The vacuum brazing of super duplex stainless steels creates some unique challenges.
Langley Alloys worked together with Krohne and TWI as part of an Innovate UK project (Adv-Flow). The objective of the project was to develop a process for the production of advanced Coriolis mass flow meters in super duplex stainless steels, opening up applications in more challenging environments such as Oil and Gas.
Vacuum brazing allows the precise assembly of the Coriolis flow meters with a high degree of accuracy and repeatability. However, both the heating rate and cooling rate are relatively slow. For standard meters constructed in Alloy 316L this is not an issue. When implementing a solution for Alloy 2205 duplex stainless steel, it became a little more challenging as there is the potential to form deleterious secondary phases during the slow cooling experienced in a vacuum. Super duplex stainless steels have a great sensitivity still to the formation of phases such as ‘sigma’, creating a challenge for the project team.
TWI, as the leading authority on joining and fabrication, were able to replicate possible process settings in their vacuum furnaces. These settings could then be replicated, and validated, by Krohne in their large-scale production facility. Furnace parameters were optimised to allow the processing of super duplex stainless steels without affecting their mechanical properties or corrosion resistance. Once the furnace parameters were identified, the project team could shortlist suitable braze alloys, based upon their wettability and the strength of the joint at the given temperatures. The project successfully produced Coriolis flow meters using super duplex stainless steel suitable for more challenging applications.
Langley Alloys carries an extensive stock of super duplex stainless steels, as bars, plates and pipes.
The available grades include:
Ferralium 255 ( S32550, 1.4507, F61).
Alloy 32760 (S32760, 1.4501, F55, Zeron 100).
Alloy 32750 (S32750, 1.4410, F53, SAF2507).