M7 Welding: Union Metallurgy

Topic

The welding source—whether an electric arc or any other concentrated heat process—delivers a heat input to the metal that moves along with the source as the weld progresses; this heat input imposes the transient thermal cycle described by the model. For a given power level, if the welding source moves more slowly, the heat input received by any specific point on the metal as the source passes by is greater.

According to the Rosenthal model, this heat input determines the transient temperature field established within the metal around the moving welding source. The model describes how the temperature at any point in the metal rises rapidly as the welding source approaches, reaches a maximum as the source passes that point, and subsequently falls as the source moves away and heat dissipates into the surrounding metal. A higher heat input results—according to the Rosenthal model—in a higher peak temperature at each point and causes that elevated temperature to be maintained for a longer period.

This transient temperature field defines, at every point in the metal, both the cooling rate (the speed at which the point returns to ambient temperature after the welding source has passed) and the width of the heat-affected zone (HAZ)—the region surrounding the weld that reached a temperature high enough to alter its original microstructure without melting. A higher heat input results in a slower cooling rate and, consequently, a wider heat-affected zone. Consequently, given the transient thermal cycle and the heat-affected zone associated with the welding source used in this model, the cooling rate within that zone is ultimately what drives unwanted phase transformations in the metal: an excessively rapid cooling rate can produce—without any deliberate quenching—an unintended fraction of brittle martensite, whereas an excessively slow cooling rate can allow for excessive grain growth within that same heat-affected zone. Since the welding source ultimately governs this cooling rate throughout the process chain described by the model, adjusting the heat input and the travel speed of the source is, in practice, the way to prevent these unwanted phase transformations within the heat-affected zone.

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