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Metallurgy Matters: From one extreme to the other
- By Bob Capudean
- March 19, 2015
- Article
- Metals/Materials
In terms of temperatures, the heat-affected zone (HAZ) has it all—from near liquidus to near ambient and everything in between—a fact that makes the HAZ a tricky piece of real estate to understand the predict. Why? Remember, a number of metallurgical changes can take place in hot metal, and changes that may proceed slowly at low temperatures can happen fast at temperatures near the liquidus.
While the welding process and procedure used, coupled with base metal type, determine the strength and toughness of the HAZ, welding heat also can have a significant and often detrimental influence. Base metals affected most by welding are those strengthened or annealed by heat treatments. This makes sense, considering the high temperatures in a weld thermal cycle and the range of temperatures in a HAZ.
This also means that how welding heat influences the HAZ depends on the initial strengthening process. And the easiest way to understand it all, as well as make viable estimates of postweld strength, is to consider weld heat influences based on the strengthening process used in the first place.
The four basic strengthening processes are:
- Solid-solution hardening
- Cold working (strain hardening)
- Precipitation hardening
- Transformation (martensite) hardening
You can use more than one of these processes on some alloys, but we’ll consider each separately
Alloys Strengthened by Solid Solution
These alloys are the best, at least in terms of the HAZ, because they usually have the fewest problems in terms of losing toughness, strength, or hardness. Aluminum and copper alloys, as well as hot-rolled, low-carbon steels, are commonly solid solution-hardened. For this discussion, you can also put both ferritic and austenitic stainless steels in the same category. With all of these alloys, the thermal cycle’s effect is minimal, and the HAZ properties are largely unaffected by welding, assuming there’s no solid-state transformation.
Because there’s a high peak temperature, there will be some grain growth next to the fusion line, but this won’t significantly affect mechanical properties, assuming the zone where the grain has become coarser is only a few grains wide, which is usually the case.
Alloys Strengthened by Cold Working (Strain Hardening)
These alloys can prove to be a bit of a headache. For starters, remember that if you heat the metal above the recrystallization temperature, a temperature easily reached in the HAZ, it recrystallizes. That means the heat of welding can cause portions of the HAZ to recrystallize, making it substantially softer—and weaker—than the cold-worked base metal. In essence, the heat of welding causes the HAZ to undergo an annealing cycle. To make matters worse, you can’t regain the lost strength by heat treatment.
The unaffected cold-worked base metal has elongated grains typical of mechanical deformation. But at the point in the HAZ where the recrystallization temperature is exceeded, fine grains form. And as you get closer to the fusion zone, the higher temperatures cause the grains to get larger.
Some metals, like titanium, go through allotropic transformation, which really complicates matters. These metals may have two recrystallized zones, one fine-grained zone that results from recrystallization of the cold-worked alpha phase, and a second fine-grained zone resulting from the allotropic transformation of the high-temperature phase.
In either case, the HAZ of metals strengthened by cold working is significantly softer and weaker than the unaffected base metal.
Alloys Strengthened by Precipitation Hardening
These alloys react much the same way as work-hardened alloys—the HAZ goes through an annealing cycle. It’s more complicated—different effects take place in different regions during the weld thermal cycle—but the result is the same: the HAZ areas closest to the weld are relatively soft and weak. Fortunately, with these metals, some of the lost strength can be regained.
Let’s back up a moment. The basic precipitation-hardening heat-treatment cycle is solution-treat, quench, age. The welding heat will re-solution-treat the HAZ areas closest to the weld, creating a soft, single-phase solid solution with some coarse grains. This is good, because this area can be rehardened by a postweld aging treatment. But welding heat will also overage the HAZ areas heated to temperatures below the solution treatment temperature, and a postweld aging treatment will not reharden these areas. As you may suspect, mechanical properties are unaffected in areas where the welding heat doesn’t raise the HAZ above the original aging temperature.
It’s tough to weld precipitation-hardenable alloys without losing some strength. But there are postweld processes that can help minimize those losses. The most effective is also the most expensive and often the least practical; it involves re-solution-treating, quenching, and aging the weldment.
An alternative is simply to reage the weldment. This will bring back some of the strength lost in the solution-treated area of the HAZ, but does nothing to help the strength of the overaged zone. If this isn’t practical, try welding the base metal in the solution-treated stage, then age the completed weldment. Again, it’s no help for the overaged zone, but it should offer some improvement over more complicated approaches, or no approach at all.
Now you should realize just how critical minimizing heat input can be when dealing with heat-treated base metals. Like kryptonite to Superman, the weld thermal cycle saps the heat-treated base metal of its strength. And getting that strength back can prove challenging, if not impossible. Low heat input keeps the HAZ width to a minimum, reducing the amount of softened base metal.
We’ve given you an idea of what it takes to keep the strength you started with in the base metal. Next time we’ll finish what we started by looking at the influences of weld heat on the HAZ of alloys hardened by transformation. We’ll also begin a close look at base metal weldability, including a number of specific commercial alloys and weldability tests. In future issues, we’ll also get into some specifics about various strengthening processes.
About the Author
Bob Capudean
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