Two Nickel-Chromium Alloys, Different Performance Peaks
nickel alloy UNS N06600 (UNS N06600) and nickel alloy UNS N06601 (UNS N06601) are both nickel-chromium alloys renowned for excellent oxidation resistance and high-temperature performance, but their compositions target different environments. nickel alloy UNS N06600 is the higher-nickel alloy, with about 72-76% nickel and 14-17% chromium, which makes it exceptionally resistant in reducing and halide-containing environments and almost immune to chloride stress corrosion cracking. nickel alloy UNS N06601 contains about 58-63% nickel and roughly 21-25% chromium, with a deliberate aluminum addition of about 1.0-1.7%; the aluminum forms a stable, adherent alumina scale that gives 601 its signature resistance to high-temperature oxidation and its superior creep resistance above about 1000 degC. In simple terms, 600 is the choice for corrosive chemical service and 601 is the choice for the hottest furnace and heat exchanger service.
Chemical Composition
| Element | nickel alloy UNS N06600 (%) | nickel alloy UNS N06601 (%) |
|---|---|---|
| Nickel | 72-76 | 58-63 |
| Chromium | 14-17 | 21-25 |
| Iron | Balance | Balance |
| Carbon | 0.15 max | 0.10 max |
| Aluminum | Not specified | 1.0-1.7 |
| Molybdenum | None specified | None specified |
Composition ranges follow the applicable ASTM specifications, ASTM B168 for plate, sheet, and strip and B166 for rod and bar; verify the current specification for the exact limits.
Mechanical and High-Temperature Properties
| Property | nickel alloy UNS N06600 | nickel alloy UNS N06601 |
|---|---|---|
| Yield strength, annealed (MPa) | 200-310 typical | 250-450 typical |
| Ultimate tensile strength (MPa) | 450-700 typical | 550-750 typical |
| Elongation | 30% min | 30% min |
| Service temperature limit | Up to about 1038 degC | Up to about 1149 degC |
| Oxidation resistance | Good | Excellent |
| Creep resistance | Moderate | Very good |
Both alloys are suitable for high-temperature service, but 601's aluminum-stabilized oxide film gives it a measurable advantage in continuous oxidation resistance and in creep at the top of its range, which is why it is preferred for superheaters, reheaters, and furnace components exposed to the most severe oxidizing conditions.
Corrosion Resistance
In general corrosion and pitting, both alloys perform well, with 600 rated very good in general chemical service and 601 rated very good in pitting resistance. The defining differences appear in specific mechanisms. nickel alloy UNS N06600's very high nickel content gives it excellent resistance to chloride stress corrosion cracking and to non-oxidizing acids, which is why it remains a standard for chemical processing equipment, reactors, and heat treating equipment in halide service. nickel alloy UNS N06601 also resists stress corrosion cracking well, and its higher chromium plus aluminum content gives it excellent high-temperature acid and oxidizing-salt resistance; where the environment is a hot oxidizing gas rather than a liquid corrodent, 601 is the stronger material. Weldability is good for both: 600 normally requires no preheating and no post-weld heat treatment, while 601 is also weldable without mandatory PWHT, with matching filler metals selected for each alloy.
| Property | nickel alloy UNS N06600 | nickel alloy UNS N06601 |
|---|---|---|
| Density (g/cm3) | 8.47 | 8.11 |
| Melting range (degC) | 1354-1413 | 1360-1411 |
| Permeability at 200 oersted | 1.010 | 1.003 |
Applications and Selection Guidance
nickel alloy UNS N06600 is used for heat treating equipment, reactors, gas turbine components, oil refining systems, boiler tubes, and furnace and kiln components where corrosion resistance in chemical service and good high-temperature strength are required. nickel alloy UNS N06601 is used for chemical plant heat exchangers, high-temperature aerospace components, superheaters and reheaters in power generation, and furnace components where enhanced high-temperature stability and oxidation resistance are the dominant requirements. As a practical rule, select 600 when the service is corrosive and halide-rich, and select 601 when the service is a hot oxidizing atmosphere near the top of the temperature range.
Frequently Asked Questions
What is the main difference between nickel alloy UNS N06600 and 601? Composition and the resulting performance. 600 has 72-76% nickel for maximum resistance in reducing and chloride environments, while 601 has about 58-63% nickel plus 1.0-1.7% aluminum for superior oxidation resistance at very high temperature.
Which alloy resists oxidation better? nickel alloy UNS N06601. Its aluminum addition forms a stable alumina scale beneath the chromia layer, giving excellent oxidation resistance up to about 1149 degC compared with about 1038 degC for 600.
Is nickel alloy UNS N06600 resistant to chloride stress corrosion cracking? Yes. Its very high nickel content makes it almost completely immune to chloride stress corrosion cracking, which is why it is used for chemical and heat treating equipment in halide service.
Which alloy is better for superheaters and reheaters? nickel alloy UNS N06601, because of its better oxidation resistance and creep resistance at the highest service temperatures encountered in power generation.
Do these alloys require post-weld heat treatment? Generally not. Both weld well with matching nickel-based filler metals, and PWHT is not normally required for standard corrosion and high-temperature service.
How machinable is nickel alloy UNS N06601? nickel alloy UNS N06601 is considered easy to form, machine, and weld for a nickel alloy, with good mechanical strength and water corrosion resistance, though standard nickel alloy machining practice with rigid tooling and positive feeds should be followed.
