Three alloys, three different design objectives
The Inconel family is not a single material but a group of nickel-based superalloy grades developed for different problems. 625, 617 and 718 are often confused, yet they are not interchangeable — each is optimised for a specific mechanical/chemical requirement.
Comparison
| Grade | Main strength | Typical use |
|---|---|---|
| Inconel 625 | Broad corrosion resistance + good weldability | Chemical, oil & gas, marine, weld overlay |
| Inconel 617 | High-temperature strength + oxidation/carburisation resistance | Furnace components, high-temperature process equipment |
| Inconel 718 | High mechanical strength through age hardening | Precision parts carrying structural/mechanical loads |
625 — a strong candidate for corrosion and weld overlay
Thanks to its Ni-Cr-Mo-Nb chemistry, Alloy 625 offers resistance across a wide range of corrosive environments together with good weldability. This makes it the most widely chosen grade for chemical, oil & gas and marine applications and for weld overlays used as a corrosion barrier. Its good performance without additional post-weld heat treatment is a major advantage in field applications.
617 — the high-temperature option
Alloy 617 is considered for hot process equipment where high-temperature strength and oxidation/carburisation resistance are both important. In furnace components, high-temperature piping and thermally cycling environments (frequent heating and cooling), it serves a different design objective from 625 — 625 is chosen with corrosion as the priority, while 617 is chosen with temperature resistance as the priority.
718 — mechanical strength first
Alloy 718 is the only one of these grades that can reach very high mechanical strength after heat treatment through precipitation (age) hardening. It is preferred for structural/mechanical components where load-bearing capacity and fatigue strength take priority over corrosion resistance. Rather than in weld overlay/coating applications, it is mainly encountered as a component material in its own right or in limited special build-up applications.
Selection framework
The key question should be: is the primary threat corrosion (625), temperature (617) or mechanical load (718)? These three are not direct alternatives to one another; choosing the wrong grade can lead to premature failure even with a correctly applied coating/overlay.
Frequently Asked Questions
Can a cheaper alloy be used instead of Inconel 625?
If the corrosive environment is mild (e.g. atmospheric corrosion only), yes — more economical stainless steel grades may be sufficient. In aggressive chemical or marine environments, however, there is generally no economical alternative that can replace 625.
Which processes are used to apply these alloys?
For weld overlay purposes, PTA or similar welding-based processes are used; for thin protective coatings, thermal spray (arc spray/HVOF) can be considered with a suitable powder/wire form.
Which process is right for your part?
Send us photos, technical drawings and operating conditions. We will evaluate material and process selection for your specific part.
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