ESAB Knowledge center.

Understanding Heat Input and its Limitations

Q: We manufacture structures for the utility industry and primarily weld A36, A572-Gr. 50 and A871-Gr. 65. Recently we converted some of our welding processes to higher-deposition submerged-arc welding (SAW) but are concerned about the amount of heat we are adding from welding. What is the recommended maximum heat input for this welding process?

A: In order to answer your question, we need more information about the process, including flux and wire classifications, number of electrodes, material thicknesses, and mechanical requirements.

In general, it's important to understand how heat input affects mechanical performance.

When it comes to heat input, travel speed is typically the biggest contributing factor. Since SAW in the 2F position is fairly difficult to perform with weld parameters that produce high heat input, we will ignore this case and focus instead on flat-position welding.

In the 1F/1G position, voltage and current that is set relatively high paired with a slow travel speed (high heat input value) can produce a visually acceptable weld, but it may not meet mechanical requirements.

Considering your base material types, we could assume you are using a standard F7A2-EM12K or similar flux and wire combination. A single electrode setup typically doesn't cause excessive heat input, so we will assume that you are using a tandem setup of two or more electrodes.

Thinner material will be self-limiting to heat input. Material thicknesses less than 3/8 inch thick generally will not support a large or excessive weld bead and will tend to blow through if an attempt is made to overweld them.

You can address your weld metal mechanical requirements by selecting consumables that match the tensile and yield strength of the base material. In most cases, these values are not adversely affected by high-heat-input welding operations. Weld toughness, which generally is a requirement for applications with low service temperatures, measured by Charpy V-notch testing, is adversely affected by weld heat input and usually is the limiting factor for determining the maximum welding heat input.

Both welding consumables and base material can be sensitive to high heat generating weld parameters, contributing to the need to identify the upper heat input value threshold. Low-carbon or basic mild steel typically does not have cold weather impact requirements; therefore, the consequence impact of high heat inputs, well above 100 kilojoules per inch (kJ/in.), is not a concern. Conversely, materials such as A572 and A851 are selected for their higher strength and good impact properties and require the use of caution when you are attempting to apply parameters for high heat input.

Many welding consumables, regardless of the process, tend to lose their ability to provide adequate impact values as heat inputs increase and service/design temperatures decrease. However, many factors affect this maximum heat input, so procedure qualifications are necessary to determine the limit for the specific application.

Base materials tend to have problems in the heat-affected zone (HAZ) with high or excessive heat input. Insufficient preheat and rapid cooling upon weld completion can produce high hardness values in the HAZ, which may cause martensite, a brittle microstructure, to form. Some materials can handle heat inputs of more than 110 kJ/in., while others may max out around 80 kJ/in.

For your specific base materials, a good heat input range is 35 to 80 kJ/in., but there is no guarantee. The best option is to perform a procedure qualification test to identify what your best welding parameters are. Doing this will give you a large operating range to maximize productivity and still deliver the required properties.


This article originally appeared in The WELDER magazine.
It is reprinted here with permission of the Fabricators & Manufacturers Association, Intl.

Posted in Filler Metals , Tagged with Gas, Steel, Stick