What is austenitic stainless steel
Austenitic stainless steel refers to stainless steel with austenitic structure at room 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 well-known 18Cr-8Ni steel and high Cr-Ni series steel developed on this basis by increasing the content of Cr and Ni and adding Mo, Cu, Si, Nb, Ti and other elements. Austenitic stainless steel is non-magnetic and has high toughness and plasticity, but its strength is low. It is impossible to strengthen it through phase transformation. It can only be strengthened through cold working. For example, adding S, Ca, Se, Te and other elements, it has good Machinability.
Composition characteristics of austenitic stainless steel
The most basic alloying elements of austenitic stainless steel are chromium and nickel. The representative grades are chromium-nickel austenitic stainless steel containing about 18% chromium and about 8% nickel, often called 18-8 stainless steel. The element ratio of chromium and nickel basically ensures that the structure of the steel is stable austenite.
The development of austenitic stainless steel is rapid. In order to meet the needs of different conditions, on the original basis, changing the carbon content or adding other alloying elements endows this type of stainless steel with better properties. (1) Reducing the carbon content or adding titanium and chromium elements can improve the resistance to intergranular corrosion. The amount of titanium or chromium added has certain requirements. Adding copper to chromium-nickel austenitic stainless steel will increase the Corrosion resistance in sulfuric acid, but when added to some chromium-manganese-nitrogen stainless steels, the sensitization temperature range for intergranular corrosion may be expanded, and the sensitization time of intergranular corrosion may be shortened, resulting in adverse effects. Care should be taken during heat treatment or use.
(2) Adding silicon and increasing the content of chromium, nickel and other elements can improve the corrosion resistance of stainless steel in concentrated nitric acid.
(3) The addition of platinum or nitrogen to austenitic stainless steel can improve the pitting corrosion resistance in the medium containing ionized ions. Comparing two austenitic stainless steels, OCr18 NO and OCr17Ni12Mo2N, the former has a pitting corrosion resistance index of only 18. , while the latter's anti-pitting index can reach 27, and the anti-pitting ability of OCr17Ni12Mo2N is much higher than that of OCr18Ni9 steel.
Comparison of austenitic stainless steel grades
GB(China) ASTM(USA) JIS(Japan) DIN(Germany)
1Cr17Ni7 301 SUS301 X12CrNi177
1Cr18Ni9 302 SUS302 X12CrNi188
1Cr18Ni10 303 SUS303 X12CrNiS188
00Cr19Ni10 304L SUS304L X2CrNi189
0Cr17Ni12Mo2 316 SUS316 X5CrNiMo1810
00Cr17Ni14Mo2 316L SUS316L X2CrNiMo1810
0Cr18Ni10Ti 321 SUS321 X10CrNiTi189
0Cr19Ni13Mo3 317 SUS317 X2CrNiMo1816
0Cr18Ni9 304 SUS304 X5CrNi189
Austenitic stainless steel brand composition
On the basis of the composition of 18-8 type stainless steel, there are important developments in the following aspects:
1) Adding Mo to improve pitting corrosion and crevice corrosion resistance
2) Reduce C or add Ti and Nb to reduce the tendency of intergranular corrosion
3) Add Ni and Cr to improve high temperature oxidation resistance and strength
4) Add Ni to improve stress corrosion resistance.
5) Add S and Se to improve machinability and surface accuracy of components.
Production Process
Austenitic stainless steel has good production process performance, especially chrome-nickel austenitic stainless steel. By using the conventional means of producing special steel, various commonly used specifications of plates, tubes, strips, wires, bars, forgings and castings can be successfully produced. Due to the high content of alloying elements (especially chromium) and low carbon content, electric arc furnace plus argon oxygen decarburization (AOD) or vacuum deoxygenation decarburization (VOD) methods are used to mass-produce this type of stainless steel. Bulk products can be smelted in vacuum or non-vacuum non-induction furnaces, and electroslag remelting is added if necessary.
The excellent thermoplastic properties of chromium-nickel austenitic stainless steel make it easy to perform hot processing such as forging, rolling, hot piercing and extrusion. Nitrogen and steel stabilized with titanium and niobium tend to be low temperature, while high chromium and molybdenum steel grades tend to be high temperature. Due to poor thermal conductivity, the holding time should be longer. After hot working, the workpiece can be air-cooled. Chromium-manganese austenitic stainless steel has strong sensitivity to hot cracks. When the ingot is billeted, it needs small deformation and multiple passes, and the forgings should be piled cold. Cold working processes such as cold rolling, cold drawing and spinning and forming operations such as stamping, bending, hemming and folding can be performed. The work hardening tendency of chromium-nickel austenitic stainless steel is weaker than that of chromium-manganese steel, and the amount of cold deformation after one annealing can reach 70% to 90%. Annealing times. Generally, the intermediate softening annealing treatment is water cooling at 1050-1100 °C.
Austenitic stainless steel can also produce castings. In order to improve the fluidity of molten steel and improve the casting performance, the alloy composition of the cast steel should be adjusted: increase the silicon content, relax the range of chromium and nickel content, and increase the upper limit of the content of impurity element sulfur.
Austenitic stainless steel should be solution treated before use, in order to maximize the solid solution of various precipitation phases such as carbides in the steel into the austenitic matrix, and also to homogenize the structure and eliminate stress, so as to ensure excellent quality. Corrosion resistance and mechanical properties. The correct solution treatment system is water cooling after heating at 1050-1150 °C (thin and thin parts can also be air-cooled). The solution treatment temperature depends on the degree of alloying of the steel: the steel without molybdenum or low molybdenum should be lower (≤1100℃), and the higher alloyed grades such as 00Cr20Ni18Mo-6CuN, 00Cr25Ni22Mo2N, etc. should be higher (1080～1150 °C).
Advanced technologies are widely used in production, such as the refining rate outside the furnace is over 95%, the continuous casting ratio is over 80%, and high-speed rolling mills and fine and fast forging machines are widely promoted. Especially in the process of smelting and processing, electronic computer control is realized, which ensures the reliability and stability of product quality and performance.
