Four Alloys, Four Strengths
nickel alloy UNS N06600, 601, 625, and 718 round bars each have their advantages. nickel alloy UNS N06600 offers excellent resistance to chloride stress-corrosion cracking. nickel alloy UNS N06601 provides superior high-temperature oxidation resistance. nickel alloy UNS N06625 offers high strength and corrosion resistance, particularly suited to marine and oil and gas applications. nickel alloy UNS N07718 is the best choice for high-strength applications below its melting point, with excellent yield strength, tensile strength, and creep-rupture properties. The specific choice depends on the combination of temperature, stress, and corrosive environment in the service.
Chemical Composition Comparison
The key composition ranges of the four grades are compared below. Note that 625 and 718 additionally rely on niobium and titanium additions, and 601 carries a deliberate aluminum addition of 1.0-1.7% that drives its oxidation resistance.
| Element | nickel alloy UNS N06600 | nickel alloy UNS N06601 | nickel alloy UNS N06625 | nickel alloy UNS N07718 |
|---|---|---|---|---|
| Nickel (Ni) | 72.0 min | 58.0-63.0 | 58.0 min | 50.0-55.0 |
| Chromium (Cr) | 14.0-17.0 | 21.0-25.0 | 20.0-23.0 | 17.0-21.0 |
| Molybdenum (Mo) | - | - | 8.0-10.0 | 2.8-3.3 |
| Copper (Cu) | 0.5 max | 1.0 max | - | 0.3 max |
| Carbon (C) | 0.15 max | 0.10 max | 0.10 max | 0.08 max |
| Manganese (Mn) | 1.0 max | 1.0 max | 0.50 max | 0.35 max |
| Sulfur (S) | 0.015 max | 0.015 max | 0.015 max | 0.015 max |
| Silicon (Si) | 0.5 max | 0.5 max | 0.50 max | 0.35 max |
| Cobalt (Co) | - | - | 1.0 max | 1.0 max |
Mechanical Properties at a Glance
Typical room-temperature values for annealed round bar are summarized below.
| Grade | Density | Melting Point | Tensile Strength | Yield Strength (0.2%) | Elongation |
|---|---|---|---|---|---|
| nickel alloy UNS N06600 | 8.47 g/cm3 | 1413°C (2580°F) | 95,000 psi (655 MPa) | 45,000 psi (310 MPa) | 40% |
| nickel alloy UNS N06601 | 8.1 g/cm3 | 1411°C (2571°F) | 80,000 psi (550 MPa) | 30,000 psi (205 MPa) | 30% |
| nickel alloy UNS N06625 | 8.4 g/cm3 | 1350°C (2460°F) | 135,000 psi (930 MPa) | 75,000 psi (517 MPa) | 42.5% |
| nickel alloy UNS N07718 | 8.2 g/cm3 | 1350°C (2460°F) | 135,000 psi (930 MPa) | 70,000 psi (482 MPa) | 45% |
While 625 and 718 show similar annealed tensile values, 718 is age-hardenable: after the standard solution and aging treatment its yield strength rises well above the annealed value, which is why it dominates high-stress rotating and structural applications.
International Equivalent Grades
The established international designations for the four grades are listed below.
| Grade | Werkstoff Nr. | UNS | AFNOR | JIS | BS | EN |
|---|---|---|---|---|---|---|
| nickel alloy UNS N06600 | 2.4816 | N06600 | NC15FE11M | NCF 600 | NA 13 | NiCr15Fe |
| nickel alloy UNS N06601 | 2.4851 | N06601 | NC23FeA | NCF 601 | NA 49 | NiCr23Fe |
| nickel alloy UNS N06625 | 2.4856 | N06625 | NC22DNB4M | NCF 625 | NA 21 | NiCr22Mo9Nb |
| nickel alloy UNS N07718 | 2.4668 | N07718 | - | - | - | - |
Application Guidance and Selection
nickel alloy UNS N06600 is the choice when resistance to specific corrosive environments matters more than overall high-temperature strength: it resists chloride stress-corrosion cracking, and dry chlorine and hydrogen chloride up to 650°C, and it is generally the most economical of the four thanks to wider availability. nickel alloy UNS N06601 is selected for high-temperature oxidation service such as furnaces and engine components; its high chromium content and aluminum addition give it oxidation resistance far superior to 600, and it keeps good ductility after prolonged high-temperature exposure. nickel alloy UNS N06625 serves corrosive, high-stress applications such as deep-sea equipment in oil and gas and aerospace exhaust systems, with high strength and excellent resistance to water-based corrosion, its molybdenum content providing good resistance to reducing media. nickel alloy UNS N07718 is specified for high-stress, high-temperature components such as jet engine parts, where its exceptionally high yield, tensile, and creep-rupture strength, tunable by aging treatment, are required.
Selection should follow the service envelope: temperature ceiling, applied stress, and the specific corrosive media. Where oxidation dominates and strength demand is moderate, 601 wins; where chloride stress-corrosion is the risk, 600; where seawater and high stress combine, 625; and where maximum strength at temperature is required, 718 after full aging.
Frequently Asked Questions
What is the difference between nickel alloy UNS N06601 and nickel alloy UNS N06600? 601 carries much more chromium (21-25% versus 14-17%) plus an aluminum addition of 1.0-1.7%. That combination gives 601 far superior high-temperature oxidation resistance, at the cost of lower room-temperature tensile strength and higher cost than 600.
Which nickel-based alloy is strongest at high temperature? nickel alloy UNS N07718, in the age-hardened condition, offers the highest yield, tensile, and creep-rupture strength below its melting point, which is why it is the standard for jet engine rotating and structural parts.
Which nickel-based alloy is best for marine and oil and gas service? nickel alloy UNS N06625. Its high molybdenum content provides excellent resistance to water-based corrosion, pitting, and reducing media, making it the typical choice for deep-sea equipment and aggressive offshore service.
What is the most economical of the four alloys? nickel alloy UNS N06600 is generally the most economical because it is the most widely produced of the four. Its strength lies in specific corrosion resistance, such as chloride stress-corrosion cracking and dry chlorine service, rather than maximum high-temperature strength.
Can nickel alloy UNS N07718 replace nickel alloy UNS N06625? Not as a direct drop-in. 718 is stronger after aging but is not a solid-solution alloy and has different corrosion behavior and welding requirements. 625 remains the choice where corrosion resistance and fabricability are the priority; 718 wins where strength at temperature is the design limit.
What standards govern these round bars? Common specifications include ASTM B166 for 600 and 601 bar, ASTM B446 for 625 bar, and ASTM B637 for 718 bar, with corresponding ASME and AMS versions for pressure and aerospace service respectively.
