Definition of stainless steel
A type of high alloy steel that can resist corrosion in air or chemical corrosive media. Stainless steel is a type of steel that has a beautiful surface and good corrosion resistance, and does not require surface treatment such as color plating, while exerting the inherent surface properties of stainless steel. It is commonly referred to as stainless steel, and is used in various types of steel. Representative properties include high alloy steels such as 13 chromium steel and 18 chromium nickel steel.
From a metallographic point of view, stainless steel contains chromium and forms a very thin chromium film on the surface, which acts as a corrosion resistant film to isolate oxygen from the steel.
To maintain the inherent corrosion resistance of stainless steel, steel must contain more than 12% chromium
Stainless steel terminology
stainless steel
Generally speaking, stainless steel is steel that is not easy to rust. In fact, some stainless steels have both rust resistance and acid resistance (corrosion resistance). The rust resistance and corrosion resistance of stainless steel are due to the formation of a chromium rich oxide film (passivation film) on its surface. This rust resistance and corrosion resistance are relative. The test shows that the corrosion resistance of steel in weak media such as atmosphere and water, as well as in oxidizing media such as nitric acid, increases with the increase of the chromium content in the steel. When the chromium content reaches a certain percentage, the corrosion resistance of steel undergoes a sudden change, that is, from being easy to rust to not easy to rust, and from being resistant to corrosion. There are many ways to classify stainless steel. According to the classification of microstructure at room temperature, there are martensitic, austenitic, ferrite, and duplex stainless steels; According to the classification of main chemical components, it can be basically divided into two systems: chromium stainless steel and chromium nickel stainless steel; According to the purpose, there are nitric acid resistant stainless steel, sulfuric acid resistant stainless steel, seawater resistant stainless steel, etc. According to the type of corrosion resistance, it can be divided into pitting resistant stainless steel, stress corrosion resistant stainless steel, intergranular corrosion resistant stainless steel, etc; According to functional characteristics, it can be divided into non magnetic stainless steel, free cutting stainless steel, low-temperature stainless steel, high-strength stainless steel, and so on. Due to its excellent corrosion resistance, formability, compatibility, and strength and toughness over a wide temperature range, stainless steel has been widely used in heavy industry, light industry, household goods industry, and architectural decoration industries.
austenitic stainless steel
Stainless steel having an austenitic structure at room temperature. When the steel contains about 18% Cr, 8% - 10% Ni, and about 0.1% C, it has a stable austenitic structure. Austenitic chromium nickel stainless steels include 18Cr-8Ni steels and high Cr Ni series steels developed by increasing Cr and Ni content and adding elements such as Mo, Cu, Si, Nb, and Ti. Austenitic stainless steel is nonmagnetic and has high toughness and plasticity, but its strength is relatively low. It cannot be strengthened through phase transformation, and can only be strengthened through cold working. If elements such as S, Ca, Se, Te are added, it has good machinability. In addition to resisting the corrosion of oxidizing acid media, such steels can also resist the corrosion of sulfuric acid, phosphoric acid, formic acid, acetic acid, urea, etc. if they contain elements such as Mo and Cu. If the carbon content in such steels is lower than 0.03% or if they contain Ti and Ni, their intergranular corrosion resistance can be significantly improved. High silicon austenitic stainless steel with concentrated nitric acid has good corrosion resistance. Austenitic stainless steel has been widely used in various industries due to its comprehensive and good comprehensive properties.
Ferritic stainless steel
Stainless steel with a predominantly ferrite structure in use. The chromium content ranges from 11% to 30% and has a body centered cubic crystal structure. This type of steel generally does not contain nickel, and sometimes contains a small amount of elements such as Mo, Ti, and Nb. This type of steel has characteristics such as high thermal conductivity, low expansion coefficient, good oxidation resistance, and excellent resistance to stress corrosion. It is mostly used to manufacture components that are resistant to atmospheric, steam, water, and oxidative acid corrosion. This type of steel has shortcomings such as poor plasticity, significantly reduced plasticity and corrosion resistance after welding, which limit its application. The application of outside furnace refining technology (AOD or VOD) can greatly reduce interstitial elements such as carbon and nitrogen, making this type of steel widely used.
Austenitic Ferritic Duplex Stainless Steel
It is a stainless steel with about half austenite and about half ferrite. When the content of C is low, the content of Cr is between 18% and 28%, and the content of Ni is between 3% and 10%. Some steels also contain alloying elements such as Mo, Cu, Si, Nb, Ti, and N. This type of steel combines the characteristics of austenitic and ferritic stainless steel. Compared to ferrite, it has higher plasticity and toughness, no room temperature brittleness, significantly improved intergranular corrosion resistance, and welding performance. At the same time, it also maintains the 475 ℃ brittleness of ferritic stainless steel, high thermal conductivity, and superplasticity. Compared with austenitic stainless steel, it has high strength and significantly improved resistance to intergranular corrosion and chloride stress corrosion. Duplex stainless steel has excellent pitting resistance and is also a nickel saving stainless steel.
Martensitic stainless steel
Stainless steel whose mechanical properties can be adjusted through heat treatment is, colloquially speaking, a type of hardenable stainless steel. Typical grades are Cr13 type, such as 2Cr13, 3Cr13, 4Cr13, etc. After quenching, the hardness is higher, and different tempering temperatures have different combinations of strength and toughness. It is mainly used for steam turbine blades, tableware, and surgical instruments. According to the differences in chemical composition, martensitic stainless steel can be divided into two categories: martensitic chromium steel and martensitic chromium nickel steel. According to the different structures and strengthening mechanisms, they can also be divided into martensitic stainless steels, martensitic and semi austenitic (or semi martensitic) precipitation hardening stainless steels, and martensitic aging stainless steels.
Surface Identification Methods for Stainless Steel (No.1, 2B, BA, NO4)
In general atmospheric oxidation environments, the corrosion rate of stainless steel is very low, and it is by no means completely free of rust. Because chromium (Cr) is contained, a very dense and thin chromium oxide (Cr2O3) film is formed on the surface, which can prevent continuous corrosion and rust, hence the name. It has characteristics such as corrosion resistance, oxidation resistance, low heat conduction, and good processability.
Original surface: Hot rolled, heat treated, and pickled, commonly used for general heat and corrosion resistance, such as chemical tanks, with a thickness ranging from 2.0MM to 8.0MM.
Blunt surface: 2D cold rolled, heat treated, and pickled. Its material is soft and has a silver white luster on its surface. It is used for deep stamping, such as automotive components, water pipes, and so on.
Mist surface: 2B cold rolled, heat treated, pickled, and then finished to give a moderately bright surface. Due to its smooth surface, it is easy to regrind, making the surface brighter and widely used, such as tableware and building materials.
Bright surface: After cold rolling, BA undergoes a glossy heat treatment, resulting in a brighter surface. It is commonly used in electrical appliances, kitchenware, and architectural decoration. Hairy surface: HAIR LINE is a type of abrasive material with appropriate particle size to create continuous stripes. It is commonly used in construction and decoration, such as elevators and escalators, and has a wide range of uses.
Fine sand: No. 4 abrasive material processor with a particle size of 150-180, commonly used in milk and food processing equipment, medical equipment, etc.
Coarse sand: No. 3 abrasive material processor with a particle size of 100-120, commonly used in milk food processing equipment and kitchen utensils.
Corrosion Resistance and Types of Stainless Steel
1. Types and definitions of corrosion
In many industrial applications, stainless steel can provide today's satisfactory corrosion resistance. According to experience in use, in addition to mechanical failure, corrosion of stainless steel is mainly manifested in: a serious form of corrosion of stainless steel is localized corrosion (i.e., stress corrosion cracking, pitting corrosion, intergranular corrosion, corrosion fatigue, and crevice corrosion). The failure cases caused by these localized corrosion accounts for almost half of the failure cases. In fact, many failure accidents can be avoided through reasonable material selection.
Stress Corrosion Cracking (SCC): A generic term used to refer to the failure of stressed alloys due to the propagation of sharp lines in corrosive environments. Stress corrosion cracking (SCC) has a brittle fracture morphology, but it can also occur in materials with high toughness. The necessary conditions for stress corrosion cracking to occur are the presence of tensile stress (either residual or applied stress, or both) and a specific corrosion medium. The formation and expansion of the pattern is approximately perpendicular to the direction of tensile stress. The stress value that causes stress corrosion cracking is much smaller than the stress value required for material fracture in the absence of corrosive media. On the microscopic level, cracks passing through grains are called transgranular cracks, while cracks extending along grain boundaries are called intergranular cracks. When stress corrosion cracking extends to a depth (where the stress on the section of the material under load reaches its breaking stress in air), the material is broken as normal cracks (in ductile materials, typically through the aggregation of microscopic defects). Therefore, the cross-section of a part that fails due to stress corrosion cracking will contain characteristic areas of stress corrosion cracking and "dimple" areas associated with the aggregation of microdefects.
Spot corrosion: A form of localized corrosion that causes corrosion.
Intergranular corrosion: Intergranular boundaries are boundaries of disorderly dislocations between grains with different crystallographic orientations. Therefore, they are the segregation of various solute elements or metal compounds (such as carbides and δ Favorable areas for precipitation and precipitation. Therefore, it is not surprising that in some corrosive media, the intergranular boundaries may be corroded first. This type of corrosion is known as intergranular corrosion, and most metals and alloys may exhibit intergranular corrosion in specific corrosive media.
Crevice corrosion: a form of localized corrosion that may occur in crevices where the solution is stagnant or within shielded surfaces. Such gaps can be formed at metal to metal or metal to non-metal joints, for example, at joints with rivets, bolts, gaskets, valve seats, loose surface deposits, and marine organisms.
Total corrosion: A term used to describe corrosion that occurs in a relatively uniform manner across the entire alloy surface. When comprehensive corrosion occurs, the village material gradually becomes thinner due to corrosion, and even the material becomes corroded and fails. Stainless steel may exhibit overall corrosion in strong acids and alkalis. The failure problem caused by overall corrosion is not much of a concern, as this type of corrosion can usually be predicted through simple immersion tests or by consulting the literature on corrosion.
2. Corrosion resistance of various stainless steels
304 is a versatile stainless steel widely used for manufacturing equipment and components that require good comprehensive performance (corrosion resistance and formability).
301 stainless steel exhibits significant work hardening during deformation, and is used in various applications that require high strength.
302 stainless steel is essentially a variant of 304 stainless steel with a higher carbon content, which can achieve higher strength through cold rolling.
302B is a stainless steel with high silicon content, which has high resistance to high-temperature oxidation.
303 and 303Se are free-cutting stainless steels that contain sulfur and selenium, respectively, and are used in applications that primarily require easy cutting and high apparent brightness. 303Se stainless steel is also used for making parts that require hot upsetting, because under such conditions, this stainless steel has good hot workability.
304L is a variant of 304 stainless steel with a low carbon content, used in situations where welding is required. Low carbon content reduces the amount of carbide precipitated in the heat affected zone adjacent to the weld, which can lead to intergranular corrosion (welding erosion) of stainless steel in certain environments.
304N is a stainless steel containing nitrogen, which is added to improve the strength of the steel.
305 and 384 stainless steels contain high nickel content and have a low work hardening rate, making them suitable for various occasions with high requirements for cold formability.
308 stainless steel is used for making welding rods.
Stainless steels 309, 310, 314, and 330 have relatively high nickel and chromium contents in order to improve the oxidation resistance and creep strength of the steel at high temperatures. "While 30S5 and 310S are variants of 309 and 310 stainless steels, the only difference is that the carbon content is low, in order to reduce at least the carbide precipitated near the weld.". 330 stainless steel has particularly high resistance to carburization and thermal shock
Type 316 and 317 stainless steels contain aluminum, which greatly outperforms 304 stainless steel in resistance to spot corrosion in marine and chemical industrial environments. Among them, variants of 316 type stainless steel include low-carbon stainless steel 316L, high-strength stainless steel 316N containing nitrogen, and free-cutting stainless steel 316F with a high sulfur content.
321, 347, and 348 are stainless steels stabilized with titanium, niobium, tantalum, and niobium, respectively, and are suitable for welding components used at high temperatures. 348 is a stainless steel suitable for the nuclear power industry, with certain limitations on the amount of tantalum and drill.
304, 430, 410 stainless steel processing and hot processing
Generally, when selecting stainless steel, the first consideration is corrosion resistance, followed by strength and other mechanical properties, and the third consideration is processing and manufacturing performance. The three dominant common types of stainless steel are 304, 430, and 410. These different types of steel are more suitable for a certain manufacturing process. (Service requirements may limit the use of these different types of steel, so it is necessary to be familiar with all stainless steels in order to easily process and manufacture them using traditional manufacturing methods.) Stainless steel is easily formed by hot working such as binding, extrusion, and forging, which can produce finished or semi-finished parts. Hot banding is usually performed using a single banding machine to produce standard and special shapes. Extrusion processes are commonly associated with softer nonferrous metals and are also widely used for stainless steel. During the extrusion process, the shape of a component is obtained by applying a force to a bar or billet section to pass it through the mold, and the cross-section of the outlet coincides with the opening of the mold. Several companies produce hot extrusion parts. The relatively small number of extrusion pairs is reasonable and economical. Stainless steel profiles typically weigh a small amount of 1000 pounds. Any shape with an effective cross-section of 165.1 mm can be extruded. The weight of a large hot extrusion product is approximately 30 pounds per foot. On the other hand, the small allowable cross-section is 1.8063mm2, and the small web thickness is 3.175mm. Hollow shapes and cube shapes are easy to process. Forging is widely used in all types of stainless steel, ranging in size from a few ounces to several thousand pounds, and can be less than an inch to several feet long. Special processes. For example, drawing, punching, and precision pressing further improve the ability of forgings. The designer should consult the forging worker for design guidelines related to molding, tolerances, and boundaries. See the attached table for the relevant manufacturing process performance of austenitic, martensitic, and ferritic stainless steels.
Exploration and Practice of High Strength Stainless Steel in China
Stainless steel is one of the rapidly developing and widely used steels in special steels, and high-strength stainless steel is an important branch of stainless steel. With the rapid development of China's energy development, petrochemical industry, and aviation and aerospace industries, higher requirements are put forward for the performance of high-strength stainless steel. Therefore, it is urgent to develop high-strength stainless steel with high toughness, high corrosion resistance, easy processing, molding, and welding, and good comprehensive performance.
（1） Development Process of High Strength Stainless Steel in China
In the early 1960s, China systematically studied precipitation hardened stainless steels such as PH15-7Mo, 17-7PH in the 1970s, 17-4PH in the 1980s, and 15-5PH in the 1990s, and obtained practical applications. At the same time, since the early 1970s, China has developed a series of research and development projects σ The research and development of martensitic aged stainless steel with b=1200MPa~1700MPa began in the 1990s, and the steel grades developed basically meet the needs of China's national defense construction and national economic development.
（2） Development and application of new high-strength stainless steel grades
1. Precipitation hardened stainless steel
1.1. Semi austenitic precipitation hardened stainless steel
Semi-austenitic precipitation hardened stainless steel can be heated by varying the temperature within an optimized chemical composition range