01 Why This Comparison Matters
Procurement engineers ordering nickel alloy forgings face a version of this question constantly: the project spec says "nickel alloy forging, corrosion-resistant grade," and three alloys appear on the shortlist. The wrong choice ranges from a modest material premium to a catastrophic field failure in a sour service wellhead.
Alloy 625 (2.4856 / UNS N06625): Ni-Cr-Mo-Nb alloy. Maximum corrosion capability. PREN ~51. For extreme chloride, deepwater, and ultra-sour service. Cost index: ●●●●●
Alloy 718 (2.4668 / UNS N07718): Precipitation-hardened Ni superalloy. Minimum tensile 1035 MPa (aged). For high-strength structural applications, not corrosion resistance. Cost index: ●●●●●
This guide compares the three alloys most commonly specified for industrial open die forgings: 2.4858 (Alloy 825 / UNS N08825), Alloy 625 (2.4856 / UNS N06625), and Alloy 718 (2.4668 / UNS N07718). Each serves a fundamentally different engineering purpose — understanding that distinction, rather than chasing the highest nickel content, is the key to optimal material selection. If you have already confirmed that 2.4858 is the right choice and need to source 2.4858 custom forging parts, see the product page linked below for specifications and quote request.
02 Chemical Composition Compared
Composition determines everything downstream — corrosion resistance, strength mechanism, weldability, and price. The three alloys share a nickel base but diverge dramatically in their alloying strategies.
| Element | 2.4858 / Alloy 825 | Alloy 625 | Alloy 718 | Role |
|---|---|---|---|---|
| Nickel (Ni) | 38–46% | ≥ 58% | 50–55% | Matrix; SCC immunity |
| Chromium (Cr) | 19.5–23.5% | 20–23% | 17–21% | Passive film; oxidation |
| Molybdenum (Mo) | 2.5–3.5% | 8–10% | 2.8–3.3% | Pitting resistance (PREN) |
| Niobium (Nb) | — | 3.15–4.15% | 4.75–5.5% | Precipitation hardening |
| Copper (Cu) | 1.5–3.0% | ≤ 0.5% | — | Reducing acid resistance |
| Titanium (Ti) | 0.6–1.2% | ≤ 0.4% | 0.65–1.15% | Stabilization / aging |
| Iron (Fe) | Balance | ≤ 5% | Balance | Matrix filler / cost |
| Carbon (C) | ≤ 0.025% | ≤ 0.10% | ≤ 0.08% | Low = less sensitization |
2.4858 uniquely adds Cu for reducing acid resistance. Alloy 625 loads Mo (9%) and Nb for maximum corrosion and solid-solution strength. Alloy 718 uses Nb + Ti + Al for precipitation hardening (γ″/γ' phases) — its primary purpose is high strength, not corrosion resistance.
03 Mechanical Properties
The three alloys represent three distinct strength tiers. This matters not just for the design safety margin, but for the cost-weight trade-off in heavy forgings where material cost scales with weight.
| Property | 2.4858 / Alloy 825 | Alloy 625 (Ann.) | Alloy 718 (Aged) |
|---|---|---|---|
| Tensile Strength (Rm) | ≥ 585 MPa | ≥ 830 MPa | ≥ 1035 MPa (1380 typ.) |
| 0.2% Proof Stress (Rp0.2) | ≥ 240 MPa | ≥ 415 MPa | ≥ 862 MPa |
| Elongation (A5) | ≥ 30% | ≥ 30% | ≥ 12% |
| Charpy Impact (−196°C) | ≥ 60 J | Excellent | Moderate |
| Density | 8.14 g/cm³ | 8.44 g/cm³ | 8.19 g/cm³ |
| Max Service Temp. | ~450°C | ~600°C | ~650°C (struct.) |
Alloy 718's strength (≥1035 MPa) is only achieved in the precipitation-hardened (aged) condition: solution anneal at 980°C + double age at 720°C/620°C. In the annealed condition alone, strength is similar to Alloy 625. Always specify the delivery condition when ordering Alloy 718 forgings — the difference is dramatic.
04 Corrosion Resistance — The Decisive Factor
2.4858 is best for reducing acids (H₂SO₄, HF, H₃PO₄) — its unique Cu addition gives it an edge no other alloy in this comparison has. Alloy 625 leads on pitting/crevice resistance in chloride media (PREN ~51 vs ~28). Alloy 718 has the lowest corrosion resistance of the three (PREN ~19) — it is not a corrosion-resistant grade.
For most industrial applications, corrosion resistance — not strength — drives alloy selection. The three alloys behave very differently across corrosion modes, and understanding these differences prevents costly misspecification.
2.4858 / Alloy 825 — Reducing Acid Specialist
The copper addition (1.5–3.0%) makes 2.4858 uniquely effective in reducing acid environments — sulfuric acid (H₂SO₄), hydrofluoric acid (HF), and phosphoric acid (H₃PO₄) — where both 316L stainless and Alloy 625 perform poorly. Immune to chloride stress corrosion cracking (Cl⁻ SCC). Qualifies for NACE MR0175 / ISO 15156 sour service. PREN of ~28 is adequate for standard seawater and offshore conditions.
Alloy 625 — Maximum Corrosion Capability
The combination of 9% Mo, 3.5% Nb, and ≥58% Ni delivers the highest general corrosion resistance of the three alloys — PREN of ~51, immunity to Cl⁻ SCC, outstanding pitting and crevice corrosion resistance in high-chloride, high-temperature environments. It is the step-up from 2.4858 when environments exceed 2.4858's corrosion capability: deepwater subsea above 150°C, ultra-sour wells, hot seawater with high velocity, and chemical environments with mixed acid cycles.
Alloy 718 — Structural Grade, Not a Corrosion Alloy
Alloy 718 is frequently misunderstood as corrosion-resistant because of its high nickel content. Its PREN of ~19 is lower than Super Duplex stainless steel. It has no copper addition for reducing acids, and its high iron content reduces overall corrosion performance. Choose Alloy 718 for mechanical performance at elevated temperature — not for chemical or sour gas resistance.
05 Weldability & Fabrication
| Parameter | 2.4858 | Alloy 625 | Alloy 718 |
|---|---|---|---|
| General weldability | Good | Excellent | Good (PWHT req.) |
| Sensitization risk | Low (Ti-stabilized) | Low | Moderate |
| Post-weld anneal needed? | Usually not | Usually not | Yes (re-age for strength) |
| Filler metal (AWS) | ERNiFeCr-1 | ERNiCrMo-3 (625) | ERNiFeCr-2 (718) |
| Hot cracking risk | Low | Low | Moderate (Nb segregation) |
| Weld HAZ corrosion | Maintained (Ti) | Maintained | Reduced vs. base metal |
06 Application Selection Guide
The decision cards below show which alloy is optimal for each application category. For borderline cases, cost, lead time, and your supplier's certification capability become the deciding factors.
Once 2.4858 is confirmed as the right alloy for your service conditions, the next step is production. Our 2.4858 open die forgings product page covers the full product range — forged bars, seamless rolled rings, tube sheets, valve bodies, shafts, and more — with a direct quote form and 24-hour response.
07 International Standards
| Standard | 2.4858 / Alloy 825 | Alloy 625 | Alloy 718 |
|---|---|---|---|
| EN designation | 2.4858 / NiCr21Mo | 2.4856 / NiCr22Mo9Nb | 2.4668 / NiCr19Fe19Nb5Mo3 |
| UNS designation | N08825 | N06625 | N07718 |
| ASTM forgings | B564 | B564 | B637 |
| ASTM bar / rod | B425 | B446 | B637 |
| ASME pressure vessels | SB-564 | SB-564 | SB-637 |
| Sour service | NACE MR0175 | NACE MR0175 | NACE MR0175 (limited) |
| API 6A | Approved | Approved | Approved (select grades) |
| AMS (aerospace) | — | AMS 5666 | AMS 5662 / 5664 |
08 Cost & Lead Time
Material selection cannot ignore economics. The price difference between these alloys is significant enough to materially change the project budget for large or multiple forgings.
2.4858 / Alloy 825
Premium vs. 316L stainless: ~3–5×
Forging lead time: 20–35 working days
Ingot stock availability: Good — most forge shops stock
Alloy 625
Premium vs. 2.4858: 2–3×
Forging lead time: 25–45 working days
Ingot stock availability: Good — widely stocked globally
Alloy 718
Premium vs. 2.4858: 2–3× (plus aging heat treatment cost)
Forging lead time: 30–50 working days (incl. double aging)
Ingot stock availability: Good for standard; variable for AMS cert.
// All three available: open die forgings 30 kg – 30 tons, Jiangsu Liangyi Co., Limited
If 2.4858 meets the technical specification, the premium for Alloy 625 or 718 adds cost without engineering value. Over-specifying nickel alloy grade is as costly an error as under-specifying. Confirm the environment first — not the alloy prestige.
09 The 60-Second Decision Framework
High strength >800 MPa at >500°C → Alloy 718 | Reducing acids (H₂SO₄/HF/H₃PO₄) → 2.4858 | Extreme chloride / deepwater >150°C → Alloy 625 | Standard sour/corrosive service → 2.4858 (optimal cost)
Answer these four questions in order. Stop at the first question where the answer uniquely identifies one alloy.
- Is the primary requirement high strength (>800 MPa) at elevated temperature (>500°C)?
Yes → Alloy 718. No → Continue. - Does the environment involve reducing acids (H₂SO₄, HF, H₃PO₄) as the primary corrosive?
Yes → 2.4858 (unique Cu addition). No → Continue. - Does the spec exceed 2.4858's corrosion capability? (T >150°C in chloride, PREN >28 needed, ultra-sour H₂S >10%, deepwater subsea)
Yes → Alloy 625. No → Continue. - Does the environment fall within 2.4858's qualified range (NACE MR0175, standard seawater, moderate chloride, sour service)?
Yes → 2.4858 — optimal cost-performance choice.
In over 25 years supplying nickel alloy forgings globally, the vast majority of applications that go to alloy selection review land on 2.4858 as the optimal choice — providing sufficient corrosion resistance at the lowest material cost and shortest lead time. Alloy 625 is the right step-up when 2.4858 genuinely cannot meet the environmental demands of the service.
10 Frequently Asked Questions
2.4858 (Alloy 825 / UNS N08825) contains 38–46% Ni and provides excellent corrosion resistance in reducing and oxidizing acid environments at moderate cost. Alloy 625 (2.4856 / UNS N06625) contains over 58% Ni plus 9% Mo and 3.5% Nb, giving significantly higher corrosion resistance (PREN ~51 vs ~28), higher elevated-temperature strength, and deeper immunity to pitting and crevice corrosion — at 2–3× higher cost. Choose 2.4858 for standard corrosive environments; choose Alloy 625 for extreme chloride, deepwater offshore, or high-temperature service above 450°C.
No — they solve different problems. Alloy 718 (2.4668 / UNS N07718) is a precipitation-hardened nickel superalloy optimized for high-temperature structural applications with tensile strength up to 1380 MPa. Its corrosion resistance is significantly lower than 2.4858 — PREN of ~19 vs ~28, no copper addition for reducing acids, and higher susceptibility to pitting. Alloy 718 is chosen for aerospace, gas turbines, and downhole tooling where mechanical strength is paramount — not for acid or sour gas corrosion resistance.
2.4858 (Alloy 825) is the most cost-effective for general corrosion-resistant applications — typically 2–3× cheaper per kg than Alloy 625 or Alloy 718. Alloy 625 and 718 are comparable in price to each other but serve different purposes. Over-specifying to Alloy 625 when 2.4858 meets the technical requirement adds material cost with no engineering benefit. Alloy 718 also adds the cost of a precipitation hardening heat treatment (double aging cycle) on top of the material premium.
2.4858 (Alloy 825) is the industry-standard choice for oil and gas sour service (H₂S + Cl⁻ environments) per NACE MR0175 / ISO 15156 at standard wellhead conditions — wellhead components, casing heads, tubing spools, valve bodies, and DSA flanges. For more aggressive conditions — deepwater above 150°C, ultra-sour HPHT wells, or subsea equipment with extreme pressure and chloride — Alloy 625 provides the additional corrosion margin. Alloy 718 is used for downhole mechanical components (drill collars, packers, mandrels) where high strength under HPHT loading is the primary requirement, not corrosion resistance.
11 Summary
Three alloys, three distinct value propositions. The selection is rarely about which alloy is "better" — it is about which alloy is right for the specific engineering problem:
- 2.4858 (Alloy 825): The rational default for corrosion-resistant industrial forgings. Unique copper addition gives an edge in reducing acid environments. Cost-effective, well-standardized, NACE-qualified. Use it whenever it qualifies technically.
- Alloy 625: When the corrosion bar is higher than 2.4858 can clear — deepwater, ultra-sour, extreme chloride at temperature. The premium is warranted; do not apply it where 2.4858 suffices.
- Alloy 718: When strength is the design driver, not corrosion. A fundamentally different engineering context — do not specify it as a "premium" corrosion-resistant alloy.
For engineering consultation on alloy selection for your specific application — including free technical review of drawings and specifications — the team at Jiangsu Liangyi Co., Limited responds within 24 hours. All three alloys are available in open die forgings from 30 kg to 30 tons. When your selection process confirms 2.4858 as the right grade, see our custom 2.4858 (NiCr21Mo) forgings page for product range, EN 10204 MTC, and quote submission. For the complete 2.4858 material reference, see: What Is 2.4858 / Alloy 825? A Complete Material Guide for Engineers.
Need 2.4858, Alloy 625, or Alloy 718 Forgings?
Send your drawing, specification, and application details — our engineering team confirms the optimal alloy grade and responds with a competitive quotation within 24 hours. No commitment required.