Inconel 600 vs 625: Which to Choose | Hangbo

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What is Inconel 600 vs 625?

Quick answer: Inconel 600 and Inconel 625 are both nickel-chromium alloys from the same product family, but they are not interchangeable. Inconel 600 (UNS N06600) is a nickel-chromium-iron solid-solution alloy valued for high-temperature oxidation resistance and resistance to chloride stress-corrosion cracking; Inconel 625 (UNS N06625) adds molybdenum and niobium, which raise both strength and resistance to pitting, crevice corrosion and a wide range of acids. Choose 600 when the duty is high-heat and moderate-corrosion service; choose 625 when the environment is aggressive seawater, acidic chloride or high mechanical load.

Most buyers meet these two grades when a quote comes back naming one of them, or when an existing drawing calls for the other, and the natural question is whether the swap matters. It does. The chemical difference is small in headline terms — both are nickel-rich — but the molybdenum and niobium in 625 change the corrosion behaviour enough that substituting 600 into a seawater or sour-gas duty can mean the part survives only months instead of decades. Equally, paying for 625 in a duty that only needed 600’s oxidation resistance is money spent for no return.

At Shanghai Hangbo Alloy Group we produce both grades from 100% virgin nickel, chromium and molybdenum feed, refine by vacuum induction melting (VIM) followed by electroslag remelting (ESR) or vacuum arc remelting (VAR) to keep tramp elements low, strip the oxide scale thoroughly before working, and finish with the annealing cycle the specification names. Every heat is analysed by spectrometry plus wet chemistry, mechanically tested and dimensionally inspected before shipment, so the grade on the certificate is the grade you can re-verify on arrival rather than trust on paper.

What Actually Sets Inconel 600 and 625 Apart

Quick answer: The metallurgical difference is the alloying beyond chromium. Inconel 600 is nickel-chromium-iron with no deliberate molybdenum or niobium; Inconel 625 carries roughly 8–10% molybdenum and 3.1–4.1% niobium (columbium) plus tantalum, which both strengthen the matrix and give it a pitting-resistance equivalent number (PREN) well above 600’s. That single change is why 625 resists acids and chlorides that 600 cannot.

Both alloys are face-centred-cubic austenitic structures strengthened only by solid solution — neither hardens by heat treatment — so the strength gap between them comes entirely from alloy content, not from a hardening reaction. Inconel 600 relies on chromium for oxidation and on its high nickel content for resistance to chloride stress-corrosion cracking and for service in caustic soda. Inconel 625 leans on the same chromium plus a substantial molybdenum and niobium addition that attacks localized corrosion and raises strength at once.

The practical reading for a buyer is simple: 600 is the high-temperature and chloride-SCC-resistant workhorse, while 625 is the corrosion-and-strength workhorse. Where the pool’s grade articles describe each alloy on its own, the procurement risk is in the gap between them, and that is what this comparison is built to close. Our Inconel 625 properties guide and the Inconel 600 oxidation-resistance guide give the deeper single-alloy detail.

Chemical Composition Side by Side

Quick answer: Inconel 600 (UNS N06600) is roughly 72% nickel minimum, 14–17% chromium and 6–10% iron, with no molybdenum and no niobium specified. Inconel 625 (UNS N06625) is 58% nickel minimum, 20–23% chromium, 8–10% molybdenum and 3.15–4.15% niobium plus tantalum, with iron capped at 5%. The molybdenum and niobium are the markers that separate the two on a chemical test.

The limits above are the compositional ranges most commonly cited from ASTM B168/B166 (for 600) and ASTM B443/B446 (for 625); confirm the exact edition and product form on your purchase order. For verification, the decisive lines are molybdenum and niobium: a certificate that shows zero molybdenum and no niobium for a “625” line item is reporting 600, and that single check catches most grade-downgrade attempts before the metal is cut.

Mechanical Properties: 600 vs 625

Quick answer: In the annealed condition Inconel 625 is markedly stronger than 600 because molybdenum and niobium harden the matrix: typical annealed tensile is around 830–1050 MPa (120–150 ksi) for 625 against roughly 550–655 MPa (80–95 ksi) for 600, with yield strength roughly 415 MPa (60 ksi) versus 240 MPa (35 ksi). Density is close for both, about 8.4–8.5 g/cm³.

These figures are orientation values for comparison only; the numbers that govern acceptance are the minimums on the EN 10204 certificate and the governing ASTM product standard, and those minimums vary with section size and temper. The point for procurement is that 625’s higher strength lets a smaller section carry the same load, but it also means 625 cannot simply be swapped into a design that was analysed for 600 without re-checking stresses — the part may be over-designed and over-costed, or, if the swap went the other way, under-designed and unsafe. Either direction is a specification question, not a brand question.

When to Choose 600 vs 625

Quick answer: Choose Inconel 600 for furnace and heat-treating components, nuclear steam-generator tubing, caustic soda service and duties where chloride stress-corrosion cracking is the dominant threat; choose Inconel 625 for seawater, sour-gas and acidic-chloride service, high-strength fasteners and shafts, and aerospace structures where strength and fatigue resistance matter as much as corrosion.

Where the environment is genuinely aggressive, the PREN and pitting-resistance guide shows why 625’s molybdenum-and-niobium chemistry pushes its resistance so far above 600’s. For heat-exchanger tubing specifically, our heat-exchanger tube selection guide places the two grades against the other candidate alloys.

Corrosion Performance in the Real Service Environments

Quick answer: Inconel 600 resists oxidation at high temperature and resists chloride-induced stress-corrosion cracking, and it handles caustic soda well; it is, however, vulnerable to pitting and crevice attack in acidic chloride and to attack by reducing acids. Inconel 625 resists oxidizing and reducing acids, seawater pitting and crevice corrosion far better because of its molybdenum and niobium, though neither alloy is a substitute for a dedicated molybdenum-rich grade in the most aggressive reducing acids.

The single most useful number here is PREN. Crude pitting-resistance equivalent — PREN ≈ %Cr + 3.3×%Mo + 16×%N — puts 600 in the low-to-mid 30s and 625 in the high-40s, a gap large enough to decide whether a flange survives a tidal zone. Inconel 600’s strength is its resistance to chloride stress-corrosion cracking and its behaviour above 1000°C, where 625’s strength starts to soften. Inconel 625’s strength is its all-round acid and seawater resistance and its higher mechanical load capacity.

Buyers should also weigh the acids they actually face. Inconel 600 is the traditional choice for caustic soda service and does well in many non-oxidizing environments, while 625 is preferred where hydrochloric, phosphoric or mixed acids dominate and where chlorides are present. The hydrochloric-acid selection guide and the Hastelloy C-276 vs Inconel 625 comparison put 625 in context against the more acid-resistant grades when the duty is strongly reducing.

Procurement Pitfalls: Grade Swaps, Wrong Filler and Over-Specifying

Quick answer: The three ways this comparison becomes a loss are (1) 600 shipped or invoiced as 625, or vice versa, on a certificate that only a chemist would question; (2) welding with the wrong filler — ERNiCr-3 (82) for 600 but ERNiCrMo-3 (625) for 625 — so a correct base metal is undermined at the joint; and (3) over-specifying 625 where 600 would have done, quietly inflating cost with no service benefit.

None of these shows up on a visual inspection, which is exactly why the ppm-level chemistry line and the welding-procedure clause matter. The procurement pitfalls buyer’s guide collects the cross-cutting purchase-order clauses that make a result like this auditable before you accept the shipment, and our nickel-alloy welding guide covers filler selection in more depth.

Which standards cover Inconel 600 vs 625?

Quick answer: Inconel 600 is covered by ASTM B168 (plate/sheet/strip), B166 (rod/bar), B167 (seamless pipe/tube), B163 (condenser & heat-exchanger tube) and B516/B517 (welded tube); Inconel 625 by ASTM B443 (plate/sheet), B446 (rod/bar), B444 (seamless pipe/tube) and B704/B705 (welded tube). Welding uses ERNiCr-3 (82) filler for 600 and ERNiCrMo-3 (625) filler for 625, both per AWS A5.14; UNS numbers are N06600 and N06625.

Inconel is a registered trademark of Special Metals Corporation; the alloys here are defined by their UNS numbers and the ASTM standards above. Always cite the UNS and the ASTM product standard with its edition year on the order, because temper definitions and tolerances shift between revisions and “Inconel” alone is not a specification.

Pre-Order Checklist

• Name the UNS explicitly — N06600 or N06625 — not just the trade name.
• State the form and the ASTM product standard with its edition year.
• Require the molybdenum and niobium lines on the mill test certificate for any 625 order.
• Specify the welding filler: ERNiCr-3 for 600, ERNiCrMo-3 for 625, on the WPS.
• Require EN 10204 3.1 (or 3.2 with a third party) and full heat traceability to the melt.
• Match the alloy to the duty using the selection table before approving the quote.
• For seawater or sour service, confirm PREN and chloride resistance expectations in writing.
• Plan an arrival verification — PMI or wet chemistry — before the material is cut or welded.

FAQ

What is the main difference between Inconel 600 and 625?

The decisive difference is alloy content beyond chromium. Inconel 600 (UNS N06600) is nickel-chromium-iron with no deliberate molybdenum or niobium, while Inconel 625 (UNS N06625) adds roughly 8–10% molybdenum and 3.1–4.1% niobium plus tantalum. Those additions raise 625’s strength and its resistance to pitting, crevice corrosion and many acids, while 600 keeps an edge in very high-temperature oxidation and in resistance to chloride stress-corrosion cracking. If a certificate for “625” shows no molybdenum and no niobium, you are almost certainly holding 600.

Is Inconel 625 stronger than 600, and by how much?

Yes, meaningfully. In the annealed condition 625 typically shows a tensile strength around 830–1050 MPa (120–150 ksi) and a yield near 415 MPa (60 ksi), against roughly 550–655 MPa (80–95 ksi) tensile and 240 MPa (35 ksi) yield for 600 — a yield roughly 70% higher. Density is nearly identical at about 8.4–8.5 g/cm³. These are typical orientation values; your enforceable numbers are the minimums on the EN 10204 certificate and the governing ASTM standard. The strength gap is exactly why 625 must be verified as the named grade rather than assumed from a similar-looking bar.

Can Inconel 600 be used in seawater like 625?

Not as well. Inconel 600 resists chloride stress-corrosion cracking, which is valuable, but it lacks the molybdenum and niobium that give 625 its strong resistance to seawater pitting and crevice corrosion. In a tidal zone, splash zone or stagnant seawater, 600 is far more likely to develop localized pitting than 625, whose PREN sits in the high-40s. For serious marine duty, 625 (or a higher-molybdenum grade) is the safer choice; 600 belongs in that service only where the design has accounted for its weaker localized-corrosion resistance. Our PREN guide explains the rating.

How do I verify I received 625 and not 600?

The fastest check is chemistry: 625 must report molybdenum in the 8–10% range and niobium plus tantalum around 3.15–4.15%, whereas 600 reports neither. A handheld XRF or a laboratory wet-chemistry and optical-emission check against the EN 10204 certificate will show the difference immediately, and positive material identification (PMI) at goods-in catches a downgrade before the bar is cut. The procurement pitfalls guide lists the purchase-order clauses — naming the UNS, requiring Mo/Nb on the MTC, heat traceability — that make this auditable.

Do 600 and 625 use the same welding filler?

No. Inconel 600 is welded with ERNiCr-3 (82) filler and Inconel 625 with ERNiCrMo-3 (625) filler, both per AWS A5.14. Using 82 filler on a 625 joint dilutes the molybdenum at the weld and lowers corrosion resistance exactly where it matters most, so the filler must be named on the welding procedure specification. Both alloys still need the usual nickel-alloy discipline of clean joints, low heat input and controlled interpass temperature. Our nickel-alloy welding guide covers the procedure detail buyers should reference.

Which is better for high-temperature furnace service, 600 or 625?

Inconel 600 is generally the better choice for continuous high-temperature furnace service above roughly 1000°C, where its oxidation resistance and lower cost suit trays, mufflers and radiant tubing; 625 is strong but its mechanical advantage fades at the top of that range and it costs more. Below about 980°C, 625’s strength can be useful, but for pure heat-resistance duty 600 is the traditional, economical pick. The Inconel 600 vs 601 oxidation comparison extends the high-temperature picture if you are weighing 600 against 601 instead.

What purchase-order clauses prevent Inconel grade substitution?

Name the UNS (N06600 or N06625) and the ASTM product standard with edition on the order; for 625 require molybdenum and niobium to appear on the mill test certificate; specify the welding filler (ERNiCr-3 for 600, ERNiCrMo-3 for 625) on the WPS; require EN 10204 3.1 or 3.2 with full heat traceability to the melt; and add an arrival-verification step — PMI or wet chemistry — before cutting. Vague “Inconel as required” language is the loophole substitution hides in, so the clauses must be specific and auditable.

Contact Us for Inconel 600 & 625 Selection

Choosing between Inconel 600 and 625 is a duty-and-environment decision, not a brand decision, and the cost of guessing shows up only after the part is in service. Tell us the form, the temperature, the corrosive environment and whether the part will be welded, and we will melt from 100% virgin nickel, chromium and molybdenum feed, refine by VIM plus ESR or VAR, and deliver the exact grade your specification names — with chemistry and mechanical results you can re-verify on arrival.

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