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304 and 316 Stainless Steel

The main types of stainless steel are: austenitic stainless steel, ferritic stainless steel, duplex stainless steel (duplex usually refers to: austenite-ferrite two phases), martensitic and precipitation hardening stainless steel.

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What do the codes for stainless steel 304 and 316 mean?

The main types of stainless steel are: austenitic stainless steel, ferritic stainless steel, duplex stainless steel (duplex usually refers to: austenite-ferrite two phases), martensitic and precipitation hardening stainless steel. Among them, austenitic stainless steel is the most widely used, accounting for more than 2/3 of the total amount of stainless steel produced in the world. This is because compared with other types of stainless steel, it has the characteristics of corrosion resistance, good ductility, high and low temperature resistance and good weldability, and is usually non-magnetic.

Different types of stainless steel

Different types of stainless steel

So what do the codes for the most common 304 and 316 stainless steels mean?

Before understanding their meaning, it is necessary to understand their origin. In fact, this three-digit name originated from the American Iron and Steel Institute (AISI), which published these numbers in its “Steel Products Handbook” in 1974. The material number can be used to understand the type of different stainless steel materials, and also provides important information about their chemical composition, physical properties and scope of application. Afterwards, these grade numbering systems were widely used in the United States, where they originated, and were recognized by most stainless steel specification setters and users. Then they were widely used in other countries. For example, stainless steel in Japan also uses the AISI name, but with the letters SUS (steel use stainless) in front, such as SUS 304.

Later, the introduction of the “Uniform Numbering System” jointly formulated by ASTM (American Society for Testing and Materials) and SAE (American Society of Automotive Engineers) made the AISI grade classification more specific, but the numbering used the initial letter S + five-digit identification, such as UNS S30400 for 304, UNS S30403 for 304L.

Europe has another system, following the EN standard formulated by the European Committee for Standardization (CEN), such as EN 1.4301 for 304 stainless steel. Finally, the ISO standard took into account the differences between the two major factions in Europe and the United States, and combined the two. For example, the material number of 304 stainless steel is: 4301-304-00-I, with the European number in the front and the American number in the middle.

Chemical Composition Comparison Between 304 and 316 Stainless Steel

Element 304 (% wt) 316 (% wt) Key Differences
Carbon (C) ≤ 0.08 ≤ 0.08 Both have low carbon content, improving corrosion resistance.
Silicon (Si) ≤ 1.00 ≤ 1.00 Similar levels.
Manganese (Mn) ≤ 2.00 ≤ 2.00 Similar levels.
Phosphorus (P) ≤ 0.045 ≤ 0.045 Similar levels.
Sulfur (S) ≤ 0.030 ≤ 0.030 Similar levels.
Chromium (Cr) 18.0 – 20.0 16.0 – 18.0 304 contains slightly more chromium, enhancing oxidation resistance.
Nickel (Ni) 8.0 – 10.5 10.0 – 14.0 316 has higher nickel content, improving corrosion resistance.
Molybdenum (Mo) 2.0 – 3.0 316 contains molybdenum, significantly enhancing resistance to pitting and crevice corrosion.
Nitrogen (N) ≤ 0.10 ≤ 0.10 Similar levels.

Key Differences

  1. Molybdenum Content: 316 contains 2.0-3.0% molybdenum, which significantly improves resistance to pitting and crevice corrosion, especially in chloride-rich environments.
  2. Nickel Content: 316 stainless steel has a higher nickel content, enhancing its resistance to acids, bases, and corrosion.
  3. Chromium Content: 304 has slightly more chromium, but its overall contribution to corrosion resistance is less significant compared to molybdenum and nickel in 316.

Applications

  • 304: Commonly used for everyday kitchenware, utensils, and decorative architectural materials.
  • 316: Ideal for environments requiring higher corrosion resistance, such as marine applications, chemical processing equipment, and medical instruments.
304 and 316 Stainless Steel

304 and 316 Stainless Steel

Let’s go back to the meaning of the code. Numbers starting with “2” and “3” usually represent austenitic stainless steel, while martensitic and ferritic stainless steel usually start with 4. Therefore, when we see the “3” at the beginning of 304 and 316, we know that they are both austenitic stainless steel. However, the two digits after “3” have no special meaning, they are just the serial numbers of different materials. This is why we have been mentioning “numbers” instead of “grades” in the previous section. If you want to understand the specific chemical composition, you have to look at the grade. For example, the corresponding grade of 304 stainless steel in the ISO standard is X5CrNi18-10. In China, according to the national standard (GB/T 20878), the grade of 304 stainless steel is 06Cr19Ni10.

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