AISI 318LN Duplex Forgings

01At a glance

Key figures for 318LN in the solution-annealed condition, to DIN EN 10088-3.

Structure

Austenitic-ferritic duplex

Proof strength Rp0.2

450 MPa min

Tensile strength Rm

650 to 880 MPa

Elongation A

25 % min

Hardness

270 HB max

PREN

30.9 to 38.0

Service range

-100 to 250 C

Magnetic

Yes, ferromagnetic

Figures per AGST technical data sheet for 1.4462 to DIN EN 10088-3 and Metalcor datasheet 1.4462 / AISI 318 LN. Mechanical values apply to sizes up to 160 mm diameter at room temperature.

02Forged forms we supply

Open-die forging and ring rolling, solution annealed after hot work.

Duplex grades are only moderately forgeable, and the usable temperature window is narrower than for austenitic stainless. The press work has to finish hot and the piece has to come off the dies into a fast cool, which is why duplex is better suited to open-die forging and ring rolling than to long multi-pass sequences that lose heat. Every piece ships with a mill test certificate covering heat number, chemical analysis, mechanical results and delivered condition. Sizes depend on geometry, so send a drawing and we will confirm what the shop can take.

Standard delivery condition is solution annealed, as duplex properties and corrosion resistance both depend on the correct ferrite-austenite balance being restored after forging. Ultrasonic testing class, ferrite count and certification level are agreed per order.

03Chemical composition

Mass fraction in %, to DIN EN 10088-3, balance iron.

Sources: AGST data sheet for 1.4462 to DIN EN 10088-3; Metalcor datasheet 1.4462 / AISI 318 LN. The 0.10 to 0.22 % nitrogen range is what the LN suffix refers to.

04Mechanical and physical properties

Solution-annealed condition, room temperature.

Sources: AGST 1.4462 data sheet to DIN EN 10088-3; Metalcor 1.4462 datasheet. Thermal conductivity of 15 W/m·K is well below that of carbon steel, which affects welding heat input and machining heat removal.

05Forging and heat treatment route

The sequence we follow on 318LN open-die and ring-rolled work.

1. Heat slowly to 1200 CDuplex has lower thermal conductivity than carbon steel, so a slow, even soak avoids thermal gradients and cracking.
2. Forge between 1200 and about 950 CPublished guidance gives the hot working range as 1200 to 950 C, with the forging window sometimes stated down to 900 C. Finish hot rather than squeezing out the last reduction cold.
3. Cool rapidly in water or airFast cooling carries the piece through the sigma-phase band instead of letting it dwell there.
4. Solution anneal at 1020 to 1100 CThis restores the ferrite-austenite balance and dissolves any intermetallic phases formed during forging.
5. Quench from the solution annealCool in air or water. A slow cool after annealing undoes the anneal.
6. Test and certifyMechanical testing, and where specified ferrite count and corrosion testing, before release with the mill certificate.

Parameters per AGST 1.4462 forging and heat treatment data. Open-die forging and joint forging are both noted as suitable for this grade.

06Sigma phase, the main processing risk

The single issue that ruins duplex forgings.

Sigma phase precipitates in duplex stainless steels in roughly the 600 to 900 C range. It is hard and brittle, and it strips both corrosion resistance and toughness from the material. Nothing about the chemistry on the certificate will warn you: a piece can be fully compliant on analysis and still be compromised by how it was cooled.

Practically, that means three things. Hot work finishes above the band rather than in it. Cooling through the band is fast, not still-air on a thick section. And a slow furnace cool after solution annealing defeats the purpose of annealing in the first place. If a part has an uncertain thermal history, a solution anneal and quench is the recovery route, not a temper.

07Corrosion resistance

Where 318LN earns its place, and where it does not.

With a PREN of 30.9 to 38.0, 318LN resists pitting well, and it performs strongly in organic acids, phosphoric acid and chloride-bearing media. The duplex structure gives it resistance to chloride stress corrosion cracking, which is the failure mode that defeats 304 and 316L in warm chloride service, and it resists intergranular corrosion with that resistance retained after welding. At room temperature it is also resistant to seawater, water vapour and nitric acid.

The limits are real. Pitting can still occur in highly concentrated sodium chloride, and the 250 C service ceiling rules out hot applications. If your duty is hotter, more aggressive, or involves higher chloride concentration and temperature together, step up to a super duplex such as A182 F53 or F55, or to a super austenitic such as 904L or F44.

PREN range and corrosion behaviour per AGST 1.4462 data sheet; pitting and stress corrosion cracking resistance also noted in the Metalcor datasheet.

08S31803 or S32205, which to order

Two UNS numbers cover what is loosely called 2205.

In practice much duplex material is certified to both numbers at once. The thing to avoid is assuming it. If your specification says 2205 without a UNS number, ask which one the design is based on, and state dual certification on the purchase order if you need it. We will confirm what the heat we are offering can be certified to before you commit.

The relationship between the two UNS numbers, including the 1996 endorsement of S32205, is set out in the AZoM datasheet for grade 2205 duplex. Composition limits as cited above.

09Equivalent designations

The same material under other naming systems.

Sources: AGST comparable standards list for 1.4462; Metalcor standards table. If your drawing calls out A182 F51, that is this material under its ASTM forging designation.

10Applications

Where forged 318LN is specified.

318LN goes where chloride exposure and mechanical loading arrive together. Shipbuilding and offshore installations use it for components exposed to seawater and weather. The chemical and petrochemical industries use it for plant components, pipelines and vessels, and the oil industry for pipeline parts. It is used in seawater desalination plants and heat exchangers, in the cellulose and pulp industry, in food processing, and for architectural elements that have to last outdoors. In machinery and electronics it appears in weather-exposed enclosures.

Forged and ring-rolled 318LN is chosen over plate or bar stock when the section is heavy, when grain flow should follow the contour of a flange or body, or when a pressure-containing part needs forged properties rather than a machined weldment.

Application ranges per AGST and Metalcor datasheets for 1.4462.

11Questions we get asked