The properties and structure of stainless steel are mainly determined by various elements. At present, there are more than 100 known chemical elements. The elements that have the greatest impact on the performance and structure of stainless steel include: carbon, chromium, nickel, manganese, nitrogen, titanium, niobium, molybdenum, copper, aluminum, silicon, vanadium, tungsten, boron, etc. Due to the addition of these elements, the internal structure of the steel changes, so that the steel has special properties. The effects of various elements on the properties and structures of stainless steel are as follows:
Chromium - the basic element of stainless steel
Chromium is the most basic element that determines the corrosion resistance of stainless steel. In the oxidation medium, chromium can quickly form a layer of dense chromium rich oxide film on the steel surface that can not be penetrated and dissolved by the corrosion medium, and it is basically firmly combined with the metal, so as to protect the steel from further oxidation and erosion by external media; Chromium can also effectively increase the electrode potential of steel. When the chromium content is not less than 12.5%, the electrode potential of the steel will suddenly change from negative potential to positive electrode potential, thus significantly improving the corrosion resistance of the steel. The higher the chromium content, the better the corrosion resistance of the steel. When the chromium content reaches 25% and 37.5%, the second and third abrupt changes will occur, making the steel have higher corrosion resistance.
Nickel - does not constitute stainless steel by itself alone
The effect of nickel on the corrosion resistance of stainless steel can only be fully demonstrated when it is combined with chromium. Because if the low carbon nickel steel obtains a pure austenite structure, the nickel content needs to be 24%; to make the corrosion resistance of the steel in some media changed significantly, the nickel content needs to be more than 27%. Therefore, nickel alone cannot constitute stainless steel. The addition of 9% nickel to the steel containing 18% chromium can make the steel obtain the single austenite structure at room temperature, and it can improve the steel's resistance to non-oxidizing media (such as dilute sulfuric acid, hydrochloric acid, phosphoric acid, etc.) corrosion resistance, and improve the process performance of steel welding and cold bending.
Manganese and Nitrogen--Substitute Nickel in Chromium-Nickel Stainless Steel
Manganese and nitrogen have similar effects to nickel in stainless steel. The stabilized austenite effect of manganese is 1/2 of nickel. The effect of nitrogen is much larger than that of nickel, about 40 times of nickel. Therefore, manganese and nitrogen can replace nickel to obtain the single austenite structure. However, the addition of manganese would reduce the corrosion resistance of stainless steel with low chromium content. At the same time, high manganese austenitic steel is not easy to process. Therefore, manganese is not used alone in stainless steel, it just partially replaces nickel.
Carbon - with Duality Character in Stainless Steel
The content of carbon in stainless steel and its distribution form, to a large extent, determine the performance and structure of stainless steel: on the one hand, carbon is a stable austenite element, and it acts to a great extent, about 30 times that of nickel; on the one hand, due to the great affinity of carbon and chromium, chromium, which occupies 17 times the amount of carbon in stainless steel, is combined with it to form chromium carbide. As the carbon content increases in the steel, more chromium forms carbides with carbon, thereby the corrosion resistance of the steel is significantly reduced. Therefore, in terms of strength and corrosion resistance, the role of carbon in stainless steel is contradictory. In practical applications, in order to achieve the purpose of corrosion resistance, the carbon content of stainless steel is generally low, mostly around 0.1%. In order to further improve the corrosion resistance of steel, especially the ability to resist intergranular corrosion, ultra-low carbon stainless steel is often used. And the carbon content is 0.03% or even lower.
Titanium and niobium - prevent intergranular corrosion in stainless steel
When stainless steel is heated to 450~800 ℃, the chromium content near the grain boundary is often reduced due to the carbide of chromium precipitated at the grain boundary, resulting in intergranular corrosion. Common examples include intergranular corrosion that occurs in the heat-affected zone near the weld. And titanium and niobium is a strong carbide forming element, its affinity with carbon is much greater than chromium, steel plus human titanium or niobium, can make the carbon in the steel first with titanium or niobium to form carbides, without forming carbides with chromium, so as to ensure that the grain boundary near the chromium will not produce intergranular corrosion. Therefore, titanium and niobium are commonly used to fix the carbon in steel, improve the ability of stainless steel to resist intergranular corrosion, and improve the welding performance of steel.
Molybdenum and copper - improve the corrosion resistance of some stainless steels to certain media
Molybdenum and copper can improve the corrosion resistance of stainless steel to corrosive media such as sulfuric acid and acetic acid. Molybdenum also significantly improves corrosion resistance to chloride-containing media such as hydrochloric acid, as well as in organic acids. However, molybdenum-containing stainless steel should not be applied in nitric acid, and its corrosion rate in boiling 65% nitric acid is doubled compared with that of molybdenum-free; In copper plus human chromium manganese nitrogen stainless steel, the intergranular corrosion of stainless steel will be accelerated. Molybdenum has an adverse effect on steel to obtain a single austenitic structure, so in molybdenum-containing steel, in order to make the steel have a single austenitic structure after heat treatment, the content of nickel and manganese and other elements should be correspondingly increased.
Silicon and aluminum - improve the oxidation resistance of stainless steel
The purpose of adding silicon and aluminum to high chromium steel; One is to further improve the oxidation resistance of steel; The second is to save chromium. Although the role of silicon and aluminum in improving the oxidation resistance of chromium steel is great, there are also many disadvantages, the most important thing is that it makes the grain coarseization and brittleness tendency of steel increase.
Tungsten and vanadium
Tungsten and vanadium are added to steel, and its main role is to improve the thermal strength of steel.
The addition of 0005% boron to high chromium ferritic stainless steel (Cr17Mo2Ti) can improve the corrosion resistance of steel in boiling 65% acetic acid; The addition of a trace amount of boron (00006~0.0007%) in austenitic stainless steel can improve the thermoplasticity of steel; Boron has a good effect on improving the thermal strength of steel, which can significantly improve the thermal strength of stainless steel; Boron-containing chrome-nickel austenitic stainless steels have special uses in the atomic energy industry, but the boron content of stainless steel will reduce the plasticity and impact toughness of steel.
SINOSTEEL STAINLESS STEEL PIPE TECHNOLOGY (SHANXI) CO., LTD.
Address：No.7, Huitong Industrial Park, Jinzhong Development Zone, Shanxi Comprehensive Reform Demonstration Zone, Jinzhong, Shanxi, China