Currently, more than 100 chemical elements are known, and about 20 of these elements can be found in steel materials commonly used in industry. Stainless steel is a special type of steel that has evolved over time as a result of humanity's relentless pursuit of corrosion resistance, and it typically contains over ten common elements. In addition to iron, which is the fundamental element of steel, the elements that significantly influence the performance and structure of stainless steel include carbon, chromium, nickel, manganese, silicon, molybdenum, titanium, niobium, nitrogen, copper, and cobalt. Notably, with the exception of carbon, silicon, and nitrogen, all these elements are transition metals from the periodic table.
　　In fact, the stainless steel used in industry contains multiple elements simultaneously. When several elements coexist as a unified whole within the steel, their interactions become far more complex than if they existed individually. In such cases, it’s essential to consider not only the individual effects of each element but also how they influence one another. As a result, the structure of stainless steel is determined by the combined effects of its various elements—essentially, the sum of their individual and mutual influences.
　　1. The Influence of Different Elements on the Properties and Microstructure of Stainless Steel
　　1-1. The Decisive Role of Chromium in Stainless Steel
　　Only one element determines the properties of chromium in stainless steel. Every type of stainless steel contains a certain amount of chromium. As of now, there is no stainless steel that lacks chromium altogether. The fundamental reason chromium becomes the key element determining the performance of stainless steel is that, when chromium is added to steel as an alloying element, it triggers an internal balancing act among chromium atoms—enhancing the material's resistance to corrosion. This phenomenon can be explained from the following perspectives:
　　(1) Chromium enhances the electrode potential of the iron-based solid solution
　　(2) Chromium absorbs iron's electrons, passivating the iron.
　　Passivation is the phenomenon where the corrosion resistance of metals and alloys is enhanced due to the inhibition of anodic reactions. There are several theories explaining metal and alloy passivation, including the film theory, adsorption theory, and electron-arrangement theory.
　　The Duality of Medium Carbon in Stainless Steel
　　Carbon is one of the primary elements in industrial steel. The performance and microstructure of steel—particularly stainless steel—are heavily influenced by the carbon content and its distribution within the alloy. Carbon's impact on the structure of stainless steel manifests primarily in two key ways. First, carbon acts as a potent stabilizer of the austenitic phase, with an effect roughly 30 times stronger than that of nickel. Second, due to carbon's strong affinity for chromium, it readily forms a variety of complex carbides with the element. As a result, carbon's role in enhancing both the strength and corrosion resistance of stainless steel often presents a conflicting dynamic.
　　Understanding the principles of this effect allows you to select stainless steels with varying carbon content based on different requirements.
　　For example, the standard chromium content for five stainless steel grades—0Crl3 and 4Cr13, which are widely and less commonly used in industry—is set at 12–14%. This value is determined after considering how carbon and chromium combine to form chromium carbide. The goal is to ensure that, once carbon and chromium have formed chromium carbide, the chromium content remaining in the solid solution does not fall below 11.7%.
　　For these five steel grades, differences in carbon content result in varying levels of strength and corrosion resistance. Among them, 0Cr13 and 2Cr3 steels exhibit superior corrosion resistance compared to 3Cr3 and 4Cr13 steels, though the latter two have lower strength and are primarily used for manufacturing structural components. The last two steels, due to their higher carbon content, achieve greater strength, making them ideal for producing high-strength, wear-resistant parts such as springs and cutting tools. For instance, to mitigate intergranular corrosion in 18-8 chromium-nickel stainless steel, the carbon content can be reduced to below 0.03%, or elements with a stronger affinity for carbon than chromium—such as titanium or niobium—can be added to prevent the formation of chromium carbides. When high hardness and wear resistance are critical requirements, we can simultaneously increase both the chromium content and the carbon level of the steel to meet these demands while also addressing the need for excellent wear and corrosion resistance. Industrial bearings, measuring instruments, and blades are commonly made from 9Cr18 and 9Cr17Movco stainless steels. Despite their relatively high carbon content—ranging from 0.85% to 0.95%—these steels still maintain satisfactory corrosion resistance thanks to the corresponding increase in chromium content.
　　Generally speaking, industrial-grade stainless steel has a relatively low carbon content. Most stainless steels contain between 0.1% and 0.4% carbon—noticeably white in appearance.