Inconel 625 Chemical Composition

• Inconel 625's chemical composition (Ni 58%+ / Cr 20-23% / Mo 8-10% / Nb+Ta 3.15-4.15%) is a non-substitutable system — each element serves a specific metallurgical function that no other element can replace.

• The alloy derives its strength from solid-solution hardening (Mo + Nb), not precipitation hardening — no post-fabrication heat treatment is required.

• Niobium stabilization (3.15-4.15%) is what allows Inconel 625 to be welded without sensitization, a key advantage over non-stabilized nickel alloys.

• PREN of 46-52 places Inconel 625 above the seawater threshold (40) and between super duplex (42-43) and Hastelloy C276 (65-68) in pitting resistance.

• Grade 1 vs Grade 2 is a heat-treatment distinction, not a composition difference; the 625 LCF variant has genuinely modified composition (C ≤0.03%, Si ≤0.15%, N ≤0.02%).

• Within the ASTM B446 composition window, actual heats can vary by up to 12% in PREN and 19% in yield strength — specifying supplementary composition requirements for critical applications is recommended.

Introduction

Inconel 625 (UNS N06625 / W.Nr. 2.4856) is a nickel-chromium-molybdenum-niobium superalloy whose chemical composition delivers three simultaneous benefits: high-temperature strength from solid-solution hardening (no precipitation hardening needed), outstanding corrosion resistance in chloride and acid environments (PREN approximately 47-51), and excellent weldability with no post-weld heat treatment required.

The four primary alloying elements — nickel (58% min), chromium (20-23%), molybdenum (8-10%), and niobium+tantalum (3.15-4.15%).

For procurement professionals, the key insight is this: within the ASTM B446 composition window, minor variations in chromium, molybdenum, and carbon content can shift the alloy's corrosion resistance by up to 20% and its yield strength by up to 15%.

What Is the Chemical Composition of Inconel 625?

According to ASTM B443, ASTM B446, ASTM B444, ASTM B705, ASME SB-443, and UNS N06625, the nominal chemical composition of Inconel 625 is shown below.

What Each Element Does?

The following breakdown explains the metallurgical role of each element.

Nickel (Ni ≥58%): The Foundation

Nickel is the matrix element — it constitutes the majority of the alloy and defines its fundamental crystal structure (face-centered cubic, FCC). This structure is non-magnetic, ductile, and stable from cryogenic temperatures up to the melting point. Nickel's primary contributions are:

• General corrosion resistance, especially in reducing environments (caustic soda, alkaline solutions, organic acids).

• Immunity to chloride stress corrosion cracking (SCC) — the failure mode that plagues austenitic stainless steels like 304 and 316.

• Toughness retention at cryogenic temperatures; Inconel 625 maintains impact toughness at -196 °C (liquid nitrogen temperature).

• Compatibility with all major welding processes (GTAW, GMAW, SMAW, EBW) without preheat or post-weld heat treatment.

The 58% minimum is critical. Below this threshold, the alloy's resistance to chloride SCC begins to degrade, and the FCC matrix becomes less stable, potentially promoting deleterious phase transformations at elevated temperatures.

Chromium (Cr 20–23%): The Oxidation Shield

Chromium is the element that allows Inconel 625 to survive in oxidizing environments. When exposed to oxygen at any temperature, chromium migrates to the surface and forms a thin, adherent layer of chromium oxide (Cr₂O₃). This passive film is self-healing — if scratched or mechanically damaged, it reforms instantly in the presence of oxygen.

Key functions of chromium in Inconel 625:

• Forms the Cr₂O₃ passive layer (2-3 nm thick) that provides oxidation resistance up to 980 °C (1800 °F).

• Resists oxidizing acid environments (nitric acid, chromic acid, wet chlorine at elevated temperatures).

• Provides the base for pitting resistance — the chromium term in the PREN formula (Cr + 3.3Mo + 16N).

• Stabilizes the austenitic matrix against sigma-phase precipitation at temperatures of 600-900 °C.

The 20-23% range is deliberately broad. At the low end (20%), the alloy achieves adequate oxidation resistance for most chemical processing applications. At the high end (23%), it maximizes resistance to aggressive oxidizing media and high-temperature scaling. The trade-off: higher chromium slightly reduces ductility and increases the risk of intermetallic phase precipitation during long-term high-temperature exposure.

Molybdenum (Mo 8–10%): The Pitting Fighter and Strength Source

Molybdenum serves two critical and distinct functions in Inconel 625: corrosion resistance and mechanical strength. No other element in the composition wears two hats so prominently.

Corrosion role:

• Provides resistance to pitting and crevice corrosion in chloride-containing environments (seawater, brine, acid chlorides).

• Resists reducing acid environments (hydrochloric acid, sulfuric acid, phosphoric acid, organic acids).

• In the PREN formula, molybdenum is weighted 3.3x — making it the single most influential element for pitting resistance on a per-percentage basis.

Strength role:

• Molybdenum atoms are large (atomic radius 139 pm vs 124 pm for nickel) and create lattice strain when dissolved in the nickel matrix. This solid-solution strengthening is the primary source of Inconel 625's high yield strength (414 MPa min in annealed condition).

• Unlike precipitation-hardening alloys (e.g., Inconel 718), this strengthening mechanism does not require heat treatment — it is inherent in the chemistry.

The 8-10% range is the engineering sweet spot. Below 8%, pitting resistance drops below the PREN 40 threshold needed for reliable seawater service. Above 10%, the alloy becomes susceptible to TCP (topologically close-packed) phase precipitation — particularly mu-phase and sigma-phase — which embrittle the material during prolonged high-temperature exposure.

Niobium + Tantalum (Nb+Ta 3.15–4.15%): The Stabilizer

Niobium (historically called columbium in ASTM standards) and tantalum are grouped together in the specification because they are chemically similar and often co-occur in ore sources. Their combined content is controlled as a single parameter.

Key functions:

• Stabilization: Niobium has a stronger affinity for carbon than chromium. By preferentially forming niobium carbides (NbC), it prevents chromium carbide precipitation at grain boundaries — the mechanism that causes sensitization and intergranular corrosion in non-stabilized alloys.

• Solid-solution strengthening: Niobium and tantalum atoms are significantly larger than nickel (Nb: 146 pm, Ta: 146 pm vs Ni: 124 pm), creating substantial lattice strain that complements molybdenum's strengthening effect.

• High-temperature stability: Niobium slows grain growth at elevated temperatures, maintaining creep resistance during service above 600 °C.

• Weldability: The stabilization effect means Inconel 625 can be welded without losing corrosion resistance in the heat-affected zone (HAZ) — a critical advantage over non-stabilized alloys.

The 3.15-4.15% range is tightly controlled. Below 3.15%, stabilization becomes incomplete and the alloy becomes susceptible to sensitization during welding or service in the 600-900 °C range. Above 4.15%, excess niobium promotes formation of delta-phase (Ni₃Nb), which can reduce room-temperature ductility by 10-15%.

Iron (Fe ≤5.0%): The Controlled Residual

Iron is not an intentional alloying element in Inconel 625 — it is a residual from raw material inputs (nickel ores, ferroalloys). The 5.0% maximum is a ceiling, not a target. In practice, well-manufactured heats typically contain 2-3% iron.

At low levels (<5%), iron has minimal effect on corrosion or mechanical properties. It slightly improves hot workability by reducing the alloy's tendency to stick to forging dies. Above 5%, iron begins to dilute the nickel matrix, reducing resistance to chloride SCC and potentially promoting unwanted phase transformations during long-term thermal exposure.

Carbon (C ≤0.10%): Low by Design

Carbon is the element that engineers love to hate. In small quantities, it is unavoidable; in excess, it is destructive. The 0.10% maximum in ASTM B446 is a deliberate ceiling designed to minimize carbide precipitation.

The danger of carbon in Inconel 625:

• During welding or service in the 600-900 °C range (the sensitization window), carbon can combine with chromium to form Cr₂₃C₆ at grain boundaries, depleting the adjacent matrix of chromium and creating a path for intergranular corrosion.

• In practice, niobium stabilization largely prevents this by capturing carbon as NbC. However, maintaining carbon below 0.10% provides an additional safety margin — particularly in thick sections where cooling through the sensitization window is slow.

• For applications requiring maximum corrosion resistance in the as-welded condition, some manufacturers offer a low-carbon variant (C ≤0.03%) similar to the 316L approach in stainless steels.

Silicon and Manganese (Si ≤0.50%, Mn ≤0.50%): Deoxidizers

Both elements serve primarily as deoxidizers during the melting process. They scavenge dissolved oxygen from the melt, preventing porosity and oxide inclusions. Their 0.50% maximums are set to balance deoxidation needs against potential side effects:

• Excessive silicon (>0.50%) increases the risk of hot cracking during welding by lowering the solidus temperature of interdendritic regions.

• Excessive manganese (>0.50%) can promote the formation of complex sulfide inclusions that act as pit-initiation sites.

In the 625 LCF (Low Cycle Fatigue) variant, silicon is further restricted to ≤0.15% to minimize fatigue crack initiation sites.

Phosphorus and Sulfur (P ≤0.015%, S ≤0.015%): The Trouble Pair

These two elements are the most tightly controlled impurities in the specification. The 0.015% limits (15 ppm) are approximately 3x stricter than typical austenitic stainless steel limits.

• Phosphorus segregates to grain boundaries during solidification, reducing cohesion and promoting hot cracking during welding.

• Sulfur forms low-melting-point manganese sulfides that liquefy during welding, causing hot shortness (cracking during solidification).

• Both elements also act as pit-initiation sites in chloride environments, reducing the effective pitting resistance of the alloy.

The 0.015% dual limit is one of the defining quality markers of Inconel 625. It ensures weldability in thick sections and maximum corrosion resistance in aggressive media.

Aluminum and Titanium (Al ≤0.40%, Ti ≤0.40%): Minor Players

These elements are present as residuals from the melting process rather than intentional additions. At their 0.40% maximums, they have minor effects:

• Aluminum improves the adhesion of the surface oxide layer, slightly enhancing oxidation resistance at elevated temperatures.

• Titanium can form minor gamma-prime (Ni₃(Al,Ti)) precipitates, contributing marginally to high-temperature strength.

• Both are controlled to prevent excessive hardening or unwanted phase formation during thermal processing.

How Does Inconel 625 Chemical Composition Affect Mechanical Properties?

Inconel 625 achieves its strength through solid solution strengthening, a mechanism in which large alloying atoms (Mo, Nb, Ta) distort the nickel lattice and impede dislocation movement.

Yield Strength: The Mo + Nb Synergy

The yield strength of annealed Inconel 625 is specified at 414 MPa (60 ksi) minimum. In practice, typical values range from 460-550 MPa depending on where the composition falls within the ASTM B446 window:

•At the low end of Mo (8%) and Nb+Ta (3.15%): yield strength ≈ 420-450 MPa

•At the high end of Mo (10%) and Nb+Ta (4.15%): yield strength ≈ 500-550 MPa

•This represents a 10-15% variation driven solely by composition within specification limits.

The synergistic effect of Mo and Nb is non-linear. Each element strengthens the matrix independently, but their combined effect is greater than the sum of individual contributions — a phenomenon known as synergistic solid-solution hardening. This is why both elements must be present simultaneously; substituting one for the other would not produce equivalent properties.

Tensile Strength and Elongation

Source: ASTM B446-19 and Special Metals Corporation INCONEL alloy 625 Technical Bulletin. Typical values are from published mill test data.

High-Temperature Strength: The Nb Advantage

Above 600 °C, the strength contribution from molybdenum begins to decrease due to thermally-activated dislocation recovery. Niobium, however, maintains its strengthening effect through a different mechanism — it slows grain boundary sliding, which is the dominant creep mechanism at elevated temperatures. This is why Grade 2 (solution-annealed) material, with its larger grain size and more complete dissolution of Nb in the matrix, is specified for high-temperature service.

The niobium content (3.15-4.15%) is the critical variable for applications above 600 °C. At the low end of the range, creep resistance drops noticeably. At the high end, room-temperature ductility is slightly reduced due to incipient delta-phase formation.

Inconel 625 Composition vs Nickel Alloys

Source: ASTM B446 (N06625), ASTM B575 (N10276), ASTM B425 (N08825), ASTM B127 (N04400). PREN values calculated from mid-range composition.

Inconel 625 Composition Across Product Standards

The chemical composition of Inconel 625 is defined consistently across multiple international standards. However, the product form and applicable specification vary depending on whether you are purchasing bar, plate, pipe, tube, or fittings.

Additional standards: EN 10095 (European designation 2.4856), NACE MR0175 / ISO 15156 (sour service qualification), NORSOK M-630 (offshore qualification with supplementary chemistry requirements for Nb and N).

NACE MR0175 / ISO 15156: Sour Service Compliance

For oil and gas applications involving wet H₂S (sour service), NACE MR0175 / ISO 15156 defines additional requirements beyond ASTM B446. Inconel 625 is listed as an acceptable material provided:

•Carbon content does not exceed 0.10% (already satisfied by ASTM B446)

•Material is supplied in the solution-annealed condition (Grade 2)

•Hardness does not exceed 35 HRC (inherently satisfied by the annealed condition)

•No cold working that increases hardness beyond 35 HRC

No additional composition modification is required for NACE compliance — Inconel 625's standard chemistry is already compatible with sour service requirements. This is a significant advantage over age-hardened nickel alloys, which require careful heat treatment control to meet NACE hardness limits.

Common Composition Misconceptions

Several persistent myths about Inconel 625's chemical composition lead to suboptimal material selection and procurement decisions. This section addresses the most consequential ones.

Misconception 1: "Higher Nickel Always Means Better Corrosion Resistance"

Reality: Nickel is necessary for the matrix, but it does not directly drive pitting or crevice corrosion resistance.

Within the 58-66% range, moving nickel up while correspondingly reducing chromium or molybdenum would actually reduce pitting resistance. The elements that directly matter for localized corrosion are chromium and molybdenum — as reflected in the PREN formula.

Misconception 2: "Inconel 625 and Hastelloy C276 Are Interchangeable"

Reality: Their compositions differ fundamentally. C276 has 15-17% Mo (vs 625's 8-10%) and 3-4.5% W (absent in 625), giving it far superior reducing-acid resistance and a PREN of 65-68.

But 625 has 20-23% Cr (vs C276's 14.5-16.5%) and 3.15-4.15% Nb (absent in C276), giving it better oxidation resistance, higher yield strength, and superior weldability. They serve overlapping but distinct application spaces.

Misconception 3: "Low Carbon Means Better Corrosion Resistance"

Reality: For Inconel 625, the standard 0.10% carbon maximum is already adequate due to niobium stabilization. The low-carbon approach that works for 316 stainless steel (where 316L replaces 316) is less critical for 625 because Nb captures carbon as NbC, preventing chromium carbide precipitation. Specifying C ≤0.03% is only necessary for 625 LCF fatigue applications or for thick-section welding where slow cooling through the sensitization window is a concern.

Misconception 4: "All Heats Within Spec Are Equivalent"

Reality: A heat at the minimum end of Cr (20%) and Mo (8%) has a PREN of ~46 and a yield strength of ~420 MPa. A heat at the maximum end of Cr (23%) and Mo (10%) has a PREN of ~52 and a yield strength of ~500 MPa.

That is a 12% difference in pitting resistance and a 19% difference in strength — both from material that passes the same ASTM B446 specification. For critical applications, specify the minimum composition you need, not just the spec.

FAQ

Q1: What is the chemical composition of Inconel 625 per ASTM B446?

Inconel 625 (UNS N06625) has the following composition: nickel 58% min (balance), chromium 20-23%, molybdenum 8-10%, niobium+tantalum 3.15-4.15%, iron 5% max, cobalt 1% max, carbon 0.10% max, manganese 0.50% max, silicon 0.50% max, phosphorus 0.015% max, sulfur 0.015% max, aluminum 0.40% max, and titanium 0.40% max. These limits are defined in ASTM B446 Table 1 and are identical across all product forms (bar, plate, pipe, tube, fittings).

Q2: What is the difference between Inconel 625 Grade 1 and Grade 2?

Grade 1 and Grade 2 have identical chemical composition limits per ASTM B446. The difference is the thermal processing condition: Grade 1 is annealed at 871-982 °C (1600-1800 °F) for service up to 593 °C (1100 °F); Grade 2 is solution-annealed at 1080-1160 °C (1976-2120 °F) for service above 593 °C where creep and rupture resistance are required. The higher solution-annealing temperature produces a coarser grain structure that is more resistant to creep but slightly lower in room-temperature strength.

Q3: Why does Inconel 625 contain niobium, and what happens without it?

Niobium (3.15-4.15%) serves two critical functions: (1) stabilization — it preferentially forms NbC, preventing chromium carbide precipitation at grain boundaries that causes sensitization and intergranular corrosion, and (2) solid-solution strengthening — niobium atoms are large (146 pm vs 124 pm for Ni) and create lattice strain that strengthens the matrix. Without niobium, Inconel 625 would be susceptible to sensitization during welding and would lose approximately 15-20% of its yield strength.

Q4: How does Inconel 625's composition compare to Hastelloy C276?

The key composition differences are: Inconel 625 has higher chromium (20-23% vs 14.5-16.5%) and contains niobium (3.15-4.15%, absent in C276). Hastelloy C276 has higher molybdenum (15-17% vs 8-10%) and contains tungsten (3-4.5%, absent in 625). These differences give C276 better reducing-acid resistance (PREN ~65-68 vs ~46-52) but give 625 better oxidation resistance, higher yield strength (due to Nb strengthening), and superior weldability without post-weld heat treatment.

Q5: What is Inconel 625 LCF and how does its composition differ?

Inconel 625 LCF (Low Cycle Fatigue) is a proprietary variant with three composition modifications beyond standard ASTM B446 limits: carbon reduced to 0.03% max (from 0.10%), silicon reduced to 0.15% max (from 0.50%), and nitrogen controlled to 0.02% max (not specified in standard B446). These tighter limits minimize carbide particles, non-metallic inclusions, and nitrides that act as fatigue crack initiation sites, improving low-cycle fatigue life by 2-3x compared to standard Grade 1 material.

Q6: What PREN does Inconel 625 achieve, and is it suitable for seawater?

Inconel 625 achieves a PREN of approximately 46-52, calculated as PREN = %Cr + 3.3×%Mo + 16×%N using the ASTM B446 composition range. This is well above the 40 threshold generally required for reliable seawater service. Inconel 625 is widely used in seawater intake systems, offshore platform components, and desalination plant equipment.

Q7: Can the carbon content in Inconel 625 cause sensitization during welding?

The risk is low but not zero. Inconel 625's niobium content (3.15-4.15%) stabilizes carbon by forming NbC, which is the primary defense against sensitization. The 0.10% carbon maximum provides an adequate safety margin. However, for thick-section welding where the heat-affected zone cools slowly through the 600-900 °C sensitization window, specifying carbon at 0.05% max or lower provides additional protection. For extreme corrosion-critical welded applications, the 625 LCF variant (C ≤0.03%) offers the highest margin.