Definition of stainless steel
A high-alloy steel that resists corrosion in air or chemically corrosive media. Stainless steel has an aesthetically pleasing surface and good corrosion resistance. It does not require surface treatment such as plating, but also exhibits the inherent surface properties of stainless steel. One of the aspects of steel, commonly referred to as stainless steel. Representative properties include high-alloy steels such as 13 chrome steel and 18-chrome nickel steel.
From the perspective of metallography, because stainless steel contains chromium and the surface forms a very thin chromium film, this film is isolated from the intrusion of oxygen in the steel to resist corrosion.
In order to maintain the corrosion resistance inherent in stainless steel, steel must contain more than 12% chromium.
Stainless steel term
In layman's terms, stainless steel is a steel that is not easily rusted. In fact, some stainless steels have both rust and acid resistance (corrosion resistance). The stainless steel's rust and corrosion resistance is due to the formation of a chromium-rich oxide film (passivation film) on its surface. This rust and corrosion resistance are relative. The test shows that the corrosion resistance of steel in the weak medium such as atmosphere and water and the oxidizing medium such as nitric acid increases with the increase of the water content of chromium in the steel. When the chromium content reaches a certain percentage, the corrosion resistance of the steel occurs. Mutations, from rust to rust, from corrosion to corrosion. There are many ways to classify stainless steel. Classified according to the structure of the room temperature, there are martensite, austenitic, ferrite and duplex stainless steel; according to the main chemical composition, can be divided into two major systems: chromium stainless steel and chromium nickel stainless steel; 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. Magnetic stainless steel, free-cutting stainless steel, low-temperature stainless steel, high-strength stainless steel, etc. Due to its excellent corrosion resistance, formability, compatibility and toughness over a wide temperature range, stainless steel is widely used in heavy industry, light industry, household goods industry and architectural decoration industries. .
Austenitic stainless steel
Stainless steel having an austenitic structure at normal temperature. When the steel contains about 18% Cr, 8%-10% Ni, and about 0.1% C, it has a stable austenite structure. Austenitic chromium-nickel stainless steel includes the famous 18Cr-8Ni steel and high Cr-Ni series steel which is added with elements of Cr and Ni and added with elements such as Mo, Cu, Si, Nb and Ti. Austenitic stainless steel is non-magnetic and has high toughness and plasticity, but its strength is low, it is impossible to strengthen it by phase transformation, and it can only be strengthened by cold working. If S, Ca, Se, Te and other elements are added, it has good machinability. In addition to the corrosion resistance of the oxidizing acid medium, such steel can also resist the corrosion of sulfuric acid, phosphoric acid, formic acid, acetic acid, urea, etc. if it contains elements such as Mo and Cu. If the carbon content of such steel is less than 0.03% or contains Ti or Ni, the intergranular corrosion resistance can be remarkably improved. High silicon austenitic stainless steel concentrated nitric acid has good corrosion resistance. Due to the comprehensive and good comprehensive properties of austenitic stainless steel, it has been widely used in various industries.
Ferritic stainless steel
Stainless steel mainly composed of ferrite in use. The chromium content is between 11% and 30%, and has a body-centered cubic crystal structure. These steels generally do not contain nickel, and sometimes contain a small amount of elements such as Mo, Ti, Nb, etc. These steels are characterized by large thermal conductivity, small expansion coefficient, good oxidation resistance and excellent resistance to stress corrosion. Parts that are corroded by water vapor, water, and oxidizing acids. Such steels have the disadvantages of poor plasticity, post-weld plasticity and corrosion resistance, which limit their application. The application of refining technology (AOD or VOD) can greatly reduce the gap elements such as carbon and nitrogen, thus making this type of steel widely used.
Austenite-ferrite duplex stainless steel
It is a stainless steel that accounts for about half of the austenite and ferrite structures. In the case of a lower C content, the Cr content is between 18% and 28%, and the Ni content 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 with ferrite, it has higher plasticity, toughness, no room temperature brittleness, resistance to intergranular corrosion and weldability, and iron retention. The 475 ° C brittleness and high thermal conductivity of the stainless steel are characterized by superplasticity. Compared with austenitic stainless steel, it has high strength and 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 by heat treatment is, in a nutshell, a type of hardenable stainless steel. Typical grades are Cr13, such as 2Cr13, 3Cr13, 4Cr13, etc. After fire, the hardness is higher. Different tempering temperatures have different combinations of toughness and are mainly used for steam turbine blades, tableware and surgical instruments. According to the difference in chemical composition, martensitic stainless steel can be divided into martensitic chromium steel and martensitic chromium nickel steel. According to the organization and strengthening mechanism, it can be divided into martensitic stainless steel, martensite and semi-austenitic (or semi-martensitic) precipitation hardening stainless steel and maraging stainless steel.
Stainless steel surface marking method (No. 1, 2B, BA, NO4)
In the general atmospheric oxidation environment, the corrosion rate of stainless steel is very low, and it is not completely rust-free. Because chromium (Cr) is formed, a very thin and thin chromium oxide (Cr2O3) film is formed on the surface to prevent corrosion and rust. name. It has properties such as corrosion resistance, oxidation resistance, low heat conduction and good processability.
Original surface: After hot rolling, after heat treatment, pickling is often used for general heat and corrosion resistance, such as chemical tanks, thicker than 2.0MM-8.0MM.
Blunt surface: 2D After cold rolling, after heat treatment and pickling, the material is soft and the surface is silvery white, which is used for deep drawing processing, such as automobile parts and water pipes.
Matte: 2B After cold rolling, it is heat treated, pickled, and finished by finish rolling to make the surface light. Because the surface is smooth, it is easy to re-grind, making the surface brighter and widely used, such as tableware and building materials.
Glossy: After the cold rolling of BA, the surface is brighter, and it is often used in electrical appliances, kitchen utensils and decoration. Hairline: HAIR LINE is a kind of abrasive material with appropriate particle size to cause continuous stripe processing. It is often used for building decoration such as elevators, escalators, etc. It is extremely versatile.
Fine sand: NO.4 is processed by abrasive materials with a particle size of 150-180. It is often used in milk food processing equipment and medical equipment.
Coarse sand: NO.3 is processed by abrasive materials with a particle size of 100-120. It is commonly used in milk food processing equipment and kitchen utensils.
Corrosion resistance of stainless steel and its kind
1. Types and definitions of corrosion
In many industrial applications, stainless steel can provide satisfactory corrosion resistance. According to the experience of use, in addition to mechanical failure, the corrosion of stainless steel is mainly manifested in: a serious form of corrosion of stainless steel is local corrosion (ie stress corrosion cracking, pitting corrosion, intergranular corrosion, corrosion fatigue and crevice corrosion) . The failure cases caused by these localized corrosions account for almost half of the failure cases. In fact, many failures can be avoided through reasonable selection.
Stress Corrosion Cracking (SCC): A general term for stress-bearing alloys that fail alternately in corrosive environments due to the expansion of the striated lines. Stress corrosion cracking has a brittle fracture morphology, but it can also occur in materials with high toughness. The necessary conditions for stress corrosion cracking are tensile stress (whether residual stress or applied stress, or both) and the presence of specific corrosive media. The formation and expansion of the profile is approximately perpendicular to the direction of the tensile stress. This stress value that causes stress corrosion cracking is much smaller than the stress value required for material fracture without the presence of corrosive media. At the microscopic level, the crack passing through the grain is called a transgranular crack, and the crack along the grain boundary is called an intergranular crack. When the stress corrosion cracking spreads to a depth (here, the load is on the material section) The stress reaches its breaking stress in the air), and the material is broken by normal cracking (in the ductile material, usually by polymerization of microscopic defects). Thus, the section of the part that fails due to stress corrosion cracking will include a characteristic region of stress corrosion cracking and a "dimple" region associated with the polymerization of the microdefect.
Pitting corrosion: A form of localized corrosion that causes corrosion.
Intergranular corrosion: Grain boundaries are the boundary between crystal grains with different crystallographic orientations. Therefore, they are the segregation of various solute elements in steel or the precipitation of metal compounds (such as carbides and δ phases). District City. Therefore, in some corrosive media, it is not surprising that the grain boundaries may be corroded first. This type of corrosion is known as intergranular corrosion, and most metals and alloys may exhibit intergranular corrosion in certain corrosive media.
Crevice Corrosion: A form of localized corrosion that may occur in the gap where the solution stagnates or in the surface of the shield. Such gaps may be formed at the junction of metal and metal or metal and non-metal, for example, where rivets, bolts, gaskets, valve seats, loose surface deposits, and marine organisms are attached to the candle.
General Corrosion: A term used to describe the corrosion phenomena that occur in a uniform manner across the entire surface of an alloy. When total corrosion occurs, the village material becomes thinner due to corrosion, and even the material corrosion fails. Stainless steel may exhibit general corrosion in strong acids and bases. The failure problem caused by general corrosion is not very worrying because it can usually be predicted by simple immersion tests or by consulting the literature on corrosion.
2. Corrosion resistance of various stainless steel
304 is a versatile stainless steel that is widely used to make equipment and parts that require good overall performance (corrosion resistance and formability).
301 stainless steel exhibits significant work hardening during deformation and is used in a variety of applications where higher strength is required.
302 stainless steel is essentially a variant of 304 stainless steel with a higher carbon content, which gives it a higher strength by cold rolling.
302B is a high silicon-containing stainless steel with high resistance to high temperature oxidation.
303 and 303Se are free-cutting stainless steels containing sulfur and selenium, respectively, for applications where the main requirements are easy cutting and high gloss. 303Se stainless steel is also used to make parts that require enthusiasm, because under these conditions, this stainless steel has good hot workability.
304L is a variant of 304 stainless steel with a lower carbon content for applications requiring soldering. The lower carbon content minimizes the precipitation of carbides in the heat affected zone near the weld, which may result in intergranular corrosion (weld erosion) in certain environments.
304N is a nitrogen-containing stainless steel that is added to increase the strength of the steel.
305 and 384 stainless steels contain high nickel and have a low work hardening rate, making them suitable for a wide range of applications where cold formability is critical.
308 stainless steel is used to make electrodes.
The nickel and chromium contents of 309, 310, 314 and 330 stainless steel are relatively high in order to improve the oxidation resistance and creep strength of steel at high temperatures. The 30S5 and 310S are variants of the 309 and 310 stainless steels, except for the lower carbon content, in order to minimize the carbides precipitated near the weld. 330 stainless steel has a particularly high resistance to carburizing and thermal shock resistance.
Types 316 and 317 stainless steel contain aluminum and are therefore much more resistant to pitting corrosion in marine and chemical industrial environments than 304 stainless steel. Among them, 316 stainless steel variants include low carbon stainless steel 316L, nitrogen-containing high-strength stainless steel 316N and high-sulfur free-cutting stainless steel 316F.
321 , 347 and 348 are stainless steel stabilized by titanium, niobium and tantalum, respectively, and are suitable for use as welded members at high temperatures. 348 is a stainless steel suitable for the nuclear power industry, which has certain restrictions on the combination of boring and drilling.
304, 430, 410 stainless steel processing and manufacturing
Stainless steel is usually chosen first to consider corrosion resistance, secondly to consider strength and other mechanical properties, and third to consider manufacturing performance. The three main types of common stainless steels 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 steel grades, so all stainless steels must be familiar with and can be easily fabricated using conventional manufacturing methods.) Stainless steel is easily formed by hot forming such as staking, extrusion and forging. Get finished or semi-finished parts. Hot-tie is usually a standard machined shape and special shape for a machine. Extrusion processes are often associated with softer non-ferrous metals and are also widely used in stainless steel. In the extrusion process, the shape of a part is obtained by applying a force to a bar or billet section through a form, and the cross section of the outlet is identical to the opening of the form. Several companies produce hot extrusions. The relatively small amount of extrusion is reasonable and economical. Stainless steel profiles typically have a minimum volume of 1000 pounds. Any shape with an effective section of 165.1 mm can be extruded. The maximum product weight for hot extrusion is approximately 30 pounds per foot. On the other hand, the minimum allowable cross section is 1.8063 mm2 and the minimum web thickness is 3.175 mm. The hollow shape and the cube shape are easy to process. Forging is the most widely used of all types of stainless steel, ranging in size from a few ounces to a few thousand pounds, and can be less than 1 inch to a few feet long. Special craftsmanship. For example, drawing, punching, and precision pressing further enhance the ability of the forging. Designers should consult with forging workers on design guidelines for forming, tolerances, and boundaries. The manufacturing process performance of austenite, martensite and ferritic stainless steel is detailed in the attached table.
Exploration and practice of high strength stainless steel in China
Stainless steel is one of the fastest growing and most 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 and aerospace and aerospace industries, higher requirements are imposed on the performance of high-strength stainless steel, thus developing high strength, high toughness, high corrosion resistance, easy processing and welding, and integration. High-strength stainless steel with good performance is imminent.
(1) Development process of high-strength stainless steel in China
In the early 1960s, China conducted systematic research on PH15-7Mo, 17-7PH in the 1970s, 17-4PH in the 1980s, and 15-5PH precipitation hardening stainless steel in the 1990s. At the same time, since the early 1970s, China has developed a series of maraging stainless steels with σ b=1200MPa～1700MPa. In the 1990s, the research and development of ferritic aged stainless steels began. The need for China's national defense construction and national economic development.
(II) Development and application of new high-strength stainless steel grades
1. Precipitation hardening stainless steel
1.1. Semi-austenitic precipitation hardening stainless steel
Semi-austenitic precipitation hardening stainless steel in optimized