November 15, 2001
This article discusses using induction heating for preheating and (postheating) stress relief of welds. It focuses on what this technology is, how it works, and how it can be used in an industrial setting. This article also gives several real-life examples of how the technology has been used in actual applications.
Although many industries have used induction heating for decades (see Sidebar), it is a newcomer to industrial and construction applications involving welding. Some companies with welding-intensive operations now use induction heating for preheating before welding and stress relieving (i.e., postheating) after welding.
Induction heating systems use noncontact heating. They induce heat electromagnetically rather than using a heating element in contact with a part to conduct heat, as does resistance heating. Induction heating acts more like a microwave oven — the appliance remains cool while the food cooks from within.
In an industrial example of induction heating, heat is induced in the part by placing it in a high-frequency magnetic field. The magnetic field creates eddy currents inside the part, exciting the part's molecules and generating heat. Because heating occurs slightly below the metal surface, no heat is wasted.
Induction heating's similarity to resistance heating is that conduction is required to heat through the section or part. The only difference is the source of heat and the temperatures of the tool. The induction process heats within the part, and the resistance process heats on the surface of the part. The depth of heating depends on the frequency. High-frequency (e.g., 50 kHz) heats close to the surface, while low-frequency (e.g., 60 Hz) penetrates deeper into the part, placing the heating source up to 3 mm deep, which allows heating of thicker parts. The induction coil does not heat up because the conductor is large for the current being carried. In other words, the coil does not need to heat up to heat the workpiece.
Induction heating systems can be air- or liquid-cooled, depending on application requirements. A key component common to both systems is the induction coil used to generate heat within the part.
Air-cooled System. A typical air-cooled system consists of a power source (5 kW or 25 kW), induction blanket, and associated cables. The induction blanket consists of an induction coil surrounded by insulation and sewn into a high-temperature, replaceable Kevlar® sleeve.
This type of induction system can include a controller to monitor and automatically control temperature. A system not equipped with a controller requires the use of a temperature indicator. The system also could include a remote on-off switch. Air-cooled systems can be used for applications up to 400 degrees F, designating it as a preheat-only system.
Liquid-cooled System. Because liquid cools more efficiently than air, this type of induction heating system is suitable for applications requiring higher temperatures, such as high-temperature preheating and stress relieving. The principal differences from an air-cooled system are the addition of a water cooler and the use of a flexible, liquid-cooled hose that houses the induction coil. Liquid-cooled systems also generally use a temperature controller and built-in temperature recorder, particularly important components in stress-relieving applications.
The typical stress-relieving procedure requires a step to 600 to 800 degrees F, followed by a ramp or controlled temperature rise to a soak temperature of approximately 1,250 degrees. After a hold time, the part is control-cooled to between 600 and 800 degrees. The temperature recorder collects data on the part's actual temperature profile based on a thermocouple input, a quality assurance requirement for stress-relieving applications. The type of work and the applicable code determine the actual procedure.
Induction heating offers numerous benefits, including good heat uniformity and quality, reduced cycle time, and long-lasting consumables. Induction heating is also safe, reliable, easy to use, power-efficient, and versatile.
Uniformity and Quality. Induction heating is not particularly sensitive to coil placement or spacing. Generally, the coils should be spaced evenly and centered on the weld joint. On systems so equipped, a temperature controller can establish the power requirement in an analog fashion, providing just enough power to maintain the temperature profile. The power source provides power during the entire process.
Cycle Time. The induction method of preheating and stress relieving provides relatively quick time-to-temperature. On thicker applications, such as high-pressure steam lines, induction heating can slash two hours from cycle time. It is possible to reduce cycle time from the control temperature to soak temperature.
Consumables. The insulation used in induction heating is easy to attach to workpieces and can be reused many times. In addition, induction coils are robust and do not require fragile wire or ceramic materials. Also, because the induction coils and connectors do not operate at high temperatures, they are not subject to degradation.
Ease of Use. A major benefit of induction preheating and stress relieving is its simplicity. Insulation and cables are simple to install, usually taking less than 15 minutes. In some cases, how to use the induction equipment can be taught in one day.
Power Efficiency. The inverter power source is 92 percent efficient, a critical advantage in an era of skyrocketing energy costs. Additionally, the induction heating process is more than 80 percent efficient. Regarding power input, the induction process requires only a 40-amp line for 25 kW of power.
Safety. Preheating and stress relieving through the induction method is worker-friendly. Induction heating does not require hot heating elements and connectors. Very little airborne particulate is associated with the insulation blankets, and the insulation itself is not exposed to temperatures higher than 1,800 degrees, which can cause insulation to break down into dust that workers may inhale.
Reliability. One of the most important factors impacting productivity in stress relieving is an uninterrupted cycle. In most instances cycle interruption means the heat treat will need to be rerun, which is significant when a thermal cycle can take a day to complete. The induction heating system components make cycle interruptions unlikely. The cabling for induction is simple, making it less likely to fail. Also, no contactors are used to control the heat input to the part.
Versatility. In addition to using induction heating systems to preheat and stress relieve pipe, users have adapted the process for weldolets, elbows, valves, and other parts. One of the aspects of induction heating that makes it attractive for complex shapes is the ability to adjust the coils during the heating process to accommodate unique parts and heat sinks. The operator can start the process, determine the effects of the heating process in real time, and modify the coil position to change the result. The induction cables can be moved without waiting for air cooling at the end of the cycle.
Induction heating maintains the desired preheat temperature throughout the welding process while several welders work to complete the girth welds.
This technology has proved itself on a number of projects, including oil and gas pipelines, heavy equipment construction, and maintenance and repair of mining equipment.
Oil Pipeline. A North American oil pipeline maintenance operation needed to heat pipe before welding encirclement repair sleeves or STOPPLE® fittings to the pipeline's 48-in. girth. While workers could make many repairs without having to stop oil flow or drain it from the pipe, the presence of the crude itself hampered welding efficiency because the flowing oil absorbed the heat. Propane torches required constant interruption of welding to maintain heat, and resistance heating — while providing continuous heat — often could not meet required weld temperatures.
The maintenance company turned to induction heating as a solution (see Figure 1). Workers used two 25-kW systems with parallel blankets to obtain a preheat temperature of 125 degrees on encirclement sleeve repairs. As a result, they reduced cycle time from eight to 12 hours to four hours per girth weld.
Preheating for a STOPPLE fitting (a T junction with valve) repair was even more challenging because of the fitting's greater wall thickness. With induction heating, however, the company used four 25-kW systems with a paralleled blanket setup. They used two systems on each side of the T. One system was used on the main line to preheat the oil, and the second was used to preheat the T at the circumferential weld joint. The preheat temperature was 125 degrees. This reduced the weld time from 12 to 18 hours to seven hours per girth weld.
The induction preheat blanket maintains the temperature of the bucket edge during the attachment of teeth.
Natural Gas Pipeline. A natural gas pipeline construction project entailed building a 36-in.-diameter, 0.633-in.-thick pipeline from Alberta, Canada, to Chicago. On one stretch of this pipeline, the welding contractor used two 25-kW power sources mounted on a tractor with the induction blankets attached to booms for speed and convenience. The power sources preheated both sides of the pipe joint. Critical to this process were speed and reliable temperature control. As alloy content increases in materials to reduce weight and weld time, and to increase part life, controlling preheat temperatures becomes more critical. This induction heating application it required less than three minutes to obtain the 250-degree preheat temperature.
Heavy Equipment. A heavy equipment manufacturer often welded adapter teeth onto its loader bucket edges. The tack-welded assembly had been moved back and forth to a large furnace, requiring the welding operator to wait while the part was reheated repeatedly. The manufacturer opted to try induction heating to preheat the assembly to prevent movement of the product (see Figure 2).
The material was 4 in. thick with a high required preheat temperature because of alloy content. Customized induction blankets were developed to meet the application requirements. The insulation and coil design provided the added benefit of shielding the operator from the part's radiant heat. Overall, operations were considerably more efficient, reducing welding time and maintaining temperature throughout the welding process.
Mining Equipment. A mine had been experiencing cold-cracking problems and preheating inefficiency using propane heaters in its repair operations of mining equipment. Welding operators had to remove a conventional insulating blanket from the thick part frequently to apply heat and keep the part at the correct temperature.
The simplicity of wrapping the induction blankets around complex shapes, such as at this natural gas power plant, can reduce heat-treating time.
The mine opted to try induction heating using flat, air-cooled blankets to preheat the parts before welding. The induction process applied heat to the part quickly. It also could be used continuously during the welding process. Weld repair time was reduced by 50 percent. In addition, the power source was equipped with a temperature controller to keep the part at the target temperature. This almost eliminated rework caused by cold cracking. The company reported an annual savings of $80,000.
On a typical 16-in. weldolet with a 2-in. wall thickness, induction heating was able to shave two hours off the time-to-temperature (600 degrees) and another hour to reach soak temperature (600 degrees to 1,350 degrees) for stress relieving. The simplicity of wrapping the induction blankets around complex shapes further reduced the time to perform the heat treat (see Figure 3). It took the fitters 15 minutes to wrap a joint that previously had required two workers two hours to prepare.