Super-Austenitic Stainless Steel Alloys: A Comparison

Many industries are already using corrosion-resistant alloys beyond traditional stainless steel. In the high-purity and sanitary industries, products such as shampoos, conditioners, deodorants, sports drinks, ketchup, salsa, active pharmaceutical ingredients, etc. require extremely corrosion-resistant material to withstand the high concentration of chemicals (chlorides), high temperatures, and low pH. AL-6XN® is one of the materials that has been successfully used in these applications.

Aside from AL-6XN, other alloys that consistently come up in conversations of material construction are 254SMO® and 904L®. These are all considered super-austenitic stainless steels and are often used in similar environments. The physical and chemical properties of super-austenitic alloys (particularly AL-6XN, 904L, and 254SMO) are very similar, and because of these similarities, they may be used interchangeably or used together. When all three materials are this similar, your selection should primarily be made based on commercial availability. Of these three materials, AL-6XN is the only one that has been in stock for the past 20 years, as well as being continually available for same-day shipping.

Physical properties

Some selected physical properties are shown below to get an idea about the similarities among these materials:

Physical Properties of AL-6XN, 254SMO, and 904L

Chemical comparison of all the alloys

Chemical comparison of all the alloys

Corrosion Resistance

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

PREN Number vs Alloy

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

Welding

All super-austenitic stainless steels are weldable and call for the conventional welding methods typically used when welding austenitic stainless steels such as 304L and 316L material. However, since the percentage of alloying elements in the super-austenitics varies, the weld appearance and the behavior of the alloying elements at the weld are different.

The simple rule of thumb is that Fe-Ni-Cr-Mo alloys have about 2/3 the Mo content of the base metal. This is approximate, and for 316L, weld metal Mo of about 2.5% matches the corrosion resistance of base metal of just over 2% Mo. So, technically, AL-6XN has about 6% Mo and by the rule of thumb (2/3 Mo) the welded AL-6XN metal will have about 4% Mo which is less than the base metal and 904L will be more close to 316L.

In all cases, autogenously welded material has a lower corrosion resistance than base metal. For autogenous welds in super-austenitics, it is recommended that all the super-austenitic 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 tubing 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.

Since these alloys are so similar, in real world applications they are very commonly welded to each other. Again, immediate availability is the deciding factor when choosing which alloy to select.

Product Availability

A particular alloy may be better for specific applications, but sometimes finding 100 ft. of tubing with the required accessories can be a challenge. So, 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.

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 still a viable method, since all of these alloys have very similar properties. Once again, the only one of these alloys that is available to ship today for piping & components is AL-6XN, making it a prime candidate for material selection.

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