Introduction to Nickel-Based Alloy Fasteners

Nickel-based alloy fasteners — stud bolts, nuts and bolting for high-integrity flanged joints — are specified when stainless steel no longer offers enough corrosion or temperature margin. The fastener carries the joint load while the flange gaskets seal the process medium, so the alloy grade has to suit both the service environment and the sealing element.

Composition and Strengthening Decide the Category

Nickel materials are grouped first by chemistry. Grades with a nickel plus cobalt content of 99% or more and cobalt at 1.5% or less are treated as industrial pure nickel, and materials with 50% or more nickel are called nickel-based alloys. Iron-nickel-based alloys cover 30% to 50% nickel with nickel and iron together at 60% or more. Stainless steels sit on the other side of the boundary: iron at 50% or more and nickel below 30%.

ASME collects pure nickel, nickel-based and iron-nickel-based alloys under the term high-nickel alloys, and its boundary is not the same as the GB/T one. Grade 904L shows the difference: UNS N08904, or 03Cr20Ni25Mo5Cu, is filed as a nickel-based alloy in the US system and as a stainless steel under GB/T.

• Industrial pure nickel such as N5, N6 and N7, plus nickel-copper alloys such as NCu30 and NCu28-2.5-1.5.
• Nickel-chromium, nickel-molybdenum, nickel-chromium-molybdenum and nickel-chromium-molybdenum-copper systems; nickel-molybdenum grades are the usual answer for strongly reducing acids.
• Solid solution strengthened and precipitation strengthened (age hardened) alloys. Fastener standards basically use solid solution grades, while precipitation grades are rarely specified for bolting.
• Wrought forms follow the same logic: bar and tube for stud bolts and nuts are supplied in solid solution strengthened nickel-based materials.

How Grade Designations Appear on Fastener Stock

Nickel alloys are internationally traded commodities, so one bolt can carry several designation systems at once. In the UNS system a nickel alloy is N followed by five digits, N×××××: the first two digits group the chemistry, the last three follow the sequence of common commercial grades. International designations use NW plus four digits — NW6600, for example — and those four digits track the UNS number.

In the Chinese system, pure nickel and nickel-copper products are designated under GB/T340 (N5, N6, NCu30), other corrosion-resistant alloy products use the NS××× pattern of GB/T15007, and heat-resistant alloys use the GH××× pattern of GB/T14992. Corrosion-resistant alloys may also carry the unified digital code of GB/T17616.

Table 1. NS designation pattern under GB/T15007: the first digit gives the base type and the strengthening, the second the alloy system, the third the sequence of the individual grade.

Selecting Bolting Alloys for a Flanged Joint

A bolted joint is only as corrosion resistant as its weakest part: pipe flanges, gasket, fastener and process medium have to be selected as one system.

• Match the alloy system to the medium: nickel-molybdenum for reducing acids, nickel-chromium-molybdenum for oxidizing and mixed media, nickel-copper for hydrofluoric acid and salt water.
• Prefer solid solution strengthened grades for stud bolts and nuts, because precipitation hardened grades are rarely covered by fastener standards.
• Check galvanic compatibility with the flange and gasket materials so the bolting does not become the anode of the joint.
• Read the certificate as a set: UNS or NW number, the GB/T NS grade where applicable, and the ASTM standard for the fastener form.

Where a project quotes only a trade name, ask for the UNS number as well. Trade names vary between mills, while the UNS designation fixes the chemistry and lets the buyer compare the fastener with the rest of the joint.

Frequently Asked Questions

Which nickel alloys are normally used for stud bolts and nuts?

Most fastener standards specify solid solution strengthened nickel alloys in bar and tube form, and only rarely precipitation hardened grades.

What does a code such as NS111 or NS322 tell me?

The first digit shows the base type — NS1 is an iron-nickel-based alloy, NS3 a nickel alloy. The second digit gives the alloy system (1 for nickel-chromium, 2 for nickel-molybdenum, 3 for nickel-chromium-molybdenum), and the third digit is the grade sequence number.

Is 904L a stainless steel or a nickel-based alloy?

Both answers are correct, depending on the system: UNS N08904 is grouped with nickel-based alloys in the United States, while GB/T classifies 03Cr20Ni25Mo5Cu as a stainless steel.

Why does a certificate show both a UNS number and a national grade?

Nickel alloys are traded worldwide, so mills mark the fastener with the UNS or NW designation next to the national grade; codes of the NW6600 type follow the UNS sequence.

Do nickel-based alloy fasteners need special gaskets?

A joint is only as corrosion resistant as its weakest part, so the gasket material and the flange face material should be selected for the same medium and temperature as the bolting.