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Understanding Weld Heat Input and its Effects on Base Materials

What guidelines should a job shop follow in regards to heat input if there is no welding procedure to fall back on? The Consumables Corner team has the answer.

Q: Our shop primarily manufactures structural steel products, although we do produce some agricultural equipment and tanks for various liquids. In some cases, we have no information from the customer regarding welding guidelines, and in others we have detailed welding procedures. If there are no welding procedures, how important is heat input, and are there any general guidelines to follow?

A: In almost any welding application, heat input is an important variable, and you should follow a couple of guidelines to help prevent problems.

The two primary reasons for controlling heat input, specifically high heat input, are the detrimental effect on toughness or impact properties (for carbon steels) and corrosion resistance for certain types of stainless steel.

The basic heat input calculation is based on voltage, amperage, and travel speed:

[(Volts x Amp x 60)/(Travel speed IPM x 1,000)] x (Process efficiency)=Kilojoules/inch

Welding process efficiency is also figured into that calculation if you are trying to maximize deposition for processes such as shielded metal arc welding (SMAW) that have low deposition efficiency. In many cases, it isn't necessary to include efficiency, though.

The key to understanding the equation is that in welding the changes in voltage and/or amperage are going to be small and will have a minimal effect on the overall heat input. However, changes in travel speed can have significant effects on that value. A slow travel speed for a set voltage and amperage can result in a drastically higher heat input. For example, if a WPS specifies a travel speed of 12 to 15 inches per minute (IPM) and the welder decides to slow his travel speed to 6 IPM to fill in a large groove weld, then he has effectively doubled the heat input from the specified upper limit of the WPS. This change could result in degraded impact toughness and cause a welded structure to fail in cold-weather applications.

With stainless steels, excessive heat input or prolonged periods of high heat can cause sensitization. Depending on the grade and thickness, the ideal heat input range for stainless steel may typically fall in the 25 to 45 kJ/in. range.

Some steels such as A514, known also as T1, get their enhanced mechanical properties from a quench-and-temper heat treatment. Certain grades of these materials are also susceptible to reduced tensile and yield strength if the welding heat input is excessive.

In general, the sweet spot for welding heat input is between 35 and 65 kJ/in. Keep in mind, though, that you need to review each application to determine the specific limitations. Many of the mild steels on the market, such as A36, aren't greatly affected by high heat input. Some steels can withstand values well over 120 kJ/in. while others may have an upper limit of 45 kJ/in.

Likewise, many filler metals, and especially those developed for improved cold weather impact toughness, are also susceptible to degraded performance.

Conversely, welding heat input that is too low can also cause problems. Welding on heavy sections of steel with low heat input or small welding beads can cause the welds to cool too quickly, producing above-normal hardness values in the heat-affected zone (HAZ) or in the weld bead. Some welding codes have minimum heat input requirements to ensure proper weld fusion and prevent excessive hardness in the weld metal or HAZ.

Overall, the base material will be the primary consideration for setting limits on heat input. Beyond that the code requirements, material thickness, and in-service applications will also help guide you in determining heat input ranges.


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 Steel