Views: 2 Author: Claire Publish Time: 2026-08-03 Origin: Site
Table of Contents
Before diving into the technical details, here is the essential decision matrix that engineers and procurement teams need most often:
Strengthening Mechanism | Solid-solution | Precipitation (gamma prime/double-prime) |
Yield Strength (RT) | ~417 MPa (annealed) | ~1,035 MPa (aged) |
UTS (RT) | ~890 MPa (annealed) | ~1,380 MPa (aged) |
Max Useful Temp (strength) | ~815°C | ~700°C (long-term) |
Max Oxidation Temp | ~980°C | ~980°C |
Corrosion Resistance | Excellent (PREN 46-52) | Good (PREN 26-31) |
Weldability | Excellent (no PWHT) | Good (requires PWHT) |
Heat Treatment Required | No (annealed only) | Yes (solution + double age) |
Relative Cost Index | 1.25-1.4× (baseline) | 1.0× (baseline) |
Primary Domain | Corrosion / Weldability | High-Temp Strength |
Inconel 625 (UNS N06625, W.Nr. 2.4856) is a solid-solution strengthened alloy designed for maximum corrosion resistance in the harshest environments on earth — deep-sea seawater at 3,000 metres depth, sour gas wells at 200 bar H₂S partial pressure, flue gas desulfurization absorbers processing 15% HCl at 120°C. Its high chromium and molybdenum content make it the default choice for any application where corrosion resistance is the primary driver.
Inconel 718 (UNS N07718, W.Nr. 2.4668) is a precipitation-hardened alloy designed for maximum strength — specifically the strength needed to survive the combined heat, pressure, and vibration loads inside a jet engine turbine disk spinning at 10,000 rpm, or to contain cryogenic liquid hydrogen at −253°C inside a rocket motor casing. Its aluminum and titanium additions create a nano-scale gamma-prime precipitate that gives 718 roughly double the yield strength of 625.
This single distinction — solid-solution vs. precipitation hardening — cascades into every other performance difference discussed in this guide: weldability, thermal stability, corrosion behavior, cost, and application.
Inconel 625 is a solid-solution strengthened alloy — strength comes from molybdenum and niobium atoms dissolved in the nickel-chromium matrix. Inconel 718 is a precipitation-hardened alloy — strength comes from nano-scale gamma-prime (Ni₃Nb) and gamma-double-prime (Ni₃Nb) precipitates formed by controlled heat treatment.
Element (wt%) | Inconel 625 | Inconel 718 |
Nickel (Ni) | ≥ 58.0% (Balance) | 50.0–55.0% |
Chromium (Cr) | 20.0–23.0% | 17.0–21.0% |
Molybdenum (Mo) | 8.0–10.0% | 2.8–3.3% |
Niobium + Tantalum | 3.15–4.15% | 4.75–5.50% |
Iron (Fe) | ≤ 5.0% | Balance (~17–21%) |
Aluminum (Al) | ≤ 0.40% | 0.30–0.70% |
Titanium (Ti) | ≤ 0.40% | 0.65–1.15% |
Carbon (C) | ≤ 0.10% | ≤ 0.08% |
Manganese (Mn) | ≤ 0.50% | ≤ 0.35% |
Silicon (Si) | ≤ 0.50% | ≤ 0.35% |
Phosphorus (P) | ≤ 0.015% | ≤ 0.015% |
Sulfur (S) | ≤ 0.015% | ≤ 0.015% |
Strengthening Mechanism Difference
Inconel 625 (Solid-Solution Strengthening):
Alloying elements (Mo, Cr, Nb) are distributed atom-by-atom throughout the nickel matrix. Strength is generated by the lattice distortion these atoms create — every atom slightly impedes dislocation movement. The more Mo and Cr you add, the stronger the matrix — but this also changes corrosion behavior. 625 in the annealed condition reaches yield strength of ~450 MPa (65 ksi); cold working (20–30% reduction) can push it to ~760 MPa (110 ksi) but reduces ductility and corrosion resistance in the worked direction.
Inconel 718 (Precipitation Hardening):
Al and Ti are added in precise quantities. During solution treatment (980–1,010°C), Al and Ti dissolve completely. During the subsequent age cycle (720°C ± 5°C for 8 hours, furnace cool to 620°C, hold 8 hours), nano-scale gamma-prime (Ni₃Nb) and gamma-double-prime (Ni₃Nb) precipitates form coherently within the matrix. These precipitates are 10–50 nm in diameter — invisible to the naked eye but powerful barriers to dislocation movement. The result: yield strength of 1,035 MPa (150 ksi) — roughly 2.3× that of annealed 625, and 1.5× that of heavily cold-worked 625.
⚠️ Critical implication of this difference: The strength of 625 is achieved in the annealed condition — no heat treatment is required. 625 arrives at its as-received strength without any customer-side heat treatment. In contrast, 718 arrives in the solution-treated condition and must be aged by the customer (or the fabricator) to achieve its rated strength. The aging cycle is typically: 720°C × 8h → furnace cool to 620°C × 8h → air cool. Skipping or mistiming the age cycle leaves 718 at only ~380 MPa yield — no stronger than mild steel.
Inconel 718 in the peak-aged condition has approximately double the room-temperature yield strength of Inconel 625 in the annealed condition. At 650°C (1,200°F), 718 retains more strength than 625 in the annealed condition — making 718 the dominant choice for aerospace turbine disks and rocket motors.
However, 625 outperforms 718 at temperatures above 700°C in both strength and corrosion resistance, and 625's annealed strength is immediately available without heat treatment.
Property | Inconel 625 | Inconel 625 | Inconel 718 | Inconel 718 |
Yield Strength, 0.2% offset | 450 / 65 | 760 / 110 | 380 / 55 | 1,035 / 150 |
Tensile Strength, UTS | 880 / 128 | 1,000 / 145 | 965 / 140 | 1,240 / 180 |
Elongation (% in 4D or 50mm) | 30% | 15% | 12% | 12% |
Hardness (typical, HRC) | HRB 89–95 | HRC 25–33 | HRB 90–100 | HRC 36–44 |
Elastic Modulus | 208 GPa | 208 GPa | 200 GPa | 200 GPa |
Density | 8.44 g/cm³ | 8.44 g/cm³ | 8.19 g/cm³ | 8.19 g/cm³ |
Creep and Stress Rupture
Property | Inconel 625 (Annealed) | Inconel 718 (STA) |
100,000h Rupture Strength at 600°C | ~250 MPa | ~690 MPa |
100,000h Rupture Strength at 650°C | ~120 MPa (low; solid-solution alloys have limited creep resistance) | ~450 MPa (still significant; 718 holds creep advantage up to ~700°C) |
Maximum Recommended Service Temp | 980°C air (continuous oxidizing) | 700°C (gamma-prime overages above this; strength drops rapidly) |
Cryogenic Performance | Excellent; -196°C CVN > 100 J | Good to -196°C; -253°C (LH2) requires special grade (718L for liquid hydrogen) |
Key takeaway: 718 is the aerospace high-temperature strength champion. 625 is the corrosion champion across the full temperature range.
Inconel 625 is decisively superior in virtually all corrosive environments, its higher molybdenum content, which elevates its Pitting Resistance Equivalent Number (PREN) to approximately 46-52 compared to 26-31 for Inconel 718.
Key advantages include:
(1) excellent resistance to pitting and crevice corrosion in chloride-containing environments including seawater
(2) outstanding immunity to stress corrosion cracking (SCC) in chloride media even at elevated temperatures
(3) strong performance in reducing acids (sulfuric, phosphoric, hydrochloric)
(4) minimal sensitization risk during welding.
Inconel 718 offers adequate corrosion resistance for many aerospace and general industrial applications but should not be specified for aggressive chemical processing or marine service where 625 is available.
Environment / Medium | Inconel 625 | Inconel 718 | Notes |
Seawater (ambient) | A – Excellent | C – Fair | 625: immune to pitting/SCC; 718: pitting risk >30°C |
Seawater (elevated T) | A – Excellent | D – Poor | 625 proven to 200°C+; 718 not recommended |
H2SO4 (dilute, <10%) | A – Excellent | B – Good | Mo content drives difference |
H2SO4 (conc., >60%) | C – Fair | D – Poor | Neither ideal; consider B-3, tantalized steel |
HCl (dilute, <2%) | B – Good | NR | 625 usable briefly; 718 attacks rapidly |
H3PO4 (all conc.) | A – Excellent | B-C | 625 industry standard for phosphoric acid plants |
NaOH (caustic) | A – Excellent | B – Good | Caustic cracking possible in 718 at high T |
Chloride SCC | Immune | Susceptible | Critical differentiator for oil & gas |
Oxidation (to 980°C) | A – Excellent | A – Excellent | Comparable; both form protective Cr2O3 |
Sulfidation | B – Good | B – Good | Similar; neither matches Hastelloy X |
Table: Comparative corrosion resistance ratings based on laboratory testing and field experience. Ratings: A=Excellent, B=Good, C=Fair/Acceptable, D=Limited/Poor, NR=Not Recommended.
Inconel 625 is one of the most weldable nickel superalloys available — it can be welded using GTAW (TIG), GMAW (MIG), SMAW (stick), PAW (plasma), EBW, and resistance methods without preheating or post-weld heat treatment (PWHT), and the welded joint retains essentially the same properties as the base metal.
Inconel 718 is weldable but requires careful procedure control: welds must be made in the solution-annealed condition (not the aged condition), followed by a complete re-solution treatment plus re-aging to restore properties in the heat-affected zone (HAZ).
Improperly welded or post-treated 718 develops strain-age cracking susceptibility and HAZ softening that can reduce joint efficiency to 70-85% of base metal strength.
Welding Characteristic | Inconel 625 | Inconel 718 | Practical Impact |
Preheat Required | No (RT – 150°C max) | No (RT – 150°C max) | Neither needs significant preheat |
PWHT Required | NO | YES (mandatory) | Major cost/schedule driver for 718 |
Recommended Filler | ERNiCrMo-3 / ERNiCr-3 | ERNiFeCr-2 | Filler selection affects joint properties |
HAZ Property Retention | ~95-100% | 70-85%* (*without PWHT) | 625 wins hands-down for as-welded use |
Strain-Age Crack Risk | Very Low | Moderate-High | Constraint + aging = risk for 718 |
Compatible Processes | GTAW/GMAW/SMAW/PAW/EBW/SAW | GTAW/GMAW/PAW/EBW | 718: avoid SMAW for critical apps |
Interpass Temp Limit | ≤150°C | ≤150°C | Same practical guideline |
Post-Weld Machining | Same as BM | May vary (HAZ soft spots) | 718: check hardness map before finish machining |
Inconel 625 — Corrosion-Limited Applications
Offshore oil and gas: subsea piping, Christmas tree components, flowlines, umbilical tubes, and manifolds in deepwater (> 1,500 m water depth) where seawater corrosion is the primary threat.
Sour gas wells: downhole tubulars, wellhead equipment, and flowlines for H₂S-containing gas per NACE MR0175 / ISO 15156-3; 625 is specified in preference to 718 in sour gas because of its much better resistance to SSC and general corrosion in H₂S/CO₂/Cl⁻ environments.
Seawater cooling systems: heat exchanger tubes, piping, and condensers in LNG, petrochemical, and power plant seawater cooling circuits; 625 tubes replace Cu-Ni 90/10 or 70/30 where higher temperature or chloride levels exceed Cu-Ni's capability.
Flue gas desulfurization (FGD): absorber vessels, slurry piping, and mist eliminator hardware in wet FGD scrubbers processing coal-fired boiler flue gas containing HCl, HF, and SO₂.
Chemical processing: reactors, columns, and piping for acetic acid, phosphoric acid, and mixed acid environments where 316L or duplex stainless fails within months.
Pollution control: scrubbers, ducting, and stacks handling halogenated compounds and acidic gases.
Nuclear: fuel reprocessing plant piping where nitric acid resistance is required.
Marine: propellers, shafts, and hardware in seawater service; often used as cladding on carbon steel for seawater applications.
Desalination: brine heaters, evaporator tubes, and preheaters in multi-stage flash (MSF) and multi-effect distillation (MED) plants.
Inconel 718 — Strength-Limited Applications
Aero engine turbine disks and cases: the single largest application — 718 is the dominant alloy for high-pressure compressor disks, turbine disks, and cases in Rolls-Royce, GE, Pratt & Whitney, and Safran jet engines; the combination of high strength-to-weight ratio, fatigue resistance, and creep resistance at 650°C makes it irreplaceable in this application.
Liquid rocket motor casings and cryogenic tanks: SpaceX Merlin engine injector plates, Saturn V S-IC interstage structure, and current Space Launch System (SLS) components; 718L (low-interstitial) is used in liquid hydrogen (−253°C) and liquid oxygen (−183°C) propellant tanks.
Gas turbine hot section components: combustor cans, transition ducts, and turbine blade attachments in land-based gas turbines (GE LM series, Siemens SGT); 718 operates up to 700°C in these applications.
Nuclear reactor internal components: core shrouds, control rod drive mechanisms, and pressure vessel internals where the combination of neutron irradiation resistance and high strength is needed.
Downhole drilling tools: drill collars and mud motor stators for HPHT (high-pressure, high-temperature) wells where both strength and moderate corrosion resistance are required.
Bolting and fasteners: 718 fasteners (nuts, bolts, studs) for aerospace and high-pressure vessel flanges where combined bolt preload capacity and temperature capability are critical.
Medical devices: MRI superconducting magnet cryostat structural supports where 718's combination of cryogenic toughness, non-magnetic behavior, and strength is valued.
Oil and gas wellhead subsea Christmas tree valves: bodies and bonnets for API 6A Class 4,000– 15,000 psi equipment where high strength is needed for pressure containment.
Inconel 718 typically costs 15-30% less than Inconel 625 per kilogram in equivalent product forms.
Cost Factor | Inconel 625 | Inconel 718 | Commentary |
Raw Material ($/kg)* | $80 – 150 | $65 – 120 | 718 cheaper due to higher Fe content, volume |
Plate (10mm, commercial) | $95 – 170/kg | $75 – 140/kg | Varies with thickness, quantity, mill source |
Bar (dia. 25mm, commercial) | $85 – 155/kg | $70 – 130/kg | Aerospace (AMS) grade: add 20-40% |
Sheet (1mm, foil) | $120 – 220/kg | $100 – 190/kg | Thin-gauge premium applies to both |
Heat Treatment (per lot) | Not required | $500 – 2,000 | Solution + double aging for 718 |
Machining Cost Index | 1.0× (baseline) | 1.2 – 1.4× | Aged 718 harder on tools |
Typical Scrap Rate | 1 – 3% | 3 – 8% | 718 more sensitive to process deviation |
Global Supply Base | 8-12 major producers | 12+ major producers | 718: larger, more competitive market |
Table: Indicative pricing for plate/bar products in common sizes. Actual prices vary with quantity, supplier, market conditions, and product form. Premium surcharges apply for aerospace-grade (AMS) material vs. commercial (ASTM) grades.
Q: Can I substitute Inconel 625 for Inconel 718 (or vice versa) in an existing design?
A: Generally no. Substituting 625 for 718 in a strength-critical design will likely result in insufficient load capacity.
Q: Is Inconel 625 magnetic? Is Inconel 718 magnetic?
A: In the annealed condition, both alloys are essentially non-magnetic. However, Inconel 718 in the fully aged condition may exhibit slight ferromagnetism due to the presence of precipitate phases.
Q: What is the maximum service temperature for each alloy?
A: Inconel 625: Continuous oxidation-limited service up to 980°C; useful mechanical strength retained to approximately 815°C for static loads, lower for creep-limited applications. Inconel 718: Optimal strength window is 540-700°C; short-term excursions to 870°C are permissible but cause permanent microstructural degradation.
Q: Can Inconel 625 and 718 be welded together (dissimilar weld)?
A: Yes, this is a common practice in repair scenarios and hybrid designs. Use <b>ERNiCrMo-3 (Inconel 625 filler metal)</b> as the universal recommendation — it provides good compatibility with both base metals, excellent weldability, and corrosion resistance matching the 625 side. Alternative fillers include ERNiCr-3 for slightly higher strength.
Q: Which is more corrosion resistant: Inconel 625 or Inconel 718?
A: Inconel 625 is significantly more corrosion resistant than Inconel 718. 625's PREN of 45–52 (vs ~20–28 for 718) gives it immunity to seawater pitting, chloride SCC, and most acid media.
Q: Which alloy is stronger: Inconel 625 or Inconel 718?
A: Inconel 718 in the peak-aged condition is approximately twice as strong as annealed Inconel 625 at room temperature: yield strength of 1,035 MPa vs 450 MPa.
Q: Which alloy has higher maximum service temperature?
A: Inconel 625 has higher temperature limits in both oxidizing and reducing environments. 625 operates continuously to 980°C in air and to 650°C in reducing media. 718 is limited to 700°C by the overaging temperature of its gamma-prime phase — above 700°C, the precipitates coarsen and strength drops rapidly. For temperatures between 700°C and 980°C in a corrosion environment, 625 is the correct choice.
Q: Can Inconel 718 be used as a substitute for Inconel 625 in corrosion applications?
A: No — substituting 718 for 625 to save cost in a corrosion application is a serious engineering error. 718 has roughly one-third the molybdenum content of 625, giving it PREN ~20–28 vs 625's 45–52. In seawater, sour gas, or acid service, 718 will fail by pitting, crevice attack, or SSC where 625 would survive.
Q: Is Inconel 625 or Inconel 718 easier to weld?
A: Inconel 625 is significantly easier to weld. It is welded using standard GTAW or GMAW with ERNiCrMo-3 filler and interpass temperature ≤ 150°C. Inconel 718, while weldable, requires matching filler, very tight interpass temperature control, and a mandatory post-weld solution treatment + aging cycle to restore HAZ properties and strength.
Q: Which alloy is more expensive?
A: Inconel 718 is 10–30% more expensive per kilogram for standard mill product, and 40–60% more for aerospace-grade VIM+VAR melted, ultrasonically inspected material.
Q: What is the shelf life of aged Inconel 718?
A: There is no practical shelf life limitation for either alloy in standard product forms. Inconel 625 in the annealed condition is metallurgically stable and requires no shelf-life consideration. Inconel 718 in the aged (STA) condition is also stable at room temperature.
