The importance of the delta ferrite content (BN2) for pharmaceutical plant engineering.
Technical tenders for pharmaceutical manufacturing equipment repeatedly include the requirement to comply with Basel Standard 2 (BN2) and thus a specification for the delta ferrite content in stainless steel of less than 0.5%. The delta ferrite content has an influence on corrosion resistance - but it was also assumed that the low content would minimize the formation of rouge. But what is the current view on compliance with BN2?
Basics1
Ferrite is a phase that may precipitate during solidification of austenitic stainless steels depending on the ratios of the alloying elements. The ferritic phase consists of crystals with a body-centered cubic (bcc) lattice in contrast to the face-centered cubic (fcc) lattice of the austenitic matrix. The presence of ferrite in austenitic stainless steel welds may reduce the corrosion resistance in some corrosive environments. However, a minimum ferrite level may be required to maintain specific properties of particular product forms (e.g., castings) or is deemed necessary to prevent hot cracking of heavy wall weldments (e.g., vessels made from plate).
Fig. 1
The ferrite level of austenitic stainless steel base metal strongly depends on heat analysis, primarily the chromium to nickel ratio, product form, and final heat treatment. Whereas wrought 316L-type stainless steel materials UNS S31603, 1.4404, and 1.4435 in the solution annealed condition typically show very low ferrite levels of 0 vol. % to 3 vol.%, 1.4408 and CF8M or 1.4409 and CF3M stainless steel castings may contain 10 vol. % to 20 vol.% of ferritic phase in the austenitic matrix.
As-solidified austenitic stainless steel welds typically have higher ferrite levels than the base metal. This is caused by rapid cooling that prevents the ferrite to austenite transformation from proceeding to thermodynamic equilibrium. The ferrite level of as-solidified austenitic stainless steel welds can be determined from diagrams according to Schaeffler2, DeLong3 or the Welding Research Council (WRC) using chromium and nickel equivalents. The WRC-1992 diagram4 uses the chromium equivalent Cr (eq) = %Cr + %Mo + 0.7 %Nb and the nickel equivalent Ni (eq) = % Ni + 35 %C + 20 %N + 0.25 %Cu. Post-weld heat treatment (e.g., solution annealing of welded tubing) reduces the amount of ferrite in the weld. It should be noted that austenitic stainless steels with a high nickel content, e.g. 1.4539, and nickel alloys do not contain any ferrite in as-solidified welds.
The ferrite content in welds is measured magnetically inductively in accordance with AWS A4.2M5 or DIN EN ISO 82496.
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Influence of ferrite on the construction of pharmaceutical equipment and systems
Ferrite in the base metal and welds can have a positive or negative effect depending on the type of application, but generally offers little cause for concern in pharmaceutical engineering. Laboratory corrosion tests in highly corrosive media, e.g. 10 % hydrochloric acid, have shown that increased amounts of ferrite in the weld seam structure can reduce corrosion resistance.7 Investigations of welds with different delta ferrite contents showed no drop-in corrosion resistance up to 3 % ferrite. Only from approx. 5 % delta ferrite, a decrease in resistance to chloride-induced pitting corrosion could be demonstrated.8 However, in high-purity water systems, there have been no reported corrosion failures related to delta ferrite content in welds.9
Control of ferrite content in welds of austenitic stainless steels
In the past, numerous specifications required a low delta ferrite content of max. 0.5 % in the base metal and welds. In welds, this could usually only be achieved by one or more of the following measures:
- Post-weld solution annealing
- Use of weld filler or consumable inserts with increased nickel content
- Increase of nickel equivalent by addition of approximately 1 vol.% to 3 vol.% nitrogen to the shielding gas (argon)
These methods and the associated quality assurance measures led to increased production time and high costs. According to the current state of knowledge, ferrite control of max. 0.5 % in welds in the construction of pharmaceutical equipment and systems seems not necessary.
To avoid ferrite contents of more than 5 % in welds of austenitic stainless steels, material 1.4435 with additional restriction of analysis according to BN2 has proven itself.10 Although this works standard of the Basle Chemical Industry was withdrawn without replacement by its publishers in 2012, all major steel manufacturers have adapted to this restriction and included the material 1.4435/BN2 in their standard range. Orbital welds of tubes made of this material show on average a delta ferrite content of 3 %, i.e. not critical in terms of corrosion for pharmaceutical equipment and systems. Additional measures to reduce the ferrite content in welds, as described above, are not necessary.
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Conclusion
Worldwide, the material UNS S31603 (316L) is the most widely used material in pharmaceutical equipment and systems. The European materials 1.4404 and 1.4435 are both within the analysis limits of UNS S31603. The material 1.4404 is more in the typical lower range of the main alloying elements chromium, molybdenum and nickel. 1.4435/BN2, on the other hand, contains significantly higher contents of the three main alloying elements and shows lower ferrite contents in welds. However, this advantage is only significant in terms of minimizing the risk of localized corrosion, such as chloride-induced pitting corrosion.
About the Author
Dr Jan Rau is responsible for quality management and research & development at Dockweiler with main focus on metallurgy and surface analysis.
Noten:
1 ASME BPE-2024 Bioprocessing Equipment, Nonmandatory Appendix G - Ferrite, The American Scociety of Mechanical Engineers, Two Park Avenue, New York, NY 10016-5990, 2024.
2 A. L. Schaeffler, "Constitution diagram for stainless steel weld metal," Metal Progress, Bd. 56 (11), pp. 680-680B, 1949.
3 W. T. DeLong, "Ferrite in austenitic stainless steel weld metal," Welding Journal, Bd. 53 (7), pp. 273-s - 286-s, 1974.
4 D. J. Kotecki und T. A. Siewert, "WRC-1992 Constitution Diagram for Stainless Steel Weld Metals: A Modification of the WRC-1988 Diagram," Welding Journal, Bd. 71 (5), pp. 171-s - 178-s, 1992.
5 AWS A4.2M:2020 (ISO 8249:2018MOD) Standard Procedures for Calibrating Magnetic Instruments to Measure the Delta Ferrite Content of Austenitic and Duplex Ferritic-Austenitic Stainless Steel Weld Metal, American Welding Society, 8669 NW 36 Street, #130 Miami, FL 33166, 2020.
6 DIN EN ISO 8249 Schweißen - Bestimmung der Ferritnummer (FN) in austenitischen und ferritisch-austenitischen (Duplex-)Schweißgut von Cr-Ni-Stählen, DIN Deutsches Institut für Normung e. V. (Beuth Verlag GmbH, 10772 Berlin), 2018.
7 R. Morach, "Einfluß des ?-Ferritgehaltes auf das Korrosionsverhalten," in Conference Proceedings International Symposium on Orbital Welding, La Baule, France, 24./25. April 1997.
8 S. R. Collins und P. C. Williams, "Weldability and Corrosion Studies of AISI 316L Electropolished Tubing," in Conference Proceedings INTERPHEX, S-29, New York, March 22, 2000.
9 T. Mathiesen, J. Rau, J. E. Frantsen, J. Terävä, P.-A. Björnstedt und B. Henkel, Using exposure tests to examine rouging of stainless steel, Pharm. Eng. 21, 90-97, 2002.
10 Basler Norm BN 2: Nichtrostender Stahl nach BN 2, BCI Basler Chemische Industrie, 2006.
