Stainless steel
(metal material)
Stainless Steel is the abbreviation of stainless acid-resistant steel, which is resistant to weak corrosive media such as air, steam, water, etc. The type of steel that is stainless is called stainless steel; the type of steel that is resistant to chemical corrosion media (chemical corrosion such as acids, alkalis, salts, etc.) 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 corrosion by chemical media, while acid-resistant steel is generally stainless.
Historical origins
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 Military Arsenal to study the improvement of weapons. At that time, the rifle bores used by soldiers were very easy to wear, and Brearley wanted to invent an alloy steel that was not easy to wear.
The stainless steel invented by Brearley obtained a British patent in 1916 and began mass production. Since then, stainless steel accidentally discovered in garbage dumps has become popular all over the world. Henry Brearley is also known as "Stainless Steel father". During World War I, British guns on the battlefield were always shipped back to the rear because the barrels were worn and unusable. The military production department ordered Brearley, who developed high-strength and wear-resistant alloy steel, to specialize in solving the problem of gun bore wear. Brearley and his assistants collected various types of steel produced at home and abroad, as well as various alloy steels with different properties, conducted performance experiments on various machines with different 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 a wear-resistant test, they found that this alloy was not wear-resistant, indicating that it could not be used to make guns. So, they recorded the experimental results and threw them into the corner. thing. One day a few months later, an assistant came excitedly to Brearley with a piece of shiny steel and said: "Sir, this is the alloy steel sent by Mr. Maura that I found when I was cleaning the warehouse. Do you want to experiment and see what special effects it has? "Okay!" Brearley said happily, looking at the bright steel.
The experimental results prove that it is a piece of stainless steel that is not afraid of acid, alkali and salt. This kind of stainless steel was invented by Mullah in Germany in 1912. However, Mullah did not know what the use of this stainless steel was.
Brillie thought in his mind: "This kind of steel that is not wear-resistant but corrosion-resistant cannot be used to make guns. Can it be used to make tableware?" He went ahead and made stainless steel fruits. Knives, forks, spoons, fruit plates and folding knives, etc.
Main Types
The word "stainless steel" does not simply refer to one kind of stainless steel, but refers to more than one hundred industrial stainless steels. Each stainless steel developed is unique in its specific It has good performance in the application fields. The key to success is to first understand the application and then determine the correct steel grade. There are usually only six types of steel relevant to building construction applications. They all contain 17 to 22% chromium, and the better steel types also contain nickel. Adding molybdenum can further improve the atmospheric corrosion resistance, especially the corrosion resistance of chloride-containing atmosphere.
Stainless steel is often divided according to its organizational state: martensitic steel, ferritic steel, austenitic steel, austenitic-ferritic (duplex) stainless steel and precipitation hardened stainless steel, etc. In addition, it can be divided according to its composition: chromium stainless steel, chromium-nickel stainless steel, chromium-manganese-nitrogen stainless steel, etc.
Stainless steel
Ferritic stainless steel
Contains 15% to 30% chromium. Its corrosion resistance, toughness and weldability increase with the increase of chromium content, and its chloride stress corrosion resistance is better than other types of stainless steel. Those belonging to this category include Crl7, Cr17Mo2Ti, Cr25, Cr25Mo3Ti, Cr28, etc. Due to its high chromium content, ferritic stainless steel has relatively good corrosion resistance and oxidation resistance, but poor mechanical and process properties. It is mostly used in 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 water solutions, and has good high-temperature oxidation resistance and small 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. .
Austenitic stainless steel
Contains more than 18% chromium, and also contains about 8% nickel and small amounts of molybdenum, titanium, nitrogen and other elements. It has good comprehensive performance and can withstand corrosion from various media. Commonly used grades of austenitic stainless steel include 1Cr18Ni9, 0Cr19Ni9, etc.
The Wc of 0Cr19Ni9 steel is <0.08%, and the steel number is marked as "0". This type of steel contains a large amount of Ni and Cr, which makes the steel in an austenitic state at room temperature. This type of steel has good plasticity, toughness, weldability, corrosion resistance and non-magnetic or weak magnetism. It has good corrosion resistance in both oxidizing and reducing media. It is used to make acid-resistant equipment, such as corrosion-resistant containers and equipment. Linings, pipelines, nitric acid-resistant equipment parts, etc., and can also be used as the main material of stainless steel clocks and jewelry. 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 combines the advantages of austenitic and ferritic stainless steel and has superplasticity. Austenite and ferrite structures
each account for about half of stainless steel. In the case of low C content, the Cr content is 18%~28% and the Ni content is 3%~10%. Some steels also contain alloying elements such as Mo, Cu, Si, Nb, Ti, and N. This type of steel has the characteristics of both austenitic and ferritic stainless steel. Compared with ferrite, it has higher plasticity and toughness, no room temperature brittleness, significantly improved intergranular corrosion resistance and welding performance, while maintaining iron content. The solid stainless steel is brittle at 475°C, has high thermal conductivity, and has superplasticity and other characteristics. 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 structure. Commonly used grades of precipitation hardening stainless steel include 04Cr13Ni8Mo2Al, etc. It is stainless steel that can be hardened (strengthened) through precipitation hardening (also known as age hardening).
Stainless steel disinfection cabinet
Martensitic stainless steel
High strength, but poor plasticity and weldability. Commonly used grades of martensitic stainless steel include 1Cr13, 3Cr13, etc. Because of their high carbon content, they have high strength, hardness and wear resistance, but slightly poor corrosion resistance. They are used for applications with higher requirements on mechanical properties and corrosion resistance. Some parts with general requirements, such as springs, turbine blades, hydraulic valves, etc. This type of steel is used after quenching and tempering. Annealing is required after forging and stamping.
Main Characteristics
Weldability
Different product uses have different requirements for welding performance. Category I tableware generally does not require welding performance, even for some pot-type companies. But most products require raw materials with good welding performance, such as second-class tableware, thermos cups, steel pipes, water heaters, water dispensers, etc.
Stainless steel sculptures
Corrosion resistance
Most stainless steel products require good corrosion resistance, such as Class I and II tableware, kitchen utensils, water heaters, and drinking fountains Etc., some foreign businessmen also conduct corrosion resistance tests on products: heat NACL aqueous solution to boiling, pour out the solution after a period of time, wash and dry, and weigh the weight loss to determine the degree of corrosion (note: when polishing the product, Because emery cloth or sandpaper contains Fe, it will cause rust spots to appear on the surface during testing)
Polishing performance
In today's society, stainless steel products generally go through the polishing process during production. Only A few products such as water heaters and water dispenser liner do not require polishing. Therefore, this requires the raw material to have good polishing performance. The factors that affect polishing performance mainly include the following points:
① Surface defects of raw materials. Such as scratches, pitting, pickling, etc.
② Raw material problem. If the hardness is too low, it will be difficult to polish during polishing (poor BQ properties). Moreover, if the hardness is too low, orange peel phenomenon will easily appear on the surface during deep drawing, thus affecting the BQ properties. BQ with high hardness is relatively good.
③For products that have undergone deep drawing, small black spots and RIDGING will appear on the surface in areas with great deformation, thus affecting BQ properties.
Heat resistance
Heat resistance means that stainless steel can still maintain its excellent physical and mechanical properties under high temperatures.
The influence of carbon: Carbon is strongly formed and stable in austenitic stainless steel. Elements that determine austenite and expand the austenite zone. The ability of carbon to form austenite is about 30 times that of nickel. Carbon is an interstitial element and can significantly improve the strength of austenitic stainless steel through solid solution strengthening. Carbon can also improve the stress-corrosion resistance of austenitic stainless steel in highly concentrated chlorides (such as 42% MgCl2 boiling solution).
However, in austenitic stainless steel, carbon is often regarded as a harmful element, mainly due to some conditions in the corrosion resistance of stainless steel (such as welding or heating at 450~850℃) , Carbon can form high-chromium Cr23C6 carbon compounds with chromium in steel, leading to local depletion of chromium and reducing the corrosion resistance of steel, especially intergranular corrosion resistance. therefore. Most of the newly developed chromium-nickel austenitic stainless steels since the 1960s are ultra-low carbon types 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 the steel decreases. When the carbon content is lower than Only 0.02% has the most obvious effect. Some experiments also pointed out that carbon will also increase the tendency of pitting corrosion of chromium austenitic stainless steel. Due to the harmful effects of carbon, not only should the carbon content be controlled as low as possible during the smelting and processing of austenitic stainless steel, but also during the subsequent hot, cold working and heat treatment processes, the surface of the stainless steel should also be prevented from carburizing, and chromium-free Carbide precipitates.
Corrosion resistance
When the number of chromium atoms in the steel is not less than 12.5%, the electrode potential of the steel can suddenly change from negative potential to positive electrode potential. Prevent electrochemical corrosion.