What materials can replace Inconel 718

Inconel 718 is the reference precipitation-hardening nickel alloy for high-strength fasteners — stud bolts, bolts and nuts — in turbines, wellhead equipment and chemical plant, but its price and long mill production schedules push buyers to look for a replacement. The closest drop-in substitute is the Chinese grade GH4169: the two alloys carry almost identical chemistry, and equivalent designations exist under the French, German, British and ISO systems. Fasteners and gaskets are selected together, so the same review usually covers the flange gaskets that the bolted joint has to compress.

Inconel 718 and the GH4169 Equivalent

Alloy 718 is a precipitation-hardenable nickel-based alloy. It holds high yield strength, tensile strength and creep-rupture strength up to 704 °C. The age-hardening reaction is slow, so the alloy does not harden during the heating and cooling cycles of annealing and welding, and it welds better than aluminium- and titanium-bearing nickel superalloys.

Its structure is sensitive to hot-working practice. Supplied forms include forgings, forged bars, rolled bars, cold-drawn bars, billets, rings, plates, strips, wire and tube. From these the mill makes discs, blades, shafts, fasteners, springs, plate structures and casings.

• Stud bolts, bolts, nuts and other fasteners for high-temperature service.
• Two strength classes are commonly supplied: ASME specification 718 at 150 ksi yield strength, and NACE specification 718 at 120 ksi yield strength.
• The Chinese grade GH4169, also written GH169, is the direct structural equivalent; UNS N07718 is the unified designation of the US grade.

Equivalent Grades and Chemical Composition

One alloy is traded under different designations depending on the standard system, which is why the purchase order and the mill certificate often show different names for the same heat.

Designations for the same nickel-chromium-iron-niobium alloy in different national standard systems.

Chemistry is nearly identical: 50–55 % Ni, 17–21 % Cr, 2.80–3.30 % Mo, 4.75–5.50 % Nb with iron as the balance, carbon capped at 0.08 % and manganese at 0.35 %. The real differences sit in the aluminium and titanium windows and in the phosphorus and sulphur limits (0.010 % in 718 versus 0.015 % in GH4169).

Mechanical Properties and Heat Treatment

Typical mechanical data for bar and fastener stock in the aged condition:

Values apply to material in the aged condition; the mill certificate confirms the exact figures for each heat.

Standard heat treatment is solution annealing at 1021–1052 °C for 1–2.5 hours followed by water quench — bars under 3.5 in diameter may be air cooled — then ageing at 774–802 °C for 6–8 hours and air cooling. Heat treatment is normally carried out by the mill or under mill supervision.

• Best tensile and stress-rupture strength: 954–982 °C for 1 hour, air cool; then 718 °C for 8 hours, cool to 621 °C at 56 °C/hour, hold 8 hours, air cool.
• Best room-temperature and low-temperature tensile strength: 1066 °C for 1–2 hours, then the same double ageing at 718 °C and 621 °C.
• Hot working: furnace temperature not above 1121 °C, forging between 927 and 1010 °C, without long soaking.
• Machining and welding: annealed and aged material both machine well; 718 welds in either condition, although the heat-affected zone softens and may deform.

Where 718 Fasteners and Parts Are Used

In oil and gas service, 718 fasteners go into wellhead completion equipment, blowout preventers, valves, tubing hangers and high-pressure bolted joints on pipe flanges, where sour gas and high pressure rule out ordinary stainless grades.

• Chemical processing — valves, pump and reactor bolting, springs and shafts.
• Aerospace and gas turbines — discs, blades, wheels and high-temperature fasteners.
• Liquid-fuel rocket engine components and pollution control equipment.
• Nuclear reactors and cryogenic storage tanks.
• Wellhead completion equipment, blowout preventers, valves and tubing hangers.

When Inconel 718 is expensive or hard to obtain, GH4169 is the practical substitute in the same product forms, provided the order states the UNS N07718 chemistry and the required heat treatment. Above 704 °C the strengthening phase changes and strength falls away quickly, so the answer is not a 718 equivalent but a nickel alloy designed for that temperature range.

Frequently Asked Questions

What material can replace Inconel 718?

The Chinese grade GH4169, also written GH169, is the direct replacement: the nickel, chromium, molybdenum and niobium ranges of the two alloys are essentially the same. NC19FeNb (France), NiCr19Fe19Nb5Mo (Germany) and NA 51 (UK) are designations for that same alloy.

Are Inconel 718 and GH4169 chemically identical?

They are close but not identical. The aluminium and titanium windows differ, as do the phosphorus and sulphur limits (0.010 % against 0.015 %). Confirm the substitution on the mill certificate, not on the grade name alone.

What strength classes are supplied for 718 fasteners?

Two are common: 150 ksi yield under ASME specification 718 and 120 ksi yield under NACE specification 718. In the aged condition the alloy shows at least 150 ksi tensile strength, 120–145 ksi yield strength and 30–40 HRC hardness.

Which heat treatment gives the best high-temperature strength?

Solution anneal at 1021–1052 °C, water quench and age at 774–802 °C. For maximum tensile and stress-rupture strength, use the 954–982 °C solution treatment with double ageing at 718 °C and 621 °C.

Can Inconel 718 be welded?

Yes. The age-hardening reaction is slow, so the alloy can be welded in the annealed or the aged condition. Zones softened by weld heat may deform, so welded parts are re-checked against the specified heat treatment.