Brief description:
Super stainless steel is a special kind of stainless steel. It is different from common stainless steel 304 in terms of chemical composition. It refers to a high-alloy stainless steel containing high nickel, high chromium, and high molybdenum. Secondly, in terms of high temperature resistance or corrosion resistance, compared with 304, it has more excellent high temperature or corrosion resistance, and 304 is irreplaceable. In addition, from the classification of stainless steel, the special stainless steel metallographic structure is a stable austenite metallographic structure.
Basic introduction:
Because this kind of special stainless steel is a kind of high alloy material, it is very complicated in the manufacturing process. People usually can only rely on the traditional craft to make this kind of special stainless steel, such as pouring, forging, rolling and so on.
Development History:
The concept of super austenitic stainless steels comes with super ferritic stainless steels and super duplex stainless steels. A typical example is super austenitic stainless steel with 6% molybdenum and 7% molybdenum. These steel grades are developed for a number of harsh industrial conditions, such as petrochemical, chemical, paper and offshore systems.
The famous brands of austenitic stainless steel pipes are 18-8 (daily 18-10 or 19-9) type 304 stainless steel (00Cr19Ni10) and 18-12-2 316 (0Cr17Ni12Mo2). In order to solve the intergranular corrosion susceptibility caused by the depletion of chromium due to the precipitation of chromium carbides after austenitic stainless steel welding, the early stage was the addition of carbide-stabilizing elements titanium and tantalum. In the late 1960s, AOD and VOD etc. The advent of furnace refining technology has reduced the amount of carbon in the steel to ≤0.03%, solved the sensitivity of the sensitized state of austenitic stainless steels (after welding) to intergranular corrosion, improved the purity of the steel, and solved the problem The solubility of solid solution intergranular corrosion of steel. Therefore, the new austenitic stainless steels developed since the 1980s are basically ultra-low carbon types.
In order to meet the requirements of modern industry development for corrosion resistance to harsh media, the chromium, nickel and molybdenum contents of steels are increased on the basis of 304, 316 and other stainless steel pipes, and the addition of copper, silicon or other elements or the residual amount of impurity elements is reduced. A number of new high-alloy grades have been developed, including 317LM (00Cr18Ni16Mo5) and 904L stainless steel (00Cr20Ni25Mo4.5Cu) with approximately 4.5% Mo, and austenitic stainless steel grades with urea, nitric, nuclear, and food grades. According to the statistics of the large number of corrosion damages of stainless steel pipes from 1962 to 1997, it can be seen that the total corrosion and intergranular corrosion have been greatly reduced from 1962 to 1971, and from 1962 to 1997, stress corrosion, pitting corrosion, and clearance corrosion and Local corrosion such as corrosion fatigue still occupies a relatively high proportion in 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, it has been learned that increasing the amount of nickel in austenitic stainless steel tubes can significantly increase the stress corrosion resistance of the steel, and increasing the amount of chromium and molybdenum can significantly improve the resistance to pitting and crevice corrosion of the steel, while the Stress corrosion and corrosion fatigue are usually originated from pitting and crevice corrosion. Therefore, attention has been focused on the development of high-alloy austenitic stainless steels that are resistant to pitting and crevice corrosion.
Since 1970, the widespread application of nitrogen as an important alloying element in stainless steel tubes has made the development of stainless steel tubes a new stage, and the application of nitrogen in austenitic stainless steels has also created the creation of super austenitic stainless steels. condition. The development of super austenitic stainless steel tubes comes from many sources. For example, addition of nitrogen to the existing AL-6X (00Cr21Ni24Mo6) produces AL-6XN (00Cr21Ni24Mo6N), which increases the molybdenum content to about 6% based on a high molybdenum 904L stainless steel tube and adds nitrogen.
Characteristic:
Because super austenitic stainless steels are austenitic stainless steels with high nickel, high molybdenum and copper and nitrogen content, they are harder to be smelted and are easily segregated and cracked. Therefore, super austenitic stainless steels are the most demanding in the production process of stainless steels. The most difficult variety is the concentrated expression of the process technology of steel mills. As with other commonly used Cr-Ni austenitic steels, super austenitic stainless steels have good cold and hot workability.
1, the highest heating temperature up to 1180 degrees Celsius hot forging, the minimum stop forging temperature of not less than 900 degrees Celsius.
2, thermoforming can be carried out at 1000-1150 degrees Celsius.
3, heat treatment process is 1100 - 1150 degrees Celsius, after cooling fast heating.
4. Although general welding processes can be used for welding, the most suitable welding methods are manual arc welding and tungsten-arc welding.
Because 904L, 254SMO super austenitic stainless steel has strong resistance to pitting corrosion, crevice corrosion, chloride ion stress corrosion and resistance to intergranular corrosion, especially for sulfate ions, chloride ions and other acid ions have good corrosion resistance, you can Used in extremely harsh working conditions, so the application of super austenitic stainless steel is becoming more and more widespread.
Application area:
1. Marine: Marine structures in sea areas, seawater desalination, marine aquaculture, and seawater heat exchange.
2. Environmental protection: Flue gas desulfurization equipment for thermal power generation, wastewater treatment, etc.
3. Energy: Atomic power generation, comprehensive utilization of coal, and tidal power generation.
4, petrochemical industry: oil refining, chemical and chemical equipment.
5, the food industry: salt, soy sauce and other brewing.
Super stainless steel can be divided into super martensitic stainless steel and super austenitic stainless steel according to the amount of chromium.