If you are specifying a nickel alloy for a corrosive, high-pressure, or high-temperature forging application, there is a very high probability that Inconel 625 — formally designated UNS N06625, also called Alloy 625 or W.Nr. 2.4856 — will appear at the top of your shortlist. After two decades of manufacturing Inconel 625 open-die forgings and seamless rolled rings for customers in more than 50 countries, our engineering team has accumulated detailed, production-proven knowledge of this material: its behavior on the press, its response to heat treatment, and the failure modes that arise when specifications are misapplied.

This guide consolidates that field experience into a single authoritative reference. Unlike a generic datasheet, it covers forging-specific process parameters, quality control thresholds that most suppliers never publish, and practical selection guidance that helps you make the right material decision before issuing your purchase order. For full manufacturing capabilities, size range, and RFQ, see our custom Inconel 625 forgings and seamless rolled rings product page.

Who This Guide Is ForMechanical and metallurgical engineers, procurement specialists, and project managers who need to specify, procure, or qualify Inconel 625 (UNS N06625) forgings for oil & gas, subsea, aerospace, chemical processing, nuclear, or marine applications.

1. Definition and Designation of Inconel 625 (UNS N06625)

Inconel 625 is a nickel-chromium-molybdenum-niobium superalloy belonging to the solid-solution-strengthened family of nickel-based alloys. Unlike precipitation-hardened grades such as Inconel 718, its exceptional strength — maintained from cryogenic temperatures up to 982°C — derives primarily from the solid solution hardening effect of molybdenum and niobium, not from a secondary phase precipitate. This distinction has enormous practical consequences for forging, heat treatment, and weldability.

The same alloy is known by multiple designations worldwide. Procurement engineers must use the UNS number or recognized standard, not the trade name alone, when writing purchase specifications to ensure supplier competition:

Table 1 — Inconel 625 Designation Equivalency by Standard System
Standard SystemDesignationIssuing Body / Region
UNS (Unified Numbering System)N06625ASTM / SAE — USA
W.Nr. (Werkstoffnummer)2.4856DIN — Germany / Europe
ISO Chemical SymbolNiCr22Mo9NbISO — International
Trade name (most common)Inconel® 625Special Metals Corporation
Alternative trade namesHaynes® 625, Nicrofer® 6020, Chronin® 625Various OEMs
Chinese GB standardNS334 (approximate equivalent)GB — China
Procurement Best Practice"Inconel 625" is a registered trademark of Special Metals Corporation. Always reference the UNS number N06625 combined with a recognized material standard (e.g., ASTM B564 or AMS 5666) in purchase orders to ensure supplier competition and avoid restricting procurement to a single brand.

2. Chemical Composition of Inconel 625 (ASTM B564)

The performance envelope of Inconel 625 is defined first by its chemistry. ASTM B564 — the primary standard for nickel alloy forgings — sets the following compositional limits for UNS N06625. Every element has a specific metallurgical function:

Table 2 — Chemical Composition of Inconel 625 per ASTM B564 / UNS N06625 (weight %)
ElementSymbolMin %Max %Metallurgical Function
NickelNi58.0—Base element; austenitic matrix stability, corrosion baseline
ChromiumCr20.023.0Passivating oxide layer; oxidation and hot corrosion resistance
MolybdenumMo8.010.0Solid solution strengthening; chloride pitting resistance
Niobium + TantalumNb+Ta3.154.15Grain boundary stabilization; additional solid solution hardening
IronFe—5.0Residual; controlled to limit phase instability
CarbonC—0.10Low carbon prevents sensitization (M₂₃C₆ precipitation)
ManganeseMn—0.50Deoxidizer during melting
SiliconSi—0.50Deoxidizer; slight oxidation resistance benefit
PhosphorusP—0.015Impurity; controlled — segregates to grain boundaries
SulfurS—0.015Impurity; critical to control for hot ductility in forging
AluminumAl—0.40Minor solid solution contributor; oxide former
TitaniumTi—0.40Minor grain refinement; combines with C and N
CobaltCo—1.0Residual from raw materials; controlled

The three elements that define Inconel 625's performance are nickel (austenitic matrix and corrosion baseline), chromium (oxidation and hot corrosion protection), and the molybdenum-niobium combination — which together provide solid-solution hardening and resistance to chloride pitting and crevice corrosion that few alternative alloys can match at comparable cost.

Figure 1 — Inconel 625 Principal Element Composition (nominal weight %)

Nickel (Ni)
~62%
Chromium (Cr)
~21.5%
Molybdenum (Mo)
~9%
Niobium (Nb)
~3.6%
Iron (Fe)
≤5%

3. Mechanical Properties of Inconel 625 Forgings

ASTM B564 recognizes two heat treatment conditions for Inconel 625 forgings: annealed (solution treated) and hot-finished. Aerospace forgings under AMS 5666 always use the annealed condition with tighter tensile scatter and 100% ultrasonic testing requirements.

Table 3 — Minimum Mechanical Properties of Inconel 625 Forgings per ASTM B564 (Room Temperature, 21°C)
PropertyAnnealed ConditionHot-Finished ConditionUnit
Tensile Strength (UTS)≥ 827≥ 896MPa (min)
0.2% Offset Yield Strength≥ 414≥ 517MPa (min)
Elongation (2 in gauge length)≥ 30≥ 30% (min)
Reduction of Area≥ 35≥ 35% (min)
Hardness (typical range)≤ 25 HRC≤ 35 HRCRockwell C
Charpy Impact (typical)> 100 J at −196°C> 80 J at −196°CJ (Charpy V-notch)
Tensile Strength
≥ 827 MPa
Annealed — ASTM B564
Yield Strength
≥ 414 MPa
0.2% offset, annealed
Max Service Temp.
982 °C
Oxidation-limited in air
Density
8.44 g/cm³
Nominal value
Critical: Heavy-Section Hardness and NACE MR0175The NACE MR0175 / ISO 15156 Part 3 hardness limit of 35 HRC applies to the entire finished forging cross-section, not only the surface. For forgings exceeding 150 mm in diameter, a part measuring 30 HRC at the surface may still register 38 HRC at the core if the solution annealing cycle is not tailored to the actual section size. Always request hardness mapping data at surface, T/4, and T/2 depth positions from your supplier. Jiangsu Liangyi conducts this test as standard for all NACE-critical heavy-section forgings.

4. Physical and Thermal Properties

Table 4 — Physical and Thermal Properties of Inconel 625 (UNS N06625) — Typical Values
PropertyValueUnitNote
Density8.44g/cm³At room temperature
Melting Range1290 – 1350°CSolidus to liquidus
Specific Heat (20 °C)410J/kg·K—
Thermal Conductivity (20 °C)9.8W/m·K~⅓ of carbon steel
Thermal Expansion (20–100 °C)12.8µm/m·°C—
Modulus of Elasticity (RT)207GPa—
Electrical Resistivity1.29µΩ·m—
Magnetic Permeability~1.0006µEssentially non-magnetic

The relatively low thermal conductivity (9.8 W/m·K versus ~50 W/m·K for carbon steel) has a direct forging implication: heat does not propagate from the billet surface to the center rapidly. Heavy-section billets therefore require carefully controlled soak times at forging temperature to achieve thermal equilibration before the first press reduction. Skipping adequate soak is one of the most common root causes of center-burst defects in large Inconel 625 forgings.

5. Corrosion Resistance of Inconel 625

Inconel 625's corrosion resistance is multi-mechanism. Identifying which mechanism is active in your service environment is essential for correctly specifying this alloy versus alternatives.

5.1 Chloride Pitting and Crevice Corrosion

The molybdenum content (8–10 wt%) drives the alloy's exceptional resistance to chloride-induced pitting and crevice corrosion. The pitting resistance equivalent number (PREN) of Inconel 625 is approximately 51 — far superior to 316L stainless steel (~24) or duplex 2205 (~35). The critical pitting temperature (CPT) in 10% ferric chloride solution exceeds 85°C. This makes Inconel 625 the dominant choice for subsea wellhead components, risers, flexible pipe end fittings, and umbilicals exposed to chloride-containing production fluids.

5.2 Acid Resistance

Chromium content provides resistance to oxidizing acids including nitric acid and mixed acid environments. Inconel 625 performs well in dilute sulfuric acid (below 50% concentration at up to ~65°C), phosphoric acid, and moderate concentrations of hydrofluoric acid. Concentrated sulfuric acid above 70% or hydrochloric acid above 20% should be evaluated on a project-specific basis with coupon testing.

5.3 Sour Service — Hydrogen Sulfide (H₂S)

Compliance with NACE MR0175 / ISO 15156 Part 3 qualifies Inconel 625 forgings for sour gas service environments containing H₂S. The governing acceptance criterion is the 35 HRC hardness limit across the entire cross-section — not tensile strength. This is why hardness mapping at depth (T/4 and T/2 positions) is mandatory for heavy-section NACE-critical forgings, not just surface hardness testing.

5.4 Seawater and Marine Environments

Long-term immersion testing confirms Inconel 625 is essentially immune to general corrosion in clean natural seawater. In polluted seawater containing sulfur-reducing bacteria (SRB), the alloy shows significantly better resistance than 6Mo stainless steels and duplex grades. Cathodic protection or antifouling coatings are still recommended for permanently submerged structures exposed to severe microbiologically influenced corrosion (MIC) risk.

6. Forging Behavior and Process Parameters for Inconel 625

Inconel 625 is rated as a moderately forgeable nickel alloy — more workable than precipitation-hardened grades such as Inconel 718 or Waspaloy, but significantly more demanding than carbon or low-alloy steels. The following parameters reflect Jiangsu Liangyi's production experience across thousands of forgings from 10 kg tooling components to 8,000 kg subsea valve bodies.

01
Billet Inspection & PMI Verification
All incoming Inconel 625 bar or bloom stock undergoes 100% ultrasonic inspection per ASTM A388. Chemical positive material identification (PMI) is performed at lot level using X-ray fluorescence (XRF) or optical emission spectrometry (OES). Any heat with Mo or Nb outside the ASTM B564 compositional window is quarantined and returned before forging begins.
02
Controlled Heating and Thermal Soaking
Billets are charged to the forge furnace at a controlled rate below 150°C/hr above 650°C to avoid thermal shock cracking. Soak time at the target forging temperature must allow full thermal equilibration to the billet center — a minimum of 1 hour per 100 mm of cross-section diameter for heavy billets.
🌡 Soak Temperature: 1100–1200 °C
03
Open-Die Forging or Ring Rolling
The hot-working window for Inconel 625 is 950°C (finish lower limit) to 1200°C (start upper limit). Working below 950°C promotes delta (δ) phase precipitation that embrittles grain boundaries and degrades toughness. For complex multi-heat shapes, reheating above 1120°C is required after each significant press reduction. Die wear rate for Inconel 625 is approximately 2× that for equivalent-weight carbon steel forgings.
⚙️ Working Window: 950 – 1200 °C
04
Intermediate Dimensional & Surface Inspection
Dimensional checks are performed between forging operations for near-net shapes. Surface cracking — which can initiate from adiabatic shear bands at high strain rates — is identified and dressed before reheating to prevent defect propagation into the finished part.
05
Solution Annealing Heat Treatment
After forging, the part is solution annealed to dissolve secondary phases formed during slow cooling and to achieve the annealed microstructure required by ASTM B564 and NACE MR0175. For NACE-critical forgings, rapid quench after annealing is non-negotiable.
🔥 Anneal: 1093–1149 °C → rapid air or water quench
06
Post-Forge Testing and Certification
Full test coupon program per order specification: tensile testing (UTS, yield, elongation, RA), Charpy impact (at required temperature), hardness mapping (surface / T/4 / T/2 depth), grain size per ASTM E112, and ultrasonic re-inspection of the finished forging per ASTM A388 or customer UT procedure. MTR issued per EN 10204-3.1 or witnessed per EN 10204-3.2.

7. Heat Treatment of Inconel 625 Forgings

Unlike nickel superalloys that rely on precipitation hardening (e.g., Inconel 718 with its γ' and γ'' phases), Inconel 625 relies on solid solution strengthening. A single-step solution anneal is the standard heat treatment for forgings; no aging cycle is required or beneficial for standard structural and corrosion-service applications.

Table 5 — Heat Treatment Conditions for Inconel 625 (UNS N06625) Forgings
ConditionTemperatureCooling MethodPrimary Result / Application
Solution Anneal — Grade 1 (ASTM B564)1093 °C minimumRapid air or water quenchMaximum corrosion resistance; NACE MR0175 compliant
Solution Anneal — Grade 2 (ASTM B564)980–1038 °CRapid coolSlightly higher hardness; non-sour service applications
Post-Weld Stress Relief870–980 °C, 1 hour minimumAir coolReduces residual weld stress without over-softening
Stabilization Anneal (special cases)843 °C, 4–8 hoursAir coolStabilizes carbides before service at 593–760 °C
Sensitization Risk: The 600–800°C Danger ZoneExtended exposure in the 600–800°C temperature range promotes the precipitation of M₂₃C₆ carbides at grain boundaries and potentially delta (δ) phase, reducing ductility and toughness. Forgings intended for service at these temperatures, or forgings that will cool slowly through this range during heat treatment, must have their thermal cycle reviewed by a metallurgist — especially for cross-sections above 500 mm in diameter where natural cooling rates are slow.

8. Applicable Standards and Specifications for Inconel 625 Forgings

The specification cited in your purchase order controls the test requirements, dimensional tolerances, traceability documentation, and acceptance criteria. The following standards most frequently govern Inconel 625 (UNS N06625) forged components:

Table 6 — Key Standards for Inconel 625 (UNS N06625) Forgings
StandardFull Title / ScopeKey Requirements
ASTM B564Nickel Alloy Forgings — commercial gradeComposition, UTS/YS/elong./RA, hardness; MTR 3.1 standard
AMS 5666Aerospace Nickel Alloy Forgings — UNS N06625Tighter composition limits; grain size per AMS 2643; 100% UT; MTR 3.2 required
API 6A (PR2)Wellhead and Christmas Tree EquipmentPSL 3/4 material requirements; hardness by zone; heat traceability
API 17DSubsea Wellhead and Tree EquipmentExtended traceability; third-party inspection; serialized forgings
NACE MR0175 / ISO 15156 Part 3Sour Service — Nickel AlloysMaximum 35 HRC at any cross-section location; per-lot hardness; H₂S partial pressure limits
ASTM A388Ultrasonic Examination of Heavy Steel Forgings100% volumetric scan; calibration reflectors; reflector acceptance by class (A/B/C/D/E)
ASTM E112Grain Size DeterminationComparative or intercept methods; ASTM No. 5 or finer typical for Inconel 625
EN 10204-3.1 / 3.2Material Test Reports3.1 = manufacturer-certified; 3.2 = third-party witnessed and countersigned
Jiangsu Liangyi Compliance CapabilitiesJiangsu Liangyi Co., Limited manufactures Inconel 625 forgings to ASTM B564, AMS 5666, API 6A / 17D, and NACE MR0175. We issue MTRs per EN 10204-3.1 as standard and EN 10204-3.2 (third-party witnessed) on request. Class society witnessing (DNV, BV, Lloyd's, ABS) is available. See our our product page for the full quality documentation list.

9. Industrial Applications of Inconel 625 Forgings

9.1 Oil and Gas — Upstream and Subsea

This is the dominant market for Inconel 625 forgings by volume. HPHT (high-pressure, high-temperature) conditions — pressures above 690 bar and temperatures above 150°C — combined with H₂S and chloride-laden production fluids create an environment where few engineering alloys can perform economically. Typical forged components include christmas tree bodies, wellhead flanges, BOP (blowout preventer) components, riser connectors, spool pieces, Y-blocks, valve bodies, and nozzles. Browse our full range of Inconel 625 open-die forging capabilities, including NACE MR0175-compliant wellhead and subsea components supplied to operators and EPCs across 50+ countries.

9.2 Chemical Processing

Pressure vessels, reactor components, heat exchanger tube sheets, agitator shafts, and pump impellers in facilities processing organic acids, chlorinated solvents, fertilizers, and specialty chemicals. The alloy's immunity to stress corrosion cracking (SCC) in most aqueous environments — a failure mode common in austenitic stainless steels — makes it the preferred material for high-value reactors operating above 60°C in chloride-containing media.

9.3 Aerospace and Defense

Under AMS 5666, Inconel 625 forgings appear in exhaust system components, thrust reverser structures, fuel line fittings, heat exchanger casings, and rocket motor hardware. The alloy's strength through the 550–750°C range — where many titanium alloys lose their advantage — makes it suitable for firewall-adjacent engine structures and nacelle components that must withstand both thermal and mechanical fatigue.

9.4 Nuclear and Power Generation

Steam generator components in pressurized water reactors (PWR), control rod drive mechanism housings, pressurizer heater sleeves, and primary circuit heat exchanger heads specify Inconel 625 for its resistance to primary water corrosion, resistance to intergranular attack (IGA), and tolerance to radiation damage over decades of service.

9.5 Marine and Offshore Structures

Propeller shafts, seawater pump impellers, tidal turbine hubs, hydrofoil components, and submersible pressure housings leverage the alloy's near-immunity to seawater corrosion and excellent cavitation-erosion resistance. Inconel 625 is also extensively used for thermal spray coatings on marine structural steel in splash zones.

10. Inconel 625 vs. Common Alternative Alloys

Material selection for high-performance forgings requires a multi-criteria evaluation. The table below compares Inconel 625 against its four most frequently specified alternatives, including forging-specific parameters that standard datasheets never address:

Table 7 — Alloy Selection Comparison for Forged Components: Inconel 625 vs. Alternatives
Selection CriterionInconel 625
(N06625)		Hastelloy C-276
(N10276)		Inconel 718
(N07718)		Super Duplex 2507
(S32750)		6Mo Stainless
(S31254)
Pitting Resistance (PREN) ~51 ~70 ~23 ~43 ~46
Heavy Forgeability (>500 kg) Excellent Poor Good Excellent Excellent
High-Temp Strength (>500°C) Good Good Excellent Poor Poor
NACE MR0175 Compliance Yes Yes Yes Limited T No
Weldability Excellent Excellent Good Good Excellent
Relative Material Cost 1.0× (base) 1.4–1.6× 1.1–1.2× 0.5–0.6× 0.7–0.8×
Manufacturer Insight: Why Inconel 625 Beats C-276 for Large HPHT ForgingsHastelloy C-276's high molybdenum content (15–17%) makes it highly susceptible to secondary sigma and mu phase precipitation during the slow cooling of heavy-section forgings exceeding 500 kg — an issue that does not occur with Inconel 625 (only 8–10% Mo). In practice, C-276 is rarely specified for large-diameter wellhead or BOP forgings, even though its pitting resistance numbers look better on a datasheet. Inconel 625 provides the optimal balance of corrosion resistance, heavy-section forgeability, and commercial availability.

11. Weldability of Inconel 625

Inconel 625 is considered one of the most weldable nickel superalloys — a direct result of its solid-solution strengthening mechanism and low carbon content. Key engineering considerations for welded assemblies using Inconel 625 forgings:

  • Recommended filler metal: ERNiCrMo-3 (AWS A5.14) is the matching consumable. It is also the most widely specified overmatching filler for joining Inconel 625 forgings to carbon steel or low-alloy steel piping systems, and for applying corrosion-resistant weld overlays (CRW / CRC) per ASME Section IX.
  • No preheat required for most section thicknesses in normal ambient conditions. A minimum interpass temperature of 10°C is recommended to minimize moisture-related hydrogen porosity risk.
  • Post-weld heat treatment (PWHT): Not mandatory for most corrosion or structural service applications. A stress-relief anneal at 870–980°C for a minimum of 1 hour is applied when weld residual stresses are a concern in high-constraint assemblies such as heavy nozzle-to-shell junctions in pressure vessels.
  • HAZ cracking resistance: Inconel 625 is notably resistant to both strain-age cracking (SAC) and liquation cracking in the heat-affected zone — the two most common weld-related failure modes in precipitation-hardened nickel superalloys such as Inconel 718. This makes Inconel 625 significantly easier to weld in field conditions.
  • Dissimilar metal welding: Inconel 625 / ERNiCrMo-3 is the industry-standard filler for joining stainless steel or low-alloy carbon steel to nickel alloys, and for buttering face sealing areas on steel flanges prior to overlay welding.

12. Frequently Asked Questions — Inconel 625 (UNS N06625)

What is Inconel 625 (UNS N06625) used for?
Inconel 625 (UNS N06625) is used primarily in oil and gas wellhead and subsea components (christmas trees, BOP components, valve bodies, riser connectors), chemical processing equipment (pressure vessels, heat exchangers, reactor components), aerospace structures (exhaust systems, fuel line fittings, rocket hardware), nuclear components (steam generator tube sheets, control rod housings), and marine applications (pump impellers, propeller shafts, pressure housings). Its combination of high strength, outstanding corrosion resistance, and excellent weldability makes it the material of choice wherever conventional stainless steels fail.
What does UNS N06625 mean?
UNS N06625 is the Unified Numbering System designation for Inconel 625, a nickel-chromium-molybdenum-niobium superalloy. The "N" prefix indicates a nickel-based alloy. The same alloy is designated W.Nr. 2.4856 under the German DIN system, NiCr22Mo9Nb under ISO chemical symbolism, and is commercially available as Inconel 625 (Special Metals), Haynes 625, Nicrofer 6020, and other trade names.
What is the maximum service temperature of Inconel 625?
Inconel 625 maintains useful mechanical properties up to approximately 982°C (1800°F). For oxidation resistance in air, the practical upper limit is around 1093°C (2000°F). Yield strength drops significantly above 815°C, and prolonged exposure in the 600–800°C range requires metallurgical evaluation for secondary-phase precipitation (M₂₃C₆ carbides and delta phase) that can reduce toughness.
Is Inconel 625 NACE MR0175 compliant for sour gas service?
Yes, Inconel 625 (UNS N06625) is qualified for sour service under NACE MR0175 / ISO 15156 Part 3, provided the finished forging meets a maximum hardness of 35 HRC throughout the entire cross-section — not only at the surface. For heavy-section forgings exceeding 150 mm in diameter, solution annealing must be tailored to the actual section size, and hardness must be mapped at T/4 and T/2 depth positions to verify NACE compliance at the core.
Is Inconel 625 magnetic?
No. Inconel 625 is essentially non-magnetic with a magnetic permeability of approximately 1.0006 µ — classified as paramagnetic. This property is important for subsea instrumentation housings, flow assurance measurement equipment, and any structural application where magnetism would interfere with sensor accuracy or electromagnetic safety systems.
What is the difference between Inconel 625 and Inconel 718?
Inconel 718 is precipitation-hardened, achieving yield strength ≥1034 MPa through a two-step aging heat treatment involving gamma-prime (γ') and gamma-double-prime (γ'') phases. Inconel 625 is solid-solution strengthened with a minimum yield of 414 MPa (annealed). Key practical differences: Inconel 625 has far superior corrosion resistance (especially in chloride and sour environments), is much easier to weld without HAZ cracking risk, and is more readily forgeable in large heavy sections. Inconel 718 is selected when very high yield strength is the primary driver and corrosion conditions are moderate.
What is the forging temperature range for Inconel 625?
Inconel 625 should be forged between 950°C and 1200°C. The lower limit of 950°C is critical: working below this temperature promotes delta phase (δ) precipitation that embrittles grain boundaries. Billets should be soaked at 1100–1200°C for a minimum of 1 hour per 100 mm of cross-section diameter to ensure full thermal equilibration before pressing. Reheating above 1120°C is required between working steps for multi-heat operations.
What are the lead times for custom Inconel 625 forgings from Jiangsu Liangyi?
Standard lead times at Jiangsu Liangyi Co., Limited are 15–25 days for standard forged bars and simple rings (ASTM B564, EN 10204-3.1 MTC). Complex custom forgings requiring CNC machining typically need 30–45 days. Orders requiring EN 10204-3.2 third-party witnessed inspection (SGS, BV, DNV, Intertek, etc.) add 5–10 working days. Rush production is available with an agreed scheduling fee. Contact us via our our product page for a project-specific schedule and quote.
Trademark Notice: Inconel® is a registered trademark of Special Metals Corporation (a Precision Castparts Corp. company). Hastelloy® is a registered trademark of Haynes International, Inc. These trade names are used solely for material identification and technical reference purposes and do not imply any affiliation with, sponsorship by, or endorsement from the respective trademark owners. Jiangsu Liangyi Co., Limited is an independent manufacturer of forged components and is not affiliated with Special Metals Corporation or Haynes International, Inc.