Technical knowledge
Inconel X-750: the alloy behind high temperature fasteners and springs
Inconel X-750 composition, age hardening response, heat treatment conditions, relaxation resistance at temperature and practical guidance for fasteners, springs and retaining rings.
Why X-750 and not 718 or stainless
Austenitic stainless fasteners lose preload rapidly above about 400 C through stress relaxation and creep. Inconel 718 is stronger at room temperature but X-750 retains useful relaxation resistance to a higher temperature and has a long service history in turbine, reactor and exhaust applications. It is also readily formed into springs and retaining rings in the solution treated condition before ageing.
Heat treatment conditions and what they give you
| Condition | Typical treatment | Where it is used |
|---|---|---|
| Solution treated | Annealed around 1150 C | Forming, machining, then aged by the fabricator |
| Age hardened, high strength | Two step age near 730 C then 620 C | Fasteners and springs for service to about 540 C |
| Equalised and aged | Intermediate step then age | Improved stress rupture and relaxation performance above 540 C |
| Hot worked and aged | Aged without full solution treatment | Higher strength, lower ductility, agreed case by case |
Practical points that cause field problems
- Thread galling. Use a nickel compatible anti-seize and control thread fit; uncontrolled torque on a clean nickel thread frequently seizes.
- Over-ageing. If a component is aged twice, or held too long, strength falls. Requiring the heat treatment chart with the certificate is the simplest control.
- Hydrogen exposure. Age hardened X-750 can be susceptible to hydrogen embrittlement in certain environments; check the project specification for plated fasteners.
- Mixed material joints. Avoid coupling X-750 fasteners directly to carbon steel without isolation, since the fastener becomes cathodic and drives corrosion of the softer member.
Questions about this topic
What temperature can Inconel X-750 fasteners reach?
Useful relaxation resistance extends to roughly 540 C in the standard aged condition, and somewhat higher with an equalising treatment. Above that range, and for long term creep limited designs, Nimonic or cobalt based alloys are usually considered.
Can X-750 be welded?
It can be welded in the solution treated condition with a matching nickel chromium filler, followed by a full age. Welding in the aged condition produces a softened heat affected zone that will not recover without re-ageing the whole component.
Is X-750 the same as Inconel 718?
No. Both are precipitation hardening nickel alloys but X-750 is a nickel chromium alloy hardened mainly by gamma prime, while 718 uses niobium to form gamma double prime and reaches higher room temperature strength. The two are not interchangeable without re-qualifying the design.
Questions about our products
What products does Hangbo Alloy supply?
Four product families, all available in nickel-based alloys, titanium, duplex and special stainless steel: flanges, butt weld pipe fittings, elbows, and seamless tube and pipe. Flanges cover weld neck, long weld neck, slip-on, blind, socket weld, threaded and lap joint patterns. Fittings and elbows cover concentric and eccentric reducers, equal and reducing tees, caps, stub ends, and 45°, 90° and 180° returns in both long and short radius, plus 3D and 5D bends for piggable lines. Seamless tube and pipe is supplied cold drawn or hot finished from 6 mm to 630 mm outside diameter.
Which alloy grades are available for these products?
More than 90 grades are published in our grade library. The most requested are Inconel 625, 718, 600 and X-750, Hastelloy C-276, C-22 and B-2, Monel 400 and K-500, Incoloy 800H, 825 and 925, duplex 2205 and super duplex 2507, 316L, 904L and 254SMO stainless steel, and titanium Grade 2 and Grade 5. Nickel-based grades can also be supplied with VIM plus ESR double melting where cleanliness or fatigue life is critical.
What sizes, wall thicknesses and pressure classes can you supply?
Flanges run from NPS ½ to NPS 24 to ASME B16.5 and from NPS 26 to NPS 60 to ASME B16.47 Series A or B, in pressure classes 150 through 2500. Butt weld fittings and elbows run from NPS ½ to NPS 48 (DN 15 to DN 1200) in wall thickness Schedule 5S through XXS. Seamless tube and pipe covers 6 mm to 630 mm outside diameter, in random lengths of about 5 to 7 metres or fixed lengths up to 12 metres. Non-standard and intermediate sizes are quoted on request.
Which material and dimensional standards do you work to?
Forged flanges to ASTM B564, B381 and A182 with dimensions to ASME B16.5 and ASME B16.47. Butt weld fittings and elbows to ASTM B366 and A403 with dimensions to ASME B16.9 and MSS SP-43. Seamless tube and pipe to ASTM B444, B423, B622, B163, B167, B861, A312, A213 and A790. Marking follows MSS SP-25 and the applicable product standard. If your project is governed by EN, DIN, JIS or a company specification, send the document and we will confirm equivalence before quoting.
Is there a minimum order quantity?
There is no minimum for standard sizes and grades held in stock, and prototypes, spares and small trial batches are accepted. Made-to-order melts and special forgings carry a minimum weight that depends on the alloy and the route, typically from about 500 kg for special grades.
What inspection and certification is issued with the goods?
Every shipment carries an EN 10204 3.1 mill test certificate quoting the actual chemical and mechanical results for the specific heat lot, together with dimensional records and heat treatment data where applicable. Testing is carried out in house by optical emission spectrometry, tensile, impact and hardness testing, plus ultrasonic and eddy current examination, with hydrostatic pressure and intergranular corrosion testing to ASTM G28 or A262 Practice E where the specification requires it. Third party inspection by SGS or another recognised agency can be arranged, and their report is issued alongside the shipment.
What are typical lead times?
Standard stock items ship within days. Common production grades such as Inconel 625 or duplex 2205 are typically a few weeks. Special chemistries, larger forgings and items that require third party inspection take longer and are quoted individually against the requirement.
How are the products packed for export?
Flanges and fittings receive a rust preventive coating, are individually wrapped and are packed in plywood cases secured with steel banding. Tube ends are capped and bundles are strapped. Packing lists quote heat numbers so every case can be cross-checked against the mill certificate on arrival. We quote EXW, FOB, CFR, CIF and DAP, and mixed-alloy orders can be consolidated into one shipment with a single documentation package.
How do I request a quotation or a sample?
Send the grade or UNS number, the dimensions including schedule or wall thickness, the governing material and dimensional standards, quantity, any heat treatment or NDT scope and the certificate type you need; a drawing is ideal for special flanges and fittings. Email [email protected] or message us on WhatsApp +86 15618279078 and we normally issue a formal quotation within 24 hours on working days. Samples and previous test reports can be arranged before the first order.
Talk to an engineer
Share your duty conditions and we will shortlist candidate alloys within one working day.