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Metallurgy Matters: The science of welding metallurgy
- By Bob Capudean
- March 9, 2015
- Article
- Metals/Materials
It’s time to narrow our focus and look at the science of welding metallurgy, a branch of metallurgy that addresses the behavior of a metal during welding and, just as important, the effects of welding on a metal’s properties.
Think about what happens when you weld together two pieces of metal—say, two pieces of a mild steel roll cage or a chrome-moly support bracket, or maybe pieces of a stainless steel countertop or an aluminum radiator. For our purposes, the material, like the process, isn’t critical. Assume you’re using any typical manual or semiautomatic gas tungsten arc welding (GTAW), gas metal arc welding (GMAW), arc, or oxyacetylene welding setup.
So, what happens? The metal melts; gas-meta reactions, slag-metal reactions, surface phenomena change, and solid-state reactions all take place; and then the metal solidifies. And this all happens very quickly—especially when compared to metallurgical reaction times during metal manufacturing, casting, forging, or heat treatment. In the end, the result is a welded joint.
From a welding metallurgy standpoint, that weld consists of metal that has been melted, the heat-affected zone (HAZ), and unaffected base metal. The metallurgy of the weld, as well as the area around it, is directly related to the composition of the base metal (the metal you started with), the weld metal (the admixture of melted base metal and deposited filler metal, if used), heat input, the size of the HAZ, and the process and procedures used.
A Brief Word on Filler Metals
Some welds are autogenous. That is, the consist only of remelted base metal because filler metal wasn’t used. But the fact is, most of the time a filler metal is used, and its chemical composition is critical because it can significantly influence the weld metal and the structural properties of the joint.
Often the filler metal is designed to produce a weld metal that’s similar in chemical composition to the base metal, for obvious reasons. But sometimes the chosen filler metal produces a weld metal that’s significantly different from the base metal. The idea is to produce a weld metal with properties that are different, but compatible, with the base metal. We’ll look more closely at filler metals in a future column.
A Closer Look at the Weld
Once you have molten metal and add filler metal, the first grains to solidify are the ones closest to the unmelted base metal. The unmelted base metal nucleates these grains, so they maintain the same crystal orientation.
But as the weld continues to solidify, it does so in either a cellular or dendritic growth mode. Either of these growth modes causes the base metal’s alloying elements to become segregated, which means the weld metal is less homogenized than the base metal.
The Heat-affected Zone
Right next to the weld metal is the HAZ. In theory, the HAZ may include all the metal raised above ambient temperature. In practice, it’s usually considered the area of the base metal that wasn’t melted but was heated to the point that its microstructure or mechanical properties were altered by the welding heat.
For example, a plain carbon steel is influenced little until the heat of welding gets higher than 1,350 degrees F. On the other hand, a heat-treated steel quenched to martensite and then tempered at 600 degrees F is affected as soon as its temperature is raised above 600 degrees F; at the very least, its mechanical properties would be influenced. Similarly, the HAZ of a heat-treated aluminum alloy age-hardened at 250 degrees F includes any area heated above 250 degrees F.
From a practical standpoint, the HAZ depends as much on the material and its accompanying treatments as it does on temperatures and heat input. Also, keep in mind that each weld pass has its own HAZ, although the weld metal from a previous pass—even though it most likely has a raised temperature—is not considered a part of the HAZ. The width of the HAZ is directly related to heat input, and adjacent the HAZ is unaffected base metal.
The Base Metal
Base metals usually are specified to take advantage of specific properties or characteristics such as tensile strength, notch toughness, yield strength, corrosion resistance, weight, and density.
It’s up to the welding engineer or—more often than not—the welder to use the right process and filler metal or consumable to ensure everything works as planned.
The Weld Metal
The mixture of filler metal and base metal found in the weld joint is called the weld metal. And while the weld metal’s chemical composition may be similar to that of the base metal (depending on the chemical composition of the filler metal), the microstructures of each are significantly different. This is because the microstructure is more closely related to the metal’s thermal and mechanical histories than to chemical compositions.
For example, consider a typical V-groove butt joint made from hot-rolled, low-alloy steel. Even if the filler metal was designed to provide a weld metal with an almost identical chemical composition, the microstructural differences would be substantial.
The base metal’s structure resulted from a hot rolling operation, which means the hot-worked metal recrystallized a number of times as it was manufactured. The weld metal, on the other hand, has an as-solidified structure; it wasn’t mechanically deformed, so its structure (and mechanical properties) resulted directly from the events that occurred as the weld metal solidified. These events include gas-metal reactions, liquid-metal reactions, and solid-state reactions.
And that sets the stage for next time, when we’ll go a little deeper into the aforementioned reactions as well as the HAZ and solidification. Over the next couple of months, we’ll get into some key metallurgical events that take place almost every time you lay a bead—reactions that can seriously impact the quality of your finished ferrous or nonferrous welds.
About the Author
Bob Capudean
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The Welder, formerly known as Practical Welding Today, is a showcase of the real people who make the products we use and work with every day. This magazine has served the welding community in North America well for more than 20 years.
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