The steel that contains more than 12.5% chromium in steel and has high resistance to corrosion by external media (acid, alkali salt) is called stainless steel. According to the structure of the steel, stainless steel can be divided into martensitic, ferritic, austenitic, ferritic-austenitic, and precipitation hardening stainless steels. According to the national standard GB3280-92, there are 55 regulations in total. .
Austenitic stainless steel (some people call it nickel stainless steel) and martensitic stainless steel (some people call it stainless iron, but it is unscientific, easy to misunderstand, and should be avoided) two categories that we come into contact with in daily life. . The typical grades of austenitic stainless steel are 0Cr18Ni9, namely "304" and 1Cr18Ni9Ti. Martensitic stainless steel is the stainless steel we make knives and scissors. The main grades are 2Cr13, 3Cr13, 6Cr13, 7Cr17 and so on. Due to the difference in the composition of the two types of stainless steel, the microstructure of the metal inside is also different.
Austenitic stainless steel has a structure called austenite due to the addition of high chromium and nickel (about 18% chromium and over 4% Ni) in the steel, the internal structure of the steel is called austenite. Without magnetic conductivity, it cannot be attracted by magnets. It is often used as decorative materials, such as stainless steel pipes, towel racks, tableware, stoves, etc.
The stainless steel for making knives and scissors should be martensitic stainless steel. Because knives and scissors have the function of cutting items, they must have sharpness, and to have sharpness, they must have a certain hardness. This type of stainless steel must undergo internal structural transformation through heat treatment. Knives and scissors can only be used after increasing the hardness. However, the internal structure of this type of stainless steel is tempered martensite, which is magnetically conductive and can be attracted by magnets. Therefore, stainless steel cannot be simply explained by whether it is magnetic.
There are many kinds of stainless steel, which can be divided into several categories according to the structure at room temperature:
1. Austenitic type: such as 304, 321, 316, 310, etc.;
2. Martensite or ferrite type: such as 430, 420, 410, etc.;
The austenitic type is non-magnetic or weakly magnetic, and the martensite or ferrite is magnetic. Most of the commonly used stainless steels are austenitic 304 materials. Generally speaking, they are non-magnetic or weakly magnetic, but magnetic properties may also appear due to fluctuations in chemical composition or different processing conditions caused by smelting, but this cannot be considered counterfeit or unqualified. , what is the reason for this? As mentioned above, austenite is non-magnetic or weakly magnetic, while martensite or ferrite is magnetic. Due to component segregation or improper heat treatment during smelting, a small amount of martensite or ferrite in austenitic 304 stainless steel will be caused. body tissue. In this way, 304 stainless steel will have weak magnetism.