What is Alloy 904L?
Alloy 904L is a more highly alloyed austenitic stainless steel with 20% chromium and 25% nickel. The alloy was developed specifically for dilute sulfuric acid applications.
Austenitic stainless steels
Austenitic stainless steels are widely used due to their availability, low cost, reasonable corrosion resistance, and good impact toughness. Increasing the chromium and molybdenum content will significantly improve its pitting resistance.
Alloy 904L (UNS N08904, 1.4539) also has 1-2% copper added, which significantly improves its resistance to strong reducing acids such as sulfuric and phosphoric acids. As a result, it has been widely used in many chemical process industry sectors, including plate and bar.
The higher nickel content means that it is much more expensive than ordinary stainless steel. As a result, it has been replaced by the more cost-effective super duplex stainless steel (SDSS). Super duplex stainless steel has a yield strength 2-3 times that of alloy 904L, which means that it can achieve the same functional performance in thinner thicknesses.
Super duplex stainless steels have a tensile strength (PREN) of greater than 40, while alloy 904L has a PREN of 43, which is roughly equivalent. However, Ferralium 255 (UNS S32550, F61, 1.4507) also utilizes the same high copper content as alloy 904L, which gives it excellent corrosion resistance in a wide range of reducing acids. In contrast, the higher strength of Ferralium 255 also makes it more wear-resistant, making it ideal for use in blenders and mixers.
Advantages of Using 904L Alloy
The main reason for continuing to use 904L alloy instead of super duplex stainless steel is that 904L alloy is still used when the operating temperature is below or above the reasonable operating range of SDSS. Since 904L alloy is austenitic, it maintains its impact toughness at cryogenic temperatures (-196 degrees Celsius).
An interesting aspect of 904L alloy is that it is used in the manufacture of Rolex watches. For this type of application, the metal selected must be non-magnetic. Therefore, duplex and super duplex stainless steels are not suitable.
Gnee Steel offers a wide range of duplex, super duplex, austenitic stainless steels and nickel alloys in diameters from 1/2" (12.7mm) to 16" (406.2mm) and plate thicknesses up to 3".
