Company: IRIS NDT
Speaker: Peter Beck and Simon Fenton

Topic: Metallographic Replication of In-Service Plant.   

Synopsis:

Metallographic surface replicas can provide important information relating to the metallurgical condition of a component, that can have a direct bearing on its fitness-for-service. Such fitness-for-service considerations can include its susceptibility to corrosion and suitability for design mechanical stresses; both of which are key factors when making run, monitor, repair or replace decisions.

The condition of metallic materials can be affected by several factors, which primarily include manufacturing defects, thermal inputs, mechanical stresses, process fluids, the environment, system design and service life consumed.

Replication is often used as part of routine inspection strategies to monitor the material condition over its service life. It is also employed to further evaluate unexpected damage revealed by NDT techniques or after unplanned events such as a fire. It can also be used as part of the quality assurance process for weld repairs.

The concept of metallographic replication is analogous to taking a fingerprint. Depending on the condition of the site to be replicated and the information required from assessment, there are two main methods utilised on plant. For deep defects where surface morphology is required a two-part putty can be used. This takes on the surface contours and sets hard for removal and assessment. The second and more common method of replication is to polish the metal surface and etch it before partially melting an acetate sheet onto the surface. Once the acetate has hardened it is peeled off and adhered to a glass microscope slide. This technique provides the full metallurgical information of the site replicated which can then be assessed remote from the component and provides a permanent physical record of the condition of the site at that particular moment in its service life.Assessment of the replicated microstructures is typically carried out by optical microscope using bright field illumination. Depending on the degradation mechanism of interest, magnifications of between x50 and x1000 are used. Some of the common degradation mechanisms that can be assessed for with respect to corrosion include the presence of sigma phase, sensitisation, stress corrosion cracking, corrosion fatigue cracking, hydrogen cracking, corrosion pitting and welding defects.