Super duplex stainless steel and duplex stainless steel differ primarily in their alloy composition and corrosion resistance. Super duplex stainless steels typically contain about 25% chromium, compared to the 22% chromium content in duplex stainless steel grades. This increased chromium content enhances the pitting corrosion resistance, making super duplex materials suitable for more aggressive environments.
Duplex Stainless Steel
Duplex stainless steel is so named because of its dual-phase microstructure, combining austenite and ferrite phases. These duplex steel grades generally feature 20–28% chromium, up to 5% molybdenum, and lower nickel content compared to traditional austenitic grades. They also include small amounts of nitrogen, which improve strength and corrosion resistance.
Super Duplex Stainless Steel
With a Pitting Resistance Equivalent Number (PREN) formula of %Cr + 3.3×%Mo + 16×%N, super duplex stainless steel ASTM grades offer a PREN value exceeding 40, compared to a maximum of 34 for standard duplex grades. This gives super duplex stainless steel enhanced resistance to pitting, making it ideal for challenging applications such as marine, offshore, and subsea environments.
By choosing the appropriate duplex material grade, industries can benefit from excellent corrosion resistance, strength, and durability tailored to their specific operating conditions.
Price Difference Between Duplex and Super Duplex Stainless Steel
While both duplex stainless steel grades and super duplex stainless steel grades offer superior corrosion resistance compared to traditional austenitic stainless steels, there is a notable price difference between them. This difference arises from the distinct chemical compositions of the two materials.
Super duplex stainless steel achieves higher resistance to pitting corrosion by increasing chromium content to approximately 27%, compared to the 22% found in standard duplex material grades. However, this enhancement affects the alloy’s dual-phase microstructure and necessitates the addition of expensive elements like nickel and molybdenum.
Nickel, a traded commodity with fluctuating prices, and molybdenum, a rare and costly element, contribute significantly to the increased cost of super duplex stainless steel ASTM grades. As a result, duplex steel grades are generally more economical, while super duplex grades provide enhanced performance at a premium price, making them suitable for more demanding applications.
Welding Duplex and Super Duplex Stainless Steels
Welding duplex stainless steels and super duplex stainless steels presents challenges that differ from those encountered with conventional austenitic stainless steel alloys. During welding, the heat-affected zone (HAZ) can face issues such as loss of toughness, post-weld cracking, and a reduction in corrosion resistance. This is why skilled welders are essential for working with these materials. Techniques like SAW (Submerged Arc Welding), TIG (Tungsten Inert Gas), and MIG (Metal Inert Gas) welding are commonly used, and proper post-welding treatment is necessary to maintain the alloy’s properties.
Duplex and Super Duplex Corrosion Resistance
The corrosion resistance of duplex stainless steel and super duplex stainless steels is enhanced by the inclusion of elements like chromium, nickel, molybdenum, nitrogen, and tungsten. This combination improves their ability to resist pitting, with values reaching up to 45 on the pitting resistance equivalent number (PREN) scale. These alloys also perform well at high temperatures without experiencing oxidation corrosion and offer strong resistance to various alkaline, acidic, and neutral environments.
Cutting Speed for Duplex and Super Duplex Stainless Steel
Due to their high strength and low thermal conductivity, duplex and super duplex stainless steels can be challenging to machine. It is recommended to use robust and rigid machines fitted with high-performance cutting inserts. Additionally, a significant amount of coolant is often required to prevent overheating and maintain efficiency. Using the right speeds and feeds is also crucial for successful machining of these materials.
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 |
Conclusion
Understanding the differences between duplex and super duplex stainless steels is essential for choosing the right material for specific applications. While both offer superior corrosion resistance compared to conventional austenitic steels, super duplex alloys provide enhanced protection, especially against pitting and stress corrosion, due to their higher chromium content. This makes them ideal for harsh environments, such as marine and subsea applications. However, their higher cost and specialized welding requirements must be considered. By knowing these distinctions, industries can make informed decisions for optimal performance and durability.
FAQs
What is the difference between duplex and super duplex stainless steel?
Duplex stainless steels have about 22% chromium, while super duplex has around 25%, offering higher corrosion resistance and strength due to the increased chromium content.
What is a super duplex stainless steel?
Super duplex stainless steel is a high-performance alloy with about 25% chromium, featuring a microstructure of both austenitic and ferritic grains, offering superior strength and corrosion resistance.
What is duplex stainless steel material?
Duplex stainless steel is a two-phase alloy with equal parts ferritic and austenitic phases, offering a blend of high corrosion resistance and increased strength.
