Metallography vs. Fractography

[ibf]

Hi There !

The Facts: Large fabricated gear presenting "throat cracks" on some of the weldments. Some cracks were through thickness (fillet welds)and some others still showed a propagation front. "In-situ" metallography has been developed ONLY at the ones which still showed a propapgation front. After finishing the replicas, the cracks were excavated , pre-heated and repaired at site due to both the size of the gear and urgency. No replicas were taken from the ones which presented through thickness propagation.

The Question: Would Fractography be the most appropriate tool for the ones which were through thickness because they did not show a propagation front to be examined via in-situ metallography ? Or should have taken in-situ replicas of those as well ?

Thank you.

 

[metengr]

ibf;
Based on the information you provided, if you wanted to characterize the mode of crack propogation in the gear base metal as either transgranular or intergranular, in-situ replicas is probably the better choice. Provided you can get to an exposed surface to locally polish, etch and extract a surface replica of the crack for microscopic examination.

For fractography, you would be looking at a more intrusive method of trying to remove a small, wedge-shaped sample containing a portion of the crack to then expose the fracture surfaces for low magnification (sterescope) or high magnification (SEM) examination.

It would appear that the orignal fabrication fillet welds having throat cracks were either undersized for the application or, most likely, were initially deposited without adequate preheat leading to excessive restraint. What was the gear base material?

 

[ibf]

Thank you for your reply metengr.

The base metal is 4340. No cracks were detected at either base metal or H.A.Z . It sounds like that I should have taken replicas from the through thickness cracks as well. F.Y.I , due to the urgency and size (38 ft. in diameter) , the gear was repaired at site and put back into service.

Thank you.