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Why are some stainless steels magnetic and some not magnetic?

Stainless steel is a high-alloy steel material with a variety of alloying elements. According to the organization, stainless steel can be divided into a variety of types such as austenitic system, ferritic system, martensitic system, biphasic system and precipitation hardening system.
Austenitic stainless steel is widely used in the market, commonly known as 304, 304 is a representative steel of austenitic stainless steel. Austenitic stainless steel is superior in processability, weldability and corrosion resistance, accounting for about 60% of the world’s stainless steel consumption. It has very economic value and use value. This stainless steel is magnetic. The representative steel of the austenitic stainless steel is SUS304 (18Cr-8Ni-0.05C). SUS304 is also the most widely used stainless steel. The crystal structure is FCC (face-centered cubic), and since the process-induced martensite is formed, the elongation is about 60%. (→Because of the material, you must have a set of martensitic stainless steel knife!)
In the daily life environment, it has sufficient corrosion resistance, but in order to further improve the corrosion resistance, Mo is often added, and the biggest disadvantage is that stress corrosion cracking is relatively easy to occur. By increasing or decreasing the Ni content, it is possible to control the formation of martensite in the process, and it is also possible to use steel grades such as SUS301 (17Cr-7Ni) which have both high strength and high toughness. In order to suppress intergranular corrosion at the welded portion, the C content was reduced to about 0.02%, and an L type steel such as SUS304L or SUS316L was developed. In addition, in recent years, the price of Ni raw materials has risen and the cost has increased. Therefore, the development of the 200-series SUS201 (17Cr-4.5Ni-6.5Mn-0.2N) in which Ni is replaced with Mn and the resource-saving stainless steel such as the high corrosion-resistant iron system SUS have been developed. This 200 series is also austenitic stainless steel and is also non-magnetic.
Another type of ferritic stainless steel is used. The representative ferritic stainless steel is SUS430 (16Cr-0.05C), which is a magnetic stainless steel, sometimes called “stainless iron”. Although magnetic, it does not mean rust. In order to improve corrosion resistance, workability and weldability, a plurality of steel grade ferritic stainless steels have been developed. This steel grade is basically free of Ni, so the price is relatively cheap, and it is widely used as ordinary stainless steel. Industrially produced stainless steel has a Cr content of 11% to 30%. SUS430 is a two-phase structure of α phase (ferrite phase) and γ phase (austenite phase) in the high temperature zone of the hot rolling process. In the cold rolling annealing process, heat treatment of α single phase region is performed, which is ferrite single phase. organization. It is also possible to pass high Cr and low C, high temperature gamma phase, and alpha single phase structure in the full temperature region. The crystal structure is BCC and the elongation is about 30%. The deep value of r is important. In order to improve the workability, low C, N (high purity) and Ti, Nb (stabilized) elements are added. The addition of Ti and Nb is also effective for improving weldability. Ferritic stainless steel is mostly used in construction. The most common ones are handrails, escalators, railings, etc. It is the eyeball of the bloggers. There are also such stainless steels marked with “304” on the stainless steel. That is the “Li Gui” stainless steel.
Types of stainless steel such as martensitic, biphasic and precipitation hardening systems or with slightly magnetic properties, containing more ferrite in the structure, are more magnetic.
Source: China Pipe Fittings Manufacturer – Yaang Pipe Industry Co., Limited (www.yaang.com)

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