Differences in selecting my valve material: Superduplex or 316?

Be careful. Everything I’m going to explain is a manual to know what material to use for your valve.

Take your notepad or iPad (for the more techno-savvy) and write it down:

One of the essential aspects to consider when choosing a valve for a specific application is the material used in its manufacture.

The valve material has important implications, such as the ability to withstand working pressures and compatibility with the fluid in the circuit.

Example! A standard ball valve for tap water circuits should not be used in a hydrogen system. A ball valve specifically manufactured to work with hydrogen should be used in this case. These are generally made of materials compatible with ISO 15916.

Among all possible materials, the most common is stainless steel. Let’s see the most common types, their differences, and when to use one or the other.

Let’s see, what material do you need for your valve?

First of all: The difference between 304 and 316

Stainless steel 304 or 316 are technically called austenitic types because of their microstructure composition. This type of stainless steel is mainly known for its high corrosion resistance due to its high chromium and nickel content.

The main difference between the two is that 316 contains molybdenum which significantly improves corrosion resistance. They are used primarily in valves exposed to the open air, in saline environments, or slightly exposed to chloride and other corrosive chemicals.

A priori 304 is more economical, but Redfluid valves are standard in 316, so as far as valves are concerned, they are usually already directly in 316. If the application requires a high level of corrosion resistance, as in the case of underwater applications or where seawater flows, then read on.

What is the difference between 316 and 316L stainless steel?

Yes, adding an “L” at the end of the number changes things. It seems silly, but it is not.

As already mentioned, 316 stainless steel is an alloy containing molybdenum, chromium, and nickel. However, there is a variation known as 316L (1.4404), whose main difference is the carbon content of this steel.

While 316 (1.4401) contains a maximum of 0.08% carbon, 316L includes a maximum of 0.03%. This fact makes 316L more recommendable for valves that will be connected to pipes or fittings by welding. In other words, the “L” facilitates welding. The lower carbon content allows it to perform better at high temperatures, and the weld will not show weld galling as can happen with 316.

What are Duplex or Superduplex steels?

Duplex steels (F53, F55, F51 …) get their name because they have a two-phase microstructure. They are composed of austenitic stainless steel and ferritic grains. This makes them almost twice as strong as regular stainless steel. In addition, this offers significantly higher corrosion resistance than stainless steels such as 304 and 316.

For its part, the superduplex, we have two grades, the F53 or 32750, SAF2507, 1.4410. On the other hand, the F55 (also called UNS 32760, 1.4501, or Zeron100) resists corrosion even better, usually with a PREN higher than 40.

For the duplex, we have a grade called F51 or 2205 or UNS S31803 or 1.4462 has better corrosion resistance than 316 but is worse than Superduplex.

Duplex or superduplex are stainless steels containing 25% chromium, 4% molybdenum, and 7% nickel. Thus, it can form an austenitic-ferritic type alloy of iron, chromium, and nickel with added molybdenum. The result is increased resistance to corrosion cracking and a high ability to withstand exposure to acids, chlorides, caustic solutions, and other corrosive environments, especially seawater.

Is the Superduplex magnetic?

An excellent trick to identify if we have 316 or Superduplex:

Yes, superduplex steel, like duplex, is magnetic steel. It is magnetic thanks to the ferritic phase of its microstructure. On the other hand, 316 is NOT magnetic.

So with a magnet, we can discard if we are in front of Duplex/Superduplex or 316.

What is the PRE or PREN in Superduplex?

It is a formula we can calculate if we have the certificate of materials 3.1 of the valve. This certificate mentions the % of each element that composes the stainless steel. You only have to fill in the following formula, and you will calculate the PRE of the material. Pren or PRe stands for Pitting Resistance Equivalent Number.

• Duplex: PREN %Cr + 3.3%Mo + 16%N) ≥ 31
• Superduplex: PREN %Cr + 3.3%Mo + 16 %N ≥ 40
• Alternative Superduplex (usually at 1.4501): PREN = %Cr + 3.3(%Mo +0.5%W) + 16%N ≥ 41

But you tell me, what does the result mean?

It is a type of localized corrosion that creates holes in metals. It is a way to compare different materials. The higher the number, the better it will withstand “pitting corrosion” or honeycomb corrosion in English.

When to use 316 or Superduplex

When determining when to use 316 or Superduplex stainless steel, you must consider the type of fluid to be handled and the environment in which the valve will operate. The option of using Superduplex stainless steel valves or fittings is justified when the fluid is not compatible with other materials. This may be due to its chemical composition or when the operating environment is extreme, marine or underwater, ideal for chlorinated seawater and acidic media with high chlorine content.

However, it is always better to consult an expert in case of doubts, so Redfluid provides a material compatibility tool.

In our tool, you can enter the fluid of your system, and it will indicate the compatibility with 316, Duplex, and Superduplex materials.

You can also contact us and give us a call. We are always happy to hear from you.

1.4404 or 316L: Table of equivalences

Steel identified as 1.4404 is equivalent to 316L steel. The difference is that the 1.4404 designations are based on the UNE-EN 10088 standard, while the 316L designation is applied by AISI (American Iron and Steel Institute). To see equivalence tables click here.