Hot tapping, or in-service welding, of C-Mn steels (i.e. welding onto a pipe while the pipe contents are still pressurised and flowing), is common in the upstream oil and gas industry. This is done for system modification and could involve addition of branch connections, valves or by-pass piping. With duplex stainless steels (DSS) now in similar service, the need for in-service welding of these steels will undoubtedly arise. However, in-service welding raises a number of technical challenges for DSS which are not present for C-Mn alloys, namely whether the phase balance of the weld deposit and the HAZ will be affected by the very rapid weld cooling experienced by hot tap welds and if so, would this also impact corrosion performance and mechanical properties.

The current study has analysed the effects of welding heat input, heat sink conditions of the run pipe, single vs multi-pass welds and the use of N₂-bearing shielding gases on the phase balance and mechanical properties and corrosion resistance of hot tap welds in 22% Cr DSS. A  laboratory parametric study was supported by full size branch connection fabrication to demonstrate the practical feasibility of the welding procedures developed and to establish the corrosion and mechanical properties.

Hot tapping of DSS under a variety of welding conditions and run pipe heat sink severity has been demonstrated to be feasible. Acceptable microstructures, corrosion performance and mechanical properties were developed for a broad range of welding conditions and run pipe heat sink severity. Some limitations have been recommended for application of the data in terms of the alloying content of the steels to be welded. Nevertheless, the data do have widespread applicability and should enhance long term component reliability and safety of hot tap connections.