Everyone knows about stainless steel, but non-professionals generally don't know that stainless steel is divided into different types and even hundreds of grades.
"Stainless steel" is a very general concept. According to the European standard EN 10088, stainless steel refers to a category of metal materials with a chromium (Cr) content of more than 10.5% and anti-corrosion function. Other alloying elements may also be added for other performance considerations.
Generally speaking, stainless steel is divided into five categories: martensitic stainless steel, ferritic stainless steel, austenitic stainless steel, duplex stainless steel, and precipitation hardening stainless steel.
In order to quickly distinguish these five types of stainless steel, a preliminary judgment can be made based on the alloy content of the material.
Martensitic stainless steel
Figure 1 Martensitic stainless steel
Martensitic stainless steel belongs to Fe-Cr-C-(Ni-Mo) alloy - the main alloying elements are iron, chromium and carbon, and some grades contain nickel and molybdenum.
The characteristic of this type of material is that the chromium (Cr) content is between 11.5~18%. Compared with other types of stainless steel, its carbon (C) content is very high, between 0.15~1.2%. It should be noted that the carbon content of general alloy steel such as Q355B is only about 0.2%.
This type of material can be hardened by heat treatment to obtain higher strength and hardness. Martensitic stainless steel is basically magnetic, and this feature can also be used as a preliminary method to judge the type of material.
Martensitic stainless steel is often used to make products that require higher hardness, such as kitchen knives, surgical instruments or bearings.
Ferritic stainless steel
Figure 2 Ferritic stainless steel
Ferritic stainless steel belongs to Fe-Cr-(Mo) alloy - the main alloying elements are iron and chromium, and some grades contain molybdenum.
Compared with martensite, ferritic stainless steel has a lower carbon content, generally below 0.08%, but a much wider range of chromium content - 10.5~30%.
Ferrite is also magnetic. This type of material is difficult to harden by heat treatment and is generally used in situations where toughness is not a high requirement but resistance to chloride stress corrosion cracking is high, such as automotive exhaust systems, automotive trim, hot water towers and other equipment.
Austenitic stainless steel
Figure 3 Austenitic stainless steel
Austenitic stainless steel belongs to Fe-Cr-Ni (Mo) alloy - the main alloying elements are iron, chromium, nickel, and some grades contain molybdenum.
Similar to ferrite, austenitic stainless steel usually controls the carbon content below 0.08%, the chromium content is between 16 and 28%, and the nickel content range is much larger - 3.5 to 32%.
The alloy content of these materials ensures that they always maintain an austenitic structure from cryogenic temperatures to the melting point. Like martensite, austenite can also be hardened by heat treatment to change its mechanical properties.
It should be noted that austenitic stainless steel is generally non-magnetic, but it will generally become magnetic if it is cold worked without solution treatment. The biggest feature of austenite is its excellent corrosion resistance, ductility and toughness.
Based on its excellent properties, austenite is generally used to make food-grade kitchenware, chemical industrial equipment, construction products, etc.
Duplex stainless steel
Fiture 4 Duplex stainless steel
Duplex stainless steel belongs to Fe-Cr-Ni-(Mo)-N alloy - the main alloying elements are iron, chromium, nickel, nitrogen, and some grades contain molybdenum.
The so-called "dual-phase" means that the ferrite structure and the austenite structure each account for 50%, and its performance also combines the advantages of ferrite and austenite in one - it has higher yield strength and tensile strength than austenite and is magnetic.
The carbon content of duplex stainless steel can be said to be the lowest among all stainless steels - less than 0.03%. The lower limit of chromium content is higher than that of other stainless steels - 21~26%. The nickel content is 3.5~8%. Some individual grades contain molybdenum, but generally it will not exceed 4.5%.
Duplex stainless steel can generally be used in maritime equipment, petrochemical plant facilities, heat exchangers, papermaking equipment, etc.
Precipitation hardening stainless steel
Figure 5 Precipitation hardening stainless steel
Precipitation hardening stainless steel belongs to Fe-Cr-Ni-(Mo-Al-Cu-Nb)-N alloy - the main alloying elements are iron, chromium, nickel and nitrogen, and some grades contain molybdenum, aluminum, copper and niobium.
This type of stainless steel is characterized by moderate corrosion resistance, easy processing, and can produce very high strength after heat treatment at relatively low temperatures (generally 500~800℃).
Precipitation hardening stainless steel has an initial microstructure of austenite or martensite. Austenitic stainless steel can be converted into martensite by heat treatment before precipitation hardening occurs. When the martensite is tempered, the resulting heat aging treatment will cause hard intermetallic compounds to precipitate from the lattice, and then precipitation hardening will occur.
After heat treatment, this type of material can obtain a yield strength that is 3 or even 4 times that of austenitic stainless steel - 520MPa~1500MPa. Therefore, it is mostly used in the aerospace industry and other high-tech fields that require high material strength.
