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The Role of Alloying Elements in Austenitic Stainless Steel(part 1)
Time：2021-07-26
Source：Wuxi Hongwang
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The chemical composition has a great influence on the metallographic structure, mechanical properties, physical properties and corrosion resistance of steel. Whether it is intentionally added or alloying elements that are unintentionally brought in during steelmaking, it will have an impact on these properties.
Alloying elements have different effects on the properties of steel, sometimes beneficial, sometimes harmful. Choosing a certain steel composition or steel grade often requires a designer or material engineer to sacrifice a part of a certain performance in exchange for the maximization of another performance. So, what function do important alloying elements in austenitic stainless steel have?
Chromium (Cr)
Chromium is an alloying element that can make stainless steel "not rusty". At least 10.5% of chromium needs to be added to form the unique surface passivation film of stainless steel. The passivation film can effectively resist the corrosion of corrosive water, a variety of acid solutions and strong oxidizing high-temperature gases.
When the chromium content exceeds the 10.5% threshold, the corrosion resistance to various environments is enhanced. Therefore, the chromium content of many brands is much higher than this value. For example, the chromium content of 304 stainless steel is 18%, and the chromium content of high-performance austenitic stainless steel is as high as 20%-28%.
Nickel (Ni)
The main function of nickel is to form and stabilize the austenite phase. If there is no 8% Ni, 304 will not become austenitic stainless steel, nor will it have the mechanical properties of austenitic stainless steel. In that case, it is a ferritic stainless steel whose strength and toughness are not at the same level.
When more chromium or other ferrite forming elements are added to the steel, nickel needs to be added to maintain the austenitic stainless steel structure. High-performance austenitic stainless steel contains high chromium and high molybdenum, and the nickel content must reach about 20% to ensure the austenitic structure.
Nickel can improve the corrosion resistance to certain reducing acids. When the nickel content is greater than about 20%, it improves the stress corrosion cracking resistance. Nickel can also reduce the work hardening rate during cold deformation. Therefore, the nickel content of alloys used for deep drawing, spinning and cold heading is generally higher.
Molybdenum (Mo)
Molybdenum can improve the pitting and crevice corrosion resistance of steel in a chloride environment. Under the combined action of molybdenum and chromium, especially nitrogen, the performance of steel in these environments is improved. This synergy makes high-performance austenitic stainless steel have strong resistance to pitting and crevice corrosion.
Molybdenum can also improve the corrosion resistance of steel in reducing environments such as hydrochloric acid and dilute sulfuric acid. The minimum molybdenum content of austenitic stainless steel is about 2%, such as 316 stainless steel. The high-performance austenitic stainless steel with the highest alloy content has a molybdenum content of up to 7.5%.
Molybdenum contributes to the formation of the ferrite phase and affects the phase balance. It participates in the formation of several harmful secondary phases, and will form unstable high-temperature oxides, which will adversely affect the high-temperature oxidation resistance. These factors must be considered when using molybdenum-containing stainless steel.
Molybdenum can improve the pitting and crevice corrosion resistance of steel in a chloride environment. Under the combined action of molybdenum and chromium, especially nitrogen, the performance of steel in these environments is improved. This synergy makes high-performance austenitic stainless steel have strong resistance to pitting and crevice corrosion.
Molybdenum can also improve the corrosion resistance of steel in reducing environments such as hydrochloric acid and dilute sulfuric acid. The minimum molybdenum content of austenitic stainless steel is about 2%, such as 316 stainless steel. The high-performance austenitic stainless steel with the highest alloy content has a molybdenum content of up to 7.5%.
Molybdenum contributes to the formation of the ferrite phase and affects the phase balance. It participates in the formation of several harmful secondary phases, and will form unstable high-temperature oxides, which will adversely affect the high-temperature oxidation resistance. These factors must be considered when using molybdenum-containing stainless steel.
Carbon (C)
Carbon stabilizes and strengthens the austenite phase. Therefore, carbon is a useful alloying element for stainless steel used in high-temperature environments such as boiler tubes. In addition, carbon has no other useful functions, and in some cases (as mentioned above), it will have an adverse effect on corrosion resistance.
The carbon content of most austenitic stainless steels is usually limited to the lowest achievable level. The carbon content of the standard low-carbon grades (304L, 201L and 316L) used for welding is limited to 0.030%. The carbon content of some high-alloy high-performance grades is even limited to 0.020%.
Carbon stabilizes and strengthens the austenite phase. Therefore, carbon is a useful alloying element for stainless steel used in high-temperature environments such as boiler tubes. In addition, carbon has no other useful functions, and in some cases (as mentioned above), it will have an adverse effect on corrosion resistance.
The carbon content of most austenitic stainless steels is usually limited to the lowest achievable level. The carbon content of the standard low-carbon grades (304L, 201L and 316L) used for welding is limited to 0.030%. The carbon content of some high-alloy high-performance grades is even limited to 0.020%.
Hongwang Group is a professional supplier and manufacturer of stainless steel products. If you have any needs in this regard, please contact us.
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