Nickel-Based Superalloys: Inconel 625 vs 718 vs Hastelloy Properties

Inconel 625, Inconel 718 and Hastelloy C-276 are the three most commonly specified wrought nickel superalloys outside the turbine-blade class, yet they occupy distinct positions on the strength-corrosion-temperature map. This guide compares their compositions, strengthening mechanisms, mechanical properties, weldability and typical service envelopes to support alloy selection decisions.

Composition and Strengthening Mechanism

All three alloys are nickel-based, but they occupy different points in composition space and rely on different strengthening routes. Inconel 625 and Hastelloy C-276 are solid-solution alloys: their strength comes from substitutional alloying elements distorting the nickel lattice and impeding dislocation glide, with no aging response required. Inconel 718 is a precipitation-hardened alloy whose strength depends on a controlled thermal aging cycle following solution treatment, in the same conceptual family as the quench-and-age logic used elsewhere in physical metallurgy, though the precipitate chemistry and kinetics are unique to nickel-niobium systems.

Inconel 625

Inconel 625 derives its strength primarily from molybdenum and niobium in solid solution within the nickel-chromium matrix, supplemented by minor carbide and, in some conditions, small amounts of Ni3Nb. It is used almost exclusively in the mill-annealed condition; extended thermal exposure between roughly 550 C and 980 C instead promotes formation of delta phase, Laves phase and grain boundary carbides that reduce ductility and toughness rather than provide useful strengthening, so the alloy is deliberately not aged in service.

Inconel 718

Inconel 718 is strengthened by a fine, coherent dispersion of gamma double prime (Ni3Nb), a metastable body-centred tetragonal phase, with a smaller contribution from gamma prime (Ni3(Al,Ti)). The defining metallurgical feature of 718 is the sluggish nucleation and growth kinetics of gamma double prime relative to the gamma prime that strengthens most other superalloys — this slow response is what makes 718 weldable without the strain-age cracking that limits fusion welding of faster-aging alloys.

Hastelloy C-276

Hastelloy C-276 relies on molybdenum and tungsten in solid solution for strength, with chromium providing oxidation and general corrosion resistance. Its defining feature is an extremely low carbon ceiling (0.01% maximum), developed specifically to suppress the grain-boundary carbide precipitation that sensitized the original Hastelloy C alloy during welding, a mechanism closely related to the intergranular corrosion mechanisms seen in sensitized austenitic stainless steels.

Mechanical Properties Comparison

Weldability and Corrosion Resistance

Weldability

Inconel 625, being solid-solution strengthened, welds readily with matching filler metal and requires no post-weld heat treatment to restore base metal properties. Inconel 718 is unusual among precipitation-hardened superalloys in also being highly weldable, specifically because of the slow gamma double prime precipitation kinetics described above; most fabricators still recommend a post-weld solution and age cycle to restore full design strength across the weld and heat-affected zone. Hastelloy C-276 is weldable with low heat input and matching or slightly overalloyed filler, but excessive heat input or multiple reheats can precipitate intermetallic mu and P phases in the heat-affected zone, locally reducing corrosion resistance even though the ultra-low carbon prevents classical sensitization.

Corrosion Resistance

Hastelloy C-276 leads the group in resistance to reducing environments such as hydrochloric acid, wet chlorine and mixed oxidizing-reducing acid streams, owing to its high combined molybdenum and tungsten content. Inconel 625 offers excellent resistance to seawater, marine chloride environments and moderately oxidizing media, making it the standard choice for subsea flexible risers, umbilicals and marine hardware, an application area closely tied to pitting corrosion resistance under chloride attack. Inconel 718’s corrosion resistance is good but secondary to its role as a structural, strength-driven alloy; it is not typically the first choice purely for aggressive chemical service.

Approximate pitting resistance equivalent number (PREN):

  PREN = %Cr + 3.3 x (%Mo + 0.5 x %W) + 16 x %N

  Inconel 625  (Cr 21.5, Mo 9,  W 0):    PREN ≈ 21.5 + 3.3(9)      ≈ 51
  Hastelloy C-276 (Cr 15.5, Mo 16, W 3.7): PREN ≈ 15.5 + 3.3(16+1.85) ≈ 74
  Inconel 718  (Cr 19, Mo 3, W 0):       PREN ≈ 19 + 3.3(3)       ≈ 29

Typical Applications

Selection Logic

Treat these three alloys as answers to different questions rather than direct substitutes. If the governing requirement is aggressive chemical corrosion resistance, start with Hastelloy C-276 and only step down to 625 if cost or availability dictates. If the governing requirement is high strength with reasonable corrosion resistance and good weldability at moderate temperature, 718 is the default. If the requirement spans high-temperature oxidation resistance with good but not extreme corrosion resistance and easy fabrication, 625 covers the widest envelope of the three.

Frequently Asked Questions

Reference Reading

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Further Reading