Radosław Sieczkowski, PhD Eng., Head of SPETECH^® Technical Department

Introduction

Modern chemical industry imposes increasingly stringent material requirements on installations operating in highly corrosive environments. The production of urea and caustic soda involves processes in which installation components are exposed to high temperatures, pressures, and aggressive corrosive media such as ammonium carbamate, sodium hydroxide, and chlorides. Under such conditions, conventional acid-resistant steels do not provide sufficient corrosion resistance and durability. The solution to these challenges is the use of specialized duplex (austenitic-ferritic) steels, such as SAF^TM2906 (1.4477), especially in critical components like gaskets, fittings, plugs, valve parts, etc.

Material Characteristics of SAF^TM2906

SAF^TM2906 (1.4477) is a special duplex steel manufactured by Alleima. This material was originally developed under the name SAFUREX in 1996 by Stamicarbon in collaboration with Sandvik Materials Technology (now part of Alleima) as a response to the limitations of traditional steels used in the fertilizer industry for urea production. SAFUREX remains a trademark of Stamicarbon. SAF^TM2906 exhibits exceptional resistance to all forms of corrosion. Only significantly more expensive nickel-, zirconium-, or noble metal-based alloys demonstrate superior chemical resistance in such applications. Thanks to its unique chemical composition (Table 1), including a very high chromium content, the material offers both outstanding corrosion resistance and very high mechanical strength (Table 2).

Key Properties:

• Very high resistance to general, intergranular, stress, and pitting corrosion,
• Excellent chloride corrosion resistance,
• Good weldability,
• High mechanical strength (Table 2),
• High impact strength at sub-zero temperatures (Fig. 1),
• Lower thermal expansion compared to austenitic steels, comparable to carbon steels.

Fig. 1 Fracture toughness curve

Application of SAF^TM2906 Products in Chemical Installations

In engineering practice, components made of this material are successfully used in the following applications:

• Urea production installations, especially for ammonium carbamate exposure,
• Caustic soda (sodium hydroxide) production plants – see Fig. 2,
• Installations requiring exceptional resistance to pitting and crevice corrosion, particularly due to chloride content.

Fig. 2 Corrosion rate of SAF^TM2906

SPETECH^® supplies the following products made from SAF^TM2906:

• Metal gaskets including flat, RTJ, lens-type, ballistic, double-cone type, and others,
• Welded membrane gaskets,
• Spectacle and paddle blinds,
• Diaphragms (protective elements for pressure vessel covers),
• Flanges, filter-gaskets, valve components, and other pressure equipment parts,
• Flanges, couplings, reducers, covers, etc.

 
Fig. 3 Example SAF^TM2906 Products

Machining and Welding

The aforementioned components typically require precise machining, which is relatively demanding, especially as duplex steels exhibit poor chip breakability, leading to chip removal issues. Machining also requires continuous and intense cooling, preferably under elevated pressure. Selecting machining parameters in terms of cutting speed and feed rate is also challenging. For applications requiring low surface roughness, relatively high cutting speeds are preferred, which conflicts with the lower parameters required for this steel due to its high heat generation. Therefore, parameters must be a compromise, typically falling between those for heat-resistant nickel-based alloys and stainless steels.
Welding this steel also requires high skill. A critical factor is ensuring the formation of a duplex ferritic-austenitic structure in the weld upon cooling. Welding is performed in an argon-nitrogen shielding gas atmosphere using appropriately selected welding parameters, wire diameter, current, and travel speed. Often, multiple trials are necessary to optimize parameters, and the welder’s experience is particularly valuable.

Conclusion

The use of metal gaskets made from SAFT^M2906 addresses modern challenges in the chemical industry—especially where high pressure, temperature, and aggressive media are involved. With its high corrosion resistance and excellent mechanical properties, SAF^TM2906 proves suitable for gaskets and other components. Urea and caustic soda producers are encouraged to consider standardizing such gaskets at critical points in installations, potentially leading to significant operational and economic benefits.