Classification of stainless steel

The number of types of stainless steel depends on which classification method is used, such as based on chemical composition or functional characteristics, or metallographic structure or heat treatment characteristics. (Right). So this cannot be generalized. However, from the perspective of heat treatment, based on the metallographic structure and heat treatment characteristics, it can be divided into five categories: ferritic stainless steel, austenitic stainless steel, martensitic stainless steel, ferritic austenitic duplex stainless steel, and precipitation hardening stainless steel. Below, I will introduce the composition and properties of specific types of stainless steel.
1. Ferritic stainless steel.
The main alloying element is Cr, or a small amount of stable iron element, such as Al.Mo, is added, and the microstructure is composed of iron element. The strength is not high and the performance cannot be adjusted through heat treatment. It has a certain degree of plasticity and high brittleness. Oxidizing media (such as nitric acid) have good corrosion resistance, but poor corrosion resistance in reducing media.
2. Austenitic stainless steel.
The Cr content is relatively high, generally greater than 18%, and the Ni content is about 8%. Some use Mn instead of Ni. In order to further improve corrosion resistance, elements such as Mo. Cu. Si. Ti. Nb must be added. There is no phase change during heating and cooling, and heat treatment cannot be used to strengthen it. It has low strength, high plasticity, and high toughness. It has strong corrosion resistance to oxidizing media. After adding Ti. Nb, it has good intergranular corrosion resistance.
3. Martensitic stainless steel.
Martensitic stainless steel mainly contains 12-18% Cr, and the amount of C is adjusted according to needs, generally ranging from 0.1 to 0.4%. When producing tools, C can reach 0.8~1.0%. In order to improve tempering stability, some people have added Mo.V.NB and others. After high-temperature heating and cooling at a certain rate, the microstructure is basically martensite. Depending on the differences in C and alloy elements, some may contain a small amount of ferrite. Residual austenite or alloy carbides. Changes occur during heating and cooling. Therefore, the organizational structure and form can be greatly adjusted to change performance. The corrosion resistance is not as good as that of austenite. Ferrite and duplex stainless steel have good corrosion resistance in organic acids, but poor corrosion resistance in media such as sulfuric acid and hydrochloric acid.
4. Ferritic austenitic duplex stainless steel.
The general Cr content is 17-30%, and the Ni content is 3-13%. In addition, alloy elements such as Mo. Cu. Nb. N. W were also added. The control of C content is very low. According to the different proportions of alloy elements, some are mainly composed of iron, while others are mainly composed of austenite, forming a dual phase stainless steel where both phases coexist. Due to the presence of ferrite and strengthening elements, the strength after heat treatment is slightly higher than that of austenitic stainless steel, exhibiting good plasticity and toughness. Basically, performance cannot be adjusted through heat treatment. High corrosion resistance, especially in Cl - media. Seawater has good resistance to pitting corrosion and interstitial corrosion. The characteristics of stress corrosion.
5. Precipitation hardened stainless steel.
The characteristic of the composition is that in addition to elements such as C.Cr. Ni, it also contains elements such as Cu. Al. Ti that can precipitate precipitates in a timely manner. The mechanical properties can be adjusted through heat treatment, but its strengthening mechanism is different from martensitic stainless steel. Due to its precipitation strengthening, C can be controlled very low, and its corrosion resistance is superior to martensitic stainless steel, equivalent to Cr Ni austenitic stainless steel.