Can you mix stainless alloys in the same construction? What guidelines are there for mixing and matching alloys for success? Let’s look at a few cases.
You work for an equipment fabricator and have quoted a stainless job. When purchasing, not every required piece (plate, pipe, fitting, etc.) is available in the designated alloy, or the delivery time on a few required pieces is too long to satisfy your customer. Or perhaps you are the customer, a processing plant and the fabricator comes to you with this dilemma.
Some factors to consider include: corrosion compatibility, weldability, strength, toughness, hardness, etc.
This is commonly done. Type 316L, in most cases, is at least as corrosion resistant as 304L although not in all cases. For example, in nitric acid, 316L may be less corrosion resistant than 304L. If both alloys are sufficiently corrosion-resistant, there is no concern for galvanic corrosion. The mechanical properties are identical, both are austenitic, so no issues of concern there. Welding is simple, either 308L or 316L filler metal could be used.
However, using 304L components in a 316L piping system is a different story, especially if 304L has a significantly higher corrosion rate than the 316L. If there is a galvanic effect, the area ratio is poor, with a small anode (more active 304L) and a large cathode (more noble 316L). So great care must be taken in such cases.
There are a few composition variations with the 6%Mo alloys – UNS S31254, N08367, N08926 etc. These have similar corrosion resistance and mechanical properties, and are welded with the same filler metals. In general, these alloys can be mixed in the same construction. However, a few somewhat similar alloys, such as S34565, have similar or higher corrosion resistance, but much higher strength. Welding a much higher strength alloy to a lower strength alloy may be a challenge to prevent distortion.
Lean duplex alloys are often available as plate for use in tanks, whereas the attachments (pipe & fittings, nozzles, flanges, bar, etc.) are much harder to source. In general, it is possible to mix different versions of lean duplex, at least if their corrosion resistance and mechanical properties are suitable. Duplex 2205 piping components are more likely to be available, and they are often used in conjunction with a lean duplex tank or vessel. Some care must be taken as 2205 has lower strength than some of the lean duplex alloys, even if close. It may be possible to use an austenitic alloy for some parts mixed with duplex alloys, but a full evaluation of all the requirements including welding properties and the effect of the difference in thermal expansion would be necessary before approving. Remember that the weld metal will be a different composition than either of the base metals.
if considering joining components in a ferritic stainless steel to an austenitic one, again it may be possible, but the large difference in thermal expansion, especially important during welding, must be considered. In addition, the ferritic alloy is much weaker than the austenitic at the high temperatures during welding, so distortion of the ferritic grade will commonly occur on cooling. Similarly, if equipment is being used at low or high temperatures, differences in toughness and elevated temperature strength need to be considered. This includes the weld metal, which will be a different composition than either base metal. And from a corrosion viewpoint, while the standard austenitic alloys may be susceptible to chloride stress corrosion cracking, the ferritic alloys are highly susceptible to hydrogen cracking.
Dissimilar metal welding is covered in several publications available on the Nickel Institute website:
Practical Guidelines for the Fabrication of Duplex Stainless Steels
Practical Guidelines for the Fabrication of Austenitic Stainless Steels
If in evaluating a particular combination of alloys you have any doubts or questions, please use the free Nickel Institute Technical Inquiry service. www.inquiries.nickelinstitute.org
This is the fifth article in a series of communications comparing possible alternative grades to 300 series stainless steels.
