Specifications

Chemical composition

Nickel includes cobalt. Limits as given in ASTM B127, B164 and B165 (alloy 400) and B865 (alloy K-500).

Fasteners keep the composition of the bar from which they are made. Background on the alloys is on the Monel page.

Monel 400 or K-500 fasteners?

Monel 400 fasteners are used where corrosion resistance matters more than strength: bolting for Monel equipment in chemical and HF service, marine fittings and general fastening in seawater. Annealed alloy 400 has a modest yield strength, so cold-worked bar is often used to raise fastener strength.

Monel K-500 fasteners are age hardened to a yield strength roughly three times that of annealed 400, which allows much higher bolt preload. K-500 is the usual choice for structural and pressure bolting in seawater – flange studs, pump and valve bolting, propeller and rudder fastenings, subsea equipment – where the bolt must be both strong and resistant to seawater.

For sour service, the hardness and condition limits of NACE MR0175 / ISO 15156 apply to age-hardened K-500 bolting, and fasteners can be supplied to the standard on request. In seawater systems protected by cathodic protection, the risk of hydrogen embrittlement of high-strength K-500 bolts should be considered in the design, and hardness controls may be specified for this reason as well.

Why Monel bolting in seawater

In seawater, carbon steel and low-alloy steel bolts corrode quickly, and stainless steel bolts can suffer pitting and crevice corrosion under the nut and washer, where oxygen is excluded. Monel resists flowing seawater and is not prone to chloride stress corrosion cracking. Galvanic compatibility is also important: Monel bolts are slightly more noble than many structural materials, so the bolt – which has a small area – is protected rather than attacked when fastening larger components of a less noble metal. Bolting in a less noble metal than the parts it joins should be avoided.

How we manufacture Monel fasteners

We manufacture Monel fasteners from certified bar. Studs are cut and threaded, with rolled threads preferred for fatigue strength; bolts are hot forged or machined; nuts are machined from hexagonal or round bar. K-500 fasteners are usually machined and threaded in the annealed condition and then age hardened, or finish machined after ageing where tolerances require. Each fastener is marked with alloy and heat number, inspected for thread fit and dimensions, and supplied with the mill test certificate.

Galling and installation

Monel threads can gall when nuts and bolts of the same alloy are tightened under load. Galling is reduced by using an anti-seize lubricant suitable for marine use, rolling the threads, tightening at a steady speed, and combining a K-500 stud with an alloy 400 nut, as the different hardnesses reduce seizing. Torque values should be calculated for the lubricant and alloy.

Ordering Monel fasteners

Please give the alloy, fastener type, diameter and thread, length, number of nuts per stud, washers, condition, quantity and any testing or inspection requirement. For flange bolting, give the flange size, class and facing and we can calculate stud lengths.

Applications and related products

Monel fasteners are used for marine and offshore equipment, seawater pumps and valves, desalination plants, propeller and rudder fastenings, HF alkylation and chemical plants, and Monel flanged piping. Matching products include Monel flanges, Monel pipe fittings, Monel pipes and tubes, Monel sheets and plates and Monel round bars. Fasteners in other materials are on the fasteners page.