What is super austenitic stainless steel

What is super austenitic stainless steel

super austenitic
Development History
The concept of super austenitic stainless steel appeared together with super ferritic stainless steel and super duplex stainless steel. Typical examples are super austenitic stainless steels containing 6% molybdenum and 7% molybdenum. These steel grades are developed for industries with harsh working conditions, such as petrochemical, chemical, papermaking and offshore systems, etc.
The famous grades of austenitic stainless steel pipes are 18-8 (commonly known as 18-10 or 19-9) type 304 stainless steel (00Cr19Ni10 in China) and 18-12-2 316 (0Cr17Ni12Mo2). In order to solve the intergranular corrosion sensitivity caused by chromium depletion due to chromium carbide precipitation after welding of austenitic stainless steel, carbide stabilizing elements titanium and niobium were added in the early days. In the late 1960s, AOD and VOD, etc. The advent of refining technology outside the furnace has reduced the carbon content in steel to ≤0.03%, solved the sensitivity of austenitic stainless steel to intergranular corrosion in the sensitized state (after welding), improved the purity of steel, and also solved the problem of Susceptibility of steel to solid solution intergranular corrosion. Therefore, the new austenitic stainless steels developed since the 1980s are basically ultra-low carbon.
In order to meet the needs of comprehensive corrosion resistance in harsh media in the development of modern industry, on the basis of 304, 316 and other stainless steel pipes, the chromium, nickel, and molybdenum content of steel is increased, and elements such as copper and silicon are added or the residual amounts of impurity elements are reduced, and Many new grades of high alloys have been developed, such as 317LM (00Cr18Ni16Mo5) and 904L stainless steel (00Cr20Ni25Mo4.5Cu) containing about 4.5% Mo, as well as urea grade, nitric acid grade, nuclear grade, food grade and other types of austenitic stainless steel pipes. According to statistics on a large number of corrosion damage forms of stainless steel pipes from 1962 to 1997, it can be seen that general corrosion and intergranular corrosion have been greatly reduced from 1962 to 1971, while from 1962 to 1997, stress corrosion, pitting corrosion, crevice corrosion and Local corrosion such as corrosion fatigue still accounts for a relatively high proportion of corrosion damage. Among them, pitting corrosion and crevice corrosion still account for more than 20%, and stress corrosion and corrosion fatigue still account for more than 10%. Through research, people have learned that increasing the nickel content in austenitic stainless steel pipes can significantly improve the stress corrosion resistance of the steel, and increasing the chromium and molybdenum content can significantly improve the pitting corrosion and crevice corrosion resistance of the steel, while rigid Stress corrosion and corrosion fatigue usually originate from pitting corrosion and crevice corrosion. Therefore, people have begun to pay attention to the development of high-alloy austenitic stainless steel with excellent pitting corrosion and crevice corrosion resistance.
Since 1970, the widespread application of nitrogen as an important alloying element in stainless steel pipes has brought the development of stainless steel pipes to a new stage. The application of nitrogen in austenitic stainless steel has also created the basis for the birth of super austenitic stainless steel. condition. The development of super austenitic stainless steel pipes comes from many avenues. For example, nitrogen was added to the existing AL-6X (00Cr21Ni24Mo6) to produce AL-6XN (00Cr21Ni24Mo6N). Based on the high-molybdenum 904L stainless steel pipe, the molybdenum content was increased to about 6% and nitrogen was added.
characteristic
Since super austenitic stainless steel is an austenitic stainless steel with high nickel and molybdenum and contains copper and nitrogen, it is a relatively difficult steel to smelt and is prone to segregation and cracking. Therefore, super austenitic stainless steel has the highest production process requirements among stainless steels. , the most difficult variety, it is a concentrated expression of the steel plant's process technology. Like other commonly used Cr-Ni austenitic steels, super austenitic stainless steel has good cold and hot working properties.
1. During hot forging, the maximum heating temperature can reach 1180 degrees Celsius, and the minimum forging stop temperature is not less than 900 degrees Celsius.
2. Thermoforming can be performed at 1000-1150 degrees Celsius.
3. The heat treatment process is 1100-1150 degrees Celsius, and it cools quickly after heating.
4. Although general welding techniques can be used for welding, the most appropriate welding methods are manual arc welding and tungsten arc welding.
Since 904L and 254SMO super austenitic stainless steel have strong resistance to pitting corrosion, crevice corrosion, chloride ion stress corrosion and intergranular corrosion, they are particularly resistant to acid ions such as sulfate ions and chloride ions. Used in extremely harsh working environments, super austenitic stainless steel is increasingly used in applications.
Application areas
1. Petroleum and petrochemical equipment, such as bellows in petrochemical equipment.
2. Pulp and paper bleaching equipment, such as pulp digesters and bleaching equipment.
3. The main parts used in power plant flue gas desulfurization equipment are: the tower body of the absorption tower, the flue, the baffle plate, the internal parts, the spray system, etc.
4. Offshore systems or seawater treatment, such as thin-walled condensation pipes cooled by seawater and seawater desalination equipment in power plants.
5. Salt industry, such as salt production or desalination equipment.
6. Heat exchangers, especially heat exchangers in working environments with chloride ions.