The influence of alloying elements on the properties of stainless steel

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The effect of alloying elements: The effect of alloying elements on steel is neither a simple addition nor an offsetting effect between them. New physicochemical reactions sometimes occur between them, often leading to an enhancement of their mechanical properties. The effects of alloying elements on the microstructure of stainless steel can be broadly divided into three categories: the first category consists of elements that form ferrite, such as chromium, silicon, aluminum, molybdenum, titanium, and niobium ； The second category consists of elements that form austenite, such as carbon, nitrogen, nickel, manganese, and copper. Among these, carbon and nitrogen have the greatest effect ； The third category consists of elements that form carbides, such as niobium, titanium, carbon, chromium, tungsten, manganese, molybdenum, etc. The addition of elements such as copper, aluminum, titanium, niobium, and nitrogen can induce dispersion hardening in steel, thereby enhancing its heat strength. Chromium, silicon, aluminum – Chromium, silicon, and aluminum are elements that contribute to the formation of ferrite, and they are the main alloying elements that grant stainless steel its corrosion resistance. By adding an adequate amount of chromium to carbon steel, it is possible to create a passivation film of ferric chromite (FeCr)2O3 in oxidizing media, which bonds firmly with the matrix structure of the steel ； It can also increase the electrode potential of steel in dielectrics, thereby enhancing its chemical stability. Both silicon and aluminum can enable steel to form a dense protective film in oxidizing media, with aluminum having an even more pronounced effect than chromium. In austenitic heat-resistant steels, all these elements can improve their oxidation resistance. In 18-8 stainless steel, as the mass fraction of silicon increases from 0.4% to 2.4%, the oxidation resistance of the steel at 980°C increases by 22 times. If the silicon content is too high, it will severely degrade the weldability of stabilized austenitic steels; therefore, the silicon content in the steel must be strictly controlled. Aluminum in precipitation-hardening stainless steels can enhance their strength at room temperature and high temperatures. Nickel: Nickel is an element that contributes to the formation of austenite. It can passivate the alloy surface and expand the passivation range of steel in acids, but it cannot improve its corrosion resistance to dilute nitric acid. It can improve the resistance of stainless steel to corrosive agents such as sulfuric acid and hydrochloric acid, and is a major alloying element in corrosion-resistant steels. If nickel is used alone as an alloying element in stainless steel, a mass fraction as high as 24% is required to obtain a fully austenitic structure, but this is extremely uneconomical. Based on low-carbon chromium stainless steel, the addition of 9% by mass of nickel is sufficient to obtain a stable austenitic structure at room temperature, which possesses good corrosion resistance as well as excellent comprehensive mechanical properties. This approach not only meets the requirements for corrosion resistance but also enhances the steel’s high-temperature strength and oxidation resistance, making it a steel grade with outstanding overall performance. Molybdenum and copper: Molybdenum is an element that contributes to the formation of ferrite. Adding molybdenum to chromium stainless steel can improve the steel’s stability in non-oxidizing media. Its uniqueness lies in its ability to resist pitting corrosion caused by chloride ions (Cl-) ； It can also enhance the heat strength of austenitic steels, improve their short-term and long-term plasticity, which is beneficial for welding. However, the addition of molybdenum will reduce the austenite region in the steel, resulting in the formation of ferrite phases in austenitic stainless steels. To this end, in molybdenum-containing single-phase austenitic stainless steels, the contents of corresponding austenite-forming elements such as nickel, manganese, and nitrogen are slightly increased in order to maintain their fully austenitic structure. In duplex stainless steels, molybdenum promotes ferrite, which is beneficial for improving both pitting resistance and stress corrosion resistance. However, an excessive molybdenum content reduces the toughness of austenitic stainless steels. Adding copper to chromium-nickel stainless steel promotes the formation of a dispersedly hardened microstructure, thereby enhancing the steel’s heat strength. Used in combination with molybdenum, it can further improve the corrosion resistance of chromium-nickel stainless steel in dilute sulfuric acid. Manganese and nitrogen: Manganese and nitrogen have no direct effect on improving the corrosion resistance of stainless steel, but they are both effective elements in promoting and stabilizing austenite, with nitrogen having a more pronounced effect than manganese. When the manganese content is too high, it is detrimental to the corrosion resistance of stainless steels with low chromium content; it can also cause pores in cast steel parts, while increasing hardness and making cold working of the steel more difficult. The combined action of nitrogen and carbon can enhance the heat strength of austenitic steels; the strengthening effect of nitrogen arises from the formation of nitrogen compounds and carbon-nitrogen compounds during aging. Titanium and niobium are elements that combine more easily with carbon than chromium to form stable carbides. In chromium-nickel stainless steels, when the amount of titanium added is more than 5 times the carbon content, or when the amount of niobium added is more than 8 times the carbon content, it is possible to have most of the carbon present in the carbides of titanium or niobium, thereby reducing the mass fraction of dissolved carbon to below 0.03%. This ensures an effective solubility concentration of chromium in the steel. The effective solubility of chromium in steel is ensured, thereby improving the steel’s resistance to intergranular corrosion. When the mass fraction of niobium ranges from 0.5% to 2.0%, it can enhance both the heat strength of austenitic steel and its ductility under stress. In austenitic steels with low carbon content, niobium promotes crack formation in the near-weld zone and weld metal; in chromium-nickel austenitic steels, the mass fraction of niobium should be kept within 1.0%.