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Preheating can be critical to welding success

Preheating, a crucial step in many welding applications, slows the rate of cooling in a finished weld, lowers the amount of hydrogen in it, and reduces the risk of cracking.

A crucial step in many welding applications, preheating slows the rate of cooling in a finished weld, lowers the amount of hydrogen in it, and reduces the risk of cracking. Artificially introducing heat into the base metal — by means of an external heat source — adds a step to the welding process; however, it can save you time and money in the long term by reducing the potential for a failed weld that requires rework.

You have numerous methods for preheating the base material. Each has benefits and drawbacks. The best choice for a specific application depends on several factors, including any code requirements that may apply. Consider these tips and best practices that contribute to proper preheating and a high-quality weld.

Why Is Preheating Important?

Preheating minimizes the temperature difference between the welding arc and the base material. This benefits the weldment in several ways.

First, it helps to lessen shrinkage stresses that can lead to cracking and distortion. Because hot materials expand and cool ones contract, a large temperature variance between the molten weld pool and the relatively cool base material can result in internal stresses as the weldment tries to normalize those temperature differences. These internal stresses increase the risk of cracking and distortion.

Second, proper preheating helps to slow the cooling rate of the finished weld and reduce hardness in the heat-affected zone (HAZ), which creates a weld that is less brittle and more ductile. These characteristics are especially important for materials more susceptible to hardness at elevated temperatures, such as cast iron, medium- and high-carbon steel, or high- carbon-equivalency steel.

Slowing the cooling rate also allows hydrogen to escape the weld puddle as it hardens to help minimize cracking.

Last, preheating introduces the necessary heat into the weld area to ensure proper penetration. This benefits thick materials and those that conduct heat quickly. By preheating, you can use less heat in the welding arc and still achieve optimal penetration, because the base material starts out at an elevated temperature.

When Should You Preheat?

Preheating is especially important when welding:

  • Highly restrained weld joints.
  • Thick materials (the rule of thumb on thickness and when to preheat varies by material type).
  • Base materials that tend to be more brittle, such as cast iron, and when welding dissimilar materials.
  • When recommended by the base material manufacturer. This information often can be found in a table that specifies preheat temperature ranges for a given material thickness.

Preheating also can be good for materials with a high-carbon equivalency, such as AISI 4130 and 4140. High carbon levels and/or additional alloys can make the material stronger and harder, but also more brittle and less ductile, which can lead to potential cracking issues.

How Are Parts Preheated?

Once you have determined that the welding application requires preheating, consider the best method to use.

Induction heating is one preheat option that provides consistent heat throughout the weldment. It offers fast time-to-temperature and is considered a very safe option for preheating.
Photo courtesy of Miller Electric Mfg. Co.

Preheating with an open flame from a torch is a common method, as it is easy to use and offers simple setup and portability. Also, the initial investment cost is low, and the process is one welders usually know. However, preheating with an open flame can be inefficient compared to other options because much of the heat is lost into the surrounding air. It also can be difficult to ensure consistent temperature levels throughout the part.

Flame preheating also poses safety hazards, including an increased burn potential, and has special storage requirements for explosive gases, usually propane or propylene.

An oven or furnace also can be used for preheating, particularly for small parts. Large models are available for large weldments.

Induction heating, another preheat option that provides more consistent heat throughout the weldment, offers fast time-to-temperature. Some equipment can document preheat temperatures via digital recording capabilities.

Induction heating is considered a very safe option for preheat and postweld heat treatment (PWHT) and is commonly used for pipe welding. It also offers benefits for other part geometries, including flat plates for structural steel applications, as well as for shrink-fit applications.

Induction heating systems operate by creating localized eddy currents within a conductive part. This is accomplished by passing alternating current through a coil positioned very near or around the part. As a result, alternating magnetic fields produced near the coil induce eddy currents within the part. The material’s own resistance to this current causes the part to rapidly heat, essentially becoming its own heating element. It is a very efficient method because it eliminates or minimizes heat loss typical with other heating methods.

Liquid-cooled, air-cooled, and rolling options are available for induction heating.

Considerations for Preheating

As with any welding procedure, it’s important to follow the preheating guidelines from the material manufacturer, as well as some general best practices.

First, when using an open-flame method, consider the distance from the joint to achieve proper preheating. The correct distance from the joint varies based on the base material and any welding codes or procedures for the application.

Preheat a large enough area around the weld joint to ensure the proper temperature is maintained throughout welding. Preheating a wider area minimizes the risk of colder areas in the material sucking away the heat.

Preheat measuring often is done with Tempilstik®s, infrared thermometers, or other heat-measuring devices. Generally, the preheat temperature should be measured at least 3 inches from the joint. The preheat temperature should be verified directly before welding begins.

Induction heating systems often feature a built-in heat controller to monitor temperatures using feedback from thermocouples mounted on the weldment. It typically works best to place the thermocouple toward the center of a coil configuration, which tends to be the warmest spot.

Low-hydrogen filler metals reduce cracking risks

Controlling the amount of hydrogen in the weld through other means, such as using a low-hydrogen filler metal, is a good complement to proper preheating. Both contribute to a reduced risk of weld failure and cracking.

Many filler metal manufacturers offer low-hydrogen options in tubular wires (metal-cored and flux-cored arc welding, or FCAW, wires) and shielded metal arc welding (SMAW) electrodes.

An optional designation for diffusible hydrogen is included in the American Welding Society (AWS) classification for low-hydrogen filler metals. Among welding filler metals, H4 and H8 are common designations that indicate the filler metal contains low levels of diffusible hydrogen. The numeral in the designator signifies the amount of hydrogen, in milliliters, for each 100-gram amount of weld metal. For example, filler metals with H4 designation consistently produce weld metal deposits containing less than 4 ml of hydrogen per 100 g of deposited weld metal.

SMAW electrodes can have an “R” designator signifying that the electrode is resistant to absorbing moisture, which also helps control hydrogen levels.

Solid wires in general tend to have low hydrogen levels. Because they are solid metal, they do not readily absorb moisture, and thus minimize the risk of hydrogen-induced cracking. Be aware that the AWS classification for hydrogen — which is optional — typically is not included for solid wires.

Proper storage and handling of filler metals is also important to control hydrogen in the weld and reduce cracking risks. Improper filler metal storage can result in moisture or other contaminants being picked up on the surface of the product and entering the weld. This issue can negate the gains made by proper preheating and selecting low-hydrogen filler metals.

Store filler metals in a clean, dry area and keep them in their original packaging until they are used. The temperature of the storage area is also important. It should be similar in temperature to the welding environment, since storing filler metals in a cold area and then moving them to a hot one can lead to condensation forming, which increases the risk of introducing hydrogen into the weld. If a storage area with similar temperature isn’t available, be sure to acclimate the filler metal before welding. Also look for filler metal packaging that is heat- and/or vacuum-sealed, which is more likely to block moisture and hydrogen from entering the product.

These additional methods are among the best ways to control hydrogen in the welding process, which in conjunction with proper preheating, can help reduce the risk of cracking and rework.