1, alloy development
In the current American standard (ASTM), three kinds of super austenitic stainless steels can be found, as shown in table 1. Among them, only 254SMO was developed specifically to improve the pitting and crevice corrosion resistance of materials and ensure the weldability of thick plates. At the end of 1978, 254SMO began mass commercial production and was applied to parts with harsh corrosive environment in different industries.
AL-6XN(N08367) is developed on the basis of its original steel grade. Its prototype contains about 20% chromium and 6.0% molybdenum, but contains no nitrogen. Due to the lack of nitrogen, only thin steel strip and thin-walled heat exchange tube can be produced, otherwise σ phase will be precipitated, resulting in decreased toughness and corrosion resistance. The improved type with nitrogen content of 0.20% was put on the market in the early 1980s.
1.4529 stainless steel plate (N08926) is an improved model by adding nitrogen on the basis of the old steel grade 904L. The nitrogen content of the original model is also very low. It was not until the mid-1980s that the current model with nitrogen content of 0.20% began to mature gradually.
2. Implementing standards
Although all three steel grades are included in American Standard (ASTM), only 254SMO and N08926 are included in the newly implemented and agreed European Standard (EN), while N08367 is not, as shown in Table 1. The research shows that chromium, molybdenum and nitrogen play a decisive role in improving the corrosion resistance of stainless steel. Table 1 shows the contents of these elements in three super austenitic stainless steels. And their functions can be represented by the following formula. The higher the PRE value, the higher the corrosion resistance.
Formula (1): pre =% Cr+3.3% Mo+16% n.
Table 1 Contents of elements affecting corrosion resistance of three super austenitic stainless steels according to European and American standards.
This product is generally called steel grade Cr-Ni-Mo-N PRE 1.
254SMO 1.4547
S3 1254
19.5 -20.5 18 6.0-7.0 0. 18-0.25 42.2
1925 HMO/ 1.4529 1.4529
N08926
19.0-2 1.0 24~25 6.0-7.0 0. 15-0.25 4 1.2
AL-6XN n 08367 20.0-22.0 24 6.0-7.0 0. 18-0.25 42.7
It can be seen that the contents of all important chemical components are very close. PRE value is also at the same level. It is worth mentioning that the precision of modern steelmaking method is very high. In other words, it is easy to control the content of higher-priced elements at the lower limit of the standard (in order to reduce the cost).
The biggest difference in chemical composition of these three super austenitic stainless steels lies in the nickel content. The nickel content of N08926 and N08367 is 25%, while the nickel content of 254SMO is only 18%. However, nickel has little effect on the corrosion resistance of the material, as shown in the formula (1).
Different standards have different requirements for mechanical properties, but there is little difference in chemical composition.
Table 1 also lists three typical steel grades of super austenitic stainless steel popular in the market.
4. Selection of materials
The selection of stainless steel must be based on corrosive environment (such as chloride ion content, pH value and temperature, etc.). ) and the structural design of the device. Corrosion resistance is one of the main properties. To sum up, these three super austenitic stainless steels have the same chloride ion corrosion resistance. In addition, the procurement of materials and the production specifications of materials are very important factors. 254SMO can be made into coiled material. This brings great convenience to design and save materials. At the same time, because of its low nickel content, its price is less affected by nickel price.
Application performance of 5.254SMO
Because of its excellent corrosion resistance, 254SMO has been widely used in many fields such as plate heat exchangers and flue gas desulfurization devices. In the United States, Germany, Spain, Finland, Sweden, Austria and China, 254SMO was selected.
6. Conclusion
1, 6.0% Mo super austenitic stainless steels N08926, 254SMO and N08367 have little difference in chemical composition (especially the elements affecting corrosion resistance), which has little effect on their pitting corrosion resistance and crevice corrosion resistance.
2. This conclusion is the research result of different laboratories and has been published in larger international conferences and influential academic journals.
3.6.0% Mo super austenitic stainless steel 254SMO has been used in many fields for a long time and has good properties.