An Overview of Non-Destructive Field Metallography

Field metallography is the non-destructive in-situ testing of metallurgical condition of metallic components. This process can be conducted in the field without the need to remove any components. When metallographic replication is underway, the component being tested can be left in service. The process is often used to assess the high temperature damage of power generating components. Creep damage assessment (Figure 1), spheroidisation of pearlite in carbon steels, and carbide coarsening in low alloy steels are the key areas of analysis.

SEM micrographs of creep damage assessment

SEM micrographs of creep damage assessment

Figure 1. SEM micrographs of creep damage assessment

Field metallography can also be employed to measure grain size, determine the type and grades of materials (like cast irons and steel), and  to assess the heat treatment of materials. This method can be used in the field of tribology to determine wear mechanisms to evaluate fretting, abrasion and burnishing mechanisms. This technique can also be utilized to carry out criminal forensic investigations and archaeological investigations of artefacts.

Materials/Equipment/Planning

Pre-Planning

The purpose of sectioning is to expose the area of interest before field metallography is performed, planning is vital as certain aspects are required to be carried out in advance [1, 3]. While performing field metallography, a metallographer has to understand material being analyzed, with the key parameters being;

  • The type and composition of the alloy - to determine the procedures to be employed
  • The material hardness - is a guide to the type of consumables to use, e.g. abrasives, grinding papers, polishing cloths, etc.
  • The surface conditions - help to choose the initial grinding process and consumables

Equipment

The equipment and accessories required for field metallography to assess the damage of components during power generation are described below. These instruments can be applied to other applications as well.

  • Grinding/polishing
    • Motorized or portable handheld grinding/polishing device (ElecterSet)
    • Rubber stubs used for grinding and polishing paper cloths
    • Adhesive-backed grinding paper discs (60, 120, 240, 320, 400, 600 grit in separate plastic bags/containers)
    • Polishing cloth discs (VelTex)
    • Diamond paste (9, 6, 3, 1 µm)
    • Plastic bottle containing alcohol/acetone
    • Cotton balls for swabbing and cleaning
  • Etching
    • Plastic bottles containing premixed etchants (screw caps)
    • Cotton balls for swabbing
    • Rubber gloves
  • Replication
    • Cellulose acetate film sheets (thickness: 20-45 µm)
    • Bottle containing acetone or methyl acetate solvent
    • Tweezers
    • Box containing standard glass slides
    • Double-sided adhesive tape
    • Black spray paint
  • In situ examination of microstructure
    • Portable upright microscope
    • Camera attachment included
  • Miscellaneous
    • Absorbent cloths/paper towels
    • Flashlight depending on the luminescence in the work area
    • Safety equipment (glasses, safety goggles, hardhat)
    • Extension cord
    • Collapsible stool

The process of acetate sheet replication can be substituted by silicone-based two-part replication (Figure 2), which is ideal to measure surface roughness in tribological applications, or during the analysis of fracture surfaces of large components.

Silicone-based two-part replication process

Silicone-based two-part replication process

Silicone-based two-part replication process

Figure 2. Silicone-based two-part replication process

Replication

Surface Preparation (for Mirror Finish)

The selected preparation process should correspond to the material type and the microstructural features under analysis. The principles employed in usual metallographic preparation can be applied here, this includes rough grinding by coarse grit papers and the progressive use of finer grit paper prior to polishing stages (Table 1). Inter-stage etching of the surface during final polishing is specifically important in creep damage assessment of components.

Table 1. Recommended procedure using 30-32 mm discs

Surface Abrasive Size Key Parameters
CarbiMet 280grit [P320] • Electer Set settings for grinding procedure
CarbiMet P600   • Ease of grinding/polishing - Angled handle (45)
• Speed: 15,000 rpm (Motor load - green Amber)
CarbiMet P800   • Depending on area of interest, multiple SiC papers should be used
• Precaution: Dust Mask
VelTex 9 µm MetaDi Supreme Diamond* • Speed: 6000-7000 rpm with 1 cloth per each step
VelTex 3 µm MetaDi Supreme Diamond* • For 1 µm step, it should be carried out at least 2 steps with intermediate etching between the stages
VelTex 1 µm MetaDi Supreme Diamond* • Lubricant: *MetaDi fluid

Etching

Etching involves the attack of the as-polished surface with chemical etchants to reveal the alloy microstructure, where the polished surface is swabbed for up to 60 seconds. Choosing an appropriate etchant for various alloys is very important. Nital is used as a common etchant for carbon and alloy steels, Fe, and cast iron. It is a mixture of nitric acid in ethanol or methanol, where nitric acid concentration differs from 1% to 10%. The most commonly used mixture is the 2% solution, but 5-10% solution is also employed for high alloy steels. Another common etchant utilized for ferrite-carbide structures and stainless steel is Villella’s reagent. It is an ideal etchant for martensitic stainless steels and tool steels. Kalling’s No.2 is also applied by swabbing the sample to etch duplex and austenitic stainless steels. Care should be exercised when dealing with etchants, and risk assessments and COSHH evaluations should be carried out.

Application and Extraction of Replica

This involves the application of the acetate sheet on the etched surface to produce a replica of the surface (Figure 3). Before being applied to the surface, the acetate sheet must be softened using methyl acetate solvent or acetone to adhere to the surface. Black paint is sprayed on the transparent acetate sheet once it is dried so that the film becomes opaque, improving the contrast of the replica before it is peeled off with tweezers. The peeled replica is fixed on a glass slide using a double-sided adhesive tape. It is labelled to indicate the date, the details of the component being tested, and the replica number.

Schematic representation of the replication process

Figure 3. Schematic representation of the replication process

Factors such as geometry/configuration of the component being tested, film thickness, the experience of a replicator, and environmental conditions can affect the application of the acetate sheet at the time of the replication process. As thicker films may not fill the contours of the etched microstructure with a relatively low spatial resolution, they are not ideal. In contrast thin films have exceptional spatial resolution, but they will be easily torn during replica extraction, and must be handled carefully. During the process of metallurgical replication, silicone-based systems are ideal for flat surfaces and where replicas of very low volume are required. For replicated regions with complex geometry or curved surfaces, acetate sheets replica can be easily produced and flattened on a glass slide. For silicone-based systems this is not practically suited. The longer curing time taken for the two-part systems is limited in this area.

Replica Examination

The extracted replicas are normally examined with an optical microscope, like the Nikon LV 150 upright microscope with bright field illumination. Usual magnification range is 50-1000 magnifications at the eyepiece.

Improving Contrast

The contrast of the replicas is further improved by applying a carbon or metallic coating on the replica through a vacuum deposition of gold, gold-palladium alloy, or carbon by using a sputter coater. The replica can be examined by electron microscopy techniques like scanning electron microscopy (SEM). Compared with the actual microstructure, the metallic coated replicas possess exceptional spatial resolution. The contrast enhancement of the replica is not ideal for silicone-based systems, however the resolution is adequate for low magnification investigations and general applications.

Examples Of Metallography Applications

Weld Microstructural Investigations

Figure 4 displays the weld microstructure, including a base metal on both sides of the weld after being etched with 2% Nital. Weld microstructural investigations can involve the examination of inclusions, blow holes, reheat cracks at the cap or toe regions because of high residual stresses, weld slag, etc.

As prepared and etched weld component depicting the weld passes and the heat affected zone on either sides of the weld and the base metal

Figure 4. As prepared and etched weld component depicting the weld passes and the heat affected zone on either sides of the weld and the base metal

The final etched substrate and weld microstructures are displayed in Figure 5a and Figure 6a, respectively. The corresponding replicas are shown in Figure 5b and Figure 6b, exhibited as copies of the original microstructure but with reduced contrast. All of the features are shown to exhibit pearlitic regions and ferrite grains distinctly.

(a ) The base metal ferrite/pearlitic microstructure on etched sample and (b) the corresponding replica.

(a ) The base metal ferrite/pearlitic microstructure on etched sample and (b) the corresponding replica.

Figure 5. (a ) The base metal ferrite/pearlitic microstructure on etched sample and (b) the corresponding replica.

a) The weld, weld fusion line and coarse grained HAZ regions and (b) the corresponding replica.

a) The weld, weld fusion line and coarse grained HAZ regions and (b) the corresponding replica.

Figure 6. (a) The weld, weld fusion line and coarse grained HAZ regions and (b) the corresponding replica.

Creep Damage Assessment

VGBTW 507 [4] microstructure rating chart is used to evaluate the creep damage and microstructure in high temperature piping and tubing. As depicted in the replica images Figure 7 and Figure 8, this rating is based on the potential to determine the number of cavities, cavity orientations, grain boundary separations, micro-cracks and macro-cracks.

A 9Cr steel with randomly distributed cavities

Figure 7. A 9Cr steel with randomly distributed cavities

A 14MoV6-3 showing cavities aligned along grain boundaries

Figure 8. A 14MoV6-3 showing cavities aligned along grain boundaries

Tribological Investigations

 

Tribology is the study of interacting surfaces relative to motion [4]. Tribologists particularly focus on the ability to replicate the related surface damage, wear or modifications on the interacting surfaces, and usually measure the roughness parameters and surface texture both before and after the wear process. Different processes in this area include tyre tribology, metal rolling tribology, brake tribology, jet engine components, etc. The application procedure is illustrated in Figure 2, with the tribologists preferring a flexible rather than a rigid two-part system.

Figure 9 displays metal rolling tribology for an aluminum sample. Figure 10 depicts the measurements performed on a silicone-based replica of a stamping tool for aluminum sheet metal. A two-part silicone system with medium to high shore hardness was selected. Surface roughness measurements were performed on the stamping tool, and correlated to the roughness measurements on the replication of the aluminum surface and the test panel. A high spatial resolution on the replica was witnessed, with a difference of 0.05 µm between the measured roughness values.

(a) An Al rolled surface, (b) replicated region and (c) the resultant replica that can be used for surface profilometry measurements

(a) An Al rolled surface, (b) replicated region and (c) the resultant replica that can be used for surface profilometry measurements

(a) An Al rolled surface, (b) replicated region and (c) the resultant replica that can be used for surface profilometry measurements

Figure 9. (a) An Al rolled surface, (b) replicated region and (c) the resultant replica that can be used for surface profilometry measurements

(a) The steel stamping tool. (b) Optical surface roughness measured as a function of depth. (c) A 3D matrix reconstruction of the surface showing depth of penetration of the surface imperfections

(a) The steel stamping tool. (b) Optical surface roughness measured as a function of depth. (c) A 3D matrix reconstruction of the surface showing depth of penetration of the surface imperfections

(a) The steel stamping tool. (b) Optical surface roughness measured as a function of depth. (c) A 3D matrix reconstruction of the surface showing depth of penetration of the surface imperfections

Figure 10. (a) The steel stamping tool. (b) Optical surface roughness measured as a function of depth. (c) A 3D matrix reconstruction of the surface showing depth of penetration of the surface imperfections

Forensic investigations

In forensic investigations, science is applied using different methods to collect and inspect data that is admissible in court [6]. These investigations are very extensive and include; tool marks evidence, firearm evidence, tyre marks, and imprint evidence. An example is tool mark evidence, where tools have been used to commit a crime by aiding entry. These tools have unique marks related to the manufacturing procedure and defects from use that can be identified from the surface they contact. Usually tools and items bearing tool marks are submitted for examination by law enforcements. Figure 11a depicts a screwdriver used to form a tool mark on a lead sample, and Figure 11b shows the replica that is extracted utilizing silicone-based resins. Figure 11c illustrates the comparison made between the actual damage (A) and the replica (B). The characteristic manufacturing striations can be seen with defect areas on the tool.

(a) A test mark in a lead sample, (b) a two-part silicone resin applied to replicate the surface, (c) and the resultant replica with striations from the test mark

(a) A test mark in a lead sample, (b) a two-part silicone resin applied to replicate the surface, (c) and the resultant replica with striations from the test mark

(a) A test mark in a lead sample, (b) a two-part silicone resin applied to replicate the surface, (c) and the resultant replica with striations from the test mark

Figure 11. (a) A test mark in a lead sample, (b) a two-part silicone resin applied to replicate the surface, (c) and the resultant replica with striations from the test mark

Conclusion

Field metallography is a non-destructive technique that is essentially an extension of a metallographic laboratory. The principles of laboratory sample preparation are applicable in field metallographic preparation, except the use of portable tools to extract a record of alloy microstructure. This method can also be utilized to perform in situ examinations of large components that cannot be examined in a metallographic laboratory. It is mainly used to assess the lifetime of high temperature components used in petrochemical refineries, nuclear and coal fired power plants, heat recovery steam generator (HRSG), etc. The article has also described the vast applications of field metallography in forensic investigations and tribology.

References

1. Buehler® SumMet™ – The Sum Of Our Experience – A Guide to Materials Preparation & Analysis, © 2007. Buehler, a division of Illinois Tool Works Inc.

2. Vander Voort, G.F., Metallography, principles and practice. 1999, Materials Park, Ohio: ASM International.

3. American Society for, M., Metals handbook. 9th ed. ed. 1978, Metals Park, Ohio: American Society for Metals.

4. ASM International. Handbook Committee., ASM handbook. 10th edition. ed. 1990, Materials Park, Ohio: ASM International.

5. VGB-TW 507 – Guidelines for the Assessment of Microstructure and Damage Development of Creep Exposed Materials for Pipes and Boiler Components, VGB Technical Association of Large Power Plant Operators, 1992.

6. Nickell, J. and J.F. Fischer, Crime science : methods of forensic detection. 1999, Lexington, Ky.: University Press of Kentucky.

This information has been sourced, reviewed and adapted from materials provided by Buehler.

For more information on this source, please visit Buehler.

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