Difference Between Duplex and Super Duplex Material
The main difference between duplex and super duplex material is the amount of chromium & Molybdenum. The content of chromium in super duplex grade stainless steel ranges between 24% to 26%, while the content of chromium in duplex grade stainless steel could range anywhere between 19% to 32%.
Duplex stainless steel contains about 3% Molybdenum & 5% - 6% nickel in their alloy while super duplex stainless steel contains 4% Molybdenum & 7% nickel.
Another difference between duplex and super duplex steel is the addition of nitrogen. Adding Nitrogen in super duplex stainless steels improves its yield strength as well as its pitting resistance. Nitrogen also stabilizes the austenite effect in the presence of nickel.
The higher content of chromium & Molybdenum makes the super duplex grade stainless steel an increased resistance to corrosion and an increased tensile strength that is much higher as compared to duplex stainless steel.
As super duplex stainless steel grades are highly alloyed in comparison to duplex stainless steel, they offer many advantages like – excellent resistance to chlorine stress cracking, good toughness.
Welding duplex and super duplex stainless steels can be done easily, because they both have good weld able properties and can be welded by addition of filler metals. Both these stainless steel grades do not require a pre and a post heat treatment for welding.
Since both duplex and super duplex material have a high tensile strength as compared to austenitic grade stainless steels, the consumption to design & manufacture a product can be lowered. The walls of the product can be thinned, leading to reduction in both weight of the product as well as the cost of the product.
The mechanical and physical properties in both duplex vs super duplex steel can be compared to a superlative degree. For instance, if the mechanical strength of duplex steel is considered as high, the mechanical strength of super duplex steels would be considered as very high.
Although the duplex and super duplex difference is visible in terms of the content of chromium, nickel, Molybdenum & tungsten, the addition of these elements makes the processing of super duplex stainless steels a little difficult. These elements added in the alloy of super duplex stainless steel, promote the formation of intermetallic phases. Intermetallic phases formed can reduce the impact strength of this grade drastically.
While the content of the elements differ in the alloy of both duplex and super duplex grades, a major difference that stands out is the improved tensile strength and the corrosion resistance. But other mechanical and physical properties of these grades are similar to each other. Hence, both these grades of stainless steel can be considered to be the best because of their improved weld ability, ductility, corrosion resistance and high tensile strength as opposed to both Ferritic and austenitic stainless steel grades.