In situ metallography (field metallography) is used for micro-structural evaluation of plant structural components during service. Regular metallography consists of preparing a section of a material in laboratory by usual grinding, polishing and etching sequence for revealing microstructure and then examining the same under an optical microscope in the laboratory.

In the in situ metallography, instead of sectioning a component for metallograpic examination, the surface of it is metallographically prepared 'in situ' by using portable grinding, polishing and electro-polishing units and it is a non-destructive method. Microstructure can be examined on the prepared spot using a portable microscope or can be registered on a replicating tape which will reveal the microstructure on processing in the laboratory.

Non-destructive testing in conjunction with metallography is extensively used in quality evaluation of components not only during stages of manufacturing and installation, but also during in-service and maintenance. The great advantage of metallographic examination is that it can provide valuable information regarding the origin and nature of flaws and thus plays a key role in failure investigation. It also helps in determining whether indications obtained from NDT methods such as ultrasonics and magnetic particle testing are due to metallurgical structure or due to flaws present in the component being tested. Indications of magnetic particle examinations are re-checked by in situ metallographic examination to find out ghost indications due to variations in chemistry and heat treated conditions of welds. Sometimes, it becomes impractical to adopt satisfactory quality control techniques and therefore process is controlled by using combination of metallographic examination and statistical techniques.

SIGNIFICANCE OF MICROSTRUCTURAL OBSERVATION :-

Micro-structural features are very useful in assessing microstructural degradation, damage assessment and life prediction. Periodic measurements made during in-service inspection help in knowing any unanticipated degradation in components, thus avoiding premature failures. Many a time, after an accident, like accidental increase in temperature and pressure, fire accident etc., it is necessary to confirm that the metallurgical conditions of the components are not altered and the components are acceptable. This can be successfully verified using in situ metallography.

APPLICATIONS -

Metallographic examination is used for the following applications: -

1. Evaluation of material condition
2. Condition monitoring of components
3. Failure analysis
4. Assessment of creep damage, temper embrittlement etc.

CODES AND STANDARDS -

• ASTM E 340 - Macro-etching of metals and alloys
• ASTM E 380 - Preparation of metallographic specimens
• ASTM E 1351- Production and evaluation of field metallographic replicas.
• ASTM E 1180- Preparing sulphur prints for macro-structural examination