Essential Knowledge for Purchasing Stainless Steel

What is Stainless Steel?

Stainless steel is a type of steel that resists rusting and corrosion. Its resistance comes from a chromium-rich oxide film on the surface, known as a passivation film. This resistance is relative; higher chromium content increases resistance to corrosion in various environments, including weak media like air and water and strong oxidizing media like nitric acid.

Types of Stainless Steel

Stainless steel can be classified in several ways:

By Structure at Room Temperature:
- Martensitic
- Austenitic
- Ferritic
- Duplex
By Main Chemical Composition:
- Chromium stainless steel
- Chromium-nickel stainless steel
By Purpose:
- Nitric acid-resistant
- Sulfuric acid-resistant
- Seawater-resistant
By Corrosion Resistance:
- Pitting-resistant
- Stress-corrosion-resistant
- Intergranular corrosion-resistant
By Functional Characteristics:
- Non-magnetic
- Free-cutting
- Low-temperature
- High-strength

Stainless steel is favored for its excellent corrosion resistance, formability, compatibility, and toughness over a wide temperature range. Thus, it is used extensively in heavy industry, light industry, consumer goods, and architectural decoration.

Austenitic Stainless Steel

Austenitic stainless steel has an austenitic structure at room temperature, achieved with approximately 18% chromium, 8-10% nickel, and 0.1% carbon. This category includes the well-known 18Cr-8Ni steel and other high Cr-Ni series steels. Austenitic stainless steel is non-magnetic, with high toughness and plasticity but low strength. It can only be strengthened through cold working. Adding elements like sulfur (S), calcium (Ca), selenium (Se), or tellurium (Te) can improve machinability.

Austenitic stainless steel resists corrosion in oxidizing acids and can withstand sulfuric, phosphoric, formic, acetic acids, and urea if it contains molybdenum (Mo) and copper (Cu). Lower carbon content (below 0.03%) or the addition of titanium (Ti) and niobium (Nb) improves resistance to intergranular corrosion. High-silicon austenitic stainless steel resists concentrated nitric acid.

Due to its excellent properties, austenitic stainless steel is widely used across various industries.

316 and 316L Stainless Steel

316 and 317 stainless steels contain molybdenum, with 317 having slightly higher content. This improves their overall performance compared to 310 and 304 stainless steels. Under high temperatures, 316 stainless steel is effective in sulfuric acid concentrations below 15% and above 85%. It also resists chloride corrosion, making it ideal for marine environments.

The maximum carbon content of 316L stainless steel is 0.03%, making it suitable for applications requiring maximum corrosion resistance and where annealing after welding is not possible.

Conclusion

When purchasing stainless steel, understanding its various types, compositions, and applications is crucial. This knowledge ensures selecting the right stainless steel for specific needs, maximizing performance and durability.