heating methods. ArcReach Heating Systems use electricity from heating tools placed on or around magnetic metal surfaces such as iron or steel to create currents in the metal. These currents pass through the workpiece and convert to heat — so the workpiece itself is the source of the heat, not the heating tools.
Preheating for in-service pipelines
While speed is not as critical in repair or maintenance projects on in-service transmission pipelines, induction heating systems offer numerous benefits for these applications as well.
In these applications, it’s difficult to properly heat the steel with an open flame because anything flowing through the pipe has a heat sink effect, which pulls the heat from the steel. Once the operator heats the area and pulls the torch away, the steel can cool down within seconds.
In these applications, it’s often not practical or feasible to shut down the pipeline while completing repair or maintenance work.
Induction heating allows the operator to maintain the necessary level of preheat in order to slow down the cooling of the weld puddle and minimize the risk of cold cracking in the weld. This can be done without stopping the flow of oil or natural gas through the pipe.
Liquid-cooled induction coils work well for jobs on in-service transmission pipelines. Operators can arrange the coils to accommodate geometry transitions common with the split tees used for hot taps and pipe-to-valve welds. Results from testing funded by the Pipeline Research Council International (PRCI) supports the use of induction heating for welding projects on in-service pipelines to reduce risks of hydrogen cracking.
Additional benefits for weld quality
Induction heating also offers benefits when considering several other factors involved in transmission pipeline welding.
- Coated pipes: When completing transmission pipeline repair projects that require cutouts and tie-ins, there is often a need to reapply coating to protect the pipe from corrosion. Many materials used in the industry will not cure properly if the steel is not warmed up to a certain temperature. Operators can use air-cooled induction blankets to heat up zones of the pipe, creating a wide area of heated steel so the epoxy coating can be applied with the pipe at the temperature necessary for proper curing.
- Pipe segments: Some applications require welding pipe segments to valves. In these situations, induction heating can minimize the risk of damaging seals and valves, — a common issue with flame heating. Valve ends are typically much thicker steel compared to the pipe, which increases the risk of damaging the valve seals when using flame. Induction provides a much more localized heat that operators can direct to a specific area of the part, preventing damage. Because of these benefits, several major pipeline companies specify the use of induction heating for preheating in applications that involve a pipe segment or valve.
When quality is critical
More transmission pipeline companies are transitioning to induction, successfully using the process for preheating in new construction projects and repair and maintenance work.
Among its numerous benefits, induction provides great consistency in heating and faster time to temperature of the part. It also eliminates a potential hydrogen source that is a byproduct of heating with open flame. In addition, induction offers benefits for safety on the jobsite.
The result is a preheating process that contributes to improved weld quality and integrity — a critical factor in helping pipeline contractors achieve success.
To learn more about induction heating, visit https://www.millerwelds.com/products/induction-heating-systems.