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Consumables Corner: Achieving toughness in the HAZ of weld joints

Q: We’ve experienced Charpy V-notch (CVN) failures in the heat-affected zone (HAZ) while qualifying several weld plates. We have a few opinions about what produces good HAZ toughness. Would high preheat, high interpass temperature, or high heat input have more effect on HAZ performance?

A: The answer isn't so straightforward because all three of those variables plus several others can affect CVN results significantly. Just like your welding parameters, there is a range of values for each variable to be optimized for producing favorable results.

When the HAZ has poor CVN results, a majority of the time it can be attributed to excessive welding heat input, excessive interpass temperatures, or insufficient or improper preheat temperatures. The corresponding results will be a HAZ microstructure change that produces high hardness values or excessive grain growth, neither of which is conducive to good notch toughness.

Preheating weld joints before welding—including tack welding—is done primarily to slow the cooling rate of weld metal and adjacent base material. This takes place when there is an appreciable amount of mass that can rapidly transfer heat away from the weld area. Additionally, if the weld joint is highly constrained or the base material is susceptible to martensite formations, the significance of preheat increases.

To determine the value of preheat temperature, calculate the carbon equivalency (CE) of the base material and refer to standard CE preheat recommendation charts for proper temperatures. Keep in mind, some steel types such as A514 get their high-strength properties in part from the quench and temper process used to produce them. Steels in this category typically have conservative preheat temperatures, relatively low maximum interpass temperatures, and low weld heat input values. If any value or combination of values is exceeded, it is likely the steel will not retain its classified strength.

If we assume you are using common CVN requirements of at least 20 ft.-lbs. at -20 degrees F for many structural steel applications, then we can assume that higher preheat or interpass temperatures shouldn’t cause any problems with impacts. This is assuming you are staying within the recommendations of the base material and filler metal. However, if either of these values exceeds the recommendations, then you would want to have them both slightly elevated to minimize the large temperature gradient between them.

Large temperature gradients increase the cooling rate, which then increases the risk of less desirable microstructure changes. The first sign of trouble will be excessive hardness values in the HAZ on the macroetch from the qualification test plate. This usually means there has been some martensite formation.

On the other hand, if we look at high heat input, then you would want to use a preheat temperature that is near the upper limit of the recommendation. Again, this is to minimize the temperature gradient for the same reasons.

The two main concerns with high heat input are the possibility of increased cooling rates or excessive grain growth in the weld metal or HAZ. Depending on the preheat temperature and the base material used, you could end up with martensite formation or large grains. Large grain formation can lower the impact results, but more importantly, it can lower the yield and tensile strength.

From a welding variable standpoint, the only way to get high heat input in most applications is to use very high amperage and voltage or very slow travel speed, which is more common. Since most welding arcs become unstable at very high values, we can usually rule that out as a concern. Slow travel speeds are much more common, especially when the weld joint is in the flat or 1G/1F position. This allows you to slow the travel speed to fill the weld joint faster with fewer passes in a multipass weld application.

Large weld beads with high heat input are very likely to result in poor mechanical performance of the weld joint because of the slow cooling rate of the weld pool. Slow travel speed will also create a larger HAZ, causing the same adverse effect.

About the Authors

Nino Mascalco

Independent welding consumables professional

ESAB Welding & Cutting Products

Rob Koltz

Application Engineer

411 S. Ebenezer Rd.

Florence, 29501

636-485-2253