Short Answer: A Range, Not a Single Temperature
Inconel alloys melt across a temperature interval. The solidus is the temperature at which the first liquid appears, and the liquidus is the temperature at which the alloy is fully molten. For the common grades the interval falls broadly between about 1260 C and 1370 C. Typical published values are approximately 1260-1330 C for alloy 718, approximately 1290-1350 C for alloy 625, and approximately 1330-1370 C for alloy 706.
Because these are alloyed materials rather than pure metals, quoting one number is physically misleading. Any specification or quotation that cites a single melting point without a range or a reference method should be treated as an approximation only.
Melting Ranges of Common Grades
| Grade | Nominal Melting Range | Nominal Density | Notes |
|---|---|---|---|
| Alloy 625 (UNS N06625) | Approximately 1290 - 1350 C | About 8.44 g/cm3 | Niobium and molybdenum bearing, solid solution strengthened. |
| Alloy 718 (UNS N07718) | Approximately 1260 - 1330 C | About 8.19 g/cm3 | Precipitation hardening; melting range lowered by Nb, Mo and Ti additions. |
| Alloy 706 (UNS N09706) | Approximately 1330 - 1370 C | About 8.05 g/cm3 | Iron-rich chemistry raises the melting interval relative to 718. |
The trend is consistent: the more refractory and heavy elements dissolve in the nickel matrix, the wider and generally lower the melting interval becomes. Iron-rich chemistry, by contrast, tends to push the interval upward.
How Melting Behaviour Is Measured
Solidus and liquidus are determined by thermal analysis, most commonly differential thermal analysis or differential scanning calorimetry. The relevant test methods are ASTM E794 for melting and crystallisation temperatures by thermal analysis, with temperature calibration supported by ASTM E967. The measured onset corresponds to the solidus and the peak or completion of the endothermic event corresponds to the liquidus.
Three practical cautions apply. First, chemistry tolerance bands mean that two heats of the same grade can differ by tens of degrees, so melted range values should be quoted for the actual heat where the process is sensitive to them. Second, the test measures a small sample and reflects local segregation behaviour, not a large ingot. Third, a measured range is only meaningful when the heating rate and calibration are reported alongside it.
Melting Point Versus Maximum Service Temperature
The useful working range of a superalloy is far below its melting range. Two mechanisms set the practical ceiling: creep, which becomes significant as the material is held above roughly 0.6 of its absolute melting temperature, and oxidation or hot corrosion, which consumes the protective surface oxide layer. For alloy 625 and alloy 718 the generally accepted continuous service ceiling is around 650-700 C, which is several hundred degrees below the solidus.
This distinction is why a melting point comparison is a poor basis for alloy selection. A material with a slightly lower melting range may still offer far better creep resistance, because creep performance depends on the stability of the strengthening precipitates and the grain structure rather than on the melting interval itself.
Why Melting Behaviour Matters in Fabrication
Melting range data is used directly in three shop floor decisions. In welding, the liquidus influences penetration and dilution behaviour, and the solidus to liquidus gap influences solidification cracking susceptibility in the weld pool. In vacuum melting and remelting, the melting interval sets the operating window for electrode remelting processes that control cleanliness and segregation. In brazing and diffusion bonding, filler metals are selected with a liquidus below the solidus of the base alloy so the workpiece retains its structure.
Thermal cutting and welding should be carried out with a low heat input and monitored interpass temperature, because nickel alloys have low thermal conductivity and high thermal expansion relative to steel. Local overheating raises the risk of hot cracking and of liquation in the heat affected zone, particularly in precipitation-hardening grades in the aged condition.
Frequently Asked Questions
Q: Does Inconel 625 have a single melting point?
A: No. It melts over a range, approximately 1290-1350 C, with the solidus where melting begins and the liquidus where the alloy is fully liquid.
Q: Which melts at a higher temperature, Inconel 625 or 718?
A: Alloy 625 has the higher melting range, at approximately 1290-1350 C compared with approximately 1260-1330 C for alloy 718.
Q: Is the melting point the maximum service temperature?
A: No. Practical service limits are set by creep and oxidation, generally around 650-700 C for these grades, far below the melting range.
Q: How is an alloy melting range measured?
A: By thermal analysis, typically differential thermal analysis or differential scanning calorimetry, following the practice of ASTM E794 with calibration per ASTM E967.
Q: Why do two heats of the same grade melt at different temperatures?
A: Chemistry is delivered within specification tolerance bands, and small differences in refractory element and interstitial content shift both the solidus and the liquidus.
Q: Can Inconel be welded without melting the base metal?
A: Solid state processes such as friction welding can join these alloys below the solidus, but fusion welding necessarily melts both filler and a portion of base metal.
