Inconel 600 vs Inconel 625 – Composition, Heat Treatment, Properties, and Applications
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Table Of Content
Table Of Content
Introduction
Inconel 600 (UNS N06600) and Inconel 625 (UNS N06625) are two widely used nickel‑based alloys in high‑temperature, corrosive and high‑stress industrial environments. Engineers, procurement managers and manufacturing planners frequently face the choice between the two when specifying materials for heat exchangers, process piping, turbines, chemical vessels, and subsea hardware. Typical decision drivers are corrosion performance versus cost, required static or creep strength versus fabricability, and weldability versus long‑term stability.
The principal practical difference between the two alloys is their alloying strategy: Inconel 625 is deliberately enriched with molybdenum and niobium (and has a lower iron content) to provide substantially higher strength and improved resistance to localized corrosion compared with Inconel 600, which is a more chromium‑stabilized nickel–iron alloy with lower alloy content and lower room‑temperature strength. Because of this difference in alloying, these grades are commonly compared when designers need to trade off strength, localized corrosion resistance, weldability and cost.
1. Standards and Designations
Major standards/codes that cover Inconel 600 and Inconel 625 include: - ASTM / ASME: - Inconel 600: ASTM B166 / ASME SB‑166 (for sheet, strip, and plate); other product standards for bars, forgings, wire. - Inconel 625: ASTM B446 (pipe), ASTM B443/B444/B446 for various product forms, and ASME equivalents. - EN: covered under European nickel‑alloy standards (e.g., EN 2.4816 for alloys similar to 625). - JIS / GB: Japanese and Chinese standards have equivalent designations for nickel‑chromium alloys; check national tables for exact equivalence. - UNS: N06600 (Inconel 600), N06625 (Inconel 625).
Material classification: - Both are nickel‑base corrosion‑resistant alloys (not carbon steels, tool steels, or HSLA). They are typically treated as high‑performance austenitic nickel‑chromium alloys (stainless analogs in the nickel alloy family).
2. Chemical Composition and Alloying Strategy
The following table lists typical composition ranges commonly cited in UNS/ASTM product specifications. Values are given as weight percent and are typical ranges — verify against the applicable product standard or mill certificate for procurement.
| 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 min) | ≈58 min |
| Mo | — | 8.0–10.0 |
| V | trace / not specified | trace / not specified |
| Nb (Nb+Ta) | — | 3.15–4.15 (Nb predominantly) |
| Ti | ≤ 0.50 (often very low) | ≤ 0.40 |
| B | trace (very low) | trace (very low) |
| N | ≤ 0.10 (typically low) | ≤ 0.05 (typically low) |
| Fe | ≈6.0–10.0 | ≤ 5.0 |
| Cu | ≤ 0.50 | ≤ 0.50 |
How the alloying affects performance - Chromium (Cr): provides oxidation and general corrosion resistance in both alloys. 625 has higher Cr than 600, aiding passivity at some environments. - Nickel (Ni): base element providing the face‑centered cubic (austenitic) matrix and high‑temperature stability. - Molybdenum (Mo) and Niobium (Nb): present in substantial quantities in 625; Mo increases resistance to pitting and crevice corrosion and improves strength via solid‑solution effects; Nb (with Ni) contributes to solid‑solution strengthening and stabilizes the microstructure against certain carbide / intermetallic precipitates. - Iron (Fe): higher in 600; dilutes other alloying elements and reduces cost. - Carbon and trace elements affect weldability and potential for carbide formation; both have low allowable C.
In short, Inconel 625 is a higher‑alloyed, higher‑Mo/Nb grade engineered for higher strength and improved localized corrosion resistance relative to Inconel 600.
3. Microstructure and Heat Treatment Response
Microstructure (as‑manufactured) - Both alloys are essentially austenitic (FCC) nickel‑based solid solutions at room temperature. Inconel 600 typically exhibits a relatively simple, single‑phase austenitic matrix with dispersed carbides possible at grain boundaries after long exposures. Inconel 625 is also a single‑phase austenitic matrix in the solution‑annealed condition but contains higher amounts of Mo and Nb that increase solid‑solution strengthening and the potential to form intermetallic or Laves phases, or Ni3Nb (delta or gamma″/gamma′′‑like) precipitates, under prolonged thermal exposure or specific ageing treatments.
Heat treatment response - Inconel 600: Response is dominated by stress‑relief annealing and solution annealing; it is not a precipitation‑hardening alloy. Normalization/quenching concepts used for steel do not apply—thermal processing aims to restore ductility and homogenize microstructure. - Inconel 625: Typically supplied in the solution‑annealed (soft) condition. It is primarily strengthened by solid solution; controlled aging is not commonly used to produce a high‑strength precipitation‑hardened condition like Inconel 718. However, under some high‑temperature long‑term exposures, precipitates (e.g., Ni3Nb, nitrides, or Laves phases) can form, which may increase hardness but can reduce ductility and toughness. Careful thermal processing (solution anneal followed by rapid cooling) is used to avoid detrimental precipitates where ductility or weldability are priorities.
Effect of mechanical working - Both alloys can be cold worked and will work‑harden; 625 tends to work‑harden more strongly due to alloying, making forming and machining more challenging after partial hardening.
4. Mechanical Properties
Mechanical properties depend on product form (sheet, plate, bar, pipe), heat treatment, and temperature. The table below gives representative room‑temperature ranges to enable engineering comparisons; always confirm with supplier certification.
| Property (RT) | Inconel 600 (typical, annealed) | Inconel 625 (typical, solution‑annealed) |
|---|---|---|
| Tensile strength (UTS) | ~500–900 MPa | ~700–1100 MPa |
| Yield strength (0.2% proof) | ~200–400 MPa | ~350–700 MPa |
| Elongation (in 50 mm) | ~30–50% | ~30–50% |
| Impact toughness (Charpy, typical) | Good — moderate values depending on form | Good — generally comparable or higher in heavy sections |
| Hardness (HV / HRB) | Lower (softer) | Higher (depends on temp & aging) |
Interpretation - Strength: Inconel 625 is generally stronger (higher UTS and yield) than Inconel 600 because of solid‑solution strengthening from Mo and Nb and the greater total alloy content. - Ductility/toughness: Both alloys retain good ductility and toughness in the annealed/solution‑annealed condition. 625 can show reduced toughness if coarse intermetallics form after prolonged high‑temperature exposure. - For applications where higher static/creep strength and better resistance to localized corrosion or chloride stress‑corrosion cracking is required, 625 is the stronger choice. For moderate temperature/pressure service where cost and simpler fabrication are priorities, 600 is often selected.
5. Weldability
Weldability considerations hinge on carbon, alloy content, and propensity to form cracking‑prone phases. Both alloys are weldable with common nickel‑alloy welding procedures; however, differences matter.
- Inconel 600: Good weldability using nickel filler metals matched to N06600. Lower hardenability risk compared with steels; susceptibility to hot cracking is low but care is needed to avoid contamination and to control heat input on heavy sections.
- Inconel 625: Excellent weldability in many practices; matched filler metals (e.g., ERNiCrMo‑3) are common. Higher alloy content can increase risk of solidification cracking if weld composition or procedure is incorrect; 625 is generally considered weldable with proper technique and filler selection.
Weldability indices (qualitative use) - Example indices that apply to steels can be used for qualitative interpretation of weldability. For steels: $$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$ and $$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$ - While these formulas were developed for steels and are not directly applicable to nickel alloys, they illustrate the role of alloying elements—higher Nb and Mo increase the numerical indices, indicating higher propensity for weldability challenges in steels. In nickel alloys, higher Mo/Nb increase solidification range and change melting behavior; practitioners use alloy‑specific welding guidelines rather than steel CE indices.
Practical advice - Pre‑ and post‑weld heat treatments: Typically not required for 600 and 625 for most applications; stress relief or solution anneal may be used for specific duty cases. Control heat input and use appropriate filler wires to match corrosion and mechanical properties.
6. Corrosion and Surface Protection
- Inconel 600: Excellent general corrosion and oxidation resistance up to high temperatures due to Cr and Ni. Good resistance to carburization and many corrosive media; less resistant than 625 to pitting, crevice corrosion and chloride‑induced localized attack because it lacks the high Mo and Nb content.
- Inconel 625: Superior resistance to pitting, crevice corrosion and chloride stress‑corrosion cracking thanks to high Mo and Nb. Also excellent oxidation resistance at elevated temperatures.
Use of PREN - PREN (Pitting Resistance Equivalent Number) is commonly used for stainless steels: $$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$ - For nickel alloys, PREN is of limited use because the corrosion performance is governed by a different balance of elements (Ni, Mo, Nb) and their effects on passive film chemistry. However, the PREN formula highlights why higher Mo in 625 improves localized corrosion resistance.
Surface protection - For non‑stainless or lower alloy materials, galvanizing or paints are common. For Inconel grades, surface coating is rarely required for corrosion resistance; however, for wear or abrasion, hard coatings or platings may be applied for specific service needs.
7. Fabrication, Machinability, and Formability
- Machinability: Both alloys are harder to machine than common stainless steels or carbon steels. In general, Inconel 600 has slightly better machinability than 625 because 625 work‑hardens more and contains higher Mo/Nb that reduce cutability. Use sharp carbide or CBN tooling, slow speeds and heavy feed to minimize work hardening; consistent coolant and chip control are important.
- Formability: Both can be formed using standard metal‑forming techniques when annealed/solution‑annealed. 625’s greater work hardening needs to be managed—intermediate anneals may be required for extensive forming.
- Welding and thermal forming: Use qualified procedures; 625 requires attention to avoid solidification cracking in certain weld geometries, though it is commonly welded in industry.
8. Typical Applications
| Inconel 600 — Typical Uses | Inconel 625 — Typical Uses |
|---|---|
| Furnace muffles, heat‑treat baskets, industrial heaters and retorts | Chemical processing piping and vessels exposed to chlorides or sour environments |
| Appliance components, electrical resistance elements | Marine and subsea systems, including risers and umbilicals |
| Steam generator tubing (older designs), oxidation‑resistant hardware | High‑strength fasteners, rocket engine and aerospace components, exhaust systems |
| Corrosion‑resistant oven linings, heat exchangers in moderate chloride environments | High temperature flanges, columns, and process equipment where pitting/corrosion resistance and higher strength are needed |
Selection rationale - Choose Inconel 600 for moderate temperature oxidation resistance, simpler fabrication and lower cost where high localized corrosion or very high strength is not required. - Choose Inconel 625 where high static/creep strength, superior pitting and crevice corrosion resistance, and alloy stability in aggressive environments justify the higher material cost.
9. Cost and Availability
- Relative cost: Inconel 625 is typically more expensive per kilogram than Inconel 600. The higher cost reflects the content of Mo and Nb and more demanding production controls.
- Availability: Both alloys are produced globally in forms including plate, sheet, bar, pipe and welded tubing. Inconel 600 tends to be widely stocked in common forms due to its long history and broad industrial use. Inconel 625 is also widely available but specialty product forms or large‑section forgings may have longer lead times and higher minimum order quantities.
- Procurement tip: Specify exact UNS grade, product form, and mill testing requirements (e.g., heat analysis, tensile test, PMI) to avoid substitutions.
10. Summary and Recommendation
Summary table
| Characteristic | Inconel 600 | Inconel 625 |
|---|---|---|
| Weldability | Excellent in common practices; simpler filler selection | Excellent when using appropriate Ni‑Mo‑Nb fillers; more attention to procedure |
| Strength–Toughness | Moderate strength, excellent toughness | Higher strength, good toughness in solution‑annealed condition |
| Cost | Lower (more economical) | Higher (premium alloy) |
Final recommendation - Choose Inconel 600 if: - Your application requires good oxidation and general corrosion resistance at elevated temperatures but does not require the highest level of pitting/crevice resistance or elevated static/creep strength. - Fabrication simplicity, lower material cost and good weldability are priorities. - Typical uses: heat treating equipment, moderate‑temperature furnace components, and where alloy cost or availability is a constraint.
- Choose Inconel 625 if:
- The service environment contains chlorides, sulfides or other aggressive species where pitting/crevice corrosion and chloride SCC resistance is required.
- Higher static, creep or fatigue strength is required at elevated or ambient temperatures and reduced section thickness is desirable.
- Typical uses: chemical process systems, subsea applications, aerospace components exposed to aggressive fluids or mechanical stress where longer life and higher safety margins justify higher material cost.
Concluding note Always confirm final selection with a combined assessment of mechanical loads, corrosion data for the intended environment (including temperature), fabrication and welding plans, lifecycle cost and supplier certification. For critical applications, request material heat analysis, mechanical test certificates, and, if necessary, corrosion testing or engineering qualification of welded assemblies.