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Custom replica training gives insights into material microstructures

Connect, no. 164, January/February 2010, p.6

Surface of a component after replicas were taken
Surface of a component after replicas were taken

An unusual training project has given one of TWI's Industrial Members a unique insight into low cost non-destructive examination of its assets. Three members of Metalysis staff have been trained by TWI in replicating the surfaces and microstructures of in-service components for laboratory examination.

Replication of microstructures and surfaces allows many industries to derive information on the material condition of their assets without need for sampling and subsequent repair, yet there are very few opportunities to be guided by experts in the art. TWI provides customised training in a wide range of topics.

Metalysis operates a series of high temperature molten salt cells in a highly corrosive environment. It needed a quick and reliable way of examining its vessels' interiors to determine whether they had reached the end of their practical lives.

The extreme operating temperature, more than 900°C, precluded using most of the popular and available non-destructive testing techniques during operation. However if surface replicas could be made during the brief cool down periods they could be examined, away from the equipment under investigation, in the less frenetic surroundings of a materials laboratory. So Metalysis asked TWI to train its staff in two metallographic techniques.

The first involved making rubber replicas of untreated surfaces. It's a well developed technique at TWI and has been used successfully on-site for several decades. However this was the first time that TWI had provided training in reproducing rubber surface replicas to a Member company. The work went well.

It allowed Metalysis to obtain accurate three dimensional copies of a critical component's surface topography without taking it out of service. Pits and cracks were all reproduced in graphic relief in the rubber replica. The detail is sufficiently fine that, with the use of a microscope, it is a simple task to assess the extent and severity of pitting, or any other form of surface degradation. It's fast, inexpensive and useable on almost any surface, in any orientation.

The second technique taught to Metalysis involved the in-situ preparation of surfaces to reveal their underlying microstructure. The client was trained in grinding and fine polishing to a mirror finish, using standard metallographic techniques, and then schooled in how to etch the surface accurately to reveal its relevant features.

Modifications in the surface preparation can reveal a variety of different features, so a great deal of information can be obtained from a single site. This is followed by applying a specially prepared acetate which replicates the microstructure in a form that can be subjected to high power light microscopy.

The technique's versatility was demonstrated on both flat and curved surfaces, so emulating actual site conditions on plate and pipe respectively. TWI's expertise in handling, storing and interpreting the replicas was also made available to Metalysis.

The training was a complete success and TWI is keen to offer the same service to other Industrial Members. For more information, contact roger.barnett@twi.co.uk

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