Stainless steel: an all-round material for modern industry and life

In the dazzling universe of materials science, stainless steel undoubtedly shines as a dazzling star. From towering skyscrapers to microscopic medical devices, from cars zipping through city streets to the pots and pans of everyday kitchens, stainless steel is ubiquitous, profoundly influencing and transforming our lives and the way we work.

I. Definition and Composition
Stainless steel is not simply a single metal, but a carefully blended iron alloy. Its core characteristic lies in its chemical stability, which allows it to resist corrosion in natural environments and specific industrial media. It generally includes stainless steel and acid-resistant steel, a category of special-performance steels. Ordinary stainless steel is stainless, but lacks corrosion resistance; acid-resistant steel combines both excellent stainless and corrosion resistance.

Iron, chromium, and carbon constitute the primary elements of stainless steel. Chromium, among them, is considered stainless steel's "corrosion protector." When chromium is incorporated into an iron-based solid solution, increasing its content, especially to around 13%, significantly increases the electrode potential of chromium-containing steel, significantly enhancing its corrosion resistance. Chromium forms a continuous solid solution with iron, reduces the austenite phase, and forms various carbides with carbon.

In addition to the main elements, other alloying elements are often added to stainless steel to optimize its properties. Molybdenum, as a ferrite-forming element, stabilizes the metal oxides on the surface of stainless steel, significantly enhancing its corrosion resistance in reducing and chloride-containing environments, effectively improving pitting resistance, and inhibiting crevice corrosion. Silicon is a strong ferrite-forming element. Its moderate addition can increase the tensile strength and elastic limit of stainless steel, improve its corrosion resistance and pitting resistance, enhance its resistance to stress corrosion cracking in chloride-containing media, and improve its resistance to oxidation and sulfidation. Titanium and niobium are strong ferrite-forming elements in austenitic alloys. They refine the grain structure, form carbides and carbonitrides, and fix carbon in the steel, improving strength, corrosion resistance, and cold forming properties. They also contribute to weldability, but excessive amounts can affect high-temperature plasticity.

Among austenite-forming elements, carbon and nitrogen significantly contribute to the formation and stabilization of austenite. While they enhance the strength of stainless steel, they also reduce impact toughness and increase the ductile-brittle transition temperature. Nickel is a key element in austenitic stainless steel, expanding the austenite phase and stabilizing the austenite structure. Increasing nickel content reduces the hardness and tensile strength of austenitic steel, while increasing toughness and improving the alloy's resistance to stress corrosion cracking. Manganese, a weaker austenite-forming element, stabilizes austenite, improves the steel's resistance to pitting corrosion, and is also an excellent deoxidizer and desulfurizer. However, its addition can somewhat reduce corrosion resistance.

II. Diverse Classifications Demonstrate Differences in Properties
(I) Classification Dominated by Alloy Composition
Chromium stainless steels are based on an iron matrix with chromium as the primary alloying element. Some also contain one or more elements, such as silicon, aluminum, tungsten, copper, and nickel, typically ranging from 1% to 3%. Its performance characteristic is its moderate corrosion resistance. It is often used in general industrial applications where corrosion resistance requirements are relatively low and cost control is relatively strict, such as the protective housing components of some common machinery.
Chromium-nickel stainless steel: In addition to the iron base, the main alloying elements are chromium and nickel. Some also contain one or more elements such as titanium, silicon, molybdenum, tungsten, and vanadium, with content below 4%. This type of stainless steel offers excellent overall performance, outstanding corrosion resistance, good plasticity, and machinability. It is widely used in applications requiring high material properties, such as food processing equipment, where food contact parts require excellent corrosion resistance and hygienic properties. Chromium-nickel stainless steel is an excellent choice.
Chromium-manganese-nitrogen stainless steel: The main alloying elements are chromium and manganese. Most contain nitrogen below 0.5%, and some also contain nickel, silicon, copper, and other elements, with content below 5%. This type of stainless steel offers unique performance advantages, such as excellent corrosion resistance and mechanical properties in certain environments. It can be used in structural components in architectural decoration, ensuring aesthetics while meeting certain strength and corrosion resistance requirements. (II) Metallographic Structure Classification
Ferritic stainless steel: This is a high-chromium steel with a chromium content of 13% to 30%. When the chromium content is above 13%, its microstructure is stable, with no gamma-phase transformation. It maintains a ferrite morphology from high to low temperatures, and its crystal structure is primarily a body-centered cubic lattice at room temperature. It exhibits excellent corrosion and oxidation resistance, particularly in stress corrosion corrosion resistance, but its mechanical properties and processing characteristics are relatively average. It is often used in construction and oxidation-resistant steels that require high acid resistance but relatively low strength. For example, ventilation ducts in chemical plants are subject to long-term exposure to corrosive gases, and ferritic stainless steel can meet these corrosion resistance requirements.
Austenitic stainless steel: With austenite as its primary structural component, it contains approximately 18% Cr, 8% to 25% Ni, and approximately 0.1% C. The austenitic structure is stable. During room-temperature solidification, its crystal structure adopts a planar cubic lattice, with high contents of metallic components such as chromium and nickel. It offers advantages such as strong corrosion resistance, good ductility, easy forming, no homogeneous transformation during heating, excellent weldability, excellent toughness and low-temperature toughness, and generally no cold brittleness. Its recrystallization temperature is higher than that of ferrite, making it suitable for high-temperature, high-strength environments operating above 550°C. For example, austenitic stainless steel is used in the manufacture of certain high-temperature components in aircraft engines due to its high-temperature performance. Some austenitic stainless steels contain a small amount of ferrite, resulting in a metastable austenitic stainless steel. At low temperatures or after cold working, some of the austenite can transform to martensite.

Austenitic-ferritic duplex stainless steel: At room temperature, austenite and ferrite coexist, with a 1:1 ratio for optimal overall properties. Specified indicators are set for the smaller phase content. It exhibits excellent stress corrosion resistance, with austenite imparting excellent embrittlement resistance, enabling good hot and cold working and weldability. Furthermore, the presence of ferrite increases yield strength and stress corrosion susceptibility. The combined effect of the two phases reduces the alloy's tendency to crystallize and intergranular attack. In some piping systems in the petrochemical industry, this type of stainless steel can adapt to complex operating environments and ensure long-term, stable operation.

Martensitic stainless steel: It is primarily composed of martensite, contains 12%-18% chromium, and also contains some austenitic components such as carbides and nickelates. At high temperatures, it can produce a large or complete γ phase, with a magnetic martensite matrix. Mechanical properties can be adjusted through heat treatment. Under appropriate heat treatment conditions, the microstructure is primarily composed of austenite, and upon cooling, martensite forms. The high-carbon component has a body-centered tetragonal lattice structure, while the low-carbon component has a body-centered structure. It is often used in the manufacture of mechanical parts requiring high hardness and wear resistance, such as cutting tools and bearings. Heat treatment can achieve high hardness and excellent wear resistance.

III. Excellent Performance Lays the Foundation for Widespread Application
(I) Corrosion Resistance
Stainless steel's corrosion resistance is arguably its most prominent advantage. The dense chromium oxide passive film formed by the reaction of chromium with oxygen on the surface acts like a solid "protective armor" for the steel, effectively shielding it from air, water, and chemicals. Different types of stainless steel have their own unique strengths in corrosion resistance. 304 stainless steel, a common type, maintains a smooth surface for a long time, rusting and corrosion-resistant in normal atmospheric conditions and mildly corrosive media thanks to its passive film. Therefore, it is widely used in architectural decorative railings and handrails, as well as in everyday items such as kitchen utensils and utensils. 316 stainless steel, with its 2%-3% addition of molybdenum, offers excellent resistance to highly corrosive media such as seawater and salt spray. In marine engineering applications, such as ship hulls and offshore platform structural components, 316 stainless steel is an ideal material for ensuring the long-term safe operation of equipment in harsh marine environments.
(II) Strength and Toughness
Stainless steel possesses an excellent balance of strength and toughness. Its strength enables it to withstand significant pressure, tension, and impact without deformation or fracture. In architectural structures, stainless steel is used to manufacture supporting structural components of high-rise buildings, such as beams and columns. These components are able to stably bear the immense weight of buildings and withstand the impact of natural disasters such as wind and earthquakes. Furthermore, stainless steel's toughness ensures that it will not fracture as easily as some brittle materials when subjected to external forces. In automotive manufacturing, stainless steel is used in body structural components. In the event of a collision, stainless steel components absorb energy through deformation, effectively protecting the safety of passengers.
(3) Processability
Stainless steel exhibits excellent processability and is highly malleable. Through stamping, stainless steel sheets can be formed into various complex shapes, such as automotive body panels and home appliance housings. In mobile phone manufacturing, stainless steel frames undergo precision stamping and subsequent processing, ensuring both a refined and aesthetically pleasing appearance and excellent structural strength. Bending processes allow stainless steel profiles to be fashioned into various shapes, such as railings and furniture frames. Welding performance is also a crucial aspect of stainless steel processing and forming. Different types of stainless steel can be securely joined using appropriate welding methods. In the construction of large bridges, numerous stainless steel components are welded together to create a stable bridge structure, ensuring its load-bearing capacity and stability.
(IV) Beautiful and Durable Appearance
Stainless steel exhibits a distinctive metallic silver-gray luster, offering a tough, high-gloss finish. A variety of finishes are available, ranging from a glossy mirror effect to a subtle matte finish, catering to diverse design styles. Its smooth surface resists stains and is easy to clean and maintain; a simple wipe keeps it looking like new. In architectural decoration, stainless steel is often used to create modern building facades, such as the curtain walls of large commercial complexes. Its gloss and texture lend a stylish, high-end aesthetic to the buildings, while its durability ensures that the exterior remains in excellent condition despite long-term exposure to wind, sun, and rain. In furniture design, stainless steel table and chair legs, handles, and other accessories not only provide sturdy support but also enhance the overall aesthetics and quality of the furniture. (5) Heat Resistance
Under high temperatures, many metal materials will soften and deform, but stainless steel maintains its structural stability. Ferritic and austenitic stainless steels both exhibit good heat resistance to varying degrees. Industrial furnaces and kiln equipment often use stainless steel for their inner walls to withstand the heat loads of high-temperature environments and ensure proper operation. In thermal power plant boiler piping systems, heat-resistant stainless steel pipes are capable of operating stably and for extended periods in high-temperature, high-pressure steam environments, ensuring the continuity and safety of power generation.
(6) Recyclability
With the growing popularity of environmental protection, the recyclability of materials is becoming increasingly important, and stainless steel excels in this regard. At the end of their useful life, stainless steel products can be recycled and reprocessed through a series of specialized smelting and refining processes, transforming them into new stainless steel for use in production. This characteristic not only helps reduce resource waste and lowers reliance on primary mineral resources, but also effectively minimizes environmental pollution caused by the mining and production of new materials. In the steel industry, a large amount of scrapped stainless steel products are recycled, enabling resource recycling and contributing positively to sustainable development. IV. Development History Witnesses Innovation and Breakthroughs

Between 1820 and 1822, British physicist Michael Faraday first explored the rusting of steel, initiating humanity's quest to understand the corrosion resistance of steel. In the early 20th century, French scholars Gillette and Portvin, and British scientist Giessen, respectively, published reports on the metallurgy of Fe-Cr and Fe-Cr-Ni alloys, demonstrating their corrosion resistance and laying the theoretical foundation for the development of stainless steel. Between 1908 and 1911, German theoretical researchers Monnartz and Bauchers discovered the phenomenon of passivation and proposed a theory of passivation for high-chromium alloys, further advancing stainless steel research.

Between 1914 and 1916, while developing corrosion-resistant alloys, scientist Johann Sebastian Brearley stumbled upon the discovery that a steel alloy containing 12% chromium was impervious to corrosion in various acid, alkali, and salt solutions. Stainless steel was thus officially born. Subsequently, the United States and the United Kingdom pioneered the commercial production of stainless steel. Between 1924 and 1935, American researchers applied for patents for oxygen-assisted stainless steelmaking, the redox process, and improved methods for carbonizing iron ore. Oxygen-assisted stainless steelmaking gradually gained popularity, continuously improving stainless steel performance and rapidly increasing production.

In my country, stainless steel consumption grew at an average annual rate of over 21.17% from 2000 to 2006. In 2001, my country's apparent stainless steel consumption reached 2.25 million tons, surpassing the United States to become the world's largest stainless steel consumer. In 2008, China's apparent stainless steel consumption reached 6.24 million tons, a year-on-year decrease, but overall demand remained substantial. In November 2011, my country's stainless steel production increased by 11.1% to 12.5 million tons, demonstrating the vigorous development of the country's stainless steel industry.