Welding is a very important joining process and has been used extensively for at least the past 60 years. Like most processes, there is a need to control welding and ensure a high-quality end result. Over the years, there have been many spectacular failures of welded structures that emphasize this need. Many procedures involving nondestructive and destructive tests are used to study weldments. Metallographic examination can be performed by grinding a spot on the surface of a weld, its heat-affected zones or nearby base metal (the metal being joined that was unaffected by the heat of the welding process). This is a reasonably nondestructive evaluation. However, destructive examination, where a specimen is removed from either the welded assembly or test coupons, is quite commonly performed. Test coupons are often used to qualify the welder and ensure that the techniques and materials chosen will produce a weld with acceptable soundness and mechanical properties. Post mortems of failed weldments are also examined metallographically using sections removed from the welded assembly, generally after nondestructive examinations are completed.
cases, the welded structure is large and, in the case of a field failure, a
section must be removed by flame cutting. This process produces a substantial
damaged zone adjacent to the cut, perhaps as large as 10-15 mm. When the
section gets to the laboratory, the damaged cut region must be removed by a
less-damaging cutting method, such as band sawing or abrasive sectioning. Then
the metallographer will cut out coupons using a laboratory abrasive cut-off saw
that introduces less damage than production manufacturing equipment. Weld
samples often tend to be large and irregular in shape. Many will not fit within
a standard 1-, 1.25-, 1.5- or 2-inch (25-, 30-, 40- or 50-mm) diameter mold for
compression molding. In such cases, the metallographer often builds a mold
using bent sheet metal (coated perhaps with a mold-release agent), places the
specimen inside this mold (after the mold is glued to a suitable base plate)
and encapsulates the specimen with epoxy resin. After it has cured, the
specimen can be ground and polished using a wide variety of semi-automated
time, we will take a look at the microstructure of the weld.