FIELD METALLURGICAL REPLICATION

This form of non-destructive testing has been in use for several decades, and is designated by ASTM E 1351 as the Standard Practice for Production and Evaluation of Field Metallographic Replicas. Replication is used as a tool for evaluating microstructures and other surface features in lieu of laboratory evaluations. The procedure is performed by engineers and trained technicians using portable polishing equipment following modified laboratory procedures for the preparation of metallographic samples.

Producing a replica requires abrasive preparation using successively finer grits, followed by a final diamond-impregnated cloth polish. The work is done with portable polishing equipment and small diameter (1-inch or 32-mm diameter) grinding and polishing disks.

Observations are made in the field and can also be made in a laboratory after the samples are transported. For field use, a portable 400X magnification microscope is used to observe prepared metal surfaces and the quality of the replica. A portable microscope with photographic capabilities can also be used to provide digital or positive prints of the examined areas.

Metallurgical replicas can be viewed in the laboratory using either portable or laboratory light optical microscopes (LOM) or scanning electron microscopes (SEM). Additional contrast can be attained by coating the plastic film containing the structure with a conductive coating, which is required for SEM observation.

There are endless applications for replication. It is a useful tool when used in the field as in-situ metallography. This on-site evaluation of microstructures can save a lot of time and money during plant shut-downs; it can be used to examine equipment when downtime is not a viable option, or when it would be too costly to transport equipment for a laboratory evaluation because of its size. It has been used to observe features with or without metallurgical preparation, such as in a quality control application for the comparison of porosity percentages on compressor cylinders to coating types and thickness on compressor blades, and to characterize the type of crack on a fan disk.

Replication provides the metallurgical engineer a two-dimensional view of the microstructure, similar to that observed in a laboratory metallographic specimen. Features such as grain size and orientation, carbides and cracks are easily identifiable. Results can be presented to field and plant personnel during an on-site visit. It can confirm observations made by other non-destructive methods, such as acoustic emission tests, penetrant examination and ultrasonic flaw detection, and go a step further to identify the nature of the flaw. For example, to determine whether a crack is intergranular corrosion or transgranular fatigue.The accompanying graphics show typical examples of successful applications of metallurgical replication.

Replication can be used as a tool to verify microstructures of component at remote locations, shop sites and laboratories around the world. It can be used to provide meaningful metallurgical information to support ongoing nondestructive evaluation, forensic evaluations, and in any application that requires additional knowledge about the component in question. Cases when informed decisions are to be made regarding extended or continued service or characterization of the flaws that are detected, which require further confirmation of the fracture mechanism, can be provided by replication.

Stress Engineering Services has qualified personnel that have evaluated a variety of equipment and components by replication techniques and are available to travel on-site on a short notice.