In selecting fittings to be used in the pipe in critical applications in industries like oil and gas, petrochemicals industries, marine, and power generation, in most cases materials that include Duplex and Super Duplex steel are used. These materials are well known for their very good resistance to corrosion and high strength and capacity to go through extreme environments. In this blog we shall explore further the difference between Duplex and Super Duplex Pipe Fittings, the advantage and the application of the two in different industries.
What are Duplex and Super Duplex Steel?
Duplex Steel is a type of stainless steel which is a blend of austenitic and ferritic micro-structures. This composition leads to high strength, improved corrosion resistance and hardness over normal austenitic stainless steels. Chromium (22-25%), molybdenum (3-5%), and nickel (4.5-6.5%) are commonly found as duplex steel pipe fittings. This composition enables the duplex steel to be very effective in a highly aggressive environment where the conventional materials would collapse.
Super Duplex Steel is another innovation of duplex steel as it was created to provide even better anti-corrosion properties and strength. Super duplex steel pipe fittings have a higher chromium (25-30) and molybdenum (4-6) content and slightly lower levels of nickel and thus offer better pitting, crevice corrosion and stress corrosion cracking resistance. Due to its properties, it is best suited to work in environments with high levels of corrosion especially in the offshore oil and gas industry, desalination plants, and chemical processing.
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The Price difference between duplex and super duplex
Although both duplex steel grades and super duplex steel grades possess the properties of resistance to corrosion more so than conventional alloys of austenitic stainless steel, the ability to resist pitting in particular is an attribute that is superior in the super duplex grades of stainless steel. Another trait worth noting is resistance to pitting, and this is done by adding more chromium to approximately 27% in comparison with the already present 22% in standard duplex grades. The dual microstructure of the alloy however changes with increase in chromium content. Hence, the chemistry of super-duplex stainless steels will be in the form of nickel additions as well as other elements. Nickel is an expensive traded commodity metal because the price is fluctuating. Molybdenum is, on the other hand, a rare element that is costly. The price of super duplex alloys is likely to rise by adding more nickel and molybdenum contents in the alloy.
Duplicating and super duplex stainless steels through welding
Contrary to the traditional austenitic stainless steel alloys where the base metals are influenced in the process of welding, the HAZ is an issue in these alloys. As an example, the key problems that are observed in the HAZ or heat affected weld zone, during welding involve loss in toughness, post-weld cracking along with reduction in the corrosion resistance properties of alloys. This is what makes the skilled welders have to weld either grade by the use of SAW, TIG and MIG methods with the alloys receiving an appropriate post welding treatment.
Duplex and super duplex resistance to corrosion
As described earlier, the corrosion resistance behaviour of the alloys by adding chromium, nickel, molybdenum, nitrogen and tungsten is enhanced. The pitting resistance may be appreciated to be up to 45. Besides, they can be employed at high temperatures and no oxidation corrosion is experienced. The alloys are also less resistant to a variety of alkalines, acidic and neutral solutions.
Duplex and Super Duplex Chemical Composition
| Trade Name | UNS | Chemical Composition | Min.Tensile (KSI) | Min.Yield (KSI) | Elongation % | Super Duplex American Standards |
|---|---|---|---|---|---|---|
| SAF 2507® | UNS S32750 | C 0.030 max Cr 24.0-26.0 Cu 0.5 max Mn 1.20 max Mo 3.0-5.0 N 0.24-0.32 Ni 6.0-8.0 P 0.035 max S 0.020 max Si 0.8 max | 116 | 80 | 15 | ASTM A182 F53, A240, A276, A479, A789, A790, A815, A928, A988 SAE J405 |
| UNS S32760 | C 0.03 max Cr 24.0-26.0 Cu 0.5-1.0 Mn 1.0 max Mo 3.0-4.0 N 0.2-0.3 Ni 6.0-8.0 P 0.03 max S 0.01 max Si 1.0 max W 0.5-1.0 | 109 | 80 | 25 | ASTM A182 F55, A240, A276, A314, A473, A479, A789, A790, A815, A928, A988 SAE J405 | |
| SAF 2205® 22Cr | UNS S31803 | C 0.30 max Cr 21.0-23.0 Mn 2.00 max Mo 2.50-3.50 N 0.08-0.20 Ni 4.50-6.50 P 0.030 max S 0.020 max Si 1.00 max | 90 | 65 | 25 | ASME SA-182 (F51), SA-240, SA-798, SA-790, SA-815 ASTM A182 (F51),A240, A276, A479, A789, A790, A815, A928, A988 SAE J405 |
| SAF 2205® 22Cr | UNS S32205 | C 0.30 max Cr 22.0-23.0 Mn 2.00 max Mo 3.00-3.50 N 0.14-0.20 Ni 4.50-6.50 P 0.030 max S 0.020 max Si1.00 max | 95 | 65 | 25 | ASME SA-182 (F60), SA-240, SA-798, SA-790, SA-815 ASTM A182 (F60), A240, A276, A479, A480, A798, A790, A815, A928, A988 SAE J405 |
Duplex and Super Duplex Stainless Steel Equivalent Grades
| STANDARD | WERKSTOFF NR. | UNS |
|---|---|---|
| Duplex 1803 | 1.4462 | S31803 |
| Duplex 2205 | 1.4462 | S32205 |
| Super Duplex S2507 | 1.4410 | S32750 / S32760 / S32950 |
The Bottom Line
Duplex and Super Duplex pipe fittings are two very essential materials in corrosion-resistant and high-strength industries. The selection of fittings between Duplex and Super Duplex types depends more on the environmental conditions they will face and the performance requirements of the application. Although Duplex pipe fittings possess a good ratio of strength and corrosion resistance with reduced price, in case of circumstances that are more demanding like high-pressure and highly corrosive, it will be preferable to use Super Duplex pipe fittings.
With technical experience and quick service, Kalpataru Piping Solution is the solution provider of your most specific piping needs in the fields of chemicals, petrochemical, marine, and power generation. Get in touch with us and have an expert guide you, provide you with a rapid quote or talk to you about your material choice- upgrade your plant and project reliability with Kalpataru Piping Solution today.
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