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FFS Forum: Field Metallographic Replication (FMR) - When it helps, and when it doesn’t

By Greg Garic, Managing Principal at Stress Engineering Services, Inc.. This article appears in the March/April 2020 issue of Inspectioneering Journal
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Field Metallographic Replication, or “FMR”, is a nondestructive way to get a look at a material’s surface microstructure.

But before we jump into a discussion of FMR, let’s review traditional microstructural evaluation so we can better compare and contrast the two.

Traditional Microstructural Evaluation

In traditional microstructural evaluation, a piece of metal is removed from a component. It is cut down to a small size, mounted, polished, and etched to reveal the microscopic structure (microstructure) of the metal. For the convenience of handling, once the sample is cut down to final size, it is mounted in a cylinder of poured resin (Figure 1), called a “mount.” A mount is made from epoxy, Bakelite, acrylic, or melamine and is poured around the sample in a cylindrical mold.

Before a metal sample is cut from a component, we can choose where to make the cuts and in which orientation to mount the sample in order to best achieve our objectives.

Figure 1. Metal Specimen in Bakelite "Mount."
Figure 1. Metal Specimen in Bakelite "Mount."

The orientation of the specimen in the mount is selected by the metallurgist. Figure 2 illustrates a specimen taken from a pipe wall and mounted such that the through-thickness face can be examined (i.e., OD to ID).

Figure 2. Mounting a Specimen in Epoxy.
Figure 2. Mounting a Specimen in Epoxy.

In this case, the variation in microstructure can be examined and, if desired, microhardness measurements can be taken, from OD to ID.

Once the specimen is mounted as shown in Figure 1 or Figure 2c, the specimen must be polished to a mirror finish to remove all scratches. Lastly, the highly polished surface is etched to prepare it for examination under a microscope. Sometimes the etchant is an acid, sometimes it’s a base, but it is always selected for the specific material being examined. The purpose of the etch is to reveal the grain boundaries and characteristics of the grains. To understand this, you must understand a little bit about the structure of metals.

Lattice Structure of Steel

Most metals (including steel) are crystalline in their solid state. This implies a highly organized arrangement of the atoms in a lattice structure. The lattice structure may be of different forms depending on several factors, such as the material and cooling rate. The terms “face-centered cubic” and “body-centered cubic” refer to different lattice structures.

As a steel transforms from a liquid to a solid, crystals begin to form. Many crystals begin to form simultaneously throughout the material, and they grow as the metal continues to cool.

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