TWI Industrial Member Report Summary 1078/2016
By Michael Dodge
Following a number of in-service failures, there is significant interest in understanding the microstructures and environmental conditions that contribute to hydrogen cracking of subsea dissimilar metal welds. Through a programme of environmental performance tests under cathodic protection, the environmental performance of F22 and 8630M-Alloy 625 interfaces has been evaluated.
High resolution electron microscopy (SEM and TEM) was used to determine the microstructures responsible for the observed environmental performance.
Following a programme of characterisation and environmental testing the key findings were as follows:
- The resistance to hydrogen cracking of dissimilar metals welds may be optimised by balancing competition between HAZ tempering and new phase formation.
- Optimal PWHT times were found to be below those which led to the precipitation of carbides within a narrow band adjacent to the fusion line.
- Cracks found in a retrieved dissimilar joint indicate subsurface initiation, secondary crack coalescence and progressive propagation.