Two Alloys Built for Very Different Duty Cycles
Alloy 718 (UNS N07718) is a precipitation hardening nickel-based alloy containing niobium, molybdenum and titanium; 304 stainless steel (UNS S30400) is the standard austenitic chromium-nickel grade. Both resist corrosion in many of the same environments, which is why they are frequently compared, but they are not interchangeable. Alloy 718 is designed to keep high strength at elevated temperature and is supplied to ASTM B637 for bar and forgings and to the AMS 5662 and AMS 5663 specifications for aerospace stock. Type 304 is a general-purpose material supplied to ASTM A240 for plate, ASTM A276 for bar and ASTM A312 for pipe, and it derives its properties from cold work rather than from a heat treatment.
The practical consequence is a cost-and-capability trade-off: Alloy 718 costs several times more per kilogram than 304, and the extra cost is justified only where high strength at temperature, or exceptional resistance to chlorides, is genuinely required.
The table below shows the specified ranges of the two grades. Alloy 718 is nickel-based with iron as a minor addition; 304 is iron-based with nickel and chromium as alloying elements.
| Element, % | Alloy 718 (UNS N07718) | 304 (UNS S30400) |
|---|---|---|
| Nickel | 50.0–55.0 | 8.0–10.5 |
| Chromium | 17.0–21.0 | 18.0–20.0 |
| Iron | balance | balance |
| Molybdenum | 2.80–3.30 | - |
| Niobium + Tantalum | 4.75–5.50 | - |
| Titanium | 0.65–1.15 | - |
| Aluminium | 0.20–0.80 | - |
| Carbon | 0.08 max | 0.08 max |
| Manganese | 0.35 max | 2.00 max |
| Silicon | 0.35 max | 0.75 max |
| Sulfur | 0.015 max | 0.030 max |
The niobium, titanium and aluminium additions in Alloy 718 form the gamma-double-prime and gamma-prime precipitates that give the alloy its strength; their complete absence in 304 means the stainless grade can only be strengthened by cold working.
Strength from Ambient to Elevated Temperature
Typical mechanical values show how far apart the two grades are. Age-hardened Alloy 718 reaches a tensile strength of roughly 1240–1380 MPa with a 0.2% proof strength of about 1030–1170 MPa; its standard ageing cycle is 720 °C for 8 h, furnace cooled to 620 °C, held for a further 8 h and air cooled. Annealed 304, by comparison, has a specified minimum tensile strength of 515 MPa and a minimum 0.2% proof strength of only 205 MPa, with elongation of 40% or more.
At room temperature: Alloy 718 is roughly three to five times stronger than annealed 304 in proof strength.
At 600–650 °C: Alloy 718 still retains useful strength, while 304 has lost most of its load-carrying capacity and its allowable stress becomes very low.
Above about 700 °C: neither grade is a rational structural choice; oxidation-resistant grades with higher chromium, or cobalt-based materials, are normally considered instead.
Duplex and work-hardened stainless steels can be made much stronger than annealed 304, but no stainless grade approaches the combination of strength and toughness that Alloy 718 delivers after ageing.
Corrosion and Oxidation Limits
Alloy 718 resists chlorides far better than 304. It is used in seawater and salt-laden environments where 304 would pit, and it has good resistance to stress corrosion cracking in chloride media, which makes the dissimilar joint between 718 and 304 a familiar detail in oil and gas equipment. Its molybdenum content also gives useful resistance to mild reducing acids.
Type 304 performs well in atmospheric, fresh-water, food, beverage and oxidising acid service, but it has three well-known limitations: it pits in chloride-bearing water, it is prone to chloride stress corrosion cracking above roughly 60 °C, and it becomes sensitised when held in the 425–815 °C range, after which intergranular corrosion can occur at welds. The low-carbon variant 304L is specified wherever welding thick sections is unavoidable.
Oxidation behaviour follows the same pattern. Both alloys form a protective chromium oxide scale, and both may be used in oxidising atmospheres at elevated temperature, but Alloy 718 retains mechanical strength at temperatures where 304 remains only scale-resistant and structurally weak.
Welding, Fabrication and Dissimilar Joints
Dissimilar welding between Alloy 718 and 304 is routine in the oil and gas industry. The usual consumable is a matching nickel-based filler for the 718 side and a 308L-class filler for the stainless side, with the joint design arranged so that dilution of the nickel alloy by the stainless steel is limited. Pre-heat is not required, but interpass temperature must be controlled because both materials are austenitic and therefore susceptible to hot cracking if heat input and bead shape are not managed.
Machining is the other clear difference. Type 304 machines and forms easily, with conventional tooling and generous cutting speeds. Alloy 718 work-hardens rapidly, requires lower surface speeds, rigid set-ups and sharp tooling, and is usually machined in the solution-treated condition before ageing so that distortion during the ageing cycle is predictable.
Selection Guidance and Typical Applications
Choose Alloy 718 for aerospace engine and airframe hardware, gas turbine components, high-temperature fasteners and springs, pump shafts and pistons, downhole and wellhead equipment, cryogenic vessels where high strength with toughness is needed, and any part exposed to chlorides under stress.
Choose 304 for general industrial fabrication, food and beverage processing equipment, kitchen and architectural products, storage tanks, sanitary tubing, heat exchanger shells and low-temperature piping where moderate strength is sufficient.
A straightforward decision rule: if the design is governed by strength above about 400 °C, or by chloride pitting and cracking resistance, the higher-cost nickel alloy is the economical answer over the life of the equipment. If the design is governed by corrosion resistance in benign media, by cleanliness and by fabrication cost, 304 or its low-carbon variant is the better engineering and commercial choice.
Frequently Asked Questions
Q: Is Alloy 718 a stainless steel?
No. Alloy 718 is a nickel-based precipitation hardening alloy whose nickel content exceeds 50%, while 304 is an iron-based austenitic stainless steel with only 8–10.5% nickel.
Q: Does Alloy 718 rust?
In normal seawater and industrial atmospheres Alloy 718 does not rust in the way carbon steel does, but it can suffer localised pitting and crevice attack in severe marine conditions with high chloride concentration, elevated temperature and stagnant flow.
Q: What is the ASTM grade for Alloy 718?
Alloy 718 is UNS N07718, covered for bar and forgings by ASTM B637 and for aerospace stock by AMS 5662 and AMS 5663. Round bar is normally supplied in the aged condition.
Q: Can 304 be welded directly to Alloy 718?
Yes, and the combination is common in oil and gas piping. A nickel-based filler is used for the alloy side and a 308L-class filler for the stainless side, with controlled heat input and interpass temperature to avoid hot cracking.
Q: Which material is cheaper?
Type 304 is substantially cheaper per kilogram and far easier to machine. Alloy 718 costs more and is more difficult to fabricate, so it should be specified only where its strength or its chloride resistance is actually needed.
Q: How is Alloy 718 heat treated?
The standard ageing treatment is 720 °C for 8 h followed by controlled cooling to 620 °C for 8 h, then air cooling. This cycle produces the gamma-double-prime precipitates responsible for the alloy's strength.
