Metallographic Replicas on In-Service Equipment

A metallographic replica is a non-destructive test that copies the microstructure of a component surface without removing material from the wall. A delimited area receives progressive polishing and a chemical etch that reveals the grain boundaries and the phases present; an acetate film is then applied and, once cured, carries the relief of that microstructure. The film is afterwards analysed in the laboratory under an optical microscope, at whatever magnification the case requires.

What the replica shows is the actual condition of the material after thousands of hours at temperature, not the condition of a reference specimen. In the carbon and low-alloy steels of boilers and steam lines the technique identifies creep cavities aligned along the grain boundaries, spheroidisation of the pearlite, graphitisation, decarburisation and grain growth. The progression of those stages places each area on a damage scale and provides the microstructural input for a remaining-life assessment.

The test is carried out on the installed equipment: there is no need to cut the wall, weld a patch or replace the section under examination. That is why it is used on boiler superheaters and reheaters, steam headers, furnace tubes and coils, reformers and high-temperature steam sections, where extracting a specimen would force a replacement. Compañía Técnica delivers a report with the micrographs, the damage rating of each area and the exact location of every replica on the equipment.

The evaluation criteria follow ASME practice for pressure components and ASNT guidance for qualification of the method, with personnel qualified to IRAM-ISO 9712.