Flux Cored Arc Welding: The High Productivity Welding Process for P91 Steels

摘要

In the power generation industry, the benefits of using modified 9Cr1Mo (P91) steels in reducing structure weight, improving thermal efficiency and reliability, hence saving construction and operating costs are now widely appreciated, but these advantages can only be fully exploited if appropriate welding consumables and processes are available to produce weldments that will complement the integrity of the completed structures. At present, shielded metal arc welding (SMAW) and gas tungsten arc welding (GTAW) are the most commonly used welding processes in the fabrication of P91 steels, but because they are manual processes, productivity is limited. There are other processes available to improve welding deposition rate and duty cycle. For welding positions and components where mechanised welding is applicable, submerged arc welding (SAW) is a generally preferred and most productive process. However, for all-positional welding and particularly for fixed pipe or site welding, the ideal high productivity process is tubular flux cored arc welding (FCAW). Flux cored arc welding is already well established for welding 1CrMo (P11) and 2CrMo (P22) materials but this is still a relatively new process for P91 steels. Although a FCAW wire classification is in preparation by The American Welding Society (AWS), published performance data are lacking. This paper describes the potential productivity benefits of using FCAW for P91 steels and presents joint completion rates and time savings in comparison to other arc welding processes. The suitability and quality of the FCAW consumables and process is supported by the presentation of the latest available mechanical testing data, including creep stress-rupture strength, impact and fracture toughness of the weld metals, in comparison with other widely accepted arc welding processes. Using the fracture toughness data, a critical crack assessment has also been carried out to evaluate the acceptability of the FCAW weld metal in light of a fitness for purpose concept.