Inconel 600 Plate or Inconel 625 Plate: Which One Do You Actually Pay For?

Start with a number that surprises most buyers: Inconel 600 plate weighs 8.47 g/cm³. Inconel 625 plate weighs 8.44 g/cm³. On a 6 mm × 1500 mm × 3000 mm sheet - 213 kg of metal - the weight difference between the two grades is under half a kilogram. Yet the invoice for the 625 plate is routinely two to three times higher. That gap is not a density story; it is a molybdenum-and-niobium story. And whether that story pays for itself depends entirely on what your plate has to survive.

This article compares the two plates the way a fabricator should compare them: by chemistry that actually drives performance, by the strength you can use to make the plate thinner, and by the price per finished square meter - not per kilogram.

1. The chemistry that separates them

Both are nickel-chromium alloys, but 625 carries three alloying elements that 600 simply does not have.

The causal chain is short and worth memorizing. Molybdenum stabilizes the passive film against chloride attack - it is the single reason 625 plate survives seawater and acidic brines that pit 600 plate.

Niobium contributes almost nothing to corrosion but a great deal to strength: combined with Mo, it lifts the minimum yield strength of annealed 625 plate to 40 ksi against 35 ksi for annealed 600 plate under ASTM B168, and the typical annealed yield is closer to 414 MPa versus 240 MPa in practice.

2. What ASTM says about each plate

Both plates are bought to ASTM B168 / ASME SB-168 (plate, sheet and strip). The mechanical minima differ:

*Typical mill values; always confirm against the actual B168 revision you order to.

One more spec note buyers miss: B168 defines plate as material 4.76 mm (0.188 in) and over, with the same sheet/plate boundary logic B127 uses for Monel 400. If your drawing calls for anything below that, you are buying sheet, and the tolerances change.

3. The arithmetic buyers skip: thinner plate from higher yield

Here is the calculation almost no competing article runs. Suppose you need a flat bulkhead or liner plate designed to a fixed load where 600 plate at 12 mm does the job with its 240 MPa yield. Substitute 625 plate and the same load needs roughly 12 × (240/414) ≈ 7 mm. You have cut the cross-section by 42%.

Now price it. Per kilogram, 625 plate typically runs 2–3× the 600 plate price, driven by the Mo/Nb raw-material surcharge. But per square meter of finished plate:

Even with the aggressive thickness reduction, 625 plate still costs more per square meter for a pure strength swap. The premium only pays back when the environment demands Mo - chlorides, seawater, wet acid gases. If your 600 plate is failing by pitting, the swap is justified. If it is failing by nothing at all and you simply want a "better" grade, the arithmetic says keep the 600.

4. Where each plate actually belongs

Inconel 600 plate: caustic evaporator internals and vessels, furnace muffles and radiant-tube supports where oxidation resistance dominates, and high-purity water service. Its 72% nickel floor is the reason it shrugs off chloride-SCC in the right chemistries.

Inconel 625 plate: seawater-exposed fabrications, offshore cladding backing plate, chemical-process equipment with mixed acid/chloride streams, and any weld-overlay program - 625 is simultaneously a plate product and the world's most-used overlay filler chemistry (ERNiCrMo-3), which keeps its supply chain deep and pricing competitive for a Mo-bearing grade.

A boundary worth stating plainly: in stagnant crevices or under deposits at elevated temperature, 625's Mo advantage narrows; and in dry, high-temperature oxidation above roughly 1000 °C, 600's longer furnace-service record still matters. Neither plate is a universal upgrade over the other - they are tuned for different failure modes.

5. Fabrication notes that affect the invoice

Both plates weld readily; for 600 use ERNiCr-3, for 625 use ERNiCrMo-3. Cross-welding a 600 plate with 625 filler is common and sound.

 

625 plate work-hardens faster - budget for slower feeds and more rigid tooling when machining nozzles or bolting holes.

 

Both are supplied solution-annealed; 625 plate ordered in Grade 1 (annealed) versus Grade 2 changes the strength minimums, so state the grade on the PO, not just "Inconel 625".

6. Quick decision boundary

FAQ

Q1: Is Inconel 625 plate always better than 600 plate?

No. 625 wins in localized corrosion and strength; 600 keeps the edge in high-temperature oxidation service and costs far less per kilogram.

Q2: Can I order 600 and 625 plate to the same standard?

Yes - both are ASTM B168 / ASME SB-168. Note B168's plate/sheet boundary at 4.76 mm and the 625 Grade 1/Grade 2 strength split.

Q3: Why is 625 plate so much more expensive if it weighs the same?

The Mo (8–10%) and Nb (3.15–4.15%) raw-material surcharge. Weight is identical; alloy content is not.

Q4: Can 625 plate replace 12 mm 600 plate at 7 mm thickness?

Only where the design is yield-driven and code allowables permit. Per-m² cost usually still favors 600 when corrosion is not the driver.

Q5: Which filler wire matches each plate?

600 plate: ERNiCr-3 (82). 625 plate: ERNiCrMo-3 (625).

CTA

HUITONG (HT PIPE) supplies both plates to ASTM B168 with full traceability and cut-to-size service. For German fabricators and EPC buyers, send your drawing to market@htpipe.com - we will run the thickness-and-cost comparison on your actual plate sizes before you commit to a grade.

Contact now