Stainless steel is a steel material that is often exposed to in machining work, and it is very helpful for machining personnel to understand the relevant knowledge of stainless steel, which can help them better choose and use instruments.

Stainless steel is a kind of steel that can resist the corrosion of weak corrosive media such as air, steam, water and chemical etch media such as acid, alkali, salt, so it is also known as stainless acid-resistant steel. In practical applications, we usually call the steel that can resist the corrosion of weak corrosive media as stainless steel, and the steel that can resist the corrosion of chemical media as acid-resistant steel. Due to the differences in the chemical composition of these two kinds of steel, the former is not necessarily able to resist the corrosion of chemical media, while the latter usually has rust resistance. The corrosion resistance of stainless steel mainly depends on the alloying elements contained within it.

According to different classification standards, we can divide stainless steel into the following categories:

Austenitic stainless steel: The matrix of this stainless steel is mainly a face-centered cubic crystal structure of austenitic structure (CY phase), it is not magnetic, mainly through cold processing to enhance its hardness (but may lead to a certain magnetic). The American Iron and Steel Institute identifies this stainless steel with 200 and 300 series numbers, such as 304.

Ferritic stainless steel: The matrix of this stainless steel is mainly a ferrite structure of body-centered cubic crystal structure (a phase), it is magnetic, usually can not be hardened by heat treatment, but can be slightly enhanced by cold working to enhance its hardness. The American Iron and Steel Institute designates this stainless steel as 430 and 446.

Martensitic stainless steel: The matrix of this stainless steel is martensitic structure (body-centered cubic or cubic), which has magnetic properties and can be adjusted by heat treatment. The American Iron and Steel Institute uses 410, 420, and 440 to identify this stainless steel. Martensite has an austenitic structure at high temperatures, which can be converted to martensite (i.e., hardened) when cooled to room temperature at an appropriate rate.

Austenitic ferritic (biphase) stainless steel: the matrix of this stainless steel simultaneously contains austenitic and ferritic two-phase structure, of which the content of the less phase matrix is generally greater than 15%, it is magnetic, and can be enhanced by cold working to enhance its hardness, 329 is a typical biphase stainless steel. Compared with austenitic stainless steel, the strength of duplex steel is higher, and its resistance to intergranular corrosion and resistance to chloride stress corrosion and point corrosion has also been significantly improved.

Precipitation hardening stainless steel: The matrix of this stainless steel is austenitic or martensitic structure, and its hardness can be enhanced by precipitation hardening treatment. The American Iron and Steel Institute identifies this stainless steel with a 600 series number, such as 630, which is 17-4PH.

In general, in addition to alloys, the corrosion resistance of austenitic stainless steel is relatively excellent. In less corrosive environments, you can choose to use ferritic stainless steel; In mildly corrosive environments, if the material is required to have high strength or high hardness, you can choose to use martensitic stainless steel and precipitation hardened stainless steel.