Specifying X-750 spring wire: condition, hardness and relaxation

Spring wire is bought in three different conditions under one loose name. What each condition leaves to the spring shop, what the chemistry decides before ageing starts, and why hardness on its own will not accept a spring.

X-750 spring wire is usually specified in three words — “X-750, spring temper” — and those they cover at least three different products: annealed and unaged, solution treated and aged, cold drawn and aged. The grade is a precipitation-hardenable nickel-chromium alloy with titanium, aluminium and niobium additions, whose point is that strength is developed in the finished part rather than bought in the wire. A usable specification must say who performs the ageing.

Three conditions, not one

Spring temper is not a specification. On X-750 wire it can mean any of the three deliveries below, and the difference decides who holds an ageing furnace and who carries the risk of a coil that will not form. Strength and stability are rated to 704 °C, but they come from a precipitation reaction, not from the drawing pass.

	Annealed, unaged	Solution treated and aged	Cold drawn and aged
What arrives	Softest of the three; coilable as delivered	Hardened in the coil, before forming	Work hardened by drawing, then aged; least formable
Who forms it	The spring shop, before ageing	The spring shop, allowing for springback	Light forming only
Who ages it	The spring shop or a sub-contractor	The mill	The mill
Where it fits	Tight radii, small index, coils that would crack from hard wire	Simple forms, where the shop has no furnace	Simple shapes, minimal forming
What to state	Condition and the ageing you will apply	Condition and the property level you accept	Condition, drawing reduction, finished diameter

Whichever route is chosen, the coil must be traceable to a heat: two heats inside the same specification can behave as two materials at the ageing step, and a certificate that only states compliance with the range will not show it.

What the chemistry fixes

X-750 hardens through the gamma-prime reaction: titanium and aluminium in solution form the strengthening precipitate on ageing, and niobium reinforces it while also forming grain-boundary carbides. The ranges are wide enough that a heat at one end behaves differently from a heat at the other in the same furnace cycle.

	Inconel X-750
UNS	N07750
Europe	DIN 2.4669
China	GH4145
Nickel	balance
Chromium	14.0 – 17.0%
Iron	5.0 – 9.0%
Titanium	2.25 – 2.75%
Aluminium	0.40 – 1.00%
Niobium	0.70 – 1.20%
Cobalt	≤ 1.00%
Carbon	≤ 0.08%
Manganese	≤ 1.00%
Silicon	≤ 0.50%
Sulfur	≤ 0.010%
Copper	≤ 0.50%
Density	8.28 g/cm³
Melting range	1393 – 1427 °C
Wire	ASTM B639
Round bar	ASTM B637, ASME SB-637
Plate	ASTM B670, ASME SB-670
Seamless tube	ASTM B163, ASTM B407, DIN 2.4669

Titanium at 2.25–2.75% against aluminium at 0.40–1.00% is the pair to watch. A heat at the bottom of both ranges passes the certificate and still ages more slowly and to a lower level than a heat in the middle. If the spring works near its stress limit, ask for the titanium and aluminium results of the heat you will actually receive, not the range from the standard. Niobium at 0.70–1.20% refines the precipitate and forms the carbides that pin the boundaries. Iron at 5.0–9.0% is part of the balance, not contamination, and chromium at 14.0–17.0% is the oxidation reserve — lower than in the 600-series alloys, the trade this grade makes: it is bought for strength, not for scale life. Carbon at ≤0.08% and sulfur at ≤0.010% keep the carbide network from becoming a crack path in a fatigue-loaded coil.

Hardness is not acceptance on its own

A hardness reading confirms that the ageing reaction happened. It does not tell you how much load the spring will still hold after a year at temperature: two coils can read the same hardness and relax differently, because relaxation depends on the stress pattern in the wire, the surface condition and the cold work left in it. Where hardness is called out, define where it is measured — wire surface, prepared cross-section or finished spring — and whether the decarburised skin is removed before testing. Conversions between hardness scales are approximations that, on a nickel alloy, can move the acceptance line further than the tolerance around it.

Relaxation is a design input, not a material property

X-750 is chosen for spring duty because it keeps its shape under load: it retains strength and stability to 704 °C and has the creep-rupture resistance austenitic spring steels do not. But relaxation is not a single figure a mill can print. It is the loss of load at a given deflection, and it follows the initial stress, the temperature and the time. Two springs in the same alloy, one loaded near its elastic limit and one well below it, lose load at different rates.

The enquiry should therefore carry the three numbers that govern it: metal temperature, average and peak; the stress the spring carries; and the loss of load acceptable at the end of the design life. With those three we can say whether X-750 is needed, whether a lower-alloy wire would do, or whether the spring needs more wire section to allow for relaxation.

What to write on the order

• Form and standard — wire to ASTM B639; bar to ASTM B637 or ASME SB-637 for machined parts.
• Condition — annealed and unaged, solution treated and aged, or cold drawn and aged, plus who performs the ageing.
• Chemistry — the actual titanium, aluminium and niobium results of the heat, if the spring works near its limit.
• Size — wire diameter with tolerance, coil weight or length, and whether the surface is pickled, ground or coated.
• Properties — hardness with its measurement location, and any load test at a stated deflection.
• Documents — a 3.1 certificate with chemistry and mechanical results per heat, or 3.2 with witnessed testing.

What to send us

Send the diameter and tolerance, the condition you want and who will age the spring, the service temperature and the stress the spring carries, the quantity in kilograms, and the inspection document you need. If the drawing says only “X-750 spring temper”, send it as it is and we will propose the condition that fits it. Specifications and product forms are on hangbonickel.com; enquiries go to vicky@hangboalloy.com.

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.