The combination of high strength and corrosion resistance of duplex stainless steels (DSSs) has promoted their use in subsea environments, particularly for the oil and gas industry. However, in the past 20 years, costly in-service failures of DSS components have frequently been encountered, most of which were attributed to hydrogen induced stress cracking (HISC). This cracking mechanism occurs once a susceptible microstructure coexists with critical levels of hydrogen and stress. In subsea environments, the use of cathodic protection (CP) is a source of hydrogen and the critical stress levels can be reached locally by operation and residual stresses generated during the manufacturing and installation processes. In order to prevent HISC failures, the microstructure, sources of hydrogen and stress levels have to be determined and controlled.
An important factor contributing to the total stress experienced by a welded component is the residual stresses induced by welding. In a previous research projects conducted at TWI, residual stresses were measured in a full-scale girth weld between a DSS flange and pipe which had been in service, subsea, for 12 years.
This follow-on work, on the same component, tested a number of small-scale cross-weld specimens, under CP to evaluate resistance to HISC of the weld. The observations showed that the cross-weld specimens exhibited superior resistance to HISC than that of the all-parent pipe and flange specimens. This was unexpected, and it was thought that this could potentially be explained by the influence of any compressive residual stresses remained in the small-scale test specimens used for the environmental testing programme.
The present work provides the residual stress measurements in small-scale cross-weld specimens, using the neutron diffraction technique, and attempts to explain the relationship between the HISC testing results and residual stresses measured.