Cases

Field Failure Analysis

Use of Metallographic Replicas

August 2013 · Annelise Zeemann

Metallographic replica being applied in the field

TECMETAL performs metallographic analyses in the field using metallographic replicas, which are small pieces of plastic that, when pressed onto the area of interest after careful preparation, reproduce the structure by copying the topography obtained by polishing and chemically etching the surface. These replicas are taken to the laboratory and observed as if they were the material itself; in a microstructural analysis, the replica is usually complemented with hardness measurements of the analyzed region, and if the material is unknown, a chemical analysis may also be necessary — all these measurements performed with portable equipment.

Fig. 1 — Section of a line that will operate at low temperature, after metallographic preparation for the replica.

Fig. 2 — Ferritic-pearlitic microstructure of low-carbon steel, with refined structure and ASTM 9.5 grain size.

Figure 1 shows a polished surface where a metallographic analysis was performed, and figure 2 shows the appearance of the structure observed under an optical microscope after chemically etching the material (to reveal the structure) and applying the plastic.

In this case, there was doubt as to whether the material of the fitting used in a low-temperature line was a normalized carbon steel, and the grain size measurement of the ferritic structure is essential to ensure the carbon steel's resistance at low temperatures; in this case, the steel had an ASTM 9.5 grain size, corresponding to a very fine grain that withstands low temperatures well.

Fig. 3 — Crack in a heat exchanger tube sheet.

Fig. 4 — Heat exchanger tube sheet after metallographic preparation for the replica.

Fig. 5 — Replica micrograph showing the branched appearance of the cracking, typical of a stress corrosion process.

When a crack is present, analyzing its morphology reveals the cracking mechanism, whether it involves significant plastic deformation or no macroscopic deformation, associated with a corrosive process or heat.

Figure 3 shows a crack in a heat exchanger tube sheet that, after metallographic preparation (figure 4), revealed through microstructural analysis that the cracking mechanism was stress corrosion, as shown in the replica micrograph in figure 5.

These analyses can be performed on very large parts or in remote locations, as illustrated in figures 6 and 7, and if done well, it's not even possible to distinguish, based on the result alone, whether it is a replica or the material itself being analyzed.

Fig. 6 — Replica being made on equipment welding still in the manufacturing stage.

Fig. 7 — Replica being made on stainless steel, with electrolytic etching, on a flare burner.

Figures 8 and 9 show, for example, a duplex stainless steel, whose etching is carefully done to reveal the austenite and ferrite phases — in this case still showing the sigma phase — in a micrograph of the material itself (figure 8) or of a replica (figure 9).

Fig. 8 — Micrograph with electrolytic etching of a duplex stainless steel showing the austenite (lighter), ferrite and sigma (dark spots) phases. Sample analyzed in the laboratory.

Fig. 9 — Micrograph with electrolytic etching of a duplex stainless steel showing the austenite (smoother), ferrite and sigma (dark spots) phases. Replica brought from the field and analyzed in the laboratory.