Demystifying The Intricacies of Stainless Steel Classification

Stainless steel is a type of steel that is resistant to weak corrosive substances like air, steam, and water. It is also resistant to chemical substances like acids, alkalis, and salts. This type of steel is commonly known as stainless steel. On the other hand, there is another type of steel that is resistant to chemical substances but not necessarily to weak corrosive substances. This type of steel is called acid-resistant steel. The corrosion resistance of stainless steel depends on the specific alloying elements present in the steel.

There are three main types of stainless steel: austenitic, ferritic, and martensitic. These categories are based on specific needs and purposes, and they are derived from duplex steel, precipitation hardening stainless steel, and high alloy steel with less than 50% iron.

1, Let's talk about austenitic stainless steel. This type has a face-centred cubic crystal structure and is mainly non-magnetic. It can be strengthened through cold working, which may also result in a certain degree of magnetism. The American Iron and Steel Institute uses numbers like 200 and 300 series, such as 304, to label austenitic stainless steel.

2, We have ferritic stainless steel. This type has a body-centred cubic crystal structure and is predominantly magnetic. It cannot be hardened through heat treatment, but it can be slightly strengthened through cold working. The American Iron and Steel Institute labels ferritic stainless steel as 430 and 446.

3, Martensitic stainless steel is a type of stainless steel that has a martensitic structure, which is body-centered cubic or cubic in nature. It is magnetic and its mechanical properties can be adjusted through heat treatment. The American Iron and Steel Institute labels it as 410, 420, and 440. At high temperatures, martensite has an austenitic structure, but when cooled at the right rate to room temperature, it transforms into martensite, which makes it harder.

4, Austenitic stainless steel is a type of stainless steel that has a ferrite (duplex) structure. The matrix consists of both austenite and ferrite phases, with the lesser phase generally making up more than 15% of the matrix. It is magnetic and can be strengthened through cold working. An example of duplex stainless steel is 329. Compared to austenitic stainless steel, duplex steel has higher strength and improved resistance to intergranular corrosion, chloride stress corrosion, and pitting corrosion.

5. Stainless steel that achieves hardness through precipitation hardening is known as precipitation hardening stainless steel. This type of steel has a matrix structure that can be either austenitic or martensitic. The American Iron and Steel Institute classifies this steel under the 600 series, with examples such as 630, also known as 17-4PH.

In general, when it comes to corrosion resistance, austenitic stainless steel is superior to other alloys. However, in less corrosive environments, ferritic stainless steel can be used. In mildly corrosive environments, if the material requires high strength or hardness, martensitic stainless steel and precipitation hardening stainless steel can be utilized.