Alloy 600 sheet and alloy 718 sheet are both nickel-base materials, yet they are selected for opposite reasons. One is chosen for its tolerance of extreme temperature and aggressive chemistry, the other for its very high mechanical strength under load. Comparing the two on chemistry, properties and fabrication behaviour makes the choice straightforward.
Overview: Two Nickel Alloys, Two Design Philosophies
Alloy 600 sheet (UNS N06600) is a nickel-chromium-iron solid-solution alloy built on roughly 72% nickel with 14-17% chromium and 6-10% iron. The chromium-rich surface oxide gives oxidation, carburisation and nitriding resistance at very high temperature, while the high nickel content provides outstanding resistance to chloride stress corrosion cracking, to high-purity water and to caustic solutions. The grade cannot be strengthened by heat treatment, so sheet is supplied annealed with a tensile strength of about 655 MPa.
Alloy 718 sheet (UNS N07718) is a precipitation-hardening nickel-chromium-iron alloy that adds niobium, molybdenum, titanium and aluminium. A solution anneal followed by a two-step aging treatment precipitates gamma-prime and gamma-double-prime phases, raising tensile strength above 1000 MPa while preserving ductility, fatigue resistance and creep resistance to roughly 650 °C. Sheet is normally supplied to ASTM B670 or AMS 5596 in the solution-treated or solution-treated-plus-aged condition, depending on the forming and welding sequence planned in the workshop.
Chemical Composition and Equivalent Grades
ASTM B168 and ASME SB168 cover alloy 600 plate, sheet and strip; ASTM B670, ASME SB670 and AMS 5596 cover alloy 718 sheet, strip and plate. Composition limits in mass percent are listed below.
| Element | Alloy 600 (N06600) | Alloy 718 (N07718) |
|---|---|---|
| Nickel | 72.0 min | 50.0-55.0 |
| Chromium | 14.0-17.0 | 17.0-21.0 |
| Iron | 6.0-10.0 | balance |
| Molybdenum | - | 2.80-3.30 |
| Niobium plus tantalum | - | 4.75-5.50 |
| Titanium | - | 0.65-1.15 |
| Aluminium | - | 0.20-0.80 |
| Carbon | 0.15 max | 0.08 max |
| Manganese | 1.00 max | 0.35 max |
| Silicon | 0.50 max | 0.35 max |
| Copper | 0.50 max | 0.30 max |
| Sulphur | 0.015 max | 0.015 max |
Cross-reference designations used in other standards systems are summarised below.
| Standard system | Alloy 600 | Alloy 718 |
|---|---|---|
| UNS | N06600 | N07718 |
| AFNOR | NC15FE11M | - |
| GOST | EI868, MNZhMts 28-2,5-1,5 | - |
| BS | NA 13 | - |
| JIS | NCF 600 | NCF 718 |
| EN / W.Nr. | NiCr15Fe / 2.4816 | NiCr19Fe19Nb5Mo3 / 2.4668 |
Mechanical Properties at Room Temperature
The table compares typical room-temperature tensile data for annealed alloy 600 sheet and aged alloy 718 sheet.
| Condition | Tensile strength Rm | 0.2% yield strength Rp0.2 | Elongation A50 | Hardness |
|---|---|---|---|---|
| Alloy 600 sheet, annealed | 655 MPa (95 ksi) | 310 MPa (45 ksi) | 40% | approx. 85 HRB |
| Alloy 718 sheet, aged | 1034 MPa (150 ksi) | 828 MPa (120 ksi) | 20% | approx. 36 HRC |
Alloy 718 offers roughly 58% more tensile strength and about 2.6 times the yield strength of alloy 600 sheet, and it holds that advantage under sustained load. Alloy 600 compensates with far higher elongation and lower notch sensitivity, which simplifies forming, bending and field repair. Higher-strength 718 conditions are specified for bar and heavy sections, so the sheet figures above must not be transferred to other product forms without checking the governing specification.
Oxidation Resistance, High-Temperature Strength and Corrosion Behaviour
Oxidation resistance is the clearest dividing line. Alloy 600 resists scaling in air to about 1175 °C and remains stable in carburising and nitriding atmospheres where austenitic stainless steels fail, which is why it is used for furnace retorts, muffle tubes, radiant tubes and heat-treatment fixtures. Its load-bearing capability at those temperatures is limited, and continuous structural service is normally designed well below the oxidation limit.
Alloy 718 is engineered for strength retention rather than for the highest attainable temperature. It holds useful tensile and creep strength to about 650 °C and tolerates short excursions to 700 °C, above which long-term exposure transforms the strengthening gamma-double-prime phase and creep resistance falls away. At the opposite end of the scale, alloy 718 keeps its toughness in cryogenic service down to about -253 °C, which suits liquefied-gas containment and other low-temperature structures.
In aqueous service, alloy 600 is preferred where chloride stress corrosion cracking, high-purity water or caustic media dominate, while alloy 718 is chosen where the same environments must be combined with high stress. Both grades perform best with a clean, pickled and passivated surface, and both are sensitive to reducing acids, so the specific medium should be evaluated rather than the grade name alone.
Fabrication, Cost and Application Selection
Forming: alloy 600 sheet behaves much like austenitic stainless steel, with moderate work hardening; alloy 718 should be formed in the solution-treated condition and aged afterwards.
Welding: alloy 600 is welded with matching nickel-chromium filler and needs no post-weld heat treatment for most duties; alloy 718 is welded in the solution-treated condition and then aged directly, avoiding a full re-solution anneal.
Machining: alloy 718 work hardens rapidly and accelerates tool wear, so it needs carbide tooling, low surface speeds and rigid setups; alloy 600 machines at conventional austenitic stainless parameters.
Cost: alloy 718 carries a higher price because of its niobium and molybdenum content plus the extra melting, forging and aging steps; alloy 600 sheet is considerably more economical per kilogram.
| Duty | Recommended grade |
|---|---|
| Furnace retorts, radiant tubes, muffle tubes, heat-treatment fixtures | Alloy 600 sheet |
| Chemical and nuclear heat exchangers, caustic and high-purity water service | Alloy 600 sheet |
| Turbine and compressor casings, hot-section components | Alloy 718 sheet |
| High-stress fasteners, springs and structural parts to 650 °C | Alloy 718 sheet |
| Cryogenic tanks, liquefied-gas piping and low-temperature hardware | Alloy 718 sheet |
| Downhole tools exposed to high stress and sour service | Alloy 718 sheet |
Frequently Asked Questions
Q: Which sheet grade is stronger, alloy 600 or alloy 718?
Alloy 718 in the aged condition is far stronger, with a 0.2% yield strength of roughly 828 MPa against about 310 MPa for annealed alloy 600 sheet.
Q: Can alloy 600 sheet be hardened by heat treatment?
No. Alloy 600 is a solid-solution grade and can only be strengthened by cold work; alloy 718 is the precipitation-hardening option in this comparison.
Q: What is the maximum service temperature of alloy 600 sheet?
Oxidation resistance in air extends to about 1175 °C, but continuous load-bearing service is normally designed at much lower temperatures.
Q: Is alloy 718 sheet suitable for cryogenic service?
Yes. It retains toughness down to about -253 °C and is widely used for liquefied-gas storage and transfer equipment.
Q: Which grade resists chloride stress corrosion cracking better?
Alloy 600 is the classic choice for chloride and high-purity water service, while alloy 718 is selected when high mechanical stress must be carried in the same environment.
Q: Does alloy 718 sheet need a post-weld solution anneal?
No. Components are welded in the solution-treated condition and then aged, which is one of the practical advantages of the grade.
