The Pitting Resistance Equivalent Number (PREN) is usually the most crucial number when engineers are defining the fasteners to be used in high-stress, corrosive conditions. Although the basic marine environment will tolerate standard stainless steels such as 316, the specialized performance of duplex and super duplex families is required in the high-chloride environments.
The selection of these two does not solely depend on budget, but rather depends on the level of corrosion resistance and force needed by the application. Knowledge of the performance level between these two grades is the difference between a fastener that works and one that does not work, usually without having any warning.
What is Duplex Steel? The Dual-Phase Advantage
Duplex stainless steels are so called because of a two phase microstructure, which is formed by about 50% austenite and 50% ferrite. This is a metallurgical hybrid which takes on the ideal characteristics of both: the resistance to stress corrosion cracking of ferritic steel, and the simplicity of fabrication and corrosion resistance of austenitic steel.
The dual phase structure produces a yield strength of about 450 Mpa minimum which is about twice the normal 220 Mpa of 316L austenitic grades. Moreover, the materials comply with high standards of NACE MR0175/ ISO 15156, hence are likely to be the material of the choice in offshore approvals and sour gas conditions. When choosing duplex steel products, you are investing in a material that will resist pitting corrosion and crevice corrosion around the environment in which the use of 300-series steel would not survive.
Duplex 2205 Fasteners (UNS S31803 / S32205)
2205 is the most popular grade of duplex family. These fasteners are designed with a balanced chemistry of chromium, molybdenum, and nitrogen, and are commonly known by their Unified Numbering System (UNS) designations of S31803 and S32205, which are of higher purity.
Having PREN of about 35, duplex steel UNS S31803/S32205 fasteners are very efficient in preventing chloride-induced stress corrosion cracking (CSCC). Duplex 2205 has a continuous service rating of about 300degC with sigma phase embrittlement taking place above this temperature. They are usually used as duplex stud bolts in oil and gas topside applications, chemical processing facilities, and marine applications where the level of the chloride content is moderate. They offer an economical substitute of nickel alloys and possess better mechanical properties than a typical stainless steel.
These are offered in such forms as Duplex Stud Bolts, hex bolts, hex nuts, washers, and threaded rod all of which are available in ASTM A182 / ASME B18 standards.
Welding & Fabrication
Matched filler wire (ER2209) can be used in welding duplex 2205. The regulated nitrogen level in the S32205 revision assists in balancing the phases in the area of the heat. Standard qualified procedures can be used to perform field welding.
Super Duplex 2507 Fasteners (UNS S32750 / S32760)
At levels of aggression in the environment, i.e. an increase in temperatures, salinity or acidity, standard duplex is strained. This is where the Super Duplex steel fasteners can be used.
It has the UNS S32750 and UNS S32760 (2507) as the primary grades. They include more Chromium (25 percent) and Molybdenum (4 percent), which elevate the PREN number to 42 or more. This jump in PREN is essential in immersion of seawater and also in underwater applications where pitting is inevitable.
UNS S32760 (Zern 100) also pushes the PREN to over 45, and adds extra resistance in acidic and reducing chloride corrosive environments - a difference over S32750 base grade.
Super Duplex Fasteners and Super Duplex Stud Bolts have been found to be better than 2205 in desalination plants, in deep-sea exploration due to ability to resist localised corrosion in stagnant seawater settings which would soon lead to the rapid degradation of lower grades.
Welding & Fabrication
Super duplex needs to be heat fed in order to maintain phase balance - field welding requires qualified procedures and pre-qualified filler metals. Preheat and interpass temperature need to be highly controlled in order to avoid sigma phase formation.
PREN Comparison: Key Differences at a Glance
The Pitting Resistance Equivalent Number (PREN) is calculated using the formula: PREN = %Cr + 3.3(%Mo + 0.5%W) + 16(%N). This value dictates the grade's ability to resist pitting in chloride-rich media.
| Grade | UNS Number | Typical PREN | Yield Strength (MPa) | Max Service Temp (degrees C) | Max Chloride (ppm) | Typical Use Case |
|---|---|---|---|---|---|---|
| Duplex 2205 | S31803/S32205 | 34-36 | 450 min | ~300 | Moderate | Oil and gas topside, chemical plants |
| Super Duplex | S32750 | 41-43 | 550 min | ~260 | High/Extreme | Subsea, desalination, seawater piping |
| Super Duplex | S32760 | 41-45+ | 550 min | ~260 | Very High | Marine acidic environments |
Standards Compliance Reference
| Grade | ASTM Stud Standard | ASTM Nut Standard | NORSOK Approval |
|---|---|---|---|
| Duplex 2205 (S31803/S32205) | ASTM A182 / duplex-equivalent stud specifications | ASTM A194 Gr. 4 | MDS D54 |
| Super Duplex S32750 | ASTM A182 F53 | ASTM A194 Gr. 7 / matching SD nuts | MDS D56 |
| Super Duplex S32760 | ASTM A182 F55 | ASTM A194 Gr. 7 / matching SD nuts | MDS D57 |
Cost note: Super duplex alloys typically carry a 30–50% material cost premium over duplex 2205 due to higher Cr, Mo, and Ni content, offset by longer service life in aggressive environments.
Industry Applications: Where These Fasteners Excel
The failures of any structural system are fasteners. The selection of appropriate alloy will guarantee structural integrity on a long-term basis in the following industries.
- Subsea equipment and offshore platforms: Bolts connecting the offshore platforms to the land have to be able to withstand constant spray of sea water and other extreme mechanical forces. The standard product in the industry of subsea manifolds and trees is Super Duplex S32750.
- Desalination plants: Fasteners of desalination plants are subjected to brine of high concentration. The Molybdenum in super duplex is high hence crevice corrosion is avoided in such severe environments.
- Chemical and pulp/paper: Such industries are usually occupied with bleaching agents and acidic solutions. In this case, Duplex 2205 is often adequate, which offers special alloy applications with high strength-to-weight ratio.
- Marine tankers: It is used in manufacturing of cargo tanks and the piping system of chemical tankers where flexibility in corrosion resistance is needed.
- Power generation and heat exchangers: In heat exchanger tube sheets and in feed-water piping, Duplex 2205 finds wide specification in the eventuality that both corrosion and mechanical fatigue resistance is needed over moderate temperatures.
Selecting the Right Grade
The decision between duplex 2205 and super duplex 2507 is reduced to the fact that the corrosivity of the operating environment is a single variable. In moderate exposure conditions of chloride at topside installations or inland installations, duplex 2205 is economical and technically effective. In subsea, brine or high-temperature seawater applications, the greater PREN of super duplex is not specifiable, it is the specification floor.
Frequently Asked Questions
Can I replace super duplex with duplex to save cost?
It is only possible in case the environment permits it. When the service is high temperature seawater or high chloride levels, downgrading could cause pitting and disastrous failure within a short time to duplex 2205. Nevertheless, in the case of topside or inland chemicals, duplex 2205 is more cost-effective and is usually quite satisfactory.
What nut standard pairs with S32750 studs?
ASTM A194 Grade 7 or Grade 4 nuts are typically used, though, to ensure the greatest degree of consistency of corrosion, it is strongly encouraged to use UNS S32750 studs with corresponding Super Duplex nuts to eliminate galvanic corrosion and to achieve uniform thermal expansion.
Why is chloride stress corrosion cracking important?
Stainless steels of standard 300-series are very vulnerable to CSCC in warm chloride conditions. Both duplex and super duplex fasteners contain the ferritic phase which is virtually impervious to such kind of failure at up to 250degC.
What is the difference between UNS S31803 and S32205?
S32205 is a more purified version of S31803 with increased composition control, especially on nitrogen content, to cause more reliable corrosion behavior and weldability. In the case of new specifications, S32205 is desirable.
Are duplex stud bolts compatible with carbon steel flanges?
Duplex stud bolts are regularly paired with carbon steel flanges in topside assemblies. Verify torque specifications separately, as the higher yield strength of duplex requires adjusted torque values versus carbon steel fasteners.
Get Expert Guidance for Your Project
Selecting the right fastener grade requires a deep understanding of your operational environment. Whether you need the high-strength reliability of UNS S31803 or the extreme corrosion resistance of UNS S32760, Unifit Fastener’ technical team can help you confirm the right grade before you commit to a specification.
Get in touch with a question, or explore our duplex fastener range to understand what's available for your application.