Duplex 2205 Fasteners
The progress made in stainless steel metallurgy since the 1970’s has resulted in Duplex 2205 becoming a recognized cost effective and technically efficient material for many corrosion applications such as air pollution control equipment or chemical tankers. In more recent times these metallurgical advances have also supported the development of an entire family of Duplex materials including “lean”, “super” and even “hyper” duplex materials. These materials have expanded the range of Duplex Stainless Steels to applications generally reserved for the 300 series and super-austenitic grades.
Over the last 20 years, the use of duplex stainless steels has increased extensively in industrial applications particularly in the chemical process, seawater desalination and off-shore industries and also the transportation of chemicals. This is the result of continuous improvements in composition and properties. In many applications, a cost effective duplex stainless solution can be proposed as an alternative to an austenitic material with at least a similar corrosion resistance but better mechanical properties. For example, UNS S32304 replaced 316L material in evaporators for seawater desalination units and UNS S32205 was used instead of 317LMN in the absorbers of wet flue gas desulfurization systems.
History of Duplex 2205 Stainless Steel
Duplex stainless steels were initially developed in the 1930’s, first to improve intergranular corrosion resistance and later to provide stress corrosion cracking resistance. These first generations of duplex materials provided good corrosion resistance as a mill product, but the HAZ toughness and HAZ corrosion resistance were compromised by high ferrite and intermetallic contents. These materials were generally used in applications where the unwelded condition was acceptable.
In the late 1960’s the advent of AOD refining allowed better melt chemistry control and the use of nitrogen as an alloying element. This was very important for the duplex materials as the nitrogen provided increased localized corrosion resistance and strength and both stabilized the austenite and retarded the formation of inter-metallics in the micro-structure. The second generation –nitrogen alloyed – materials entered the market in the late 1970’s with 2205 emerging as the work horse of these grades. Since then several “lean” and “super” duplex materials were developed and most recently a “hyper” duplex, expanding the “family” of materials to encompass a complete range of corrosion protection comparable to the austenitic stainless steels.
The cost advantage of duplex stainless steels over austenitics (which contain more nickel and molybdenum) has become more and more evident during the last several years, with the continuous price escalation for alloying elements. Duplex grades have become increasingly more attractive when compared to 317LMN and 6 Mo in many applications. Due to their lower Ni and Mo contents, duplex stainless steels are less sensitive to raw material price fluctuations when compared to the austenitics. This allows better cost stability for long term project requirements.
Even now, when the raw material prices are de-escalating, proper grade selection and use of the corrosion, mechanical and physical property advantages will result in a more cost-effective solution process equipment construction.
Duplex 2205 Fasteners Composition
The duplex family is a group of materials which have a “roughly” balanced austenite/ferrite structure, normally expected to be in the 40/60 range but fully functional in the 30/70 range. The compositions are characterized by high Chromium (21% –27%), low Nickel (1% – 7%), moderate to no Molybdenum (0.5% – 5.0%), Nitrogen (0.10% – 0.40%) and in some materials additions of Copper and Tungsten. Proper balancing of these elements provides optimum performance and thermal stability for each alloy.
Corrosion Resistant Fasteners
The duplex materials will typically offer a comparable or improved resistance to general corrosion relative to the “equivalent” austenitic material. They provide good performance in both reducing and oxidizing acids as well as caustic environments. General corrosion resistance can vary greatly with changes in concentration, pH, temperature and impurities. It is important to discuss these variables for any application to ensure the proper grade is selected.
Localized corrosion (pitting and crevice) resistance is often characterized by the PREN (Pitting Resistance Equivalent Number). The PREN is calculated by using a regression formula which summarizes the cumulative effect of those alloying elements which affect pitting performance, commonly expressed as: PREN = Cr + 3.3Mo +16N. The duplex materials have PRENs ranging from 26 to 49. It is important to note that a PREN differential of 1-3 points will generally provide similar performance while a differential of at least 5 or more will be required to achieve significant improvement.
Chloride stress corrosion cracking resistance is one of the most important advantages of the duplex materials. The 300 series materials are susceptible to CSCC at relatively low operating temperatures and only the more costly 25% Nickel grades provide adequate protection. The duplex materials can provide significant improvement in CSCC resistance and a more cost-effective solution than the austenitics.
Mechanical and Physical Properties of Duplex Fasteners
The duplex materials have roughly twice the yield and 25 –35% higher tensile strength compared to the austenitic grades. This often allows the vessel designer to achieve significant reductions in wall thickness and resultant savings in material and fabrication costs. In addition to the strength advantages these materials have a higher hardness which can be increased further by cold working. While they are not to be considered wear resistant materials, they can provide improvements in performance compared to the austenitics in abrasive or abrasive/corrosive environments. Table 4 shows the ASTM maximum hardness.
The thermal expansion characteristics of the duplex materials are between those of the austenitics and carbon steel but significantly closer to the carbon steel materials. This allows for easier dissimilar metal joint design and fabrication and less need for expansion joints. They also provide a somewhat higher (~5%) heat transfer capability and subsequent performance improvements in heat exchanger applications.
Fatigue strength is another advantage. The higher strength means that the material can be cyclically stressed to higher levels without failure. The stress levels are approximately 1.5 times those expected for the austenitics. The only precaution is that fatigue failures will occur more easily in duplex materials if there is a notch present such as undercut along a weld. Fracture toughness is one area where the austenitics have the advantage. Duplex materials begin a ductile –brittle transformation at about -50°C while the austenitics can be used at cryogenic temperatures. With proper operating precautions the duplex materials have been used down to – 100ºC, but the generally accepted minimum operating temperature is -50°C.
Common Uses of Duplex Fasteners
- Air Pollution Control Equipment
- Chemical Tankers
- Chemical Processing
- Seawater Desalination Plants
- Off-shore Industries
- The Transportation of Chemicals
Duplex 2205 Fasteners (UNS S31803)
Standard products (hex bolts, nuts, screws, threaded rods) readily available.
Common Trade Name: Duplex 2205 Fasteners
Specification: ASTM A240/A240M
Approx. Room Temp. Mechanicals: UTS = 89.92 ksi; YS = 65.26 ksi;
Typical % Compositions Summary: C .03 max., Mn 2.0 max., Si 1.0 max. P .03 max., S .02 max., Cr 21-23, Mo 2.5-3.5, Ni 4.5-6.5, N .08-.20
Corrosion Resistance: Duplex 2205 is a (two phase) ferritic-austenitic stainless steel with high strength and excellent corrosion resistance, in annealed condition with a maximum Brinell hardness of 290 (Rc31). High yield strength, twice standard austenitic stainless steel grades, good fatigue strength plus excellent resistance to stress corrosion cracking, crevice, pitting, erosion and general corrosion in severe environments. It has better resistance to sea water than grade 316.