Inconel 718 or X-750: choosing between the two workhorse age-hardened grades

Both are nickel-chromium alloys hardened by titanium, aluminium and niobium. They are rarely interchangeable — the decision usually comes down to section size, welding and how hot the part really runs.

Inconel 718 and Inconel X-750 sit next to each other in most grade lists, and buyers who are new to nickel alloys often assume the newer designation is simply better. In practice they were developed for different duties: 718 is the large-section, weld-fabricated structural alloy, and X-750 is the spring, fastener and thin-section alloy with a longer history at higher temperature.

What the chemistry is actually doing

718 carries roughly 50–55% nickel, 17–21% chromium and 4.75–5.50% niobium, with 2.80–3.30% molybdenum added for corrosion resistance and to slow coarsening of the strengthening phase. X-750 runs a much higher nickel balance with 14–17% chromium, 2.25–2.75% titanium and 0.40–1.00% aluminium, and only 0.70–1.20% niobium.

That difference decides how each alloy ages. 718 forms its strengthening phase slowly, which is why it can be welded and then aged without cracking in thick sections — the age is sluggish enough that residual stress has somewhere to go. X-750 strengthens faster and harder, but the same kinetics make heavy sections and restrained welds far less forgiving.

	Inconel 718	Inconel X-750
UNS	N07718	N07750
Nickel	50.0 – 55.0%	balance (typically 70%+)
Chromium	17.0 – 21.0%	14.0 – 17.0%
Niobium	4.75 – 5.50%	0.70 – 1.20%
Titanium + aluminium	0.85 – 1.95% combined	2.65 – 3.75% combined
Ageing response	slow, tolerant of thick sections	fast, thin sections preferred
Welding	routine, no preheat	possible, strain-age cracking risk
Typical forms we ship	bar, plate, forging, tube, wire	bar, wire, strip, forging

Where 718 wins

• Anything welded after machining. Valve bodies, downhole tools, cryogenic tanks and instrument housings are routinely fabricated in 718 because the post-weld age does not require a full re-solution treatment.
• Sections over about 75 mm. The slow ageing response lets a heavy bar reach uniform properties through the section.
• Buyers who need one grade for everything. 718 covers bar, plate, tube, forging and wire from a single heat family, which simplifies qualification.

Where X-750 wins

• Springs and fasteners. X-750 is supplied as cold-drawn wire and bar specifically because it holds load at temperature where 718 creeps.
• Thin sections at 700–815 °C. Gas turbine seals, springs and bolting in that band are classic X-750 territory.
• Parts that need high hardness. Aged X-750 reaches higher hardness than 718 in equivalent sections.

The mistake we see in quotations

Most of the mismatches we are asked to rescue come from one of two directions. Either X-750 is specified for a welded fabrication, and the weld zone cracks during ageing — or 718 is specified for a spring that then relaxes at temperature. Neither is a material defect; both are selection errors that a two-line drawing note would have prevented.

If you are unsure, tell us the section size, the joining method and the service temperature. Those three answers usually settle it before price is even discussed.

Values quoted here follow the ASTM / EN edition named in each paragraph. For a specific heat, the governing numbers are those printed on the EN 10204 certificate that ships with the material. Written by the technical desk at Shanghai Hangbo Alloy Group.