Stainless steels are highly resistant to corrosion due to the presence of chromium in their alloy composition. Chromium forms a protective and dense layer of chromium oxide (Cr2O3), which acts as a strong adhesive barrier, preventing rust and oxidation damage.

Based on different alloy compositions, stainless steels are divided into several categories, each with distinct characteristics and industrial applications.

Stainless Steels in the 300 Series

The 300 series is the most commonly used and well-known category of stainless steels. These steels possess an austenitic structure, which is highly stable at high temperatures, making them ideal for applications in thermal and industrial environments. To stabilize the austenite at room temperature, approximately 8% nickel is added to the alloy. Due to their properties, this series is widely used in everyday life and industrial applications.

Stainless Steel 304 (18-8 Steel)

304 stainless steel, commonly known as the "all-purpose steel," is the most important member of the 300 series. It contains 18% chromium and 8% nickel, providing excellent corrosion resistance and mechanical strength. One common use of this alloy is in the production of everyday items like spoons and forks. Due to its versatility, this steel is also used in a wide range of industries.

Stainless Steel 316

316 stainless steel is another crucial member of the 300 series. In addition to nickel and chromium, it contains about 2% molybdenum, which significantly increases its resistance to pitting or crevice corrosion. This feature makes it suitable for use in corrosive environments such as marine and chemical industries.

Stainless Steel 321

In 321 stainless steel, titanium is used instead of molybdenum. This addition improves resistance to intergranular corrosion (corrosion between the grains) and enhances weldability. Due to these properties, stainless steel 321 is widely used in applications requiring welding.

Stainless Steel 347

347 stainless steel utilizes niobium (Nb) instead of titanium in its composition, offering good resistance to corrosion and weldability. Niobium is more economically viable compared to titanium, and this steel is commonly used in both Eastern and Western industries. While it has similar performance to steel 321, the lower production costs make it more industrially attractive.

Important Note on Carbon in the 300 Series

All stainless steels in the 300 series contain approximately 0.7% carbon. By reducing the carbon content to 0.03%, low-carbon steels are obtained. This reduction enhances corrosion resistance, though the steel may be softer. These grades are often labeled with an "L" suffix, such as 304L and 316L, and are preferred in applications where corrosion resistance is critical.

Stainless Steels in the 200 Series

The 200 series stainless steels are structurally similar to the 300 series but use manganese as an austenite stabilizer instead of nickel to reduce costs. Manganese offers similar properties, though to a lesser extent than nickel. These steels are less known in the market and have limited industrial applications, though they are still used in some economical sectors.

Stainless Steels in the 400 Series

The 400 series stainless steels do not contain nickel and have either a martensitic or ferritic structure. Martensitic steels typically contain a high carbon content (greater than 0.2%), while ferritic steels have a lower carbon content (less than 0.2%). One significant and noticeable difference between the 400 series and the 300 series is that the 400 series steels are magnetic, whereas the 300 series is non-magnetic. The most important member of this series is 430 stainless steel, which contains 23% chromium and iron. This alloy offers good corrosion resistance and, due to being more affordable than the 300 series, is used in certain industrial applications.

Stainless Steels in the 600 Series (Duplex)

The 600 series, also known as duplex stainless steels, combines both ferritic and austenitic structures. Ideally, these steels contain 50% ferrite and 50% austenite. This dual-phase structure enhances resistance to stress corrosion cracking (SCC) and improves mechanical properties. Another advantage of this series is its excellent weldability, making it a popular choice in highly corrosive environments, such as the oil and gas industries.

Selecting the right stainless steel depends on the specific requirements of each application. With varying alloy compositions and mechanical properties, choosing the correct grade can significantly increase the lifespan and performance of equipment across different industries.