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Primary phase structures: In ferritic, martensitic, austenitic, and ferritico-austenitic stainless steels, the primary phase structures are ferrite, martensite, austenite, and ferrite plus austenite. For example, since the ferritic phase constitutes the vast majority in stainless steel, it is called ferritic stainless steel, which endows this type of steel with different properties and applications compared to other types of stainless steel. During the melting and rolling of steel ingots, certain impurities are inevitably present to varying degrees, thereby reducing the purity of the steel. These impurities are mainly elements such as carbon and nitrogen, which precipitate primarily in the form of chromium-iron carbides (mainly M23C6). These carbides, nitrides, and various intermetallic phases become new phases in the steel, known as secondary phases. These secondary phases exist in the interstices, between dendrites, at grain boundaries, or between grains. The degree of density and quantity of their distribution in steel directly affect the mechanical properties of the steel. Effects of secondary elements on steel (1) Carbides At room temperature, the solubility of carbon in austenitic stainless steels is very low, at around 0.006%, and it is even lower in ferritic (or martensitic) stainless steels. As the carbon content in steel increases, the excess carbon will precipitate in the form of chromium-iron carbides (mainly M23C6). It sometimes also precipitates in small amounts as M7C3T and M6C. The mass fraction of chromium in M23C6 and M7C3T is approximately 42%–65%, **exceeding the normal chromium content in stainless steels. If heated to an appropriate temperature under carbon supersaturation, carbide precipitation will occur. These chromium carbides are most likely to form at the grain boundaries. Under appropriate conditions, chromium depletion can occur at the grain boundaries, that is, the effective solubility of chromium in those boundaries is reduced, leading to a decrease in the corrosion resistance of the steel. For the corrosion resistance of stainless steel, carbon is a harmful element. In stainless steel, the carbon content should be kept as low as possible. The effect of carbides on ferritic stainless steel: Since carbon diffuses more easily in ferrite than in austenite, and its solubility in ferrite is lower than that in austenite, the precipitation of carbides occurs more readily in ferrite than in austenite. Therefore, ferritic stainless steels are more prone to intergranular corrosion than austenitic stainless steels. The effect of carbides on chromium-nickel austenitic stainless steels: As the carbon content increases, it becomes easier for chromium carbides to form. An increase in these chromium carbides inevitably leads to a greater degree of chromium depletion at the grain boundaries, as well as an expansion of those chromium-depleted areas, thereby increasing susceptibility to intergranular corrosion. An increase in nickel content raises the activity of carbon and reduces its solubility in steel, which is equivalent to an increase in carbon content; as a result, the sensitivity to intergranular corrosion also increases. This situation only occurs when the mass fraction of nickel is greater than 20%; therefore, when the mass fraction of nickel in the steel is 20% or higher, the carbon content must be strictly controlled to avoid or reduce intergranular corrosion. Like nickel, silicon also increases the activity of carbon, and its effect is more pronounced than that of nickel. Another function of silicon is that it can produce carbon nitride [Mn(CN)2], whose effect on intergranular corrosion is similar to that of M23C6. When the mass fraction of silicon in austenitic stainless steel exceeds 4%, the mass fraction of carbon should be limited to below 0.02%. An increase in the chromium content in austenitic stainless steels allows for the timely supply of the necessary chromium to the chromium-deficient regions at the grain boundaries, thereby enhancing resistance to intergranular corrosion; it can be said that chromium is the key element for corrosion resistance. Both niobium and titanium can form stable carbides with carbon, which can effectively inhibit the precipitation of M23C6 and prevent intergranular corrosion. However, the titanium content must be at least 5 times that of carbon, and the niobium content must be at least 10 times that of carbon in order to effectively suppress the precipitation of M23C6. (2) Effect of nitrides: Compared to carbon, nitrogen is a more effective solid solution strengthening element, and it can also promote grain refinement ； Nitrogen is an austenite-forming element that can reduce the nickel content in the alloy, thereby decreasing the tendency to form ferrite and transformed martensite ； Although nitrogen does not significantly improve the overall corrosion resistance of the material in acids, it can greatly enhance its resistance to pitting and crevice corrosion. However, steel contains nitrogen, which, just like carbon, inevitably forms nitrides and carbides in the steel, becoming an important microstructure component. When the mass fraction of nitrogen in stainless steel exceeds 0.4%, two common forms of nitrides exist in the steel: Cr2N and CrN. With the precipitation of Cr2N, a σ phase forms in the matrix adjacent to the nitrides, and this phase is detrimental to the toughness and corrosion resistance of the material. Both of these nitrides also cause chromium-depleted zones to form at the grain boundaries, resulting in a decrease in the steel’s corrosion resistance. The chromium-depleted zones surrounding chromium nitride are a major source of pitting corrosion; the mechanism is the same as that for chromium carbide. The precipitation of nitrides is temperature-dependent: ① There is a sensitive temperature range of 600–1075°C; within this range, the tendency for nitride precipitation is relatively high, and precipitation of a second phase also occurs. Therefore, processing or service within this temperature range should be avoided as much as possible; however, high-temperature solution treatment can be used to eliminate nitrides. ②Related to alloying elements, nitrogen is an element that forms nitrides. Nitrogen interstitially dissolves in the austenitic matrix and diffuses relatively quickly. As the nitrogen content increases, the tendency for Cr2N precipitation becomes more pronounced ； When the nitrogen content is low, Cr2N does not precipitate along the grain boundaries, while nickel can promote the precipitation of nitrides. ③Depending on their original state, austenitic stainless steels have two service states: solution-treated and rolled. The precipitation behavior of nitrides varies depending on the original form of the material. After cold rolling and annealing, the precipitation rate of nitrides is delayed; as the number of cold rolling and annealing cycles increases, the sensitive temperature range becomes narrower, and the probability of nitride precipitation also decreases. Therefore, the annealed alloy is less favorable for the intragranular precipitation of nitrides. (3) Precipitation of the σ phase: In stainless steels, the σ phase is an iron-chromium compound that also contains other alloying elements such as Mo, Mn, Ni, Si, Ti, and P. The mass fraction of chromium in the σ phase is approximately 47%. The σ phase typically precipitates in steels with a chromium mass fraction of 16% or higher. Due to the high diffusivity of chromium, the precipitation of the σ phase in ferrite is faster than in austenite. The precipitation of the σ phase reduces the toughness and increases the hardness of the material; sometimes it also decreases the material’s corrosion resistance. The σ phase can form in all types of stainless steel. Carbon will slow down the precipitation of the σ phase, as carbide M23C6 will precipitate preferentially at this time, and only thereafter will the σ phase precipitate. Due to the precipitation of the carbide M23C6, the chromium content in the steel’s solid solution is reduced, which naturally delays the precipitation of the σ phase. Nitrogen has the same effect as carbon, and can also slow down the precipitation of the σ phase in steel. The precipitation of the σ phase occurs more easily in ferritic stainless steels than in austenitic stainless steels, and it becomes even easier with the addition of molybdenum. In austenite-ferrite duplex stainless steels, due to the relatively high chromium content and low carbon content, it is easier for the σ phase to precipitate. The effect of σ on the toughness of duplex stainless steels is greater than that on austenitic stainless steels. When duplex stainless steel contains 1% by volume of σ-phase, its impact value decreases by 50% ； When the material contains 10% by volume of σ-phase, it becomes completely brittle.