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Peace of mind assured using on-site metallography

On the surface, when minor mechanical damage was recorded to a stainless steel retaining band, installed in the workings of a power station and which is known to cause embrittlement of stainless steel. 

A full investigation of the splattered component was to be carried out without removing it and without using traditional destructive mechanical testing techniques. 

After removal of solidified copper deposits, these regions were ground and polished to a ¼ micron finish then etched to reveal the underlying grain structure.

Replicas of each etched surface were then taken using acetate film and returned to TWI for further analysis. Light micrographs were taken from several regions of the replicas. Detailed analysis of the grain boundaries of each replica showed no evidence of liquid metal penetration between the grains. 

The presence of a double line or an unusually thick line between grains is diagnostic of liquid metal penetration but no such features were detected despite panning across the entire replica area. 

As no evidence was found of underlying cracking, copper penetration into grain boundaries or other mechanical damage, TWI was able to conclude that liquid metal damage had not occurred. Quartzelec was thereby able to return the component to service. 

The work demonstrated TWI’s strong on-site metallographic capabilities, and its ability to take replicas from a wide variety of surfaces. 

While the information provided is not as extensive as that yielded by sectioning, namely a destructive technique, the components in question were not removed or damaged and continued to be used after metallographic replication. 

Combining metallographic replication with other non-destructive techniques, such as dye penetration or ultrasonic testing, yields a considerable amount of information in a matter of hours. 

TWI also offers a training course in basic on-site metallography. To learn more please email contactus@twi.co.uk

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