Technical knowledge
Inconel 718 vs Inconel 625 for flanges, fittings and bolting
Side by side comparison of Inconel 718 and Inconel 625 covering strength, corrosion resistance, heat treatment sensitivity, weldability, machinability and typical applications in flanged joints.
Quick comparison
| Factor | Inconel 718 | Inconel 625 |
|---|---|---|
| Strengthening mechanism | Precipitation hardening | Solid solution plus niobium |
| Yield strength | About 1034 MPa aged | 414 MPa annealed |
| Typical PREN | About 30 | 45 - 51 |
| Heat treatment sensitivity | High, cycle controls properties | Low, annealed is the default |
| Weldability | Good, needs post weld age | Excellent, no age needed |
| Chloride service | Moderate | Excellent |
| Typical role | Bolting, compact flanges, wellhead | Process flanges, fittings, overlay |
Cost per function rather than cost per kilogram
718 is often quoted at a premium because of the extra melting and heat treatment control it requires. But if the design is strength limited, the higher allowable stress can reduce the section or the bolt size, and that saving partly offsets the premium. Conversely, specifying 718 on a seawater line adds cost without adding life.
Risk notes from the field
- Wrong heat treatment. 718 delivered in the wrong condition is the most common source of dispute. Always request the heat treatment chart.
- Hardness for sour service. State the maximum hardness, typically 40 HRC, and require actual readings.
- Galling on threads. More frequent with 718 bolting; specify a suitable anti-seize compound.
- Mixed assemblies. A 718 bolt in a 625 flange is normal and acceptable; ensure the bolt remains slightly cathodic but small in area, or use isolation where the environment is conductive.
Questions about this topic
Which alloy should I use for high pressure wellhead flanges?
718 is common where very high strength and relaxation resistance are required, with 625 used where the fluid is highly corrosive. Many wellhead designs use a 625 wetted surface with 718 bolting, which is a sensible combination.
Does 718 corrode faster than 625?
In chloride containing water, yes, because its molybdenum content is much lower. In high temperature oxidising gas it can perform comparably, since the corrosion mechanism is different.
Can I mix 718 and 625 in one assembly?
Yes, and it is frequently done. Use the correct filler metal for each joint, keep the weld procedures qualified, and confirm the galvanic relationship is acceptable for the environment.
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 demi@nickel-alloy.com 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.