Comparing the Two Alloys in Sheet Form
Inconel 600 and Inconel 718 are the two nickel-chromium alloys most often requested as sheet, and buyers frequently ask for a direct comparison because both are stocked in similar gauges and both offer strong resistance to high-temperature oxidation. The distinction is in how each alloy gets its strength. Inconel 600, UNS N06600, is a solid-solution strengthened nickel-chromium-iron alloy supplied in the annealed condition, so its properties depend on chemistry and heat treatment. Inconel 718, UNS N07718, is a precipitation-hardening alloy that is solution treated and then aged, and its strength comes from fine niobium-rich phases that form inside the grains.
For sheet and plate work that difference shows up in three places: how much the material fights you when forming, whether a heat treatment is needed after welding, and how much strength the finished assembly carries in service.
Specifications and Available Forms
Inconel 600 sheet, strip and plate are covered by ASTM B168, with ASME SB168 as the code equivalent. Inconel 718 plate, sheet and strip are covered by ASTM B670, with AMS 5596 and AMS 5597 frequently added for aerospace work, since those specifications control cleanliness, grain size and the ageing response more tightly than the general specification does. Both alloys are produced by hot rolling for heavier plate and cold rolling for lighter gauges.
| Item | Inconel 600 | Inconel 718 |
|---|---|---|
| Common UNS number | N06600 | N07718 |
| Sheet and plate specification | ASTM B168 / ASME SB168 | ASTM B670 / ASME SB670, with AMS options |
| Supply condition | Annealed, pickled or bright annealed | Solution treated; aged after forming where required |
| Typical cold-rolled sheet | About 0.3 mm to 6 mm | About 0.4 mm to 6 mm |
| Typical hot-rolled plate | About 6 mm and above | About 6 mm and above |
Strength Levels and What They Mean in a Fabrication
Annealed Inconel 600 sheet typically develops a tensile strength around 550 MPa (80 ksi) with a yield strength near 240 MPa (35 ksi) and elongation of about 30 %. Inconel 718 sheet, once solution treated and aged, typically reaches a tensile strength in the region of 1240 MPa (180 ksi) and a yield strength near 1030 MPa (150 ksi), with elongation around 12 %. The yield strength difference is roughly fourfold, and the ductility difference is equally important: 600 will stretch and accommodate forming defects, while 718 will not.
A fabricator therefore treats 718 as a high-strength material that must be formed and welded in the soft condition and only then aged to full strength. If a part is aged before forming, springback rises sharply, cracking risk increases and repair is nearly impossible.
Forming Behaviour
Inconel 600 sheet forms much like a tough austenitic stainless steel. It work hardens, so tight radii and deep draws need intermediate annealing, and the tooling must be generous in radius with adequate clearance to avoid scoring. Lubrication should be heavy and formulated for high-pressure contact rather than for a general steel forming operation.
Inconel 718 sheet is harder to form in every respect. It is normally formed in the solution treated condition, where it is ductile enough for brake forming, light rolling and simple draws. Springback is significant and must be compensated in the tooling, warm forming may be needed for tight radii, and any severe forming operation should be followed by a full solution anneal before ageing so that the strain history does not corrupt the final properties.
Welding and Joining
Inconel 600 is welded with common arc processes using matching or nickel-based filler metals, and post-weld heat treatment is generally unnecessary for sheet fabrications. The alloy tolerates the thermal cycle well and retains ductility in the heat-affected zone.
Inconel 718 is also welded, but the weld and heat-affected zone of aged material lose strength, so the sequence must be planned. The practical rule is to weld in the solution treated condition, use a suitable nickel-based filler or weld autogenously where the design allows, and then age the completed assembly. Where a very high joint efficiency is needed, post-weld solution treatment followed by ageing restores properties across the joint but brings distortion risk, so fixtures and allowances must be planned before welding starts.
Corrosion and High-Temperature Service
Inconel 600 sheet is chosen for continuous service in air up to about 1093 C (2000 F), for carburizing and nitriding furnace atmospheres, and for chloride-bearing aqueous service where its high nickel content provides immunity to chloride stress corrosion cracking. It is the more forgiving of the two where the atmosphere is inconsistent or where the component experiences frequent thermal cycling.
Inconel 718 sheet is selected for strength retention in the 540 C to 650 C range combined with good fatigue and stress rupture behaviour. It resists many oxidizing and reducing environments and performs well in chloride-containing service where mechanical load is present. For sustained operation well above 650 C, the precipitation-hardening phases coarsen and a different grade is more appropriate, so 718 is not the automatic answer for the hottest furnace zones.
How to Choose Between Them
Choose Inconel 600 sheet where the fabrication involves deep forming, where field welding with minimal heat treatment is required, where the service temperature is very high but the mechanical load is moderate, or where a tolerance for thermal cycling is important. Choose Inconel 718 sheet where strength, fatigue life or weight saving governs the design, where the service window sits in the mid-temperature range, and where a controlled ageing cycle can be built into the manufacturing route. Where both could work, the deciding factor is usually whether the shop can manage the ageing sequence and whether the additional cost is justified by the section reduction the higher strength allows.
Frequently Asked Questions About Inconel 600 and 718 Sheet
Q: Can Inconel 718 sheet be bent after ageing?
A: It should not be. In the aged condition the material is strong but far less ductile, so bends crack or spring back excessively. Forming is done in the solution treated condition and the ageing treatment follows afterwards.
Q: Which specification covers Inconel 600 sheet and which covers Inconel 718 sheet?
A: Inconel 600 sheet, strip and plate fall under ASTM B168, while Inconel 718 plate, sheet and strip are covered by ASTM B670. Aerospace work often adds an AMS specification that imposes tighter controls on grain size and cleanliness.
Q: Is Inconel 600 sheet suitable for furnace muffles and retorts?
A: Yes. Its oxidation resistance in air extends to about 1093 C (2000 F), and it withstands carburizing and nitriding atmospheres, which makes it a common choice for muffles, retorts and heat-treat fixtures. Strongly sulfidizing atmospheres are the exception and need a higher-chromium alloy.
Q: How much stronger is Inconel 718 sheet than Inconel 600 sheet?
A: In the aged condition Inconel 718 typically offers about four times the yield strength of annealed Inconel 600, with lower elongation. That difference allows a thinner section for the same load but reduces the tolerance for forming and repair.
Q: Do both alloys need post-weld heat treatment?
A: Inconel 600 generally does not, because the weld and heat-affected zone keep useful ductility. Inconel 718 does need an ageing treatment after welding to restore strength in the joint region, and the welding sequence must be planned around that requirement.
Q: Can the two alloys be welded to each other?
A: They can, using a nickel-based filler metal selected for the combination, and the joint is usually designed so that the lower-strength alloy governs the allowable load. Welding procedure qualification should be completed for the specific thickness and joint design before production begins.
