Today, let me talk about stainless steel, a very important and commonly encountered alloy steel among all alloy steels.
There are over 150 types of stainless steel, but only 20 to 30 are commonly used. Most belong to the austenitic category, followed by martensitic stainless steel. Duplex and ferritic stainless steels account for less than 5% of the market share and are not the focus of our discussion.
Austenitic stainless steel is the most important and widely used series in the stainless steel family. It is mainly divided into the 200 series and 300 series. The 200 series was widely used in the last century but has gradually been phased out due to its poor corrosion resistance. The focus is now on the 300 series, with 304 stainless steel being the most representative.
The 304 grade originates from the United States and corresponds to China’s 06Cr19Ni10 standard. The main chemical composition is as follows:
Carbon (C): ~0.06%
Chromium (Cr): ~19%
Nickel (Ni): ~10%
These elements are key to 304 stainless steel’s properties, while trace elements like silicon, manganese, sulfur, and phosphorus play minor roles.
At room temperature, austenitic stainless steel maintains its austenitic structure, which may seem counterintuitive since austenite usually forms at high temperatures (727℃-1394℃). However, the high Cr and Ni content stabilizes the austenite structure, preventing it from transforming into ferrite. Chromium bonds more readily with carbon atoms, reducing carbon's destabilizing effect on austenite.
Chromium (Cr): Crucial for corrosion resistance, it forms a dense chromium oxide layer on the surface, blocking external corrosion and enhancing hardness and strength.
Nickel (Ni): Key for improving corrosion resistance, toughness, and plasticity. Unlike Cr, Ni does not rely on oxide films but stabilizes the austenitic structure. In acidic environments, Ni plays a more critical role.
Due to its low carbon content (<0.08%), 304 stainless steel cannot undergo full heat treatment and typically remains in an annealed state with a hardness of around 220HV (20HRC).
To meet industrial demands for both corrosion resistance and surface hardness, surface treatments like nitriding are preferred over carburizing, as carbon can destabilize the austenitic structure, reducing corrosion resistance. Nitriding introduces nitrogen at 540-560°C, forming a hardened layer without compromising the steel's corrosion resistance.
304 stainless steel fasteners are commonly used in the following fields:
Chemical equipment: connectors in piping systems, storage tanks, and chemical equipment housings.
Automotive industry: in parts requiring corrosion resistance, such as exhaust system connectors.
Electronic and electrical equipment: in fasteners for weather-resistant and lightweight requirements, e.g. terminal screws.
Marine and offshore engineering: corrosion-resistant bolts and connectors in wet, mild salt spray environments.
In conclusion, 304 stainless steel is the most widely used austenitic stainless steel, prized for its corrosion resistance and durability. However, compared to medium-carbon and high-carbon alloy steels, it lacks in hardness and strength, limiting its applications to areas requiring superior corrosion resistance rather than high mechanical strength.