Inconel 600, 601, 625 and 718 Nickel Alloys

Inconel 600 is about three-quarters nickel with 14–17 % chromium and 6–10 % iron. No molybdenum, no niobium, nothing fancy. That simple chemistry gives it good oxidation resistance up to furnace temperatures, very good resistance to caustic soda and other alkalis, and almost complete immunity to chloride stress-corrosion cracking, which is the failure that ends 304 and 316 in hot chloride water.

Typical jobs: furnace muffles, retorts and heat-treatment baskets, heater sheaths, caustic evaporator tubes and nuclear steam generator parts. Rod and bar are bought to ASTM B166, seamless pipe and tube to B167, and plate, sheet and strip to B168.

601 raises chromium to 21–25 % and adds 1.0–1.7 % aluminium. At high temperature the aluminium helps form a tight oxide layer that does not spall when the part is cycled from hot to cold. If your 600 muffle or radiant tube is flaking after a few months of thermal cycling, 601 is the usual next step. It has less nickel than 600 and a balance of iron.

625 is the grade that turns up on chemical, offshore and pollution-control drawings. It combines 20–23 % chromium with 8–10 % molybdenum and 3.15–4.15 % niobium. Molybdenum gives resistance to pitting and crevice corrosion in seawater and acid chlorides. Niobium stiffens the nickel matrix, so annealed 625 bar must reach 827 MPa tensile and 414 MPa proof stress, roughly two and a half times the minimum proof stress of annealed 316L.

It is also very weldable. Matching filler (ERNiCrMo-3) is used to overlay carbon steel valves and pipe for sour gas, and to join dissimilar metals. Common uses are seawater piping, bellows and expansion joints, flue gas scrubber parts, heat-exchanger tubes and marine fasteners. Plate is bought to ASTM B443, pipe and tube to B444 and bar to B446.

600, 601 and 625 get their strength from what is dissolved in the nickel. 718 is precipitation-hardened: after solution treatment it is aged, and niobium, titanium and aluminium come out as fine particles that lift yield strength well beyond any of the others. That is why it is used for gas turbine discs and casings, high-temperature bolts, springs and oilfield wellhead parts.

Because strength depends on heat treatment, always state the condition you want. Bar to ASTM B637, AMS 5662 (solution treated) or AMS 5663 (solution treated and aged) are the usual routes; oilfield buyers often call for API 6A718. Mixing up the condition is the most common mistake we see on 718 orders.

In practice, no. Inconel 600 has a Curie temperature of about −124 °C, so at room temperature it is non-magnetic, and 625 and 718 behave the same way. A magnet will not pick up any of the common grades. This is also why a magnet cannot tell you which nickel alloy you have; that needs a chemical check.

Inconel costs several times what 316L or 310S does, so it is worth asking whether the job really needs it. Rough rules we use when a customer is unsure:

If the trouble is general corrosion at room temperature, a better stainless or duplex usually fixes it. If the part runs above roughly 800 °C and cycles hot and cold, or sits in hot caustic, or cracks in hot chloride water, that is Inconel territory. If you cannot yet say what is attacking the part, pin down the temperature, the chemicals and the chloride level first, then share them with us so we can suggest grades to compare.

Tell us the grade, form, size and specification, and whether you need the ASME SB version for a coded pressure vessel. Material comes with a test certificate through a Govt. recognised test house; read our guide to test certificates if you want to check one yourself. For acid duties where even 625 struggles, look at Hastelloy C-276, and for furnace work on a tighter budget, compare Incoloy 800H.