What is duplex steel?

Duplex

The name "Duplex" refers to the material's dual-phase microstructure. Common stainless steel (such as A2/A4) has a purely austenitic structure, while carbon steels are often ferritic.

Duplex combines both worlds – approximately 50% austenite and 50% ferrite.

By combining these two steel types, properties are achieved that neither A2 nor A4 can match on their own:

• Double mechanical strength: Duplex (such as alloy SAF 2205 / 1.4462) has approximately twice the yield strength of A4 steel.

• Extreme resistance to stress corrosion: A4 steel can suffer from sudden cracks under stress in chloride-rich environments (so-called stress corrosion). Duplex is highly resistant to this due to its ferritic component.

• Reduced weight: Thanks to the high strength, you can downsize the screw dimension without losing load-bearing capacity.

Duplex vs A2 and A4: A quick comparison

See our range of duplex

When should you choose Duplex instead of A2/A4?

You don't need Duplex to put up a deck or build a fence at home – A2 or A4 will do an excellent job there. However, Duplex becomes a necessity when mistakes become expensive or outright dangerous.

1. Marine environments and subsea

A4 steel works well at the coast, but if fasteners are constantly submerged in saltwater, exposed to high mechanical loads, or situated in splash zones, Duplex provides much safer protection against pitting and crevice corrosion.

2. Chloride-rich and hot environments

In swimming pools, desalination plants, and chemical industries, A4 steel often degrades over time. Stress corrosion can occur quickly and without warning. Duplex eliminates this risk in almost all cases.

3. When you want to save weight or space

Since Duplex fasteners can withstand significantly more than A4, you can generally choose a smaller thread dimension (e.g., M10 instead of M12) and still maintain the same safety margin. This saves both weight and space in complex constructions.

In summary: If your application is exposed to saltwater, chlorides, or high pressure, or if replacing broken screws would cost enormous sums in downtime – choose Duplex immediately. It costs more per screw, but saves a lot of money over the construction's service life.

Electrochemical potential: Duplex is more noble

In the galvanic series (especially in seawater and chloride-rich environments), Duplex is more noble (cathodic) than standard metals and also slightly more noble than A2 and A4.

• Most noble (Cathode): Super Duplex (2507) > Duplex (2205) > A4 (316) > A2 (304) > Carbon steel/Zinc (Least noble / Anode)

What does this mean in practice?

If Duplex is fastened directly with A2 or A4 in an electrolyte (e.g., saltwater), the less noble material (A2/A4) will sacrifice itself (corrode faster), while the Duplex part is protected.

2. Risk of galvanic corrosion when mixing

The risk depends entirely on the surface ratio (area relation) between the materials:

                  ELECTROLYTE (Saltwater/Moisture)
                               │
       ┌───────────────────────┴───────────────────────┐
       ▼                                               ▼
┌──────────────┐                               ┌──────────────┐
│  A2/A4-steel │ ──── Gives off electrons ────► │ Duplex-steel │
│   (Anode)    │                               │   (Cathode)  │
└──────────────┘                               └──────────────┘

• Small A4 screw in a large Duplex plate (HIGH RISK):

  Because the cathode (Duplex) is enormous compared to the anode (A4 screw), the current density on the screw becomes very high. The A4 screw will corrode heavily and quickly lose its strength.

• Small Duplex screw in a large A4 plate (LOW RISK):

  Here the anode (A4 plate) is much larger than the cathode (Duplex screw). The corrosion is spread out over a large surface on the A4 plate and becomes practically negligible.

3. Passivation and oxide layer

Both Duplex and A2/A4 rely on their passive oxide layer (chromium oxide) to resist corrosion.

• In air and fresh/rainwater, both materials remain completely passive. The potential difference between them is so small that no galvanic corrosion occurs.

• In oxygen-poor environments, crevices, or saltwater, A2/A4 can lose its passive layer and become "active." When A4 becomes active, the potential difference against passive Duplex increases dramatically, which accelerates galvanic corrosion on the A4 steel.

Summary & Guidelines

Rule of thumb: Duplex works excellently as a fastener (screw/bolt) in constructions made of A4 or stainless steel, as the screw is then more noble than the base material. However, you should never use A2/A4 screws in a Duplex construction in marine or chloride-rich environments.

Duplex stainless steel is clearly magnetic – unlike common stainless fasteners in A2 (304) and A4 (316), which are generally non-magnetic in their delivery state.

This often surprises and confuses people, as many associate "stainless = non-magnetic." Using a magnet to test whether a screw is stainless therefore does not work on Duplex!

Why is Duplex magnetic?

The explanation lies in the material's microstructure, which consists of a 50/50 mix of two different steel types:

1. Ferrite (~50 %): Ferritic steel has a body-centered cubic crystal structure that is strongly magnetic.

2. Austenite (~50 %): Austenitic steel (like A2/A4) has a face-centered cubic structure that is non-magnetic.

Since about half the material consists of ferrite, Duplex steel (such as SAF 2205 / 1.4462 or Super Duplex 2507) has a noticeable magnetic attraction. It is slightly weaker than in pure carbon or ferritic steel, but perfectly sufficient for a standard magnet to attach firmly.

Duplex and strength

Comparing Duplex 1.4462 (which is an austeno-ferritic stainless steel) with standard steel such as 10.9 (which is a quenched and tempered carbon steel for high-strength bolting) is very interesting. They are actually surprisingly close in pure mechanical load-bearing capacity, but differ in plasticity and behavior.

Here is a direct comparison of strength values according to standardized measurement methods (ISO 3506-1 for stainless and ISO 898-1 for carbon steel):

Strength comparison (Mechanical properties)

*Note: The values for Duplex 1.4462 apply to machined/cold-drawn material in bolt and threaded rod form (equivalent to class D70/D80). Annealed raw material has a lower yield strength of around 450 MPa.

Important differences in practice

1. Load capacity (Clamping force and tightening torque)

• 10.9 steel wins when you want maximum clamping force per screw diameter. It has a higher yield strength, allowing you to tighten a 10.9 screw with significantly higher torque before it begins to plastically deform (stretch).

• Duplex 1.4462 falls slightly below an 8.8 or 10.9 screw in terms of yield strength, but significantly outperforms standard stainless (A4-70 / A4-80).

2. Ductility and toughness (Safety margin against brittle fracture)

• Duplex has significantly higher elongation at break than 10.9 steel. This means that a Duplex screw can absorb significantly more energy and deform plastically under extreme overload, whereas a 10.9 screw behaves more rigidly and has a more brittle failure mode.

3. Corrosion and stress corrosion (SCC)

• 10.9 steel lacks corrosion protection. If it is galvanized (hot-dip), there is also a risk of hydrogen embrittlement due to the material's high hardness.

• Duplex 1.4462 requires no surface treatment, does not suffer from hydrogen embrittlement in the same way, and withstands tough saltwater and chloride environments.

Summary

• Use 10.9: When building in dry/protected environments or indoor structures where you need maximum clamping force on the smallest possible screw dimension and where corrosion is not a factor.

• Use Duplex 1.4462: When you need near 8.8/10.9 performance in mechanical strength, but must simultaneously have complete protection against chlorides, saltwater, and crevice corrosion without the risk of hydrogen embrittlement.