In-situ Replica Metallography
In-situ Replica Metallography is an on-site, non-destructive technique employed for observing and documenting the microstructural integrity of many engineering metal parts, castings, forgings, welds and brazed joints, equipment, components and steel structures.
The abovementioned technique employs cellulose acetate foil to replicate the microstructure of equipment or components that cannot be moved or cut up, so as to conduct the conventional microscopic laboratory examination.
It has been proven that In-Situ Metallography (REPLICAS) is a very powerful technique for addressing many metallurgical problems, particularly when conducting failure investigations. There are many applications for the use of this technique. It can be employed as a non-destructive method for identifying materials and their microstructures, without having to remove or cut out samples for chemical analysis i.e., to distinguish between cast iron, cast steel and any other type of metallic material etc. The technique can also be employed as a quality control tool, in order to verify whether the heat treatment of various components or equipment is in the normalised, annealed, or quenched and tempered condition.
Recently it was utilised to inspect many tons of defective “Laminated” steel plate beams, at the site – in the stores and stacked on top of each other. Some were also in transit to their destination and many were still in the fabrication during the fabrication workshop. The major advantage in this case, was that it saved costs and time, by not having to cut out samples and also the subsequent patching / repair welding, etc and that the sides of the plates were examined, thus saving on handling costs, etc.
This technique can be employed for quality control and investigational purposes of brazed tungsten carbide percussion drills and bits, i.e the integrity of the brazing can be evaluated in a non-destructive method, without having to cut op the drilling component.
With regard to the replication and examination of High Temperature Materials, used for power plants, boilers, furnaces, pressure vessels, etc. It can be utilised for the following purposes.
- The identification and sizing of surface defects or cracks in the Parent Material, HAZ and the Weld Deposit.
- To establish the cracking characteristics / morphology of the components and of welds. Example: Cracked Turbine Casings.
- The microscopic assessment of the microstructural degradation of high temperature materials in service i.e. establishing the microvoiding and the creep status of the components / equipment material and the welds.
With regard to the last point mentioned above, it has been found to be a very cost effective and a well-proven technique. However, the most significant application for In-Situ Replica Metallography to date is related to the estimation of residual life of components and equipment operating at high temperatures. The assessment of actual accumulated creep microscopic damage, as a component reaches the end of its design life i.e for large power plants or refineries. This is also referred to as “Predicting the Components Remnant Life and the Planning of Future Maintenance Requirements”.
The replicas, once lifted from the polished surface, are correctly identified for traceability purposes and stored for future reference. They do not degenerate if stored correctly in specially designed slide boxes. This sequence of events allows a metallurgist to observe time-dependent phenomena, such as creep initiated cavities (voids), or the rate and type of crack propagation on any type of part, component or equipment.