What is Super Duplex Stainless steel? A detailed guide to Grade 2507, and other Super Duplex variants
What is Super Duplex Stainless steel?
Super Duplex Stainless steel is one of the most corrosion resistant types of Stainless steel, also possessing exceptional mechanical strength. It is used in a range of demanding environments, where corrosive chemicals such as acids or caustics or, more typically, permanent exposure to high salinity (ranging from seawater to hypersaline levels) will be required.
Super Duplex alloys are generally defined by the following qualities:
A ‘duplex’ microstructure (a microstructure comprised of both Austenitic and Ferritic phases);
At least 25% Chromium content.
A Pitting Resistance Equivalent Number (PREN) value of >40.
An example of the microstructure of Duplex Stainless steel, showing the Ferrite and Austenite structures.
Source: mdpi.com
The prime difference between Duplex and Super Duplex Stainless steel is that 2205 has lower Chromium, Molybdenum and Nitrogen contents, resulting in lower strength and corrosion resistance.
Resistance to pitting corrosion in Stainless steel can be measured by the Pitting Resistance Equivalent Number (PREN), which is based on the chemical composition of the material. The PREN of all Super Duplex Stainless steel Grades is 40-45.
The 3 most common Super Duplex variants are introduced below:
Note: The PRENs can vary based on slight composition changes.
Grade 2507 (UNS 32750);
Sandvik, now Alleima, led the development of probably the most widely used 2507 variant in the world today. Trademarked and marketed as SAF2507 (designated S32750) but now produced by many mills as a generic version, it is the grade most in the industry think of first as super-duplex. Originally developed for subsea applications (deep sea oil and gas umbilicals), it then found usage in heat exchanger tubing, process piping and pressure vessels.
Zeron 100 (UNS S32760).
A closer variant to S32750 than the Ferralium grade and often used interchangeably for many applications.
It was developed in the 1980’s in parallel with SAF2507 by Mather and Platt (now owned by Weir Minerals) as a cast (then wrought) alloy for centrifugal pumps for minerals processing and seawater transfer applications. The Zeron 100 trademark is now owned and marketed by Rolled Alloys.
While similar in nature and application, one is sometimes specified over the others based on nuanced differences. For example, as Zeron 100 has 0.7% Copper, this is helpful in further mitigating the attack of certain acids.
Ferralium 255 (UNS 32550 and S32520);
Developed by Langley Alloys, it is considered one of, if not the first, superduplex alloy, with a patent dating back to the late 1960’s.
While still produced in most product forms, in Australia it is predominantly seen in round bar and forgings for shafting applications (pumps, impellers, agitators and mixers), though in the USA it does share market awareness and usage with S32750 and S32760 more equally.
Due to the prevalence of S32750 in Australia, we will focus on this particular alloy regarding properties and applications. However most if not all these properties are inherent to all three super duplex variants.
Grade 2507 Pipe in stock at Stirlings.
Grade 2507 Stainless steel
Notable properties and characteristics:
Excellent resistance to stress corrosion cracking (SCC) in chloride-bearing environments;
Excellent resistance to pitting and crevice corrosion;
High resistance to general corrosion;
Very high mechanical strength;
Physical properties that offer design advantages;
High resistance to erosion corrosion and corrosion fatigue;
Good weldability.
Thanks to its superior mechanical properties, a light gauge of Grade 2507 can be used to achieve the same design strength of a thicker Austenitic 304/316 alloy, and this lower weight can result in a dramatically lower total lifecycle cost.
Characteristics in detail
General/uniform corrosion
Grade 2507 is highly resistant to corrosion by organic acids, e.g. formic and acetic acid, and alkaline solutions, suitable for use at high concentrations and temperatures. Resistance to inorganic acids is comparable to that of high alloy austenitic Stainless steels in certain concentration ranges.
In sulphuric acid, 2507 performs better than 304/316, 904L below about 25% acid concentration, and for milder applications is a cost-effective alternative to nickel-based alloys.
Source: Outokumpu
Pitting and crevice corrosion
Being a driver for their development, naturally resistance to localized (pitting & crevice) corrosion for 2507 is excellent.
Pitting Resistance Equivalent (PREN) is a useful tool for comparing the performance of Stainless steels in these environments. The contributing elements (shown below) directly and positively correlate to an improved resistance to pitting and crevice corrosion.
PREN is calculated using the following equation, typically at nominal values:
PREN = %Cr + 3.3 x (%Mo + 0.5 x %W) + (16 x %N).
PREN values above 40 are typical and commonly are as high as 42 for European produced plate, but can be extended to and beyond 43 for mill production orders.
Datasheets applicable to 2507 within Norsok M-630 normally mandate PREN >40, with many end user specifications (particularly in the oil and gas industry) requiring PREN values at 42 or slightly higher.
Comparing PREN values of lower and higher alloyed Austenitic and Duplex, and Super Duplex Grades.
Source: Unified Alloys
Stress corrosion cracking
Closely linked to localised corrosion resistance, Grade 2507 has excellent resistance to stress corrosion cracking which occurs in chloride-containing environments at elevated temperatures. Another type of stress corrosion cracking can be induced by hydrogen sulphide - good resistance to this is vital in the Oil & Gas industry. Grade 2507 is approved according to NACE MR0175/ISO 1515 for use in H2S-containing environments in Oil & Gas production.
SCC resistance in oxygen-bearing (about 8 ppm) neutral chloride solutions. Testing time 1000 hours.
Applied stress is equal to proof strength at testing temperature.
Source: Alleima
Very high mechanical strength
Super Duplex Stainless steel well outperforms its alternatives when it comes to both tensile and proof/yield strength. Although the PREN of high alloy Austenitics such as 254SMO can equal if not outperform 2507 (depending on the exact compositions), the strength of 2507 will make it the ideal choice for a combination of excellent corrosion resistance and toughness for the most demanding of environments.
Comparing the strength of different Stainless steel Grades at ASTM A240 (Plate only).
Source: ASTM
Proof strength graph showing the comparison of different Stainless steels.
Source: Alleima
Physical properties that offer design advantages
Duplex stainless steel applicable to all forming processes available for Stainless steel. High proof strength, work hardening rate, and elongation means that Super Duplex is particularly well suited to lightweight and cost-efficient applications with complex shapes.
Grade 2507 At 20 °C, typical values
Source: Alleima
Good weldability
The weldability of Grade 2507 is good – all filler bearing welding methods are applicable to Super Duplexes. While care is needed around controlling heat input and interpass temperatures to avoid intermetallic phase formations, these alloys are routinely welded by competent fabricators and approved for pressure bearing applications by ASME and other bodies.
Super Duplex Applications
Though certainly by nature a high performing alloy, perhaps one of the real highlights of 2507 is its incredible versatility. Used for resisting corrosion, mechanical loads, sometimes both at once, readily formable and weldable, widely stocked, and at a very high performance: cost ratio, it has seen extensive used in a multitude of applications.
Desalination Equipment
Widely used in Reverse-Osmosis plants, both in saline and hypersaline contacting areas as process pipe, fittings and flanges, as well as tanks, valves, and intakes.
Flue Gas Scrubbing equipment
Flue gas scrubbing is a process where pollutants are removed from industrial exhaust gases, normally by passing them through a chemical solution that absorbs and neutralises harmful components, "scrubbing" the gas clean before it is released into the atmosphere. It is often used in power plants to remove sulphur dioxide from flue gases produced by burning coal.
Offshore Oil & Gas production
Used for components like subsea pipelines, heat exchangers, and storage tanks due to its exceptional ability to withstand harsh marine environments.
Super Duplex’s resistance to sour gas environments and its high mechanical strength suits it for piping, tubing, and heat exchangers in offshore and onshore oil and gas installations.
Chemical process pressure vessels, piping and heat exchangers;
Super Duplex is ideal for pressure vessels, tanks, piping, and heat exchangers, especially in environments with aggressive chemicals, acid chlorides, and caustic solutions. .
Pulp & Paper Mill equipment
A pulp mill is a manufacturing facility that converts wood chips or other plant fibre sources into a thick fibre board which can be shipped to a paper mill for further processing. A paper mill produces paper from said pulp, as well as other ingredients.
Super Duplex is well suited to the digesters, bleaching equipment, and stock-handling systems in paper & pulp mills due to their use of corrosive chemicals.
Marine applications
Super Duplex is ideal for marine applications, including shipbuilding and permanent undersea fixtures, due to its excellent resistance to pitting and stress corrosion cracking.
Super Duplex is also occasionally used in the Food & Beverage industry, in Geothermal Wells and other highly corrosive environments.
Super Duplex Customer example
Stirlings Grade 2507 Super Duplex Stainless steel was used for the fabrication of a Submersible Pump Package for an offshore Floating Oil & Gas Platform, including this Caisson Cover. Caisson construction is a foundation used in deep water, involving a series of large, watertight cylinders that are sunk into the ground and filled with concrete. The base provides a stable foundation for structures built on top of it.
As this Caisson cover was destined for permanent submersion underwater, a material with a very high level of corrosion resistance was required - which 2507 suited perfectly.
Standards and specifications
Below details the ASTM specifications for different products as well as international equivalents.
ASTM Specifications for S32750.
Common equivalent designations to S32750.
Super Duplex stock at Stirlings
Stirlings has Grade 2507 in stock in our warehouses around Australia. We have the following products in stock, in various sizes:
Seamless & Welded Pipe
Pipe Fittings
Flanges
Round Bar
Plate
Anything that we don’t currently have in stock, we can obtain from our trusted global supply network. Contact us to make your order!
In addition - we can process your Plate to your custom requirements using our world-class Laser processing service - learn more here.
A rhyme for your time
Looking to get hold of some 2507? Come to Stirlings, you’ll be in heaven!
Sources:
alleima.com
outokumpu.com
langleyalloys.com
worldstainless.org
164580.com