On The Job

New induction heating equipment for the jobsite reduces costs and labor needs

YOU CAN ALSO LISTEN TO THIS ARTICLE

To reduce the chance of defects and help ensure compliance to welding codes and standards, jobsites across many industries, including construction, pipeline and shipbuilding, often require preheat and bakeout processes as part of their welding operations. 

Preheating involves heating the area surrounding the weld joint to a specific temperature prior to welding.This lowers the cooling rate of the weld and prevents hydrogen buildup and the potential for cracking. Bakeout is performed after the welding process; it eliminates hydrogen that may have been introduced during welding. 

Miller’s ArcReach heater features air-cooled cables available in lengths of 30 ft., 50 ft. and 80 ft.

“Certain codes and standards require preheating to a certain temperature so it’s driven by what you are welding and what codes you are following,” says Danny Mortensen, business development manager, Miller Electric Mfg. LLC. “Typically, if the material is more than 3/4 in. thick, it will require preheating in most code applications, especially when you are doing pipeline, refinery or power plant type work and you are following an AWS code. And then based on where the joint is located, it could require a hydrogen bakeout or, if it’s a highly restrained joint, a stress-relieving bakeout.” 

While the welding code specifies a minimum or maximum temperature, it does not typically specify what preheating process to use. Traditionally, three methods have been used for preheating in the field – open flame, resistance and induction. All three can get the job done but they all come with challenges in addition to their benefits, depending on the application. 

Flame first 

With open flame preheating, operators burn a fuel gas to create a flame and, using a torch, apply the flame directly to the workpiece. Benefits of this method are that it is relatively inexpensive and easy to perform. But it has some serious drawbacks, including inefficiency. 

“With fuel, most of the heat is refracted off the workpiece,” Mortensen says. “Some estimates say 80 percent is refracted off. Also, the heating is inconsistent and some areas on the workpiece may be hotter than others.” 

Furthermore, because the material cools down as soon as the flame is removed, it is time-consuming for operators to heat and reheat the workpiece after breaks or shift changes.  

Flame heating poses the obvious safety risks of skin burns and fires. “The challenge is you have an open flame being waved around,” Mortensen says. “Also, there isn’t a visual indicator that the workpiece has been heated so there are burn risks. The surrounding area also gets hot and affects the operator comfort level. Finally, flame heating is just plain noisy.” 

Resistance method 

With resistance preheating, ceramic pads with embedded resistance heating wire provide the heat. Like a blanket, they are wrapped around a workpiece and the heat is driven into the material being heated. It produces more uniform, consistent heat than flame heating. 

However, resistance heating is not necessarily labor-friendly. It can take hours to set up the equipment and then bring the weld joint to temperature. Also, the insulation needed for resistance heating is considered hazardous waste and proper disposal is required. 

Miller’s ArcReach heating system is a small, rugged 8-kW portable unit weighing 43 lbs. that allows preheat and bakeout applications up to temperatures of 600 degrees F.

“The ceramic pads, or the ‘chicklets’ as we call them in the industry, do wear out and break and need to be replaced,” Mortensen says. “They can result in hot and cold spots that cause inconsistency in the heating. You might have a dead area on the blanket and not know it because it’s still heating up overall.  

“You have environmental challenges, as well,” he adds. “If the operator needs to bring a workpiece up to 500 degrees F, they have to work next to that piece and it’s uncomfortable.” 

Induction heating 

The most efficient preheating method is induction heating whereby electro-magnetic fields generate eddy currents within the workpiece, heating it from within. Accessories, such as cables or blankets, placed on or near the workpiece generate the magnetic fields. They provide continuous controlled heat wherever they are located. 

Advantages include a time to temperature that is relatively quick compared to flame and resistance heating and is easy to set up. Operators simply attach the heating accessory to the workpiece and connect the power source and thermocouples and they are ready to heat. 

Induction heating provides a fast time to temperature, consistent quality and an even heat zone controlled throughout.

“You don’t have the heat losses (via refraction) like you do with resistance and flame heating,” Mortensen says. “Also, you have uniformity. The AC currents are going through the coils so you don’t have a hot spot here and a cold spot there. You get uniform heating wherever the coils are. And, because the accessories don’t get hot, operators can move them almost immediately to the next joint without cool-down time. 

“The welder doesn’t have the same risk factor or discomfort level with having a 400-degree-F chicklet or flame close by while they are working,” he notes. 

Bakeout and stress 

Unlike flame heating, resistant and induction heating can be used for bakeout and post-weld heat treatment. Mortensen explains bakeout this way. 

“Say your soak temperature is 400 degrees F and you have a 50-degree/hour ramp down maximum. After the pipe reaches the temperature for the required time, then you can start reducing the temperature over time to allow hydrogen to escape the weld. In some codes, you can cool down by 50 degrees/hour maximum, some you can drop 20 degrees/hour until you reach, say 100, degrees. 

“Post-weld heat treatment is taking the material up to almost Curie point to relieve stresses from welding,” he continues. “Post-weld treatment is typically performed after the weld has cooled down completely and the stresses have settled. Independent contractors are usually hired to perform the post-weld heat treatment.” 

A unique unit 

Unlike other general induction heating equipment, Miller has designed a heating system specifically for welding applications on the jobsite. The ArcReach heating system, which operates with ArcReach welding power sources, is a small, rugged 8-kW portable unit weighing 43 lbs. It allows preheat and bakeout applications up to temperatures of 600 degrees F. 

“One of the benefits of the ArcReach heater is the preheating and bakeout are intended to be self-performed by the welder,” Mortensen says. “They can use the same power source to do the preheating and then the welding. It’s better than relying on heating contractors and waiting on timelines from other people. 

Example of preheating a large weldment with the ArcReach heater system.

“The consistency allows you to conform to your codes and standards and, according to Miller studies, an ArcReach heater will pay for itself in about 11 weld joints,” he adds. 

The heater uses air-cooled cables available in lengths of 30 ft., 50 ft. and 80 ft. The air-cooled quick wrap can be used on pipes up to 10 in. in diameter and down to 1 1/2 in. in diameter. The maximum distance the heater can be away from the power source is 200 ft. one way or 400 ft. in a loop.  

Welders can set a temperature set point and the heater will maintain that temperature. It prominently shows the temperature on its control panel. The system also allows for automatic temperature documentation.  

The ArcReach heater can’t be used for post-weld heat treatment but Miller’s ProHeat is a versatile system that maxes out at 35 kW for preheating, bakeout and post-weld heat treatment up to 1,450 degrees F. Non-ferrous metals such as aluminum can’t be preheated using an ArcReach heater. 

Induction heating delivers many benefits that help operations optimize efficiency, decrease costs and reduce exposure to safety hazards compared to other methods for weld preheating. The ArcReach heater provides additional portability, ease of use and flexibility for preheating on the jobsite — so operations can take control of the weld preheating process and save time and money. 

Overall, Mortensen believes induction heating technology is ahead of industry and codes and standards.

Watch the video to learn how the ArcReach heating system allows users to control costs, stay on schedule and create safer jobsites.

“As we begin to see more alloy and high-strength steels coming out to meet the challenges and demands of industry, we are going to see more requirements for preheating and post-weld heat treatments,” Mortensen concludes. “We are going to see the higher scrutiny of those processes as well because the weldability of high-strength steel can be a challenge at times. Everything we can do to maintain the integrity of that weld we are going to do.”

Miller Electric Mfg. LLC

Get industry news first
Subscribe to our magazines
Your favorite
magazines
under one roof