The surface of stainless steel is protected by a thin & stable protective layer that prevents being rusted inside. It is the element Cr that generates this protective layer. After the reaction of Cr with Oxygen and water from atmosphere, the protective layer of hydrid～oxid will be generated on the surface of the steel, which prevents being rusted inside. We call this phenomenon as passive status, and the protective layer as passive film. We can not see this layer by eyesight because it is too thin (1～3nm).The Cr content must take over 11％ to generate the protective layer. So the Fe based alloy with over 11% Cr content is called stainless steel. The photo below shows the concept of thickness of passive protective layer.

 

 

Currently many kinds of stainless steel are available; there are 80 categories according to JIS standard (SUS mark), even as many as 200 categories if those unstandardized and developed by manufacturers individually are included. According to the chemical composition, there are Fe-Cr series and Fe-Cr-Ni series. According to its structure, there are martenite series, ferrite series, austenite series, austenite-ferrite series (dual-phase series) and PH series.

 

 

    Generally speaking, stainless steel is the steel that is not easy to become rusty. Actually, some stainless steel has both rust resistance and acid resistance (corrosion resistance), which result from the formation of the high Cr oxide film (passivation film) on the surface, and rust resistance and corrosion resistance are relative. The test shows: in the weak medium (atmosphere, water) and oxidation medium (nitric acid), the corrosion resistance increases as the Cr content increases, if the Cr content reaches certain percentage, the corrosion resistance has a great change, namely, from becoming rusty easily to rust resistance, from being corrosive to anticorrosive. There are many ways to classify stainless steel. According to the steel structure at room temperature, there are martensite, austenite, ferrite and dual-phase stainless steel. Classified by the chemical composition: there are Cr series and Cr-Ni series stainless steel. According to the application: there are nitric acid resistance stainless steel, vitriol acid resistance stainless steel, seawater resistance stainless steel, etc. According to the corrosion resistance, there are pitting corrosion resistance, stress corrosion resistance and inter-granular corrosion resistance, etc. According to the function: there are non-magnetic stainless steel, easy-cutting stainless steel, low temperature stainless steel, high strength stainless steel, etc. Due to its excellent corrosion resistance, formability, consistency and toughness applicable to a wide range of temperature, it is widely used in heavy industry, light industry, consumer goods, and architecture decoration, etc.

 

 

    It is the stainless steel with austenitic structure under normal temperature. It has the stable austenitic structure. Austenitic stainless steel includes the well-known 18Cr-8Ni steel and high Cr-Ni series steel with addition of Mo, Cu, Si, Nb, Ti, etc. Austenitic stainless has no magnetism, high toughness and formability, but low strength. It is impossible to be strengthened by changing its phase, but possible by cold working. It becomes easy-cutting if S, Ca, Se, Te is added. Besides oxidation resistant, it is also vitriol acid resistant, phosphoric acid resistant, formic acid resistant, acetic acid resistant, carbamide resistant if Mo, Cu is added. If the C content is lower than 0.03% or Ti, Ni is added; its inter-granular corrosion resistance will be greatly improved. And austenitic stainless steel with high Si has very good corrosion resistance. Due to its great comprehensive properties, it is widely used in various industries.

 

 

    It is the stainless steel mainly with ferritic structure under using condition. It has about 11%～30% of Cr content and body-centered cubic structure. Generally it contains no Ni, but a little bit Mo, Ti, Nb, etc. It has many good properties, such as great thermal conductivity coefficient, small thermal expansion coefficient, good oxidation resistance and stress corrosion resistance, etc. It is mostly used for components that require atmosphere resistance, steam resistance, water resistance and oxidized acid resistance. However, it has such disadvantages as poor formability, and even the formability and corrosion resistance is greatly reduced after welding, which limits its application. The application of AOD or VOD may greatly reduce some elements, such as C, N, etc. So this kind of steel is also widely used. 

 

 

    It is the stainless steel with 50% of austenitic structure and 50% of ferritic structure. With low C content, it has 18% - 28% Cr and 3%-10% Ni content, and some steel has alloy elements, such as Mo, Cu, Si, Nb, Ti, N, etc. This kind of steel has the characteristics of both austenitic and ferritic stainless steel. Compared with ferrite, it has better formability, toughness, inter-granular corrosion resistance and weldability, but no brittleness at room temperature, and keeps some properties of ferritic stainless steel: 475℃ brittleness, high thermal conductivity coefficient and ultra-formability. Compared with austenitic stainless steel, it has better strength, inter-granular corrosion resistance and chloride stress corrosion resistance. Dual phase has excellent hole corrosion resistance and it is Ni-saving as well.

 

 

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