Here are the few steps which may lead you to dig out the failure mechanism and perform analysis to avoid recurrence in future.
1. Collect the information regarding this failure from the people at the location
2. Operating conditions at the time of weld failure
3. Collect all the data related to the weld like welding procedure qualification, procedure qualification record, inspection reports (RT, MT, PT etc.)
4. Compare the intended design conditions for the weld joint and the service conditions at the time of failure
5. Start with a naked eye visual testing along with your experienced inspectors
6. Perform applicable NDTs on the fractured surfaces which may reveal presence of any crack
7. See for direction of failure (from which end it may have initiated, see chevron marks)
8. Conduct microscopic / metallography tests to evaluate the mode of weld failure
9. Compile all the information and then conclude
10. Remember, collection of data is the key to perform any failure analysis. Get as much data as you can.

Regards,
Ashfaq
http://thepetrostreet.com