TWI are managing a TSB collaborative project named 'Verified Approaches to Life Management & Improved Design of High Temperature Steels for Advanced Steam Plants - VALID'), a three-year research programme focused on the application of advanced processes for P92 pipe welding, which started in March 2011.
The research programme include the production of P92 pipe welds with the narrow-gap TIG (NG TIG), TOPTIG, variable-polarity submerged-arc (VP SAW), flux-cored (FCAW) and electron beam (EB) welding processes. The soundness of the experimental welds is verified via non-destructive testing (NDT) and mechanical testing. Following these, an extensive creep testing programme is planned to determine their high-temperature properties.
The preparation and NDT/mechanical testing of the narrow-gap TIG has been recently completed with successful results. Before commencing the welding activities, Polysoude reviewed their database of welding procedure, to determine the process parameters to be applied when welding P92. The following were selected:
- Single-pass layer welding technique
- Narrow groove joint 9mm wide with a 2° slope, accounting for cross seam shrinkage, enabling the maintenance of a constant groove width after each welding pass
- The use of pure argon welding gas, known for its universal availability
At the same time, the various phases of the heating cycle, such as pre- and post-heating, as well as the post weld heat treatment (PWHT) conditions, were agreed by the project partners, based on the current practice. A grade 92-matching filler metal was supplied by Metrode. The heating cycle is a key factor in obtaining optimum results when welding P92, in order to ensure the required high temperature properties.
Polysoude prepared a preliminary test weld in order to develop and then optimise all welding variables, in particular the operational weldability of the selected welding wire and the cross seam shrinkage curve obtained with such consumable.
A first P92 weld was then produced on a spare pipe to confirm the welding parameters and to make the necessary adjustments. Considering that high magnetisation had been observed for this test weld (+/- 15 gauss or 1200A/m), the pipe pieces to be used for the final weld were de-magnetised prior to welding. The final weld was then carried out according to the developed procedure and conforming to the required heating cycle. The soundness of this weld was confirmed through radiographic testing (RT).
The total arc time was two hours and eighteen minutes, whereas approximately six hours would be expected for a conventional weld groove, which shows the high productivity obtainable with the narrow-gap hot wire TIG process.
Following RT, the test piece was heat treated at 760°C ± 10°C for 4 hours. Four monitoring thermocouples were installed at different locations on the inside and outside surface of the welded pipe, in order to verify that a uniform temperature could be reached and maintained, throughout the pipe wall thickness.
Prior to delivery to TWI and the other project partners, Polysoude carried out the NDT and mechanical tests typically required by international standards for the qualification of welding procedures, namely penetrant testing, RT, macroscopic examination, transverse tensile testing, hardness survey, impact testing, bend testing as well as weld metal chemical analysis and all-weld-metal tensile testing. No defects were detected and the mechanical test results were in compliance with the acceptance criteria established for the project. This allowed progression to creep testing which is currently underway at the Doosan Power Systems and E.ON laboratories.
For further information contact Marcello Consonni
Project partners: TWI Ltd, Polysoude SAS. Air Liquide UK Ltd, Centrica Energy plc, Doosan Power Systems Ltd, E.ON New Build & Technology Ltd, Metrode Products Ltd and SSE plc. The Project is managed by TWI Ltd and is partly funded by the TSB