It is well known that heating of duplex, and especially super duplex, stainless steel in the temperature range 750 to 1000°C causes the precipitation of intermetallic phases (especially sigma phase, but also chi phase, nitrides and others), which can form within time periods of less than one minute. The presence of intermetallic phases can cause a severe loss in corrosion resistance and toughness.
TWI work [1] has shown that intermetallic precipitation can occur in the HAZ during welding of thin wall thickness super duplex stainless steels. There is a marked reduction of Charpy and fracture toughness when intermetallic precipitates are present. The very sharp decrease in toughness from the solution annealed (or precipitate free) state to a situation when very small amounts (less than 0.5% volume fraction) of intermetallic are present suggests that there are microstructural modifications taking place before the precipitates are visible by microscopy, see Ref. [2] and [3]. However, the correlation between Charpy energy and fracture toughness is not affected by the presence, or otherwise, of intermetallic precipitates. Charpy requirements applicable will remain therefore valid for superduplex steel joints which may contain intermetallic precipitation [4].
In conclusion, with respect to the failure modes of fracture and plastic collapse, the existence of intermetallic precipitation does not mean that the component cannot be used safely at low temperatures, as long as the toughness levels determined reach the appropriate requirements.
References
- C S Wiesner, S J Garwood and P L Bowden: 'The structural significance of HAZ sigma phase formation in 25% Cr super duplex pipework'. Proceedings of the 12 th International Conference on Offshore Mechanics and Arctic Engineering. Glasgow, Scotland, June 20-24, 1993. Vol. III A, 59 - 68. New York, ASME, 1993.
- R N Gunn and C S Wiesner, 'Effect of HAZ intermetallic precipitates on the toughness of super duplex stainless steel', TWI Research Report 624/98, September 1998 (only available to TWI Industrial Members).
- C S Wiesner, 'Toughness requirements for duplex and super duplex stainless steels'. Duplex Stainless Steels '97. Proceedings of the 5 th World Conference. Maastricht, The Netherlands, 21-23 October 1997. Book 2, p.979-990. Zutphen, KCI Publishing, 1997.
- Gunn R N: 'Intermetallic formation in super duplex stainless steel simulated HAZs'. Duplex Stainless Steels '97. Proceedings of the 5 th World Conference. Maastricht, The Netherlands, 21-23 October 1997. Book 2, p.335-345. Zutphen, KCI Publishing, 1997.