Stainless Steel Materials

Stainless steel, as defined in GB/T20878-2007, is a steel with the main characteristics of rust resistance and corrosion resistance, and a chromium content of at least 10.5% and a carbon content not exceeding 1.2%.

Stainless steel is the abbreviation for stainless and acid resistant steel. Steel that is resistant to weak corrosive media such as air, steam, water, or has rust resistance is called stainless steel; And the type of steel that is resistant to chemical corrosion media (such as acid, alkali, salt, etc.) is called acid resistant steel.

Due to the differences 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 generally has rust resistance. The term "stainless steel" refers not only to a single type of stainless steel, but also to over a hundred industrial stainless steels, each of which has excellent performance in its specific application field. The key to success is first to understand the purpose, and then determine the correct steel grade. There are usually only six types of steel related to the application field of building construction. They all contain 17-22% chromium, and better steel grades also contain nickel. Adding molybdenum can further improve atmospheric corrosion, especially resistance to chloride containing atmospheres. 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 invention and use of stainless steel can be traced back to the period of World War II. At that time, firearms in Britain on the battlefield were always transported back to the rear due to worn and unusable rifles. The military production department ordered Brelly to develop high-strength wear-resistant alloy steel, specifically to study and solve the problem of gun barrel wear. Brelly and his assistants collected various types of steel and alloy steel with different properties produced domestically and internationally, conducted performance experiments on various mechanical properties, and then selected the more suitable steel to make firearms. One day, they experimented with an alloy steel containing a large amount of chromium. After conducting a wear resistance test, it was found that this alloy was not wear-resistant, indicating that it could not manufacture firearms. Therefore, they recorded the experimental results and threw it into the corner of the wall. A few months later, an assistant ran over with a shiny piece of steel and said excitedly to Brelly, "Sir, this is the alloy steel that Mr. Mullah sent me while cleaning the warehouse. Would you like to experiment with it and see what it really does?" "Okay!" Brelly looked at the shiny steel and said happily.

The experimental results prove that it is a stainless steel that is not afraid of acid, alkali, or salt. This type of stainless steel was invented by the German Mullah in 1912, but Mullah was unaware of its purpose.

Brelly thought to himself, "This kind of non wear-resistant but corrosion-resistant steel cannot be used to make firearms. Can it be used to make tableware?" He said he could do it and made stainless steel fruit knives, forks, spoons, fruit plates, and folding knives.

The stainless steel invented by Brelly gained British rights in 1916 and began mass production. From then on, stainless steel accidentally discovered from garbage piles became popular worldwide, and Henry Brelly was also known as the "father of stainless steel".

Main types

Stainless steel is often divided into martensitic steel, ferritic steel, austenitic steel, austenitic ferrite (duplex) stainless steel, and precipitation hardening stainless steel according to their structural state. In addition, it can be divided into chromium stainless steel, chromium nickel stainless steel, and chromium manganese nitrogen stainless steel according to their composition. There is also a specialized stainless steel plate and strip for pressure vessels, GB24511_2009 Stainless Steel Plate and Strip for Pressure Equipment.

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 superior to other types of stainless steel. Crl7, Cr17Mo2Ti, Cr25, Cr25Mo3Ti, Cr28, etc. belong to this category. Ferritic stainless steel has good corrosion resistance and oxidation resistance due to its high chromium content, but its mechanical and process properties are poor. It is often used in acid resistant structures with low stress and as an oxidation resistant steel. This type of steel can resist corrosion from atmospheric, nitric acid, and saline solutions, and has characteristics such as good high-temperature oxidation resistance and low coefficient of thermal expansion. 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 

The chromium content is greater than 18%, and it also contains about 8% nickel and small amounts of elements such as molybdenum, titanium, and nitrogen. Good comprehensive performance and can withstand corrosion from various media. Common grades of austenitic stainless steel include 1Cr18Ni9, 0Cr19Ni9, etc. The Wc of 0Cr19Ni9 steel is less than 0.08%, marked as "0" in the steel grade. This type of steel contains a large amount of Ni and Cr, resulting in the steel being in an austenitic state at room temperature. This type of steel has good plasticity, toughness, weldability, corrosion resistance, and no magnetism or weak magnetism. It has good corrosion resistance in oxidizing and reducing media and is used to make acid resistant equipment, such as corrosion-resistant containers and equipment linings, conveying pipelines, nitric acid resistant equipment parts, etc. In addition, it can also be used as the main material for stainless steel watch accessories. Austenitic stainless steel is generally treated with solid solution, which involves heating the steel to 1050-1150 ℃, followed by water or air cooling to obtain a single-phase austenite structure.

It has the advantages of both austenitic and ferritic stainless steels, as well as superplasticity. Stainless steel with austenite and ferrite structures accounting for about half each. In the case of low carbon content, the chromium (Cr) content ranges from 18% to 28%, and the nickel (Ni) content ranges from 3% to 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 and toughness, no room temperature brittleness, 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 higher strength and significantly improved resistance to intergranular corrosion and chloride stress corrosion. Dual phase stainless steel has excellent resistance to pitting corrosion and is also a nickel saving stainless steel.

Precipitation hardened stainless steel

The matrix is austenitic or martensitic structure, and commonly used grades of precipitation hardened stainless steel include 04Cr13Ni8Mo2Al, etc. It is a stainless steel that can be hardened (strengthened) by precipitation hardening (also known as age hardening) treatment.

Martensitic stainless steel

High strength, but poor plasticity and weldability. The commonly used grades of martensitic stainless steel include 1Cr13, 3Cr13, etc. Due to their high carbon content, they have high strength, hardness, and wear resistance, but their corrosion resistance is slightly poor. They are used on parts with high mechanical performance requirements and general corrosion resistance requirements, such as springs, turbine blades, and water compressor valves. This type of steel is used after quenching and tempering treatment. Annealing is required after forging and stamping.

Stainless steel plates and strips for pressure equipment

Special stainless steel for pressure vessels has clear requirements for classification and code, size, shape and allowable deviation, technical requirements, test methods, inspection rules, packaging, marking, and product quality certificate. Commonly used grades include 06Cr19Ni10, 022Cr17Ni12Mo2 with numerical codes such as S30408, S31603, etc. Mainly used for hygiene level equipment such as food machinery and pharmaceutical machinery.