Welding Preheat for Tube and Pipe on the Jobsite

Critical pipe welding applications often require preheating and possibly postweld heat treatment to reduce the chances of cracking. Learning different methods and proper application of welding preheat can help save time and money.

AWS Publications | April 22, 2021 | Processes
Welding Digest ►  Welding Preheat for Tube and Pipe on the Jobsite

Critical pipe welding applications often require preheating and possibly postweld heat treatment to reduce the chances of cracking. Learning different methods and proper application of welding preheat can help save time and money.

Improper preheating will likely increase the occurrence of cracking and other weld problems that can result in costly rework — or failed welds.

An American Welding Society (AWS) resource used for reference when preheating in the field is AWS D10.10, Recommended Practices for Local Heating of Welds in Piping and Tubing. Using this document combined with real-world examples, this article discusses the primary reasons preheat is required and reviews the three heating methods typically used on jobsites today: induction, open flame, and resistance.

 

Why Welds Are Preheated

 

As detailed in D10.10, there are three reasons for welding preheat in pipe and tube applications:

-To prevent hydrogen cracking in the weld metal or the heat-affected zone. This is accomplished through preheat by driving off moisture prior to the start of welding, reducing the weld cooling rate, and increasing the rate of hydrogen diffusion;

-For the redistribution of solidification stresses, which results when there is slower cooling that allows a longer time for reduction of internal stresses to occur; and

-To reduce the cooling rate in materials that form hard or brittle microstructural constituents when cooled too rapidly from welding temperatures.

 

Methods of Welding Preheat

 

The material type and thickness, project timeline and budget, and available personnel and expertise are factors to consider when choosing a preheating method for pipe welding on a jobsite. D10.10 discusses several heating methods, but the three seen most commonly on jobsites are induction, flame, and resistance.

 

Induction Heating

 

This technology has been used for decades but is growing in popularity with portable equipment now available specifically for welding applications. Induction uses electromagnetic fields that generate eddy currents within the base metal, heating it internally from within — Fig. 1. Induction accessories, such as cables or blankets, generate the magnetic field and are placed on or near the part to provide continuous controlled and even heat wherever the accessories are placed.

Fig 1 Induction infrared tempFig. 1 — Induction heating produces a uniform heated area, making it easy to achieve and stay within even a strict temperature window.

 

D10.10 notes numerous advantages of induction heating (Fig. 2), including the following:

 

-High heating rates due to high power density;

-Rapid through-thickness heating because induction doesn’t rely solely upon conduction;

-Ability to heat a narrow band adjacent to regions that have temperature restrictions;

-More even heating that easily avoids local hot spots;

-Heating coils that offer relatively long life and are less likely to fail during heating compared to equipment in other systems; and

-Overall heating efficiency, in terms of frequency conversion and coil efficiency, can be greater than 90% with correct output circuit design.

Fig 2 Induction heating 1Fig. 2 — Some of the advantages of induction heating noted by D10.10 include more even heating that avoids local hot spots, good overall heating efficiency, and rapid through-thickness heating.

 

However, there are some disadvantages to induction, including the following:

-The equipment’s initial cost can be higher than other heating sources, but for those who heat on a regular basis, many induction systems pay for themselves in three to nine months with the savings generated.

-Some equipment can be larger and less portable than other options. Operators have used a skid or crane to move larger machines. However, portability may be less of an issue now with new inverter equipment available. Miller has had a 35-kW system available for years and has introduced a small 8-kW portable unit weighing 43 lb, delivering flexibility and portability for jobsite preheating. The new ArcReach® heating systems are powered by ArcReach-enabled welding power sources, which in many cases are already on the jobsite. New tools available are air cooled, faster to setup, and heat a wide variety of pipe sizes — Fig. 3.

Fig 3 AirCooledRope8Fig. 3 — Air-cooled cables for induction heating deliver flexibility and save time in jobsite weld preheating, eliminating the need for a cooler, insulation, or liquid-cooled cable connections. The flexible cables can be bent or shaped to best fit the pipe or part to be welded.

 

Open Flame

 

With flame heating, operators burn a fuel gas using a torch, sometimes with compressed air, and apply the flame directly to the part — Fig. 4. It remains the most used heating method seen on the jobsite, mainly because it is so familiar in the industry and the cost to start using it is relatively low. It will likely remain the prevalent method when operations are only required to preheat rarely or occasionally.

Fig 4 shutterstock_1498913279Fig. 4 — A welder uses flame heating on a pipeline before welding.

 

However, D10.10 points out that flame heating is more of an art than a science, and the method should be applied with care and only by an experienced operator, as weldments can be severely damaged by improperly applied flame heating. Uneven heating is fairly common with this method. The amount and concentration of heat transferred to the weldment depends on several factors, including the amount of fuel consumed, completeness of combustion, adjustment of the flame, distance between the flame and weldment, manipulation of the flame, and control of heat losses to the atmosphere.

 

D10.10 lists flame heating advantages as follows:

-Low cost; and

-Jobsite portability.

 

There are also disadvantages of flame heating cited in D10.10:

-Minimal precision and repeatability;

-Risk of uneven heating;

-The great amount of operator skill required; and

-Risk of damaging the material when improperly applied.

 

Additional disadvantages heard from users in the field are the extended time needed to heat a part, dealing with the supply and handling of the fuel gas, and the obvious safety risks of working around open flame and toxic gas byproducts.

 

Resistance

 

With this method, electrically heated ceramic pads are placed on the base metal. The tiles transfer heat through radiant and conductive heat where the pads touch the part. This form of electrical heat has been around for decades and is a simple technology, sometimes described as using toaster elements to heat the part. Many heating contractors still use this method and have trained personnel to use it.

 

D10.10 notes the following advantages of resistance heating:

-Standard heaters can accommodate a wide variety of part sizes and geometrical configurations;

-Ability to maintain continuous and even heat;

-Ability to adjust temperatures quickly; and

-Ability for welders to work in relative comfort without the need to stop intermittently to raise preheat temperatures.

 

Disadvantages include heating elements that may burn out during treatment and the fact that inadequate work practices may create the possibility for an element to short itself out to the pipe, producing arching spots.

Additional observations about resistance heating’s disadvantages from users in the field include the following:

-Power sources are heavy and inefficient, requiring large power drops on the site.

-Every ceramic pad group requires a wire harness and thermocouple to power and control them. On some jobsites, wire harnesses are brought in by the truckload and strung across the site.

-Setup and teardown times are longer than with other methods.

-Occasional pad failures or outputs that are stuck on can cause cold or hot spots, so the pads require monitoring to prevent part damage.

-Safety concerns from users include burn hazards due to the hot interconnect leads to the pads, as well as shock hazards when ceramic beads break off the pads and connectors have exposed electrical wire and are not repaired when necessary.

 

Proper Way to Apply Heat

 

Once the heating method is chosen, users must apply heat in a way that brings the part to temperature properly. To meet code requirements, the welding procedure specification for the job will detail minimum and maximum preheat temperatures and necessary duration of preheating. Temperature requirements are typically based on the base material composition and thickness. Although the procedure usually defines the temperature and area to be covered, D10.10 helps users with setup of the chosen heating method, temperature monitoring, and insulation, if required.

During preheating, welders or welder’s helpers must monitor the material temperature between weld passes to ensure it stays within the required range. Depending on the heating method, this measurement can be done with crayons, thermocouples, infrared thermometers, or thermal imaging cameras. Temperature recorders can also be used to chart temperatures during preheating, and there might be a requirement for documentation.

 

Welding Preheat on the Jobsite

 

Many critical pipe welds completed in the field require preheat to help reduce the risk of cracking and the potential for weld failure. As detailed in D10.10, several methods are available for jobsite preheating, though some are less efficient and flexible than others. Knowing the pros and cons of each method and choosing the one best suited to a specific application — along with properly applying the heat — can help save time and money and deliver high-quality welds. Discuss heating applications with a local distributor or manufacturer’s representative to help determine which method is best for the specific part or weld.

 

This article was written by Al Sherrill (induction sales support manager, Miller Electric Mfg. LLC) for the American Welding Society.