What Is Stainless Steel
Stainless steel is the abbreviation of stainless and acid-resistant steel. Steels that are resistant to weak corrosive media such as air, steam, and water or are stainless steel are called stainless steel; and those that are resistant to chemical corrosion media (acid, alkali, salt, etc.) The steel grade is called acid-resistant steel.
Due to the difference in chemical composition between the two, their corrosion resistance is different. Ordinary stainless steel is generally not resistant to chemical media corrosion, while acid-resistant steel is generally non-corrosive. The term “stainless steel” not only refers to a single type of stainless steel, but also refers to more than one hundred industrial stainless steels. Each stainless steel developed has good performance in its specific application field. The key to success is to understand the purpose first, and then determine the correct steel grade. There are usually only six steel grades related to the application of building construction. They all contain 17-22% chromium, and better steel grades also contain nickel. The addition of molybdenum can further improve the atmospheric corrosion, especially the corrosion resistance of the chloride-containing atmosphere.
Generally speaking, the hardness of stainless steel is higher than that of aluminum alloy, and the cost of stainless steel is higher than that of aluminum alloy.
The Historical Origin Of Stainless Steel
The invention and use of stainless steel can be traced back to the First World War. British scientist Henry Brearley was commissioned by the British government’s military arsenal to study the improvement of weapons. At that time, the barrel of the rifle used by soldiers was very easy to wear, and Blairley wanted to invent an alloy steel that was not easy to wear.
The stainless steel invented by Blairley obtained a British patent in 1916 and began mass production. So far, stainless steel accidentally found in garbage dumps has become popular all over the world, and Henry Blairley is also known as the “father of stainless steel.” During the First World War, British guns on the battlefield were always transported back to the rear because the barrel was worn out and could not be used. The military production department ordered Blairley to develop high-strength wear-resistant alloy steel, specifically researching and solving the wear problem of the barrel. Blairley and his assistants collected various types of steels and alloy steels of different properties produced at home and abroad, conducted performance experiments on machines of various properties, and then selected more suitable steels to make guns. One day, they experimented with a domestic alloy steel containing a large amount of chromium. After wear-resistant experiments, they found that this alloy was not wear-resistant, indicating that it could not make guns. So they recorded the results of the experiment and threw them into the corner. thing. One day a few months later, an assistant rushed to Blairley with a piece of shiny steel and said: “Sir, this is the alloy steel from Mr. Mullah that I found when I was cleaning the warehouse. You Do you want to experiment to see what special effect it has!” “Good!” Breerley said happily, looking at the dazzling steel.
The experimental results prove that it is a piece of stainless steel that is not afraid of acid, alkali, and salt. This stainless steel was invented by a German mullah in 1912, however, the mullah did not know what purpose this stainless steel had.
Blairli thought to himself: “This kind of non-wearing but corrosion-resistant steel can’t be used to make guns. Can it be used for tableware?” He just did it and made stainless steel fruit knives, forks, spoons, and fruit plates. And folding knives, etc.
Main Types Of Stainless Steel
Stainless steel is often divided into martensitic steel, ferritic steel, austenitic steel, austenitic-ferritic (duplex) stainless steel and precipitation hardening stainless steel according to the state of organization. In addition, it can be divided into chromium stainless steel, chromium nickel stainless steel and chromium manganese nitrogen stainless steel according to the composition. There is also special stainless steel “GB24511_2009_Stainless steel plate and steel belt for pressure equipment” for pressure vessels.
Ferritic stainless steel
Containing 15% to 30% chromium. Its corrosion resistance, toughness and weldability increase with the increase of chromium content, and its resistance to chloride stress corrosion is better than other types of stainless steels. In this category, there are Crl7, Cr17Mo2Ti, Cr25, Cr25Mo3Ti, Cr28, etc. Because of the high chromium content, ferritic stainless steel has good corrosion resistance and oxidation resistance, but its mechanical properties and process properties are poor. It is mostly used for acid-resistant structures with little stress and as anti-oxidation steel. This type of steel can resist corrosion from the atmosphere, nitric acid and salt solutions, and has the characteristics of good high-temperature oxidation resistance and low thermal expansion coefficient. It is used in nitric acid and food factory equipment, and can also be used to make parts that work at high temperatures, such as gas turbine parts, etc. .
Austenitic stainless steel
The chromium content is more than 18%, and it also contains about 8% nickel and a small amount of molybdenum, titanium, nitrogen and other elements. Good overall performance, resistant to corrosion by various media. Commonly used grades of austenitic stainless steel are 1Cr18Ni9, 0Cr19Ni9 and so on. The Wc of 0Cr19Ni9 steel is less than 0.08%, and the steel number is marked as “0”. This type of steel contains a large amount of Ni and Cr, making the steel austenitic at room temperature. This kind of steel has good plasticity, toughness, weldability, corrosion resistance and non-magnetic or weak magnetic properties. It has good corrosion resistance in oxidizing and reducing media. It is used to make acid-resistant equipment, such as corrosion-resistant containers and equipment. Lining, conveying pipes, nitric acid resistant equipment parts, etc., can also be used as the main material of stainless steel clock and watch accessories. Austenitic stainless steel generally adopts solution treatment, that is, the steel is heated to 1050~1150℃, and then water-cooled or air-cooled to obtain a single-phase austenite structure.
Austenitic-ferritic duplex stainless steel
It has the advantages of austenitic and ferritic stainless steel, and has superplasticity. Austenitic and ferritic structures each account for about half of stainless steel. In the case of low carbon content, the content of chromium (Cr) is between 18% and 28%, and the content of nickel (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 has the characteristics of austenitic and ferritic stainless steel. Compared with ferrite, it has higher plasticity and toughness, no room temperature brittleness, and significantly improved intergranular corrosion resistance and welding performance, while still maintaining iron The 475℃ brittleness, high thermal conductivity, and superplasticity of element stainless steel. 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 corrosion resistance and is also a nickel-saving stainless steel.
Precipitation hardening stainless steel
The matrix is austenite or martensite, and the commonly used grades of precipitation hardening stainless steel are 04Cr13Ni8Mo2Al and so on. Stainless steel that can be hardened (strengthened) by precipitation hardening (also known as age hardening).
Martensitic stainless steel
High strength, but poor plasticity and weldability. The commonly used grades of martensitic stainless steel are 1Cr13, 3Cr13, etc. Because of the higher carbon content, it has higher strength, hardness and wear resistance, but the corrosion resistance is slightly worse. It is used for higher mechanical properties and corrosion resistance. Some general parts are required, such as springs, steam turbine blades, hydraulic valves, etc. This type of steel is used after quenching and tempering. Annealing is required after forging and stamping.
Stainless steel plate and steel belt for pressure equipment
Special stainless steel for pressure vessels has clear requirements for its classification and code, size, shape and allowable deviation, technical requirements, test methods, inspection rules, packaging, markings and product quality certificates. Commonly used grades are 06Cr19Ni10, 022Cr17Ni12Mo2. The digital codes are: S30408, S31603, etc. Mainly used in sanitary equipment such as food machinery and pharmaceutical machinery.
The Main Features Of Stainless Steel
Different product uses have different requirements for welding performance. A type of tableware generally does not require welding performance, and even includes some pot companies. However, most products require good welding performance of raw materials, such as Class II tableware, thermos cups, steel pipes, water heaters, and water dispensers.
Most stainless steel products require good corrosion resistance, such as Class I and II tableware, kitchen utensils, water heaters, water dispensers, etc. Some foreign merchants also conduct corrosion resistance tests on the products: use NACL aqueous solution to heat it to boiling, and then pour it after a period of time. Remove the solution, wash and dry, and weigh the weight loss to determine the degree of corrosion (note: when the product is polished, the abrasive cloth or sandpaper contains Fe components, which will cause rust spots on the surface during the test)
In today’s society, stainless steel products generally go through the process of polishing during production, and only a few products such as water heaters and water dispenser inner tanks do not need to be polished. Therefore, this requires good polishing performance of the raw material. The main factors affecting polishing performance are as follows:
- ① Surface defects of raw materials. Such as scratches, pitting, pickling, etc.
- ②Material problems of raw materials. If the hardness is too low, it is not easy to polish during polishing (BQ is not good), and if the hardness is too low, orange peel will easily appear on the surface during deep drawing, which will affect the BQ. BQ with high hardness is relatively good.
- ③After deep drawing, small black spots and RIDGING will appear on the surface of the area where the deformation is extremely large, which will affect the BQ performance.
Heat resistance refers to the ability of stainless steel to maintain its excellent physical and mechanical properties at high temperatures.
Influence of carbon: Carbon is an element that strongly forms and stabilizes austenite and expands the austenite region in austenitic stainless steel. The ability of carbon to form austenite is about 30 times that of nickel. Carbon is an interstitial element and can significantly increase the strength of austenitic stainless steel through solid solution strengthening. Carbon can also improve the stress corrosion resistance of austenitic stainless steel in high concentration chloride (such as 42% MgCl2 boiling solution).
However, in austenitic stainless steel, carbon is often regarded as a harmful element. This is mainly due to the fact that under certain conditions in the corrosion resistance of stainless steel (such as welding or heating at 450 ~ 850 ℃), carbon can be combined with steel. Chromium forms a high-chromium Cr23C6 type carbon compound, which leads to local chromium depletion, which reduces the corrosion resistance of steel, especially the intergranular corrosion resistance. therefore. Since the 1960s, most of the newly developed chromium-nickel austenitic stainless steels are ultra-low carbon type with a carbon content of less than 0.03% or 0.02%. It can be known that as the carbon content decreases, the intergranular corrosion sensitivity of steel decreases. When the carbon content is lower than 0.02% has the most obvious effect. Some experiments have also pointed out that carbon will also increase the pitting corrosion tendency of chromium austenitic stainless steel. Due to the harmful effects of carbon, not only should the carbon content be controlled as low as possible in the austenitic stainless steel smelting process, but also in the subsequent heating, cold working and heat treatment processes, the surface of the stainless steel is also prevented from increasing carbon, and chromium carbides should be avoided. Precipitate out.
When the number of chromium atoms in the steel is not less than 12.5%, the electrode potential of the steel can be changed abruptly, from a negative potential to a positive electrode potential. Prevent electrochemical corrosion.
The Structural Composition Of Stainless Steel
The corrosion resistance of stainless steel decreases with the increase of carbon content. Therefore, the carbon content of most stainless steels is low, the maximum does not exceed 1.2%, and the ωc (carbon content) of some steels is even lower than 0.03% (such as 00Cr12 ). The main alloying element in stainless steel is Cr (chromium), and only when the Cr content reaches a certain value, the steel has corrosion resistance. Therefore, stainless steel generally has a Cr (chromium) content of at least 10.5%. Stainless steel also contains elements such as Ni, Ti, Mn, N, Nb, Mo, Si, and Cu.
Because of the different uses of each product, the processing technology and the quality requirements of the raw materials are also different. Generally speaking, different stainless steel products require different thickness tolerances of raw materials. For example, the thickness tolerances of the second type tableware and vacuum flasks are generally higher, which is -3~5%, while the thickness tolerances of the first type tableware are generally required- 5%, steel pipe category requires -10%, hotel refrigerator material thickness tolerance is -8%, dealers generally require thickness tolerance between -4% and 6%. At the same time, the difference between the internal and external sales of the products will also lead to the different requirements of customers on the thickness tolerance of the raw materials. Generally, customers of export products have higher requirements for thickness tolerances, while domestic sales companies have relatively low requirements for thickness tolerances (mostly due to cost considerations), and some customers even require -15%.
- ① DDQ (deep drawing quality) material: refers to the material used for deep drawing (punching) purposes, which is what everyone calls soft material. The main feature of this material is higher elongation (≧53%) and higher hardness. Low (≦170%), the internal grain grade is between 7.0 and 8.0, and the deep drawing performance is excellent. Many companies that produce thermos flasks and pots generally have relatively high product processing ratios (BLANKING SIZE/product diameter), and their processing ratios are 3.0, 1.96, 2.13, and 1.98, respectively. SUS304DDQ materials are mainly used for these products that require a higher processing ratio. Of course, products with a processing ratio of more than 2.0 generally require several passes of stretching to complete. If the extension of the raw materials cannot be achieved, the products are prone to cracks and pull-through phenomena when processing deep-drawn products, which will affect the qualified rate of finished products and of course increase the cost of the manufacturer;
- ②General materials: Mainly used for materials other than DDQ applications. This material is characterized by relatively low elongation (≧45%), and relatively high hardness (≦180HB), and the internal grain size grade is 8.0~9.0 Compared with DDQ materials, its deep drawing performance is relatively poor. It is mainly used for products that can be obtained without stretching, such as spoons, spoons, forks, electrical appliances, steel pipes and so on. But it has an advantage compared with DDQ material, that is, the BQ property is relatively better, which is mainly due to its slightly higher hardness.
Stainless steel sheet is an inexpensive material, but customers have very high requirements for its surface quality. Various defects such as scratches, pits, sand holes, dark lines, creases, and pollution will inevitably appear in the production process of stainless steel sheets. Therefore, the surface quality, such as scratches, creases, etc., is a high-grade material. It is not allowed, and defects such as pockmarks and sand holes are never allowed in the production of spoons, spoons, forks, because it is difficult to throw them away during polishing. It is necessary to determine the quality level of the table according to the degree and frequency of various defects on the surface to determine the product level.
The Physical Properties Of Stainless Steel
Compared With Carbon Steel
- Density: The density of carbon steel is slightly higher than that of ferritic and martensitic stainless steel, and slightly lower than that of austenitic stainless steel;
- Resistivity: Resistivity is in increasing order of carbon steel, ferritic, martensitic and austenitic stainless steel;
- The order of linear expansion coefficient is similar, austenitic stainless steel is the highest and carbon steel is the smallest;
- Carbon steel, ferritic and martensitic stainless steels are magnetic, austenitic stainless steels are non-magnetic, but they will produce magnetism when they are cold-worked and hardened to form martensite transformation. Heat treatment can be used to eliminate this martensite. The tissue is restored to its non-magnetic properties.
Compared with carbon steel, austenitic stainless steel has the following characteristics:
- High resistivity, about 5 times that of carbon steel.
- Large linear expansion coefficient, 40% larger than carbon steel, and as the temperature increases, the value of the linear expansion coefficient increases accordingly.
- Low thermal conductivity, about 1/3 of carbon steel.
The Typical Use Of Stainless Steel
Most of the requirements for use are to maintain the original appearance of the building for a long time. When determining the type of stainless steel to be selected, the main considerations are the required aesthetic standards, the corrosiveness of the local atmosphere, and the cleaning system to be adopted. However, other applications are increasingly seeking structural integrity or impermeability. For example, roofs and side walls of industrial buildings. In these applications, the owner’s construction cost may be more important than the aesthetics, and the surface may not be very clean. The effect of using 304 stainless steel in a dry indoor environment is quite good.
However, if you want to maintain its appearance outdoors in the countryside and cities, you need to wash it frequently. In heavily polluted industrial areas and coastal areas, the surface will be very dirty and even rusty. However, to obtain the aesthetic effect in the outdoor environment, nickel-containing stainless steel is required. Therefore, 304 stainless steel is widely used in curtain walls, side walls, roofs and other construction purposes. However, 316 stainless steel is best used in severely corrosive industries or marine atmospheres. There are several design criteria that include 304 and 316 stainless steel.
Because “duplex” stainless steel 2205 has integrated good atmospheric corrosion resistance with high tensile strength and elastic limit strength, this steel is also included in the European standards. Product shape, in fact, stainless steel is manufactured in full-standard metal shapes and sizes, and there are many special shapes. The most commonly used products are made of thin plates and strip steel, and medium and heavy plates are also used to produce special products, such as the production of hot-rolled structural steel and extruded structural steel. There are also round, oval, square, rectangular and hexagonal welded pipes or seamless steel pipes and other forms of products, including profiles, bars, wires and castings. In order to meet the aesthetic requirements of architects, many different commercial surface finishes have been developed.
3d printing field
Stainless steel itself has good corrosion resistance, and stainless steel can still maintain its excellent physical and mechanical properties at high temperatures. It is also widely used in the field of 3D printing.
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