Choosing Between Four Workhorse Grades
Inconel nickel alloys are renowned for their excellent performance in high-temperature and high-strength applications, and many grades are also widely used for their superior corrosion resistance in harsh chemical environments. Four grades cover most industrial needs: Inconel 600 for high-temperature and oxidation service, Inconel 601 for improved mechanical properties, Inconel 625 for broad corrosion resistance, and Inconel 718 for maximum strength. This guide summarizes what each grade does best.
Inconel 600: High-Temperature Resistance
Inconel 600 (UNS N06600) offers oxidation and carburization resistance at high temperatures, with good resistance to chlorine media such as dry chlorine, chloride-ion stress corrosion cracking and hydrogen chloride gases at moderate temperatures. It is excellent for furnace components and chemical processing equipment, and is the reference grade where long-term high-temperature stability with good fabricability is required.
Inconel 601: Mechanical Properties
Inconel 601 (UNS N06601) provides similar oxidation and carburization resistance to grade 600 with improved high strength and mechanical toughness, typically showing about 107 ksi tensile strength compared with about 93 ksi for grade 600. It offers good creep and rupture strength and metallurgical stability in heat, and is able to resist strain at high temperatures, making it a strong candidate for furnace rolls, radiant tubes and heat-treating fixtures where mechanical loading accompanies heat.
Inconel 625: Corrosion Resistance
Inconel 625 (UNS N06625) is best known for its high-temperature corrosion resistance and is often used in chemical processing, nuclear power plants and oil and gas applications. It shows good resistance to harsh chemicals including sulfuric, hydrofluoric and phosphoric acids, and its corrosion resistance is often compared with the C-family alloys. Its molybdenum and niobium content also provides outstanding strength and toughness from cryogenic temperatures to 1090°C.
Inconel 718: Maximum Strength
Inconel 718 (UNS N07718) far exceeds most materials available to industry in terms of high strength. Per ASTM B637 it offers a minimum of about 185 ksi ultimate tensile strength and 150 ksi yield strength, with work-hardened versions reaching a minimum of about 220 ksi ultimate tensile strength. It is ideal for high-strength, corrosive applications such as gas turbine engines, down-hole oil and gas and aerospace components, and retains excellent weldability despite its strength.
Selection Summary
Start with the service environment: for oxidation-dominated high-temperature service, consider 600 or 601, choosing 601 when mechanical strength matters; for corrosive chemical or marine service, choose 625; for strength-critical components such as fasteners, shafts and downhole tools, choose 718. In all cases, specify the grade by UNS number plus the governing ASTM or ASME standard, and confirm the required condition, such as annealed, solution-treated or age-hardened, in the purchase specification.
FAQ
Q: Which Inconel grade resists the highest temperature?
A: 600 and 601 both serve high-temperature oxidizing service; 601 adds improved mechanical strength and creep resistance.
Q: Which grade is strongest?
A: Inconel 718, with a minimum of 185 ksi ultimate tensile strength and 150 ksi yield per ASTM B637 in the age-hardened condition.
Q: Which grade is best for chemical corrosion?
A: Inconel 625, with resistance to sulfuric, hydrofluoric and phosphoric acids and performance often compared with C-family alloys.
Q: What is the difference between 600 and 601?
A: 601 adds aluminum for improved oxidation resistance and higher strength, typically about 107 ksi versus 93 ksi tensile strength for 600.
Q: Is Inconel 718 weldable?
A: Yes, it has excellent weldability despite its high strength, often welded in the solution-treated condition and aged afterward.
Q: How should I specify these alloys?
A: By UNS number plus the governing standard, such as ASTM B166, B167, B443 or B637, with the required condition stated.
