What is austenitic stainless steel

What is austenitic stainless steel?

Austenitic stainless steel refers to stainless steel with austenite structure at room temperature. When the steel contains about 18% Cr, 8% ~ 25% Ni and 0.1% C, it has stable austenite structure. Austenitic Cr Ni stainless steel includes the famous 18Cr-8Ni steel and the high Cr Ni series steel developed by adding CR and Ni contents 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 can not be strengthened by phase transformation. It can only be strengthened by cold working. If s, CA, Se, TE and other elements are added, it has good free cutting property.

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Characteristics of austenitic stainless steel

This kind of steel can resist the corrosion of sulfuric acid, phosphoric acid, formic acid, acetic acid and urea if it contains Mo, Cu and other elements. If the carbon content in this kind of steel is less than 0.03% or contains Ti and Ni, its intergranular corrosion resistance can be significantly improved. High silicon austenitic stainless steel has good corrosion resistance to concentrated nitric acid. Austenitic stainless steel has been widely used in all walks of life because of its comprehensive properties.

Types of austenitic stainless steel

Physical classification

Generally speaking, stainless steel is not easy to rust. In fact, part of stainless steel has both rust resistance and acid resistance (corrosion resistance). The rust resistance and corrosion resistance of stainless steel are due to the formation of chromium rich oxide film (passive film) on its surface. The rust resistance and corrosion resistance are relative. The results show that the corrosion resistance of steel increases with the increase of chromium content in weak medium such as atmosphere, water and oxidizing medium such as nitric acid. When the chromium content reaches a certain percentage, the corrosion resistance of steel changes abruptly, that is, from easy to not easy to rust, from never corrosion resistance to corrosion resistance. There are many classification methods of stainless steel. According to the structure at room temperature, there are martensite, austenite, ferrite and duplex stainless steel; According to the main chemical composition, it can be divided into chromium stainless steel and chromium nickel stainless steel; According to the application, there are nitric acid resistant stainless steel, sulfuric acid resistant stainless steel, seawater resistant stainless steel, etc. according to the corrosion resistance type, there are pitting corrosion resistant stainless steel, stress corrosion resistant stainless steel, intergranular corrosion resistant stainless steel, etc; According to the functional characteristics, it can be divided into non-magnetic stainless steel, free cutting stainless steel, low temperature stainless steel, high strength stainless steel and so on. Due to its excellent corrosion resistance, formability, compatibility and toughness in a wide temperature range, stainless steel has been widely used in heavy industry, light industry, daily necessities industry and building decoration industry.

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Microstructure of austenitic stainless steel

Chemical classification

Ferritic stainless steel is a kind of stainless steel with ferrite structure in service. The content of chromium is in the range of 11% ~ 30%, and it has body centered cubic crystal structure. This kind of steel generally does not contain nickel, and sometimes contains a small amount of Mo, Ti, Nb and other elements. This kind of steel has the characteristics of high thermal conductivity, small expansion coefficient, good oxidation resistance, excellent stress corrosion resistance, etc., and is mostly used to make parts resistant to atmospheric, steam, water and oxidizing acid corrosion. This kind of steel has some disadvantages, such as poor plasticity, low plasticity and corrosion resistance after welding, which limits its application. The application of out of furnace refining technology (AOD or VOD) can greatly reduce carbon, nitrogen and other interstitial elements, so this kind of steel is widely used.
Austenite ferrite duplex stainless steel is a kind of stainless steel with half of austenite and half of ferrite. In the case of low C content, Cr content is 18% ~ 28%, Ni content is 3% ~ 10%. Some steels also contain Mo, Cu, Si, Nb, Ti, N and other alloying elements. This kind of steel has the characteristics of both austenitic and ferritic stainless steel. Compared with ferrite, this kind of steel has higher plasticity and toughness, no room temperature brittleness, significantly improved intergranular corrosion resistance and weldability. At the same time, it has the characteristics of 475 ℃ brittleness, high thermal conductivity and superplasticity of ferritic stainless steel. Compared with austenitic stainless steel, it has higher strength and resistance to intergranular corrosion and chloride stress corrosion. Duplex stainless steel has excellent pitting corrosion resistance and is also a kind of nickel saving stainless steel.
Martensitic stainless steel can adjust its mechanical properties through heat treatment. Generally speaking, it is a kind of hardenable stainless steel. The typical grade is Cr13 type, such as 2Cr13, 3Cr13, 4Cr13, etc. It is mainly used for steam turbine blades, tableware and surgical instruments. According to the difference of chemical composition, martensitic stainless steel can be divided into martensitic chromium steel and martensitic chromium nickel steel. According to the different structure and strengthening mechanism, it can also be divided into martensitic stainless steel, martensitic and semi austenitic (or semi martensitic) precipitation hardening stainless steel and maraging stainless steel.
For the inspection of products, knowing the source and organization performance of products is the best means of NDT, and it is also the innovation of NDT practitioners.

Composition of austenitic stainless steel

Based on the composition evolution of 18-8 type stainless steel, there are the following important developments

  • 1) Adding Mo can improve pitting corrosion and crevice corrosion resistance;
  • 2) Decreasing C or adding Ti and Nb can reduce the tendency of intergranular corrosion;
  • 3) The addition of Ni and Cr improves the high temperature oxidation resistance and strength;
  • 4) The addition of Ni improves the stress corrosion resistance;
  • 5) S and Se are added to improve the machinability and surface accuracy.

Structure and structure of austenitic stainless steel

Formation of ferrite phase: effect on properties of austenitic stainless steel. Generally, the appearance of f-phase has a negative effect on the properties of austenitic stainless steel, such as increasing the tendency of cracks during hot working; The pitting corrosion resistance of steel is decreased, and the corrosion resistance is deteriorated in many corrosive environments (such as urea production); When heated at high temperature for a long time, the f phase will change to the σ Phase makes steel brittle and so on.

Rough determination of content

  • Creq=%Cr+1.5 ×% Si+%Mo,Nieq=%Ni+30 ×(% C+%N)+0.5 ×% Mn

Elimination of ferrite phase

The fundamental way is to increase the content of austenite forming elements in steel. Ni is the preferred element, but from an economic point of view, Mn and N are also valued. Especially n, its ability to inhibit ferrite formation is 30 times that of Ni, and it can improve corrosion resistance and strength.

Mechanical properties of austenitic stainless steel

The mechanical properties of high alloy austenitic stainless steel at 20 ℃ are as follows:

Alloy
ASTM
EN
Grade
Nitrogen content
Yield strength
Tensile strength
Elongation
GB
%
Rp0.2MPa
RmMPa
As%
316L
316L
1.4404
0.06
220
520
45
904L
NO8904
1.4539
00Cr20Ni25Mo4.5Cu
0.06
220
520
35
317LMN
317LMN
1.4439
0.15
270
580
40
254SMO
S31254
1.4547
00Cr20Ni18Mo6CuN
0.20
300
650
40
654SMO
S32654
1.4652
0.50
430
750
40

The strength (rp0.2mpa) of high alloy austenitic stainless steel at high temperature is as follows:

Alloy
ASTM
EN
GB
Nitrogen content %
100℃
200℃
400℃
316L
316L
1.4404
0.06
166
137
108
904L
N08904
1.4539
00Cr20Ni25Mo4.5Cu
0.06
225
175
125
317LMN
317LMN
1.4439
0.15
225
185
150
254SMO
S31254
1.4547
00Cr20Ni18Mo6CuN
0.20
230
190
160
654SMO
S32654
1.4652
0.50
350
315
295

Production process of austenitic stainless steel

The production process of austenitic stainless steel has good performance, especially the chromium nickel austenitic stainless steel. By using the conventional means of producing special steel, various commonly used specifications of plates, pipes, belts, wires, bars, forgings and castings can be successfully produced. Due to the high content of alloying elements (especially chromium) and low carbon content, the large-scale production of this kind of stainless steel is usually carried out by arc furnace argon oxygen decarburization (AOD) or vacuum deoxidation decarburization (VOD). For high-grade small-scale products, vacuum or non vacuum non induction furnace smelting can be adopted, and electroslag remelting can be added when necessary.
The excellent thermoplasticity of Cr Ni austenitic stainless steel makes it easy for forging, rolling, hot piercing and extrusion. The ingot heating temperature is 1150 ~ 1260 ℃, and the deformation temperature range is generally 900 ~ 1150 ℃. The steels containing copper, nitrogen and stabilized with titanium and niobium tend to be at low temperature, while the steels with high chromium and molybdenum tend to be at high temperature. Due to poor heat conduction, the holding time should be longer. After hot working, the workpiece can be air-cooled. The Cr Mn austenitic stainless steel is sensitive to hot cracking. The ingot needs small deformation and multiple passes during blooming, and the forging should be cold stacked. It can be used for cold working processes such as cold rolling, cold drawing and spinning, and forming operations such as stamping, bending, curling and folding. The work hardening tendency of Cr Ni austenitic stainless steel is weaker than that of Cr Mn steel. The cold deformation of Cr Ni austenitic stainless steel can reach 70% ~ 90% after one annealing. However, due to the high deformation resistance and strong work hardening tendency, the intermediate softening annealing times should be increased. Generally, the intermediate softening annealing treatment is 1050 ~ 1100 ℃ water cooling.
Austenitic stainless steel can also produce castings. In order to improve the fluidity of liquid steel and casting performance, the alloy composition of casting steel should be adjusted to increase the silicon content, widen the range of chromium and nickel content, and increase the upper limit of impurity element sulfur.
Austenitic stainless steel should be treated with solid solution before use, so as to dissolve carbides and other precipitates into austenitic matrix to the maximum extent, homogenize the structure and eliminate stress, so as to ensure excellent corrosion resistance and mechanical properties. The correct solution treatment system is 1050 ~ 1150 ℃ heating and water cooling (thin parts can also be air-cooled). The solution treatment temperature depends on the alloying degree of the steel: the grade of Mo free or low Mo steel should be lower (≤ 1100 ℃), while the grade of higher alloying such as 00cr20ni18mo-6cun and 00cr25ni22mo2n should be higher (1080 ~ 1150 ℃).
Advanced technology is widely used in production, such as refining rate beyond 95%, continuous casting ratio over 80%, high speed rolling mill and precision and fast forging machine. Especially in the smelting and processing process to achieve computer control, to ensure the reliability and stability of product quality and performance.

Development trend of austenitic stainless steel

  • (1) The steel grade has very low carbonization (carbon ≤ 0.02%) and high purification (as impurity elements, the content of sulfur, phosphorus, silicon and manganese is very low).
  • (2) Development of special purpose steel grades. Such as high molybdenum steel for hot seawater, high nitrogen steel with high corrosion resistance and high strength (nitrogen content up to 0.4% ~ 0.6% or even 0.8% ~ 1.0%), stainless steel functional materials (memory materials, hydrogen storage materials, etc.).
  • (3) New process development. Stainless steel composite, amorphous stainless steel, etc.

Forming elements of austenitic stainless steel pipe

Stainless steel pipes are iron-based alloys with alloying elements ranging from 12% to more than 50%. Alloy elements affect the stability of austenite, ferrite and martensite phases, thus affecting the equilibrium relationship between phases related to stability. The elements added into stainless steel can be divided into stable ferrite elements and stable austenite elements. Martensite is a kind of transformation product, which is formed when austenite is cooled from high temperature to low temperature. If austenite is not formed at high temperature, martensite will not be obtained at low temperature.
Austenitic stainless steel pipe contains high nickel content and other austenite forming elements, which promote the formation of austenite phase and make it stable at room temperature or even lower. Ferritic stainless steel seamless pipe contains elements that weaken the formation of austenite, such as high chromium content, which makes ferrite become the dominant phase component. Martensitic stainless steel tube is austenite structure at high temperature, but this austenite is unstable and transforms during cooling. The microstructure of stainless steel tube can be controlled by means of the balance between austenite forming elements and ferrite forming elements. The adjustment of the balance between the two elements plays an important role in the mechanical properties, corrosion resistance and weldability of stainless steel pipe.
The addition of Al, Ti, Cu and Mo to stainless steel seamless tube can promote precipitation reaction and strengthen the steel. The room temperature yield strength of precipitation hardening (PH) martensitic stainless steel seamless pipe containing Cu, AI and Mo can exceed 1375mpa (200k8i) after heat treatment. Ni Ti and Ni Al precipitates are often formed in austenitic stainless steel seamless pipes containing Ti and Al, which are similar to the precipitated strengthening phases in Ni based superalloys. Aluminum is a ferrite forming element in solid solution, while copper is a weak austenite forming element. The precipitates with composition close to pure copper can be used to strengthen martensitic steels such as 174PH steel. The forming elements of ferrite are Cr, Mo, Si, Nb, Ti, Al, V and W. The forming elements of austenite are nickel, manganese, carbon, nitrogen, copper and cobalt.

Application of austenitic stainless steel

Austenitic stainless steel is a kind of stainless steel with austenite structure at room temperature. When the steel contains about 18% Cr, 8% ~ 10% Ni and 0.1% C, it has stable austenite structure. Austenitic Cr Ni stainless steel includes the famous 18Cr-8Ni steel and the high Cr Ni series steel developed by adding CR and Ni contents 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, so it can not be strengthened by phase transformation, only by cold working. If s, CA, Se, TE and other elements are added, it has good machinability. This kind of steel can resist the corrosion of sulfuric acid, phosphoric acid, formic acid, acetic acid and urea if it contains Mo, Cu and other elements. If the carbon content in this kind of steel is less than 0.03% or contains Ti and Ni, its intergranular corrosion resistance can be significantly improved. The high silicon austenitic stainless steel with concentrated nitric acid has good corrosion resistance. Austenitic stainless steel has been widely used in all walks of life because of its comprehensive properties.
Characteristics and application of austenitic stainless steel:
Steel grade: 304 18cr-8ni
Properties: as a widely used steel, it has good corrosion resistance, heat resistance, low temperature strength and mechanical properties. Good hot workability of stamping and bending, no hardening phenomenon of heat treatment (no magnetism, service temperature – 196 ℃ ~ 800 ℃).
Uses: household products (Class 1 and 2 tableware, kitchen cabinet, indoor pipeline, water heater, boiler, bathtub), auto parts (windshield wiper, muffler, molded products), medical appliances, building materials, chemical, food industry, agriculture, ship parts
Steel grade: 304l 18cr-8ni-low carbon
Characteristics: as a Low-C 304 steel, its corrosion resistance is similar to that of 304 steel in general state, but its resistance to grain boundary corrosion is excellent after welding or stress relief. It can also keep good corrosion resistance without heat treatment. It is generally used below 400 (non-magnetic, service temperature – 196 ℃ ~ 800 ℃).
Application: it is used for outdoor machines, building materials, heat-resistant parts and parts with difficult heat treatment in chemical, coal and petroleum industries which require high grain boundary corrosion resistance.
Steel grade: 304j1 13cr-7.7ni-2cu
Characteristics: Cu should be added, which has good formability, especially wire drawing and aging crack resistance, and can be used for forming products with complex shapes. Its corrosion resistance is the same as that of 304 steel.
Uses: thermos, kitchen sink, pot, kettle, lunch box, door handle, textile processing machine.
Steel grade: 316 18cr-12ni-2.5mo
Characteristics: due to the addition of Mo, it has excellent corrosion resistance, atmospheric corrosion resistance and high temperature strength, and can be used under harsh conditions. Excellent work hardening (non-magnetic).
Uses: seawater equipment, chemistry, dyestuff, papermaking. Oxalic acid, fertilizer and other production equipment; Photography, food industry, coastal facilities, ropes, CD poles, bolts, nuts.
Steel grade: 316l 18cr-12ni-2.5mo low carbon
Characteristics: as a kind of 316 steel, Low-C series has the same characteristics as 316 steel, and has excellent grain boundary corrosion resistance.
Application: 316 steel products with special requirements for resistance to grain boundary corrosion.
Steel grade: 321 18cr-9ni-ti
Characteristics: adding Ti element to 304 steel to prevent grain boundary corrosion. It is suitable for use at 430 ℃~ 900 ℃.
Application: aircraft, exhaust pipe, boiler drum.

Source: China Pipe Fittings Manufacturer – Yaang Pipe Industry Co., Limited (www.steeljrv.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

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