What material is 904L alloy? Will 904L be completely replaced by duplex stainless steel?
Materials of super alloy 904l
904L super austenitic stainless steel is a low-carbon, high-nickel, molybdenum austenitic stainless acid-resistant steel. It is a proprietary material imported from French H·S Company. It has good activation-passivation transition ability, excellent corrosion resistance, good corrosion resistance in non-oxidizing acids such as sulfuric acid, acetic acid, formic acid, and phosphoric acid, and good corrosion resistance in neutral chlorine-containing media. Excellent resistance to pitting corrosion, as well as good resistance to crevice corrosion and stress corrosion. It is suitable for sulfuric acid of various concentrations below 70°C. It can withstand acetic acid of any concentration and temperature under normal pressure, and has good corrosion resistance in mixed acids of formic acid and acetic acid.
This alloy has the following characteristics: 904L is a highly alloyed austenitic stainless steel with very low carbon content. This steel is designed for use in environments with harsh corrosive conditions. This alloy was originally developed to resist corrosion in dilute sulfuric acid. This feature has been proven to be very successful in practical applications over many years. 904L is now standardized in many countries and has been approved for use in the manufacture of pressure vessels.
Will 904L be completely replaced by duplex stainless steel?
Austenitic stainless steels are widely used due to their availability, lower cost, reasonable corrosion resistance and good impact toughness. Increasing the chromium and molybdenum content will significantly improve the pitting corrosion resistance of austenitic stainless steel. In the case of the 904L alloy (UNS N08904, 1.4539), 1-2% copper is also added, which is known to be highly resistant to strong reducing acids such as leucine and phosphoric acid. Therefore, it has been widely used in many chemical process industrial applications of plates and rods.
The heavy, high nickel content of 904L alloy means it is much more expensive than regular stainless steel. As a result, 904L alloy has been replaced by the more cost-effective super duplex stainless steel (SDSS). The yield strength of super duplex stainless steel is 2-3 times that of 904L alloy, which means it can be used in smaller thicknesses to achieve the same functionality. The PREN of super duplex steel is greater than 40, while the PREN of alloy 904L is greater than 43, so they are roughly comparable. However, Ferralium 255 (UNS S32550, F61, 1.4507) also utilizes the same increased copper content as 904L alloy, resulting in excellent corrosion resistance in a variety of reducing acids. The increased strength also gives it better wear resistance compared to Ferralium 255, making it ideal for use in mixers and blenders.
The main reason to continue using 904L alloy instead of super duplex stainless steel is if the operating temperature is below or above the reasonable operating range of the SDSS. Because it is austenite, it can still maintain its impact toughness at low temperatures (-196°C).
