Inconel 625 vs 718 vs Hastelloy C-276 vs Monel 400: which to use?

At a glance

Which is stronger: Inconel 625, Inconel 718, Hastelloy C-276 or Monel 400?

Inconel 718 is stronger than the other three because it is the only one that is age hardened. Our profile gives a typical yield strength of 1,179 MPa and a tensile strength of 1,386 MPa at room temperature for one hot-rolled bar that was annealed and then aged. It is one typical value, not a minimum. Inconel 625 relies on solid-solution strengthening, and Monel 400 is hardened only by cold work. Where a profile prints only ksi or psi, we keep that unit.

Annealed Inconel 625 (grade 1) gives 414 to 655 MPa yield (60 to 95 ksi) and 827 to 1034 MPa tensile (120 to 150 ksi) for rod, bar and plate. The solution-annealed grade 2 is softer, at 290 to 414 MPa yield (42 to 60 ksi). These composite ranges are not for specification, so state the grade on any drawing.

Hastelloy C-276 gives 356 MPa yield and 792 MPa tensile for 2 mm sheet in the Haynes table. ATI gives a typical yield of 415 MPa and an ASTM B 575 minimum of 283 MPa yield and 690 MPa tensile, so grade 1 of 625 and C-276 overlap at the 414 and 415 MPa mark. A 50 percent cold reduction lifts C-276 yield from 415 MPa to 1,345 MPa while elongation falls from 50 to 7 percent (ATI).

Monel 400 is the softest, with 172 to 345 MPa yield and 483 to 621 MPa tensile when annealed, and Special Metals says it is hardened only by cold working.

Cold behaviour is only partly documented. Inconel 718 gives 237 ksi tensile at -320 °F in one test set, not like for like with 201 ksi at room temperature in another, and its yield there, 173.5 ksi, is close to the room-temperature 171 ksi. For C-276, ATI gives 965 MPa tensile at -196 °C (-320 °F). Charpy energy is 519 J at -196 °C in Haynes tests of mill-annealed plate and 245 J in ATI tests of a full-thickness 10 mm sample from annealed plate.

Which alloy can be used at the highest temperature, and which conducts heat best?

There is no like-for-like ranking, because each profile states its temperature limit on a different basis. Inconel 625 states service to 982 °C (1800 °F) in a datasheet range statement, not a specification limit, and ATI rates its oxidation resistance as excellent up to 1,093 °C (2,000 °F). It also ages: annealed rod held 2000 hours at 649 °C (1200 °F) rose from 140.0 to 176.0 ksi tensile and fell from 54 to 30 percent elongation.

Inconel 718 has a datasheet service limit of 1300 °F, and short-time tests show it keeps most of its strength there: 133 ksi yield against 171 ksi at room temperature. That is not long-hold capability. Between about 700 and 1000 °C, delta phase forms and removes the strengthening phase, and above about 1000 °C, Laves phase forms in large niobium-rich clumps. Both warnings are from Wikipedia, via the profile.

Hastelloy C-276 has an ASME Section VIII approval of 677 °C (1250 °F), a pressure-equipment rule rather than a physical limit. The ATI typical yield falls from 415 MPa at 21 °C to 270 MPa at 538 °C, and ATI says the alloy will eventually form embrittling precipitates, with no time or temperature printed.

Monel 400 has no single service temperature, but its profile names ASME allowable-stress ranges up to 800 °F (Section III) and 900 °F (Section VIII Division 1). ATI gives 21,000 psi yield at 538 °C (1000 °F), and says creep should be considered above about 343 °C (650 °F).

For heat transfer, Monel 400 conducts best of the four: 22 W/m°C at 21 °C, against 9.8 for 625 at 21 °C and 10.5 for C-276 at 50 °C.

How heavy are these alloys, and what does that do to strength per weight?

Inconel 718 is the lightest of the four and Hastelloy C-276 the heaviest, but the gap is small: 8.22 g/cm³ against 8.89 to 8.9 g/cm³. Strength therefore decides weight. On the room-temperature figures, a part in 718 needs less metal than one in annealed 625 or Monel 400.

If you do not need the nickel alloys' heat or acid resistance, Ti-6Al-4V is lighter still: C-276 is 8.89 to 8.9 g/cm³ against 4.43 g/cm³ for Ti-6Al-4V, a ratio of 2 times, with annealed yield of 830 to 950 MPa. The titanium profile says Grade 5 is rapidly attacked by hydrofluoric acid, and describes it for strength up to about 400 °C, below what 625 and 718 state.

Which alloy resists seawater and chloride attack best?

Hastelloy C-276 is the only one of the four whose own profile holds measured chloride tests, and Inconel 625 is next in the one comparison we hold. In the ASTM G 48 ferric chloride test, Haynes prints a critical pitting temperature of at least 150 °C for C-276, a lower bound rather than a measured onset, and a critical crevice temperature of 55 °C. For alloy 625 it prints 100 °C and 40 °C, and for 316L 15 °C and 0 °C. Haynes makes C-276, so this is the maker's comparison.

Both passed the stress-corrosion test: in boiling 45 percent magnesium chloride for 1,008 hours, C-276 and 625 showed no cracking while 316L cracked in 2 hours. In a 180 day U.S. Navy crevice test reported by Haynes, C-276 reached a maximum depth of 0.13 mm in still seawater and showed no attack in flowing seawater, where 316L reached 2.27 mm.

Monel 400 is good in flowing seawater and weaker in still water. Special Metals says it shows very low rates in flowing seawater, and that stagnant conditions induce crevice and pitting corrosion. No rate is printed. Wikipedia adds that Monel must be insulated from steel in salt water, because the steel corrodes. Inconel 718 has no seawater or chloride test in our profile, so we cannot rank it.

Which alloy handles acids and other chemicals?

Hastelloy C-276 is the only one of the four with measured acid rates in our data, and it handles dilute and moderate-temperature reducing acids well but not boiling or strong ones. Haynes gives 0.17 mm/y in 10 percent hydrochloric acid at 38 °C and 0.18 mm/y in boiling 10 percent sulfuric acid. Rates climb steeply with concentration and heat: 13.08 mm/y in boiling 60 percent sulfuric acid and 3.63 mm/y in boiling 5 percent hydrochloric acid, both rated severe in the profile.

Oxidising acids are the limit. Haynes gives 0.53 mm/y in 20 percent chromic acid at 66 °C, rated severe, and for 65 percent nitric acid 0.41 mm/y at 66 °C but 22.12 mm/y at the boil, against 0.26 mm/y for 10 percent at the boil. ATI says the alloy has too little chromium for the most strongly oxidising environments. In a boiling Green Death scrubber solution ATI gives 0.67 mm/y, also rated severe. For hydrofluoric acid, Haynes gives 0.41 mm/y in 10 percent acid at 52 °C, but its footnote says this covers external attack only and that the acid can also induce internal attack of nickel alloys.

Inconel 625 is described by Special Metals as resistant to both oxidising and non-oxidising chemicals, and was recommended for a storage tank for chemical wastes that include hydrochloric and nitric acid. The same datasheet notes that materials resisting one of those acids are normally severely attacked by the other. No rate is printed, so treat this as a pointer, not a rating. Its chromium is 20.0 to 23.0 percent, against about 16 percent in C-276.

Monel 400 is the hydrofluoric acid specialist on paper. Special Metals says it offers exceptional resistance in all concentrations up to the boiling point, but prints no rate and says nothing about aeration, vapour phase or stress, so ask for data for your conditions. ATI says it resists reducing acids, especially without air, and is not useful in highly oxidising acids such as nitric.

Where is each alloy used, and what goes wrong with each?

Three of the four have a rocket or rocket-plane use in our profiles. Inconel 718 forms the structural shell of the RS-25 main combustion chamber, lined with NARloy-Z, and is used for cryogenic tankage, turbine casings and fasteners. Inconel 625 is used for rocket thrust-chamber tubing, aircraft ducting and chemical plant. Hastelloy C-276 is used in chemical reactors, flue-gas desulfurization ducting and a nuclear-waste off-gas line; none of its sources names a space use. Monel 400 is used for valves, pumps, shafts, fasteners and tanks, and Wikipedia says Monel metal was used for the frames and skins of rocket planes such as the X-15, without naming the grade.

The failure modes follow the chemistry. Inconel 718 is exposed to delta and Laves phase if held in the wrong range. Our 718 profile says only that its resistance to postweld cracking is outstanding and that it is usually fabricated annealed and then heat treated, so strain-age cracking after welding is not covered. Inconel 625 ages near 649 °C (1200 °F), and its endurance limit at 10^8 cycles in reversed bending of cold-rolled annealed sheet is 35,000 psi for a notched specimen (stress concentration factor Kt 3.3) against 90,000 psi smooth. The other profiles hold no notched figures to compare.

Hastelloy C-276 loses stress-corrosion resistance after cold work that stretches the outer surface by 7 percent or more unless it is re-annealed, and its weld metal corrodes faster than wrought metal, for example 0.13 mm/y against 0.05 mm/y in 70 percent sulfuric acid at 66 °C. Monel 400 has the failure our profile rates high: caustic stress-corrosion cracking at high temperature and stress, although its corrosion list rates the same environment moderate. It is also attacked by sulfur-bearing gases above about 371 °C (700 °F) and can be permanently damaged if heated above 2150 °F.

On health, our profiles hold no safety data sheet for a solid alloy. The flags come from the PubChem nickel record (skin sensitisation, suspected cancer, organ damage from repeated exposure) and, for Monel, copper. The 625 powder SDS carries the signal word Danger and warns of an explosible dust-air mixture.

Terms used

Yield strength

the stress at which a metal starts to deform for good.

Solid-solution strengthening

strength gained because large atoms such as molybdenum and niobium sit in the nickel lattice and make it stiffer.

Age hardening

a heat treatment that grows tiny precipitate particles inside the metal.

Critical pitting temperature

the lowest temperature at which a standard chloride test makes pits form. A higher number means better pitting resistance.

Crevice corrosion

local attack in a tight gap, such as under a gasket or deposit, where oxygen runs out and the liquid turns aggressive.

Reducing and oxidising acids

hydrochloric and dilute sulfuric acid are reducing. Nitric and chromic acid are oxidising. An alloy that resists one kind may fail in the other.

What to check before you order

1. Name the grade by designation: UNS N06625 (625; plate ASTM B 443, AMS 5599), UNS N07718 (718; bar ASTM B637, AMS 5662 or 5663), UNS N10276 (C-276; sheet and plate ASTM B 575), UNS N04400 (Monel 400; plate ASTM B127, AMS 4544).
2. State the condition: for Inconel 625, grade 1 annealed or grade 2 solution annealed; for Inconel 718, the solution anneal and age schedule; for Monel 400, annealed or cold worked. Ask whether each strength figure is a minimum, a typical value or a composite range.
3. Name your fluid, its concentration, its temperature and whether it is aerated, and ask for a corrosion test in that fluid. Our data holds rates only for Hastelloy C-276.
4. For Hastelloy C-276, ask for anneal after welding if the service is severe, re-anneal after cold work, and matching filler metal. For Monel 400, stress-relieve cold-worked parts if caustic is present.
5. Ask for price, lead time and form availability. Our data holds no supply or price data.

What do we not know yet?

• Corrosion rates: our profiles hold no rates for Inconel 625, Inconel 718 or Monel 400. Their entries are qualitative statements from the makers, so only Hastelloy C-276 can be compared by number. No source tested all four alloys in one fluid.
• Creep, rupture and fatigue: the profiles hold no creep or rupture numbers for any of the four, and none of Inconel 718's fatigue values. The Inconel 625 creep curves are graphs that were not read.
• Failure modes the profiles do not cover: Monel 400 in aerated or vapour-phase hydrofluoric acid and in oxidising salts, hydrogen embrittlement of Inconel 718 in rocket hydrogen service, and strain-age cracking of 718 after welding.
• Monel 400 holds no service temperature, only ASME ranges without stress values, and Inconel 718 holds no seawater or acid behaviour.
• Cost, lead time and space environment data such as outgassing, atomic oxygen and radiation are not in the profiles.
• Age-hardened Monel K-500 and other Hastelloy grades are not covered.

Questions people ask