Inconel 600 vs Inconel 625 – Composition, Heat Treatment, Properties, and Applications

Introduction

Inconel 600 and Inconel 625 are two widely used nickel-based alloys in high-performance engineering applications. Engineers, procurement managers, and manufacturing planners commonly weigh corrosion resistance, mechanical strength, weldability, and cost when choosing between them. Typical decision contexts include high-temperature service versus aggressive corrosion environments, machinability and fabrication constraints, and the economics of alloying additions.

The principal difference between these alloys is alloying strategy: Inconel 600 is a chromium–nickel iron alloy optimized for oxidation and moderate corrosion resistance with good high-temperature stability, while Inconel 625 is a nickel–chromium–molybdenum–niobium alloy engineered for higher strength and superior resistance to localized and crevice corrosion. Because of these differing alloying approaches, the two grades are frequently compared when designers must trade off strength and localized corrosion resistance against cost and ease of fabrication.

1. Standards and Designations

• Inconel 600
• Common UNS: N06600
• Typical standards: ASTM B127/B163 (bar/rod), ASTM B168 (tube), ASTM B564 (forgings), ASME/ASTM equivalents
• International: EN (often listed in nickel alloy catalogs), JIS/GB equivalents in some product forms

• Classification: Nickel-based alloy (nickel-chromium-iron family)

• Inconel 625

• Common UNS: N06625
• Typical standards: ASTM B443/B444 (sheet/plate), ASTM B443/B444 (strip), ASTM B446 (bar), ASME/ASTM equivalents
• International: EN, JIS, GB product specifications in many supply chains
• Classification: Nickel-based alloy (nickel-chromium-molybdenum-niobium family)

Note: Both alloys are nickel-base (not carbon, alloy, tool steel, stainless steel, or HSLA); they are commonly specified by UNS numbers and covered by ASTM/ASME product specifications for nickel alloys.

2. Chemical Composition and Alloying Strategy

The following table summarizes typical compositional ranges for key elements in each alloy (ranges are representative of common commercial specifications and product forms; consult the applicable ASTM/UNS specification for exact permitted ranges).

Element	Inconel 600 (typical range, wt%)	Inconel 625 (typical range, wt%)
C	≤ 0.15	≤ 0.10
Mn	≤ 1.0	≤ 0.50
Si	≤ 0.50	≤ 0.50
P	≤ 0.015	≤ 0.015
S	≤ 0.015	≤ 0.015
Cr	14.0–17.0	20.0–23.0
Ni	Balance (~72)	Balance (~58)
Mo	—	8.0–10.0
V	—	Trace/none
Nb (and Ta)	—	3.15–4.15 (Nb+Ta)
Ti	≤ 0.40 (trace)	≤ 0.40
B	—	≤ 0.010
N	≤ 0.10 (trace)	≤ 0.05

How alloying affects performance - Nickel (Ni): Provides the base corrosion resistance, toughness, and stability of the matrix at elevated temperatures. - Chromium (Cr): Contributes oxidation and general corrosion resistance through formation of protective oxide films. - Molybdenum (Mo) and Niobium (Nb): Present in 625 to improve resistance to crevice and pitting corrosion and to provide solid-solution and precipitation strengthening; Nb stabilizes carbides and forms strengthening niobium-rich phases under certain heat treatments. - Carbon, Mn, Si, P, S: Kept low to minimize embrittlement and control weldability and corrosion behavior. Overall, 600 emphasizes a simpler Ni–Cr–Fe balance for oxidation and general corrosion resistance, while 625 uses additional Mo and Nb to achieve higher strength and localized corrosion resistance.

3. Microstructure and Heat Treatment Response

• Inconel 600:
• Typical microstructure: Single-phase austenitic nickel matrix with face-centered cubic (FCC) lattice; may contain small amounts of carbide precipitates (MC types) at higher carbon or after long exposures.

• Heat treatment response: Generally supplied annealed; does not respond to conventional quench-and-temper hardening since it is an austenitic nickel alloy. High-temperature anneals are used to relieve stresses; prolonged exposure to certain temperature ranges can promote carbide precipitation and grain boundary chromium depletion, which can influence intergranular corrosion susceptibility.

• Inconel 625:

• Typical microstructure: Primarily a solid-solution strengthened FCC matrix; the alloy is designed to remain solid-solution strengthened in the standard solution-treated condition. Under certain thermal exposures (e.g., prolonged 700–900 °C), secondary phases such as Nb-rich precipitates (γ″ or δ-like phases) and carbides can form, which increase strength but can affect ductility and corrosion resistance if uncontrolled.
• Heat treatment response: Typically supplied solution treated (stabilized) and can be age-strengthened modestly by controlled heat treatments that produce fine precipitates. It is not hardened by conventional quench-and-temper methods but can see strength increases from precipitation of Nb-rich phases.

Thermo-mechanical processing (forgings, cold working) refines grain structure in both alloys, improving toughness. However, cold work can increase susceptibility to localized corrosion in chloride environments unless followed by appropriate post-weld or post-forming stress relief.

4. Mechanical Properties

The following table provides qualitative comparative performance in common product conditions (annealed/solution-treated). Exact values depend on product form, heat treatment, and temperature of use.

Property	Inconel 600 (typical behavior)	Inconel 625 (typical behavior)
Tensile strength	Moderate — good at elevated temperature	Higher — enhanced by Mo/Nb solid-solution and precipitates
Yield strength	Moderate	Higher (significantly higher in solution-treated or aged conditions)
Elongation (ductility)	Good ductility in annealed condition	Good ductility but can be reduced if precipitation-strengthened
Impact toughness	Good over broad temperature range; retains toughness at high T	Good toughness; generally comparable or slightly lower at room T when stronger
Hardness	Moderate (relatively soft in annealed condition)	Higher (increased hardness due to alloying and possible precipitation)

Explanation - Inconel 625 is designed for higher static and creep strength than Inconel 600 owing to the combined effects of Mo and Nb. Consequently, 625 typically exhibits higher tensile and yield strengths, particularly at service temperatures and in components that receive stabilization or aging. Inconel 600, while tough and stable at high temperatures, is comparatively lower in strength but often more ductile and easier to form.

5. Weldability

Both alloys are considered weldable with appropriate procedures, but differences exist:

• Inconel 600:
• Low carbon and lack of strong carbide formers make it generally weldable with conventional nickel-alloy filler metals. It is not prone to hardening in the HAZ like carbon steels.

• Because Inconel 600 is single-phase austenitic, it has low hardenability concerns from carbon; hot cracking susceptibility is moderate and can be managed with established practice.

• Inconel 625:

• Also readily welded; however, higher strength and alloying (Mo, Nb) increase the potential for strain-age cracking and the need for controlled weld procedures and filler matching.
• Post-weld heat treatment is sometimes used to relieve residual stresses in thicker sections.

Weldability indices (qualitative interpretation) - The IIW carbon equivalent and Pcm formulas help predict hydrogen cracking/hardenability risk in steels; while they are tailored for steels, they illustrate the type of analysis used for weldability:

$$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$

$$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$

• Interpretation: For nickel alloys, direct application of these formulas is limited, but the presence of Nb and Mo in 625 increases the numerator terms analogous to higher hardenability—meaning more attention to weld heat input, filler selection, and pre/post-weld treatments is warranted. Overall, both alloys weld well when using qualified procedures and matching filler metals.

6. Corrosion and Surface Protection

• Corrosion behavior:
• Inconel 600: Good resistance to oxidation and many corrosive environments; excels in high-temperature oxidizing atmospheres and resists general corrosion in many media. It is less resistant than 625 to aggressive chloride-containing or reducing environments that promote pitting, crevice corrosion, or stress-corrosion cracking.
• Inconel 625: Superior resistance to localized corrosion (pitting and crevice) and to a range of reducing acids and chloride-containing environments due to Mo and Nb; often the preferred choice where crevice and pitting resistance is critical (e.g., seawater systems, chemical processing).
• Stainless indices:
• PREN (Pitting Resistance Equivalent Number) is typically applied to stainless steels and is calculated as:

$$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$

• PREN is not generally used for nickel-based alloys like Inconel 600/625; however, the formula illustrates the strong role of Mo and N in pitting resistance. For nickel alloys, the absolute composition (Cr, Mo, Ni, Nb) and passive film stability in a given environment determine performance.
• Surface protection:
• Galvanizing and standard paint systems are rarely used on nickel alloys in high-temp corrosive service; surface treatments focus on mechanical finishing, passivation, and appropriate coatings where needed. In non-critical applications, painting or cladding to less-expensive substrates may be used.

7. Fabrication, Machinability, and Formability

• Machinability:
• Both alloys are considered difficult to machine compared to carbon steels. Inconel 625 is generally more work-hardening and stronger, making it more challenging to machine (requires slower speeds, higher rigidity, and robust tooling). Inconel 600 is somewhat easier but still requires carbide tooling and conservative parameters.
• Formability:
• Inconel 600 is relatively ductile in the annealed condition and can be formed easily in many sheet/plate operations. Inconel 625, while formable, requires more force and can spring back more due to higher yield strength.
• Surface finish and polishing:
• Both take high-quality surface finishes and can be electro-polished or mechanically polished to improve corrosion resistance in service. Grinding and finishing should account for work-hardening in 625.

8. Typical Applications

Inconel 600	Inconel 625
Heating elements, furnace components, and thermocouple protection tubes (high-temp oxidation resistance)	Chemical process components (heat exchangers, piping) with chloride media, seawater systems, and offshore corrosion-resistant components
Steam generators, combustion liners, and high-temperature bolting where oxidation resistance matters	Gas turbine components, rocket and aerospace hardware where high strength-to-weight and corrosion resistance are required
Laboratory and food-processing equipment where general corrosion resistance at moderate cost is acceptable	Flanges, fasteners, and weld filler materials for aggressive or crevice-prone environments requiring high strength

Selection rationale - Choose Inconel 600 when oxidation resistance, thermal stability, and cost are priorities and the operating environment does not demand extreme localized corrosion resistance. - Choose Inconel 625 when higher static or cyclic strength, and resistance to pitting/crevice/stress-corrosion in chloride or reducing environments are primary requirements, justifying the higher alloy cost.

9. Cost and Availability

• Relative cost: Inconel 625 is generally more expensive than Inconel 600 due to higher content of Mo and Nb and the associated alloying cost. Pricing varies with global raw material markets (Mo, Nb, Ni).
• Availability by product form: Both alloys are widely available in pipe, tubing, plate, sheet, bar, wire, and welding consumables. Inconel 625 has extensive availability in heavy and engineered forms because of demand in aerospace and chemical processing; Inconel 600 remains common for general high-temperature hardware.
• Lead times: Specialty shapes, large forgings, or exotic heat-treated deliveries will increase lead times for both alloys; 625 sometimes has longer lead times for large, high-integrity forgings or custom wrought forms.

10. Summary and Recommendation

Summary table (relative comparison)

Metric	Inconel 600	Inconel 625
Weldability	Good — straightforward with standard Ni fillers	Good — requires controlled procedures for thicker sections
Strength–Toughness	Moderate strength, excellent toughness at high T	Higher strength, very good toughness; possible reduced ductility if precipitation-strengthened
Corrosion resistance (general)	Excellent oxidation and general corrosion	Superior pitting/crevice and chloride resistance
Cost	Lower (relative)	Higher (relative)

Final recommendations - Choose Inconel 600 if: - The application requires good high-temperature oxidation resistance and general corrosion resistance at a lower alloy cost. - Fabrication simplicity and forming ductility are important. - Service environments are not aggressively pitting or crevice-promoting (e.g., limited chlorides).

• Choose Inconel 625 if:
• The application demands higher static or creep strength, or superior resistance to pitting, crevice corrosion, and chloride-induced stress-corrosion cracking.
• The component will operate in aggressive chemical environments (seawater, reducing acids) or under severe mechanical loadings where strength-to-weight and long-term corrosion resistance justify the added cost.
• Welded constructions require a high-strength filler and resistance to localized corrosion at joints.

Closing note The choice between Inconel 600 and Inconel 625 is application-specific: evaluate environment (chlorides, reducing species, temperature), mechanical loading, fabrication constraints, and total life-cycle cost. For critical systems, confirm choices with material testing in representative service conditions and consult applicable standards and material suppliers for exact compositions and mechanical property data for the selected product form.