Duplex stainless steel features a unique austenite-ferrite dual-phase microstructure, delivering far better overall performance than conventional austenitic stainless steel for petrochemical, marine engineering, clean energy, water treatment and flue gas desulfurization industries.
It boasts high tensile yield strength, strong stress corrosion cracking resistance, and outstanding anti-pitting & anti-crevice corrosion capacity, solving defects of 304 and 316L such as low strength and easy cracking under harsh corrosive media. 2205 standard duplex, 2507 super duplex and 2707 hyper duplex are three dominant grades widely adopted in industrial equipment design and material procurement.
Manufacturers classify duplex stainless steel into three tiers (standard, super, hyper) by alloy composition and corrosion resistance. Improper material selection based solely on experience either raises unnecessary project costs or shortens equipment service life due to insufficient corrosion tolerance. This guide takes PREN (Pitting Resistance Equivalent Number) as the core evaluation index to sort grade standards, performance gaps and service environment matching rules, providing actionable material selection logic for industrial buyers and engineers.
PREN Value: Core Index for Duplex Stainless Steel Corrosion Rating
PREN is a globally recognized metric to assess stainless steel local corrosion resistance, calculated by chromium, molybdenum and nitrogen content. Higher PREN means stronger resistance to pitting, crevice corrosion in chloride-rich, high-temperature, acid/alkaline and seawater working conditions. It is the key benchmark distinguishing standard, super and hyper duplex stainless steel.
|
Grade Tier |
Main Grades | PREN Range | Alloy Features |
Minimum Yield Strength |
|
Standard Duplex |
2205, 2304 | 26–39 | 2205 contains ~3% Mo; 2304 molybdenum-free; balanced N content, cost-efficient |
≥450 MPa |
|
Super Duplex |
2507, Zeron 100 | 40–45 | 25% Cr, >3.5% Mo, high nitrogen alloy for heavy corrosion environments |
≥550 MPa |
|
Hyper Duplex |
2707, 3207 | >45 | Max Cr/Mo/N, precise dual-phase balance for extreme corrosion & ultra-high strength |
≥700 MPa |
Performance & Industrial Application of Three Duplex Steel Tiers
Standard Duplex Stainless Steel: 2205 (UNS S32205/S31803)
2205 occupies over 80% of global duplex stainless steel consumption, the most mature and cost-effective grade. Its 22Cr-3Mo composition outperforms 304/316L on anti-stress and anti-pitting corrosion. Controlled nitrogen addition eliminates weld performance loss of early duplex steel and improves weldability. High yield strength allows thinner equipment walls to cut manufacturing costs and realize lightweight design.
Chemical Composition
| CHEMICAL | C | Mn | P | S | Si | Ni | Cr | Mo | N |
| UNS S31803 | 0.03 max | 2.0 max | 0.030 max | 0.020 max | 1.0 max | 4.5-6.5 | 21.0-23.0 | 2.5-3.5 | 0.08-0.20 |
| UNS S32205 | 0.03 max | 2.0 max | 0.030 max | 0.020 max | 1.0 max | 4.5-6.5 | 22.0-23.0 | 3.0-3.5 | 0.14-0.20 |
Mechanical Properties
| MATERIAL | T.S (MPA) | Y.S (MPA) | EL % | HARDNESS |
| UNS S31803 | 620 min | 450 min | 20 min | 290 max |
| UNS S32205 | 655 min | 450 min | 20 min | 290 max |
Economical 2304 serves as a direct upgrade alternative to 304. Standard duplex steel fits oil refining, chemical processing, pulp & paper, ordinary marine structures and industrial water treatment for general mildly corrosive environments.
Super Duplex Stainless Steel: 2507 (UNS S32760)
2507 PREN exceeds 40, designed for high-temperature, high-chloride and strong acid working conditions where standard duplex steel fails long-term safety requirements. High Cr-Mo-N alloy greatly lifts anti-pitting, crevice and erosion corrosion capacity, comparable to premium austenitic steel and partial nickel-based alloys.
Chemical Composition
| CHEMICAL | C | Mn | P | S | Si | Ni | Cr | Mo | N | Cu | W |
| UNS S32760 | 0.03 max | 1.0 max | 0.030 max | 0.010 max | 1.0 max | 6.0-8.0 | 24.0-26.0 | 3.0-4.0 | 0.20-0.30 | 0.5-1.0 | 0.5-1.0 |
Mechanical Properties
| MATERIAL | T.S (MPA) | Y.S (MPA) | EL % | HARDNESS |
| UNS S32760 | 750 min | 550 min | 25 min | 270 max |
It works stably in high-temperature seawater, high-chloride acid medium, oil-gas extraction and desulfurization systems, widely used in offshore platform parts, seawater desalination devices, high-temperature chemical reactors and flue gas desulfurization units. It acts as an affordable substitute for expensive high-alloy stainless steel and nickel alloys.
Hyper Duplex Stainless Steel: 2707 HD (UNS S32707)
2707 HD is the top-tier duplex steel with PREN above 45 (near 50), developed for the harshest industrial scenarios. Optimized heat treatment balances corrosion resistance of austenite and ferrite phases to avoid local corrosion failure. Its yield strength hits over 700 MPa with critical pitting temperature (CPT) ≥90°C, enduring ultra-high temperature, concentrated strong acid and extreme chloride combined corrosion.
Chemical Composition
| CHEMICAL | C | Mn | P | S | Si | Ni | Cr | Mo | N | Cu | Co |
| UNS S32707 | 0.03 max | 1.5 max | 0.035 max | 0.010 max | 0.5 max | 5.5-9.5 | 26.0-29.0 | 4.0-5.0 | 0.30-0.50 | 1.0 max | 0.5-2.0 |
Mechanical Properties
| MATERIAL | T.S (MPA) | Y.S (MPA) | EL % | HARDNESS |
| UNS S32707 | 920 min | 700 min | 25 min | 318 max |
Typical applications include ultra-deepwater oil-gas facilities (over 2,500m depth), high-concentration acid-base reaction systems, deep-sea special equipment and high-end fine chemical plants.
|
Grade Tier |
Critical Pitting Temp (CPT) | Crevice Corrosion Resistance | Applicable Working Conditions |
|
Standard Duplex |
40–60°C | Moderate |
Ambient/mild temp, dilute acid, alkali, fresh & brackish water |
|
Super Duplex |
70–85°C | Good |
High-temp seawater heat exchange, high-chloride acid, nearshore oil-gas, desulfurization |
| Hyper Duplex | ≥90°C | Excellent |
Ultra-deepwater exploitation, concentrated strong acid, extreme high-chloride high-temp corrosion |
Scientific Duplex Stainless Steel Selection Principles
2205, 2507 and 2707 have no absolute superiority; material selection must match corrosion severity, project budget and construction standards. The core rule: avoid over-specification (wasting cost) or under-specification (short service life), balancing equipment safety, lifespan and total expenditure.
|
Grade Tier |
Cost Level | Welding Difficulty |
Core Selection Standards |
|
Standard Duplex |
Medium | Easy, good site adaptability |
Replace 304/316L, prevent stress corrosion, lightweight & low-cost general industrial use |
|
Super Duplex |
Relatively High | Moderate, professional welding procedure required |
High-temp high-chloride corrosive environments; replace high-alloy steel & nickel alloys |
|
Hyper Duplex |
Premium | Strict, advanced equipment & skilled welders needed |
Only for extreme corrosion; prioritize long-term stable operation for high-end cutting-edge equipment |
Industry Development Outlook
Booming deepwater oil-gas exploitation, chemical plant upgrading, clean energy equipment and environmental desulfurization projects drive surging demand for high-strength, anti-corrosion long-service-life duplex steel.
2205 remains the mainstream material for most conventional mild-corrosion industrial scenes;
2507 continuously replaces high-alloy stainless steel and nickel alloys for medium-severe corrosion applications;
With mature production technology and falling costs, 2707 hyper duplex will see large-scale commercial adoption in deep-sea equipment and premium fine chemical facilities.
Mastering duplex steel grading rules, PREN standards and environment matching criteria helps manufacturers optimize material selection, control engineering costs and guarantee long-term equipment safe operation. Scientific grading-based selection brings enterprises refined high-end equipment material solutions and lasting technical competitiveness.