Super Duplex Stud Bolts

The duplex and especially the super duplex steels have high strength, good weldability, good toughness and very good corrosion resistance.

In-house production

Production Steps

Through our in-house production EOT Energy A/S is able to produce up to approximately 1000 super duplex stud bolts per day (depending on size and length) from European origin base material. We have base material from 1/2″ to 1.1/2″ in stock in form of threaded rods in grade FLT and heavy hex nuts enabling us to produce super duplex stud bolts in any length (up to ~3000 mm) requested by our customers. The stud bolts can be delivered as stud bolts only or fitted with 1-2 or more super duplex heavy hex nuts according to customer specifications. Stud bolts larger than 1.1/2″ can be sourced. Please contact us for more information. 

Grade SA

​The base material for grade SA has also been solution annealed at 1120°C followed by quenching in water but with no subsequent strain hardening. This results in a lower strength compared to both grade FLT and grade FG. Charpy V Notch testing is performed at -46°C

Mechanical properties for Grade SA Stud Bolts:

  • Tensile strength min. 800 MPa
  • Yield Strength (0,2% offset) min. 550 MPa
  • Elongation in 2″ (50mm) or 4D min. 25%
  • Reduction of area min. 50%
  • Brinell Hardness max. 290

Stud Bolt Grades

Grade FLT – Fasteners Low Temperature

The base material for Zeron 100 stud bolts to grade FLT (Fasteners Low Temperature) has been solution annealed at 1120 °C followed by quenching in water. This is then followed by strain hardening to raise the mechanical values. The base material conforms to ASTM A276 condition S. The material is Charpy V Notch tested at -101°C

​Mechanical properties for Grade FLT Stud Bolts:

  • Tensile strength min. 860 MPa
  • Yield Strength (0,2% offset) min. 720 MPa
  • Elongation in 2″ (50mm) or 4D min. 16%
  • Reduction of area min. 50%
  • Brinell Hardness max. 335

Grade FG – Fasteners Grade 

The base material for super duplex stud bolts to grade FG (Fastener Grade) is produced and tested in the exact same way as for grade FLT with the only difference that the Charpy V Notch testing is performed at -46 °C. The mechanical properties of grade FG is also identical to those of grade FLT.

Heavy Hex Nuts

The Heavy Hex Nuts supplied with the super duplex stud bolts are produced from ASTM A276 UNS S32760 Condition A (Solution Annealed) material. The nuts are produced by drop forging followed by solution annealing at 1120°C and water quenching. The thread is produced by machining – UNC (Unified Coarse Thread to ASME B1.1) thread for nuts up to 1″ and UN8 (Unified 8 Thread to ASME B1.1) thread for nuts larger then 1″. Grade SA heavy hex nuts are used for all 3 grades of stud bolts FLT, FG and SA.

Super Duplex Stud Bolts Size and Length

Stud Bolts Size and Length for ANSI Flanges

In order to determine the needed size and length for your super duplex stud bolts to be used for ANSI flanges you need to know the size of the ANSI flange, the pressure class and the facing of the flange. There is a difference in the length of the stud bolts if you have a raised faced flange compared to a ring type joint (RTJ) flange.

Datasheet Stud Bolts Size and Length

To help you determine the correct size and length of your stud bolts for ANSI flanges you can download our PDF datasheet: Stud Bolt Sizes for ANSI Flanges. In the photo, you can see 1,5″ x 330MM Super Duplex Stud Bolts.

Super Duplex

What is Duplex Steel?

As the name Duplex suggests the duplex steels consists of 2 phases (ferritic and austenitic) in their metallurgical structure. In an ideal world these two phases should be present 50%-50% in the duplex steels. This is achieved by solution annealing the steel at temperatures above 1100°C (for UNS S32760). At this temperature any unwanted inter metallic phases will dissolve and the ferritic and austenitic phases be present 50%-50%. In order to luck-in this ideal structure the solution annealing must be followed by rapid quenching in water. The duplex and especially the super duplex steels have high strength, good weldability, good toughness and very good corrosion resistance. However the duplex steels are not suited for service above 300°C to avoid the formation of unwanted inter metallic phases. Although a 50%-50% share of ferritic and austenitic structure is ideal it is acceptable with a delta ferrite content of 35-65% in the finished duplex and super duplex products.

Typical applications​ for Zeron 100 bolts

Due to the excellent resistance to corrosion super duplex UNS S32760 stud bolts are especially suited for service in areas with corrosive environments:

  • Offshore Oil and Gas
  • Salt water applications
  • Paper production
  • Marine applications
  • Subsea
  • Chemical tanks and tankers
  • Desalination
  • Geothermal
  • Mining

​Zeron 100 – different names

The base material we use for stud bolts is UNS S32760 super duplex. Other names for this material are:

  • Zeron 100 
  • 25Cr Duplex
  • Alloy 100 Super Duplex
  • Alloy Z100
  • EN 1.4501
  • SAF 32760 (Sandvik trademark)
  • Alloy 1.4501
  • A182 F55 (when used as forgings)
  • X2CrNiMoCuWN25-7-4
  • FLT Stud Bolts

UNS S32760 chemical composition

The requirements to chemical composition according to ASTM A276 for UNS S32760:​

% C Si Mn S P Cr
Min 24,00
Max 0,03 1,00 1,00 0,01 0,03 26,00
& Ni MoNi N Cu W
Min 6,00 3,00 0,20 0,50 0,50
Max 8,00 4,00 0,30 1,00 1,00

PREN for UNS S32760

Pitting Resistance Equivalent Number (PREN) is a way to calculate the resistance to pitting corrosion for stainless steels enabling a theoretical comparison of different stainless steels and their resistance to pitting corrosion based on their chemical composition. There are various formulas being used for this calculation. The formular most often used is: PREN = Cr + 3.3Mo + 16N

​For materials where tungsten has been added to the composition a different formular is used to reflect the positive effect tungsten has on the resistance to pitting corrosion. UNS S32760 super duplex (Zeron 100) is one of the materials with added tungsten. See the chemical composition above. In this case this formular is used: PREN = Cr + 3.3(Mo + 0.5W) + 16N

​The super duplex base materials used for stud bolts at EOT Energy A/S will have a PREN > 40 – For comparison 316L stainless steel has a typical PREN of 23-28

QA/QC for Super Duplex Stud Bolts 


Besides the normal laboratory tests for mechanical properties the following additional tests will be performed on the super duplex base material at a ISO 17025 accredited laboratory:​

  • ASTM A923 – Test Method A – Sodium Hydroxide Etch Test for Classification of Etch Structures of Duplex Stainless Steels
  • ASTM E562 – Standard Test Method for Determining Volume Fraction by Systematic Manual Point Count
  • ASTM G48 – Test Method A – Standard Test Methods for Pitting and Crevice Corrosion Resistance of Stainless Steels
  • ASTM A923 – Test Method C – Ferric Chloride Corrosion Test for Classification of Structures of Duplex Stainless Steels

​As part of the in-house checks at EOT Energy AS we perform:

  • Visual checks of the threaded rods and nuts
  • Positive Material Identification (PMI) to ensure we have received the correct base material

  • Delta Ferrite Test to ensure the delta ferrite is within the acceptable limits


​All the EN 10204 3.1 certificates for the base material are checked to ensure all required tests have been performed and that the reported test values are with the required limits.


​At EOT Energy A/S we low stress hard stamp all super duplex stud bolts and nuts with a heat code. This heat code is also noted on the 3.1 material test certificate according to EN 10204. In this way we can ensure full traceability for each stud bolt and nut.