Definition and brief introduction of stainless steel microstructure

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The classification of stainless steels is often based on their microstructure at room temperature; this is the case for austenitic stainless steels, ferritic stainless steels, duplex stainless steels, and martensitic stainless steels. Ferrite, also known as ferrum (abbreviation: FN), refers to α-Fe and the solid solutions based on it, and it has a body-centered cubic crystal structure. Austenite with hypoeutectoid composition forms ferrite through pro-eutectoid precipitation. This portion of ferrite is known as pro-eutectoid ferrite or structurally free ferrite. Depending on the formation conditions, pro-eutectoid ferrite can take various forms, such as equiaxed, intergranular, spindle-shaped, serrated, and acicular. The matrix is still ferritic or pearlitic in structure. In the hot-rolled (normalized) and annealed microstructures of carbon steel and low-alloy steel, ferrite is the main constituent phase ； The composition and structure of ferrite have a significant impact on the processability of steel, and in some cases, they also affect its performance in practical applications. Pure iron is α-Fe with a body-centered cubic lattice (Note 1) below 912°C. The interstitial solid solution in which carbon is dissolved in α-Fe is called ferrite, denoted by the symbol F. Since α-Fe has a body-centered cubic crystal structure, its lattice gaps are very small, which results in an extremely poor ability to absorb carbon. The maximum carbon absorption occurs at 727°C, at 0.0218%; as the temperature decreases, this value gradually falls, reaching around 0.0057% at 600°C, and it is almost zero at room temperature. Therefore, its properties are almost identical to those of pure iron, with the following values: tensile strength of 180–280 MN/m², yield strength of 100–170 MN/m², elongation of 30–50%, reduction in area of 70–80%, impact toughness of 160–200 J/cm², and hardness of HB 50–80. It can be seen that ferrite has low strength and hardness, but it possesses good plasticity and toughness. The microstructure of ferrite is the same as that of pure iron, featuring a bright polygonal grain structure; sometimes, due to the different orientations of the grains, their degree of corrosion varies slightly, resulting in slight differences in brightness. Ferrite is ferromagnetic below 770°C and loses its ferromagnetism above 770°C. Note 1: The unit cell of a body-centered cubic lattice is a cube, with an atom located at each corner and at the center of the unit cell. It can be seen that the atoms at each corner of the body-centered cubic unit cell are shared by the eight adjacent unit cells; thus, each unit cell actually possesses only 1/8 of an atom. The atom at the center, however, is unique to that unit cell. Therefore, the number of atoms in a body-centered cubic unit cell is 8*1/8+1=2. Carbon atoms exist in tetrahedral and octahedral interstices. Note: Ferrite is a interstitial solid solution formed by the dissolution of C in α-Fe; it has a body-centered cubic crystal structure and is denoted by the letter F or α. Stainless steel with a ferritic microstructure as the dominant structure. It contains 11% to 30% chromium and has a body-centered cubic crystal structure. These types of steel generally do not contain Ni (nickel); sometimes they also contain small amounts of elements such as Mo, Ti, and Nb. They feature a high thermal conductivity, a low coefficient of expansion, good oxidation resistance, and excellent resistance to stress corrosion. They are often used to manufacture components that can withstand corrosion caused by the atmosphere, water vapor, water, and oxidizing acids. Such steels have disadvantages such as poor plasticity, as well as a significant reduction in plasticity and corrosion resistance after welding, which limits their application. But now, due to the soaring price of nickel and the growing awareness of environmental and health issues, the use of chromium stainless steel is being promoted in China, and this has already begun to show effects in the medical and healthcare sectors. David Austenite: The English name for austenite is austenite. Crystal structure: face-centered cubic (fcc). Letter codes: A, γ. Definition: It is a interstitial solid solution of carbon in γ-Fe. Properties: Austenite is a solid solution with good plasticity and relatively low strength, as well as a certain degree of toughness. It does not possess ferromagnetism. Therefore, one way to identify austenitic stainless steel tools (the common 18-8 type stainless steel) is to use a magnet to check whether the tool is magnetic. When ancient blacksmiths worked with hot iron, the iron was in the austenite state. Furthermore, since austenite is face-centered cubic, it has larger tetrahedral interstices that can accommodate more carbon. Note: Austenite is a interstitial solid solution of carbon dissolved in γ-Fe, commonly denoted by the symbol A. It still maintains a face-centered cubic lattice of γ-Fe. It has a high carbon solubility; at 727°C, the carbon solubility is ωc = 0.77%, while at 1148°C, it can dissolve 2.11% carbon. Austenite can exist stably only at temperatures above 727°C. Austenite has good plasticity, and it is the microstructure required for the pressure processing of the vast majority of steel grades at high temperatures. Then why can an austenitic structure be obtained even in a low-temperature environment such as room temperature? The reason is that austenitic stainless steel contains a large amount of the alloying element Ni (nickel), which expands the austenite region; nickel inhibits the formation of ferrite, thereby resulting in an austenitic microstructure of the steel at room temperature. In addition to resisting corrosion in oxidizing acidic media, such steels can also withstand corrosion by sulfuric acid, phosphoric acid, as well as formic acid, acetic acid, urea, and the like, if they contain elements such as Mo and Cu. Martensite is a term used to describe a type of microstructure in ferrous metal materials. Martensite was first discovered in steel (medium and high carbon steels): it is a type of hardened structure obtained by heating steel to a certain temperature (to form austenite) and then rapidly cooling it (quenching), which makes the steel harder and stronger. The crystal structure of martensite is body-centered tetragonal (BCT). This microstructure can usually be obtained by rapid cooling in medium and high carbon steels. High strength and hardness are among the main characteristics of martensite in steel. Note: Martensite is a metastable phase formed by the transformation of austenite through a diffusion-free phase transition; it is a interstitial solid solution of carbon in iron that is supersaturated. Its crystal structure is body-centered tetragonal. The microstructure of martensite in high-carbon steel after quenching is bamboo-leaf shaped ； The martensite in low-carbon steel after quenching appears as bundles of thin, elongated strips that are parallel to one another. High-carbon martensite is hard and brittle, while low-carbon martensite possesses higher strength and toughness. Martensitic stainless steels are primarily low-carbon or high-carbon steels with a chromium content in the range of 12%-18%. According to the Fe-Cr phase diagram, when the chromium content is greater than 13%, the γ phase does not exist; such alloys are single-phase ferritic alloys and cannot form martensite under any heat treatment conditions. Therefore, it is necessary to add austenite-forming elements to the Fe-Cr binary alloy in order to expand the γ phase region. For martensitic chromium stainless steels, C and Ni are effective elements, and the addition of these elements allows the alloy to tolerate higher chromium contents. In martensitic chromium stainless steel, apart from chromium, C is another essential element; in fact, martensitic chromium stainless heat-resistant steels are a type of ternary alloy consisting of iron, chromium, and carbon. Of course, there are other elements as well; martensitic stainless steel possesses high strength and corrosion resistance, and can be used to manufacture machine parts, medical devices, kitchen knives, measuring instruments, springs, and more. Like modified steels, martensitic stainless steels can be treated by quenching, tempering, and annealing. Its mechanical properties are also similar to those of modified steel: as hardness increases, tensile strength and yield strength rise, while elongation, area reduction, and impact energy decrease. David Duplex Stainless Steel: Duplex Stainless Steel, abbreviated as DSS, is a type of steel in which the ferritic phase and the austenitic phase are present in equal amounts within its solid solution structure; generally, the content of the minority phase should also be at least 30%. At low C levels, the Cr content ranges from 18% to 28%, while the Ni content ranges from 3% to 10%. Some steels also contain alloying elements such as Mo, Cu, Nb, Ti, and N. This type of steel possesses the characteristics of both austenitic and ferritic stainless steels. Compared to ferritic steels, it has higher plasticity and toughness, no room-temperature brittleness, and significantly improved resistance to intergranular corrosion as well as weldability. At the same time, it retains the brittleness at 475°C and high thermal conductivity associated with ferritic stainless steels, as well as superplasticity. Compared to austenitic stainless steels, it has higher strength, as well as significantly improved resistance to intergranular corrosion and chloride stress corrosion. Duplex stainless steel exhibits excellent pitting resistance and is also a nickel-saving stainless steel. Performance characteristics of duplex stainless steel: Due to its two-phase structure, and through proper control of chemical composition and heat treatment processes, duplex stainless steel combines the advantages of both ferritic and austenitic stainless steels. It merges the excellent toughness and weldability of austenitic stainless steels with the higher strength and resistance to chloride stress corrosion of ferritic stainless steels. It is these superior properties that have led to the rapid development of duplex stainless steel as a weldable structural material; since the 1980s, it has become one of the major types of stainless steel, on par with martensitic, austenitic, and ferritic stainless steels. Dual-phase stainless steel has the following performance characteristics: (1) Molybdenum-containing dual-phase stainless steel exhibits good resistance to chloride stress corrosion at low stresses. Generally, Type 18-8 austenitic stainless steel is prone to stress corrosion cracking in neutral chloride solutions at temperatures above 60°C. Heat exchangers, evaporators, and other equipment manufactured from this type of stainless steel in industrial environments containing trace amounts of chlorides and hydrogen sulfide are susceptible to stress corrosion cracking, whereas duplex stainless steel exhibits good resistance to such phenomena. (2) Mo-containing duplex stainless steels have good pitting resistance. When they have the same pitting resistance equivalent value (PRE=Cr%+3.3Mo%+16N%), the critical pitting potentials of duplex stainless steels and austenitic stainless steels are similar. The pitting resistance of duplex stainless steels and austenitic stainless steels is comparable to that of AISI 316L. The pitting and crevice corrosion resistance of 25%Cr high-chromium duplex stainless steels, especially those containing nitrogen, exceeds that of AISI 316L. (3) It has good resistance to corrosion fatigue and wear corrosion. Under certain corrosive medium conditions, it is suitable for manufacturing power equipment such as pumps and valves. (4) It has good comprehensive mechanical properties. It has high strength and fatigue resistance, with a yield strength that is twice that of 18-8 type austenitic stainless steel. The elongation rate in the solid solution state reaches 25%, and the toughness value AK (V-notch) is above 100 J. (5) It has good weldability and a low tendency to thermal cracking; preheating is generally not required before welding, nor is heat treatment necessary after welding. It can be welded to different materials such as 18-8 austenitic stainless steel or carbon steel. The hot working temperature range for David (6), a low-chromium (18%Cr) duplex stainless steel, is wider than that of 18-8 austenitic stainless steel; it has lower strength, allowing steel plates to be produced directly through rolling without the need for forging. Heat processing of duplex stainless steel with high chromium content (25% Cr) is slightly more difficult than that of austenitic stainless steel, but it can be used to produce products such as sheets, tubes, and wires. (7) During cold working, the work hardening effect is greater than that of 18-8 austenitic stainless steel; at the initial stage of deformation in tubes and plates, a higher stress must be applied to cause deformation. (8) Compared with austenitic stainless steels, it has a higher thermal conductivity and a lower linear expansion coefficient, making it suitable for use as linings in equipment and for producing composite sheets. It is also suitable for use in heat exchanger wafers, offering a higher heat exchange efficiency than austenitic stainless steel. (9) It still possesses various brittle tendencies of high-chromium ferritic stainless steels, and is not suitable for use in operating conditions above 300°C. The lower the chromium content in duplex stainless steel, the less harmful the brittle phases such as σ become. Since its invention in the United States in the 1940s, duplex stainless steel has evolved to its third generation. Its main feature is that its yield strength can reach 400–550 MPa, which is twice that of ordinary stainless steel; thus, it allows for reduced material usage and lower costs in equipment manufacturing. In terms of corrosion resistance, especially in harsh medium environments such as seawater with high chloride content, duplex stainless steel exhibits significantly better performance against pitting corrosion, crevice corrosion, stress corrosion, and corrosion fatigue compared to conventional austenitic stainless steels, and can rival highly alloyed austenitic stainless steels. Duplex stainless steel exhibits good weldability. Compared with ferritic and austenitic stainless steels, it does not suffer from a significantly reduced ductility and toughness in its weld heat-affected zone due to severe grain coarsening, as is the case with ferritic stainless steels; nor is it as sensitive to weld hot cracks as austenitic stainless steels. Due to its unique advantages, duplex stainless steel is widely used in various industrial fields such as petrochemical equipment, seawater and wastewater treatment systems, oil and gas pipelines, and papermaking machinery. In recent years, it has also been explored for use in bridge load-bearing structures, showing great potential for further development.