Specifications

Note: ASTM A193 and A194 do not list duplex grades. Purchase orders that name A193 / A194 for duplex bolting usually mean the dimensions, marking and testing practice of those standards applied to the stated duplex grade. Please send the project specification so we can confirm how it is to be met.

Chemical composition

S32760 also requires a minimum PREN of 40. Footnotes as given in the standards.

Fasteners keep the composition of the bar from which they are made. Background, equivalents and properties are on the duplex and super duplex steel page.

Why duplex bolting

Duplex stainless steels have a structure of roughly half austenite and half ferrite. This gives about twice the yield strength of 316 stainless steel, together with much better resistance to chloride pitting, crevice corrosion and stress corrosion cracking. For bolting, the higher strength allows a higher preload than B8M Class 1 stainless studs, and the corrosion resistance suits seawater spray, splash zones and process fluids containing chlorides.

Super duplex grades S32750 and S32760, with higher chromium, molybdenum and nitrogen and a pitting resistance equivalent number (PREN) of about 40 or more, are used where 2205 is not enough – in warm seawater, firewater ring mains, produced water and offshore topsides.

Avoiding galvanic corrosion

When duplex or super duplex flanges are bolted with carbon steel or alloy steel studs, the less noble bolts corrode preferentially in seawater and wet conditions. Coating the bolts helps but can be damaged during tightening. Matching the bolt material to the flange material, or using a more noble bolt, removes the galvanic couple and gives the joint the same life as the piping.

How we manufacture duplex fasteners

We manufacture duplex and super duplex fasteners from certified solution annealed bar. Studs are cut and threaded by rolling or cutting; bolts are hot forged or machined; nuts are machined from bar. Hot forging of duplex requires controlled temperatures and a solution anneal and water quench afterwards, because slow cooling through the critical range can form intermetallic phases that reduce toughness and corrosion resistance. Testing such as ferrite measurement, impact testing and ASTM A923 can be carried out on request. Each fastener is marked with grade and heat number and supplied with the mill test certificate.

Galling and installation

Duplex threads, like those of other stainless steels, can gall. Use a suitable anti-seize lubricant, tighten at a steady speed and calculate torque from the lubricant and material. Avoid reusing studs whose threads have been damaged during removal.

Ordering duplex fasteners

Please give the grade (UNS number), fastener type, diameter and thread, length, number of nuts per stud, washers, quantity, any project specification and testing such as ferrite measurement, A923, impact test or NACE MR0175 / ISO 15156. For flange bolting, give the flange size, class and facing and we can calculate the stud lengths.

Applications and related products

Duplex and super duplex fasteners are used for seawater cooling and firewater systems, offshore topsides and subsea equipment, desalination plants, chemical and pulp and paper plants, and marine structures. Matching products include duplex steel flanges, super duplex flanges, duplex steel pipe fittings, super duplex pipe fittings, duplex steel pipes and tubes, super duplex pipes and tubes, duplex steel sheets and plates, super duplex sheets and plates, duplex steel round bars and super duplex round bars. Fasteners in other materials are on the fasteners page.