How does 904L compare with super duplex stainless steels? Stainless Steels
904L (UNS N08904, 1.4539) is a highly alloyed austenitic stainless steel. Its approximate composition is 20% Cr, 25% Ni, 4.0% Mo plus 1.5% Cu, which ensures a pitting resistance equivalent number (PREN) of around 43. It was specifically developed for service in sulphuric acid, mostly in concentrations from 20% to 85%.
The corrosion resistance of 904L is equivalent to most super duplex stainless steels (SDSS) across a wide range of applications. However, Ferralium 255 provides superior performance in sulphuric acid due to its increased copper content of 2%, unique amongst super duplex stainless steels.
As an austenitic stainless steel, 904L has a wider operating temperature range and can be specified with confidence for applications <-50degC or >250degC, where SDSS are limited. At low temperatures, 904L will retain good impact toughness, and it is not at risk of forming deleterious intermetallic phases at higher temperatures. However, super duplex stainless steels have higher levels of strength, which means that thinner sections can be used to achieve the same structural performance. The higher alloy content of 904L makes it inherently more expensive than SDSS, even before the difference in strength is considered.
In summary, 904L stainless steel vs duplex stainless steel is an interesting comparison. Super duplex steels are lower cost and higher strength and will be the optimum choice for most projects. Langley Alloys has supplied Ferralium 255 to numerous projects as a replacement to 904L. However, super duplex stainless steels have some limitations in their operating temperature. Therefore, the higher cost 904L is a sensible choice in these instances.
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