Many welding applications require a predetermined amount of base material preheat before welding can commence. There are several reasons for this, but the most common reason is to minimize the risk of cold cracking and delayed hydrogen-induced cracking in the weld zone.
Preventing cracking becomes increasingly important in the welding of high-strength steels and alloys frequently used in energy infrastructure projects, including transmission pipelines. Global pipeline contractors are experiencing firsthand the benefits of high-strength steels (X-70 and above) in pipeline construction — lower cost-per-foot, less weight, reduced transportation costs, increased strength and pressure tolerances, and the ability to work with thinner wall thicknesses.
These benefits put high-strength steels in high demand in pipeline construction, but welding the materials does pose some challenges, as well. This includes the need for the weld to withstand the impact of thermal expansion and contraction, frost and other environmental loadings common in extreme landscapes that can lead
Weather is an additional consideration in the equation — projects requiring preheat in cold climates pose additional quality and safety challenges during the long winter season. Such challenges include temperature maintenance on cold and/or windy days; temperature uniformity; and safety outlined in Disadvantages of Open Flame Heating section later in this paper.
Proper and consistent preheating is one important step to help overcome these challenges.
Understanding Hydrogen-Induced Cracking
Hydrogen in outdoor work environments is difficult to avoid. Nearly all organic compounds contain hydrogen — everything from lubricants and oils to naturally occurring substances in the field and moisture in the atmosphere. Hydrogen-induced cracking is one of the greatest threats to the integrity of transmission pipeline welding applications in cold weather (as well as many other welding applications).
Hydrogen-induced cracking can be slow to take effect, and may appear hours or days after the weld has been completed, which can result in costly repairs and downtime.
Hydrogen ions are extremely small and are highly mobile, and can easily diffuse out of the weld zone and coalesce along discontinuities that are present in the microstructure. The hydrogen ions may recombine to form hydrogen gas, further stressing the microstructure. Those collections, or pockets, of hydrogen eventually build stresses that can lead to cracking.
In general, the susceptibility to hydrogen-induced cracking increases as the strength of the base metal increases. Improving pre- and post-weld heating and maintaining interpass temperatures are key ways to reduce the amount of diffusible hydrogen and reduce or eliminate residual stresses in pipeline welding.
Hydrogen optimally diffuses from the steels used in transmission pipeline construction at temperatures at or above 250 degrees Fahrenheit. The rapid heating and cooling of the base metal that takes place during welding puts stress into the part and can spur the creation of hard, strong grain structures that are susceptible to hydrogen embrittlement.
Rapid cooling provides less opportunity for hydrogen to diffuse out of the weld and heat-affected zone, and can lead to cracking.
Maintaining required preheat and interpass temperatures is critical, both for producing a softer, less crack-susceptible microstructure, and for allowing hydrogen to diffuse out of the weld metal and heat affected zone (HAZ).
There are several methods by which steel can be preheated prior to welding. Some heating methods are impractical for remote infrastructure projects, due to the transportation needs or other requirements of the method. These include the use of ovens, infrared heating, and in many cases, resistance heating.
The most common preheating method used in pipeline construction has been, and remains, open flame heating. It offers the advantage of ease of use, but comes with numerous disadvantages as well. Alternate heating methods, such as induction heating, are becoming a viable option for many pipeline projects thanks to the safety, productivity and consistency benefits they offer, and advancements in the technology that make them easier to use, even when portability is needed on jobsites.
Disadvantages of Open Flame Heating
Flame heating methods have several disadvantages, especially when used in cold weather.
In nearly all applications, open flame heating is very inefficient, namely because the heat is not uniformly distributed. Most of the energy in an open flame goes towards heating air surrounding the targeted area instead of the part itself, which can lead to inconsistent heating and hot and cold spots in the part.
Similarly, if the joint is allowed to fall below the minimum interpass temperature, welding must be stopped and the joint must be reheated by applying the torch to the joint.
Burning any fuel produces byproducts such as CO (carbon monoxide) and water vapor, which is a potential contributor of unwanted hydrogen in the weld. This is particularly an issue in cold climates where water vapor condenses on very cold steel not far from the weld zone. It freezes and accumulates, then eventually melts as the steel warms up. This situation creates a source of water (and hence hydrogen), which can flow into the weld zone while welding.
Reliability is reduced as the temperature falls. Propane torches become nearly impossible to ignite and maintain when temperatures fall below -40 degrees Celsius. At these temperatures, storage tanks/cylinders need to be warmed for proper operation.
There are safety issues anytime anyone works with an open flame. Workers must be careful not to ignite flammable sources that may also be on the jobsite. In cold weather, fuel lines and valves become stiff and can break, causing an increased risk of propane fires.
Logistic issues include the transportation and storage of large propane tanks in remote areas.
The use of induction heating eliminates many of the disadvantages and safety issues inherent with open flame heating methods. It is recommended for optimal hydrogen diffusion and uniform heating throughout the part.
Induction is a non-contact method of electromagnetically heating conductive/magnetic materials. It produces no flame, therefore eliminating the water vapor issues and hydrogen contribution from heating with a torch. It is faster, more efficient, much more uniform and very easy to use.
Induction heating has been around for decades, but was primarily used for production heating of parts in a shop environment using coils custom-made for that particular geometry. The machines were large for their output and were expensive.
Electronic power sources using inverter technology now have become much smaller and more efficient, making
induction heating better suited for jobsite use.
Flexible coils allow the same machine to be used for a variety of applications instead of having numerous fixed geometry coil configurations. This flexibility and ease of use has been embraced by job shops that need to frequently heat a variety of parts, as well as by contractors for on-site heating for projects outside.
How Induction Works
As mentioned earlier, induction heating is a noncontact method of heating. It creates heat within the part by causing localized eddy currents underneath the coils in response to the changing magnetic field.
The resistance to this eddy current flow within the steel causes the steel to heat up quickly. The high temperature application shown in the image below (left) shows the uniformity of this heat directly beneath the induction heating coil. This same heating uniformity exists at lower temperature requirements, such as preheat applications prior to welding.
The induction coils themselves do not get hot, so welding operators are able to work next to the induction heating coils while they are maintaining the desired minimum preheat requirements. They would not be able to do this with open flame heating due to the safety risks it would pose.
Heat is induced in the part by placing it in an alternating magnetic field created by liquid- or aid-cooled induction heating cables. The induction cables are wrapped around the part, with the eddy currents generated inside the part creating the heat. This eliminates the need to store and handle explosive gases that are required for open flame heating.
Induction heating maintains the proper preheat during the welding process and provides uniform heating to prevent the formation of localized hot or cold spots. This is particularly helpful in extremely cold environments where the surrounding cold air quickly pulls the heat away.
Automated recording devices also can be integrated into the system, which creates a permanent record showing that proper heating/cooling sequences were accomplished, which may be important for jobs that require review by welding inspectors and quality personnel.
To date, induction heating has been used worldwide to expedite preheating, ensure quality and establish greater safety on pipeline construction and repair jobs in cold temperature environments.
Projects in Russia include:
• South Stream
• Power of Siberia
Canadian projects include post-weld heat-treating (PWHT) a large tank application in Fort McMurray.
In Alaska, induction heating has been used to maintain the Trans-Alaska pipeline for many years, while other contractors have relied on the technology for applications on the North Slope.
As high-strength steels continue to be used more for pipeline construction, including in cold temperature environments, welding and heating requirements continue to change as well. Induction heating is relatively easy to learn and use, and can offer great improvements in productivity, quality, reliability, efficiency and safety — particularly over open flame methods.
Applying preheat via induction heating also eliminates the production of water vapor associated with open
flame heating, and with it the potential for generating hydrogen contamination that can lead to hydrogen-induced cracking.
To date, induction heating has proven successful on even the harshest jobsites in Canada and Siberia, where temperatures regularly reach as low as minus 40 degrees Celsius.
Steve Latvis is an induction heating applications specialist for the Global Onshore Pipeline Group at Miller Electric Mfg. Co. in Appleton, Wisconsin.