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| Steel | Duplex Stainless Steel | 304 Stainless Steel |
| Metallurgical Structure | Austenite and ferrite (approx. equal %) | Austenite |
| Chromium Content | 20-28% | 18-20% |
| Nickel Content | Lower, up to 9% | 8-10.5% |
| Molybdenum Content | Up to 5% | None |
| Nitrogen Content | 0.05-0.50% | None |
| Pitting Corrosion | Resistant, particularly chloride | Sensitive to warm chloride environs |
| Strength | Higher, 0.2% proof strength of 450-550 MPa | Lower, 0.2% proof strength of 280 MPa |
| Applications | Offshore oil and gas industry, petrochemical industry | Food handling and processing equipment, machinery parts, utensils |
Stainless steel type 304 is the kind of stainless steel used most often. It is constructed out of austenitic stainless steel and includes chromium (between 18% and 20%) and nickel (between 8% and 10.5%), which are the principal non-iron metals included in the alloy. It rusts less easily than conventional steel and is often favored because it can be easily shaped into various shapes, which is one of the reasons it is used. At Firth-Vickers, W. H. Hatfield created the recipe for this steel in 1924. The steel was marketed under the brand name "Staybrite 18/8."
The International Organization for Standardization recognizes this kind of steel as one of the grades available for use with steel products (ISO). It is often referred to as A2 stainless steel outside of the United States, which is also the name that is used for the material in ISO 3506 when discussing fasteners. It is often referred to as 18/8 stainless steel within the cookware business. The unified numbering system is denoted by the designation UNS S30400, whereas the designation SUS304 refers to the material grade in Japan. In addition, the European standard 1.4301 includes a set of criteria for this type of steel.
Duplex stainless steels are a kind of stainless steel whose metallurgical structure consists of two phases in about equal proportions: austenite (face-centered cubic lattice) and ferrite (body-centered cubic lattice). Since these grades include both austenitic and ferritic phases, they are referred to as "duplex" or "austenitic-ferritic." They're tougher and more corrosion-resistant than typical austenitic stainless steels like Type 304 and Type 316, and they're optimized to withstand the effects of chloride stress corrosion and chloride pitting corrosion. For one thing, they are built to withstand more chloride stress and pitting corrosion than conventional materials. The primary difference in composition between austenitic stainless steel and duplex stainless steel is that the former has 20–28% more chromium, while the latter contains up to 5% more molybdenum and up to 9% less nickel, in addition to 0.05–0.50% nitrogen. Because of its excellent strength and low nickel content, which permits thinner sections, this material may yield considerable cost savings.
In the offshore oil and gas business, duplex stainless steels are widely used for pipework systems, manifolds, risers, and other applications relevant to the industry due to their resistance to corrosion and strength. In the petrochemical sector, they are also often used to construct pipelines and pressure vessels. In addition, the corrosion resistance of duplex steels is far higher than that of stainless steels in the 300 series, and these steels are also considerably more robust. For instance, the 0.2% proof strength of stainless steel type 304 is around 280 MPa (41 ksi), but the proof strength of stainless steel type 22%Cr duplex grade is approximately 450 MPa (65 ksi), and the proof strength of super duplex grade is at least 550 MPa (80 ksi).
When choosing the right type of steel for your project, the price can be a significant factor to consider. In this regard, Duplex Steel and Super Duplex Steel are relatively more expensive than 304 Stainless Steel, with prices of ₹750 and ₹820 per kilogram, respectively. Whereas 304 will cost around ₹350 per kg.
Overall, the choice of which type of steel to use for your project will depend on your specific requirements and budget. If you require a high strength and corrosion resistance level, Duplex Steel and Super Duplex Steel may be more suitable than 304 Stainless Steel.