The nickel-based alloy Family in Plate and Sheet
The nickel-based alloy alloys are nickel-chromium-based materials developed for service that combines high temperature, corrosion, and mechanical stress. Five grades dominate plate and sheet applications: 718, 625, 601, 690, and X-750. Each belongs to a different family, with distinct compositions, strengthening mechanisms, and service limits, and each is governed by its own ASTM, ASME, and AMS specifications.
Understanding these differences is the foundation of correct selection. The precipitation-hardened grades achieve strength through aging heat treatment, while the solid-solution grades are used in the annealed condition and are preferred where weldability and fabricability dominate the design.
Compositions and Strengthening Mechanisms
nickel alloy UNS N07718 (UNS N07718) contains 50% to 55% nickel, 17% to 21% chromium, 4.75% to 5.5% niobium, 2.8% to 3.3% molybdenum, with aluminum at 0.20% to 0.80% and titanium at 0.65% to 1.15%. It is precipitation-hardened by gamma double-prime (Ni3Nb) and gamma prime phases, reaching yield strengths above 150 ksi (1034 MPa) after aging, and is the most widely used superalloy in aerospace for components loaded up to 1300°F (700°C).
nickel alloy UNS N06625 (UNS N06625) contains 58% minimum nickel, 20% to 23% chromium, 8% to 10% molybdenum, and 3.15% to 4.15% niobium. It is solid-solution strengthened, requires no aging, and offers outstanding fatigue strength and chloride pitting resistance for marine and chemical service. nickel alloy UNS N06601 (UNS N06601) adds 1.0% to 1.7% aluminum to a 58% to 63% nickel, 21% to 25% chromium base, forming a tenacious aluminum oxide scale with superior oxidation and carburization resistance to about 2200°F (1204°C). nickel-based alloy 690 (UNS N06690) carries 27% to 31% chromium for exceptional resistance to stress-corrosion cracking in high-temperature aqueous environments and is the established replacement for nickel alloy UNS N06600 in nuclear service. nickel alloy UNS N07750 (UNS N07750) is precipitation-hardened primarily by gamma prime, with 2.25% to 2.75% titanium and 0.4% to 1.0% aluminum, and serves high-temperature springs, fasteners, and gas turbine components to about 1500°F (816°C).
Heat Treatment and Mechanical Properties
Heat treatment protocols reflect the strengthening mechanisms. nickel alloy UNS N07718 is solution treated at 1700°F to 1850°F (927°C to 1010°C), rapidly cooled, then aged in two stages at 1325°F (718°C) for 8 hours and 1150°F (621°C) for 8 hours, producing tensile strength of 180 to 200 ksi and yield strength of 150 to 180 ksi. nickel alloy UNS N07750 is solution treated at 2100°F to 2150°F and aged at 1550°F for 24 hours plus 1300°F for 20 hours, reaching 150 to 180 ksi tensile with 100 to 140 ksi yield.
The solid-solution grades are used in the annealed condition. nickel alloy UNS N06625 is solution annealed and shows 120 to 140 ksi tensile with 40% to 60% elongation; nickel alloy UNS N06601 anneals with 85 to 100 ksi tensile; and nickel-based alloy 690 anneals with 85 to 115 ksi tensile. Because 718 and X-750 develop final strength through aging, they should be fabricated in the solution-treated condition and aged after forming, while 625, 601, and 690 can be fabricated and used without post-fabrication heat treatment.
Welding and Fabrication
Filler metal selection follows the base alloy: ERNiFeCr-2 for nickel alloy UNS N07718, ERNiCrMo-3 for 625, ERNiCr-3 or ERNiCrFe-6 for 601, ERNiCr-3 or ERNiCrFe-7 for 690, and ERNiCr-3 for X-750, all under AWS A5.14. Pre-weld cleaning removes oils, greases, and sulfur-bearing marking compounds, and dedicated grinding wheels and brushes prevent cross-contamination from carbon steel or copper. Edges are machined or ground to remove oxides.
Heat input is controlled with stringer beads, interpass temperatures below 200°F to 300°F (93°C to 149°C), and argon or argon-helium shielding with back purging of root passes. The precipitation-hardened grades are welded in the solution-treated condition and fully aged afterwards, with stress relief before aging where strain-age cracking is a risk. The annealed grades are typically used as-welded. All these alloys work-harden rapidly, so complex cold forming may need intermediate annealing, and their high springback and tendency to gall require compensated tooling, sharp tools, and quality lubricants.
Specifications, Certifications, and Procurement
Plate and sheet are covered by ASTM B670 and ASME SB-670 for nickel alloy UNS N07718, with AMS 5596 and 5597; ASTM B443 and SB-443 for 625, with AMS 5599; ASTM B168 and SB-168 for both 601 and 690, with AMS 5870 and 5871; and ASTM B637 and SB-637 for X-750, with AMS 5542 and 5582. Plate is defined as 0.1875 inches (4.76 mm) and thicker, sheet from 0.005 to 0.1875 inches, and standard widths range from 36 to 60 inches.
A complete mill test report must document heat analysis, mechanical properties, heat treatment details, grain size where applicable, and nondestructive test results, and each plate or sheet carries a heat number traceable to the original melt with the specification, grade, and manufacturer identification. Critical procurement practices include specifying the complete standard and condition, such as ASTM B670 UNS N07718 Condition A, requiring the MTR with shipment, allowing extra lead time for the heat treatment of precipitation-hardened grades, and establishing receiving inspection with positive material identification. Aerospace and nuclear applications often mandate vacuum induction melting followed by vacuum arc or electroslag remelting for enhanced cleanliness, and plate over 0.5 inches thick may require ultrasonic examination.
Frequently Asked Questions
Which nickel-based alloy grade offers the highest strength at temperature? nickel alloy UNS N07718, precipitation-hardened by gamma double-prime and gamma prime, delivers yield strength above 150 ksi and serves rotating aerospace components up to about 1300°F (700°C).
Which nickel-based alloy is best for chloride pitting resistance? nickel alloy UNS N06625, with 8% to 10% molybdenum and 3.15% to 4.15% niobium, is solid-solution strengthened and offers outstanding pitting and crevice corrosion resistance in chloride environments.
Which nickel-based alloy resists oxidation at the highest temperature? nickel alloy UNS N06601, whose aluminum addition forms an adherent aluminum oxide scale, resists oxidation and carburization up to about 2200°F (1204°C), including cyclic thermal service.
What filler metal is used to weld nickel alloy UNS N06625 plate? ERNiCrMo-3 (AWS A5.14) is the matching filler. nickel alloy UNS N06625 is welded in the annealed condition and typically used as-welded without post-weld heat treatment.
Which ASTM specification covers nickel alloy UNS N07718 plate? ASTM B670, adopted by ASME as SB-670, covers nickel alloy UNS N07718 plate, sheet, and strip, with AMS 5596 and 5597 applying to aerospace procurement.
What must a mill test report for nickel-based alloy plate include? complete MTR documents heat analysis, mechanical properties, heat treatment details, grain size where applicable, and NDE results, together with a heat number traceable to the original melt.
