Comparing Superaustenitic Stainless Steel Alloys

Many industries are already using higher corrosion-resistant alloys beyond traditional stainless steel. In the high-purity and sanitary industries, products such as shampoos, conditioners, deodorants, sports drinks, ketchup, salsa, and active pharmaceutical ingredients (API) require extremely corrosion-resistant material to withstand the high concentration of chemicals (chlorides), high temperatures, and low pH.

Alloys that consistently come up in conversations of material construction are AL-6XN®, 254SMO®, and 904L®. These are all considered superaustenitic stainless steels and are often used in similar environments. The physical and chemical properties of superaustenitic alloys (particularly AL-6XN, 904L, and 254SMO) are very similar, and because of these similarities, they may be used interchangeably or used together.

Physical properties

Some selected physical properties are shown below, to help demonstrate similarities among these materials.

Chemical comparison

Corrosion Resistance

Pitting resistance equivalent number (PREN) is a quantitative way of measuring corrosion resistance. The higher the number, the better the corrosion resistance.

Figure 1: PREN=Cr% + 3.3Mo% + 16N%

Welding

All superaustenitic stainless steels are weldable and call for conventional welding methods (typically used when welding austenitic stainless steels, such as 304L and 316L material). However, the percentage of alloying elements in superaustenitics vary, so the weld appearance and the behavior of the alloying elements at the weld are different.

Typically, Fe-Ni-Cr-Mo alloys have about 2/3 the Mo content of the base metal. For 316L, weld metal Mo of about 2.5% matches the corrosion resistance of base metal of just over 2% Mo. AL-6XN has about 6% Mo and 2/3 of that is close to 4% Mo, so the welded AL-6XN metal will have about 4% Mo, which is less than the base metal. Similarly, 904L has 4% Mo, and the corrosion resistance at the weld gets closer to 316L.

In all cases, autogenously welded material has a lower corrosion resistance than base metal. For autogenous welds in superaustenitics, it is recommended that all the superaustenitic stainless steel with more than 3% Molybdenum (that includes AL-6XN, 254 SMO, and 904L) should use higher alloyed consumable weld inserts with a minimum of 9% Mo to maintain the corrosion resistance of the weld.

Typically, Hastelloy® C-22® with 13% Mo weld inserts are recommended for tube welds.Welding techniques in piping that use filler wire to the weld face are not recommended, due to the possibility of insufficient alloying in the weld root.

Product Availability

Immediate availability is often a deciding factor when selecting an alloy. Product availability is an important material property. A particular alloy may be better for specific applications, but sometimes finding 100 ft. of tubing with the required accessories can be a challenge. A product must not only meet the technical standards of application, but also be on the shelf – in all the necessary forms, such as sheet, plate, pipe, tube, bar, and fittings (both sanitary and commercial grade) to complete a project. Since these alloys are so similar, in real world applications they are very commonly welded to each other.

Due to product availability, a project may use a tank of 904L, sanitary tubing and fittings in AL-6XN, and valves in 254SMO. This is a viable method, since all of these alloys have alike properties. The only alloy available to ship immediately for piping and components is AL-6XN, making it a prime candidate for material selection.