Introduction: Cast for Heat, Forged for Strength
Inconel 713C and Inconel 718 are both nickel-based superalloys, but they are optimized for fundamentally different roles. Inconel 713C is a cast alloy designed for extreme high-temperature service, typically up to about 980 C in gas-path components, and it is valued for its excellent oxidation resistance and thermal stability. Inconel 718 is an age-hardening, weldable alloy optimized for high strength and creep resistance at lower temperatures, around 700 C and below, and it is the standard material for rotating and structural components that must carry high mechanical loads. Choosing between them is not a question of which is better, but of which manufacturing route and property balance matches the operating temperature, stress state and component geometry.
Chemical Composition Comparison
The two alloys differ markedly in composition. Inconel 718 is a nickel-chromium-iron-based alloy with approximately 50-55% nickel, 17-21% chromium, 2.8-3.3% molybdenum, 4.75-5.5% niobium, 0.2-0.8% aluminum and 0.65-1.15% titanium, with carbon limited to 0.08% max. The niobium, titanium and aluminum additions support the gamma double-prime and gamma prime precipitates that give aged 718 its high strength. Inconel 713C, by contrast, is a nickel-chromium-molybdenum casting alloy with approximately 12.0-14.0% chromium, 3.8-5.2% molybdenum, 1.8-2.8% niobium, 5.5-6.4% aluminum and 0.4-1.0% titanium, with carbon 0.08-0.20%, zirconium 0.05-0.15% and boron 0.005-0.015%. The much higher aluminum content of 713C is what provides its exceptional oxidation resistance and high-temperature strength, but it is also the reason the alloy is extremely difficult to weld.
Mechanical Properties and Temperature Limits
In the forged and aged condition, Inconel 718 delivers a room-temperature tensile strength of at least 1275 MPa and a yield strength of at least 1035 MPa, with good impact toughness and excellent thermal fatigue resistance. Inconel 713C, in the cast and aged condition, shows a room-temperature tensile strength of roughly 895 MPa, with outstanding thermal fatigue resistance but relatively low impact toughness. The real difference appears at elevated temperature: 718 maintains excellent creep resistance up to about 700 C and is the preferred alloy for components such as turbine discs and fasteners, while above approximately 750 C it can no longer meet long-term creep requirements, and a cast alloy such as 713C, or a similar high-temperature cast grade, must be used. 713C retains useful rupture strength in the highest-temperature gas path, which is why it appears in turbine blades and guide vanes exposed to the hottest gas.
Manufacturing, Weldability and Machinability
The manufacturing routes of the two alloys are as different as their compositions. Inconel 718 is produced by forging, rolling, extrusion and machining, which suits billets of various specifications and parts with simple shapes; it is considered excellent to weld for a precipitation-hardening alloy, can be joined by various methods, and normally requires an aging treatment after welding to restore full strength. Its machinability is difficult but manageable with appropriate tooling. Inconel 713C is manufactured by investment precision casting, which can produce nearly any complex shape, including hollow cooled blades, with near-net-shape forming and high material utilization. Its weldability is very poor and it is generally considered unweldable: the high aluminum content makes it extremely prone to hot cracking during welding, so repair typically requires brazing or special processes. Machining of 713C is extremely difficult because of its high hardness and wear resistance, and in practice only necessary grinding, polishing and drilling are performed. Heat treatment also differs: 718 uses solution treatment followed by double aging, while 713C usually receives homogenization treatment and aging after casting to dissolve harmful phases and precipitate the strengthening phases.
Applications, Selection Logic and Cost Considerations
In aerospace engines, 718 is used for turbine discs, compressor discs, casings, shafts and fasteners, where high centrifugal stress and good overall mechanical properties are required, while 713C is used for turbine guide vanes and blades, especially high-pressure turbine blades exposed to the highest gas temperatures with complex shapes. In gas turbines and power generation, the same logic applies: 718 for discs, combustion chamber components, shafts, flanges, valves and bolts, and 713C for blades and guide vanes. In industrial applications, 718 appears in oil downhole tools, high-temperature molds and high-performance racing components, while 713C serves static castings that require extremely high heat resistance. On cost: the raw material cost per unit weight of 713C may be slightly higher, but the difference is not significant; the main cost difference lies in manufacturing, with 718 carrying higher forging and machining costs and 713C carrying higher precision-casting mold costs while saving a large amount of machining. For extremely complex shapes such as blades with internal cooling channels, casting with 713C is often the only economically feasible option, because producing the same part by forging and machining 718 would drive cost up exponentially or become impossible. The practical rule is simple: below about 700 C with high mechanical loads and a need for weldability, choose 718; above that temperature in the hottest gas path with complex geometry, choose a cast grade such as 713C.
FAQ
Q: What is the main difference between Inconel 713C and Inconel 718?
A: 713C is a cast alloy optimized for extreme high-temperature service in components such as turbine blades, while 718 is a forged, age-hardened, weldable alloy optimized for high strength and creep resistance up to about 700 C in rotating and structural parts.
Q: Can Inconel 713C be welded?
A: Very poorly. The high aluminum content makes 713C extremely prone to hot cracking during welding, and it is generally considered unweldable; repair typically requires brazing or special processes.
Q: Which alloy is stronger at room temperature?
A: Inconel 718 in the forged and aged condition has a room-temperature tensile strength of at least 1275 MPa, while cast Inconel 713C shows roughly 895 MPa. 718 is the stronger alloy at room and moderate temperatures.
Q: Why is 713C preferred for turbine blades?
A: Investment casting of 713C can produce complex shapes such as hollow cooled blades with near-net-shape forming and high material utilization, and the alloy retains useful strength and oxidation resistance at the highest gas temperatures, where forged alloys cannot meet long-term creep requirements.
Q: Above what temperature does Inconel 718 become unsuitable for long-term creep service?
A: Above approximately 750 C, 718 can no longer meet long-term creep requirements, and a cast alloy such as 713C or a similar high-temperature cast grade must be used.
Q: How do the manufacturing costs of the two alloys compare?
A: 718 carries higher forging and machining costs, while 713C carries higher precision-casting mold costs but saves significant machining. For complex shapes with internal cooling channels, casting is often the only economically feasible route.
Q: Is Inconel 718 weldable?
A: Yes. 718 has excellent weldability for a precipitation-hardening alloy and can be welded using various methods, but it normally requires an aging treatment after welding to restore full strength.
