Two Bars, Two Different Strengthening Mechanisms
Inconel 600 and Inconel 718 are both nickel-chromium alloys supplied as round bar, and both are stocked in the same size range, which is why they are so often compared. The difference that drives everything else is how each alloy develops its strength. Inconel 600, UNS N06600, is a solid-solution strengthened nickel-chromium-iron alloy: it is strong because the nickel matrix with its chromium and iron additions resists deformation, and its properties come from chemistry plus an anneal. Inconel 718, UNS N07718, is a precipitation-hardening alloy: it is solution treated and then aged, and fine niobium and aluminium-titanium phases precipitate inside the grains and block dislocation movement.
The practical consequence is a wide gap in strength and a reversal in behaviour at high temperature. Bar of 600 is ductile, easy to weld and forgiving. Bar of 718 is roughly twice as strong in the aged condition and much harder to machine, but it keeps its strength to higher temperatures than most precipitation-hardening alloys, which is why it dominates aerospace and gas turbine bolting and rotating hardware.
Chemical Composition at a Glance
| Element | Inconel 600, UNS N06600 | Inconel 718, UNS N07718 |
|---|---|---|
| Nickel | 72.0 min | 50.0 to 55.0 |
| Chromium | 14.0 to 17.0 | 17.0 to 21.0 |
| Iron | 6.0 to 10.0 | Balance |
| Niobium plus tantalum | Not specified | 4.75 to 5.50 |
| Molybdenum | Not specified | 2.80 to 3.30 |
| Titanium | Not specified | 0.65 to 1.15 |
| Aluminium | Not specified | 0.20 to 0.80 |
| Carbon | 0.15 max | 0.08 max |
| Other limits | Mn 1.0 max, Si 0.5 max, Cu 0.5 max, S 0.015 max | Co 1.0 max, B 0.006 max, Mn 0.35 max, Si 0.35 max, S 0.015 max |
The niobium level is the signature of 718 and the reason its aged strength can be developed without the severe quench sensitivity of some other precipitation-hardening grades. The relatively low carbon limit and the boron addition are both there to serve the same end: a clean grain boundary structure that supports good stress rupture life without sacrificing notch toughness.
Mechanical Properties in Bar Form
The figures below are typical minimum or typical published values for the normal supply conditions of each grade. Actual minima depend on the product form and the specification invoked.
| Property | Inconel 600, annealed | Inconel 718, solution treated and aged |
|---|---|---|
| Tensile strength | Around 550 MPa (80 ksi) | Around 1240 MPa (180 ksi) |
| Yield strength, 0.2 % offset | Around 240 MPa (35 ksi) | Around 1030 MPa (150 ksi) |
| Elongation | About 30 % | About 12 % |
| Hardness | Moderate, similar to austenitic stainless | Roughly 36 to 40 HRC |
The strength ratio is close to two to one on yield, and the ductility gap is just as important for designers. A 600 bar will bend, swage and deform before it breaks, which makes it a good choice for parts that must absorb misalignment or thermal movement. A 718 bar is much stiffer in practice and will support higher loads in the same section, but it offers far less warning before failure.
Bar Specifications That Apply
Inconel 600 bar is normally ordered to ASTM B166, with ASME SB166 as the code equivalent. Inconel 718 bar is covered by ASTM B637, with ASME SB637 as the code equivalent, and aerospace users often call for AMS 5662 or AMS 5663, which add tighter controls on cleanliness, grain size and heat treatment for rotating and critical parts. Where a bar will be machined into a threaded fastener, the buyer should also state whether the finished part will be aged and what hardness is expected, because the ageing treatment can be carried out either at the mill or after machining.
Size Range, Length and Available Forms
Both grades are produced as hot-worked or cold-finished bar in round, square, hexagonal and rectangular sections, and both are available as billet and forging stock. Round bar is commonly stocked from about 14 mm to 300 mm diameter, in random or cut lengths from one to six metres, with tolerances that depend on whether the bar is peeled, ground or turned and polished. Larger diameters and bespoke sections are produced to order, and cut-to-length service is standard.
Machining and Fabrication
This is where the two grades feel most different. Inconel 600 machines like a tough austenitic stainless steel: it work hardens, it is gummy, and it demands sharp positive-rake tooling, moderate speeds and generous feed to get under the work-hardened layer rather than rubbing on it. Inconel 718 is considerably harder in the aged condition, has low thermal conductivity so heat concentrates at the cutting edge, and work hardens rapidly, which makes it one of the more demanding common alloys to machine. The usual approach is to machine in the solution treated condition wherever possible and age afterwards, to use carbide or ceramic tooling with rigid fixturing, to keep the depth of cut above the work-hardened skin, and to accept reduced tool life.
Welding follows the same pattern. Inconel 600 is welded with common arc processes and generally needs no post-weld heat treatment. Inconel 718 is also weldable, but welded joints in aged material lose strength in the heat-affected zone, so fabrication is normally done in the solution treated condition and the assembly is aged afterwards, which means the heat treatment sequence must be planned before the first cut is made.
Corrosion and High-Temperature Behaviour
Inconel 600 resists oxidation in air to about 1093 C (2000 F), is essentially immune to chloride stress corrosion cracking, and tolerates carburizing and nitriding atmospheres. It is the better performer in strongly oxidizing aqueous service and in caustic environments, and it is the more forgiving material where the process chemistry is variable or not fully characterised.
Inconel 718 is designed for strength retention rather than for extreme oxidation resistance. It performs well in many oxidizing and reducing aqueous environments and holds useful properties to around 650 C, but for sustained service far above that temperature the gamma double prime strengthening phase coarsens and other grades become more appropriate. Where chlorides and seawater are combined with high mechanical load, 718 bar is often selected for its combination of strength and pitting resistance rather than for scaling resistance.
How to Choose
Choose Inconel 600 bar where ductility, weldability and a forgiving corrosion performance are the priorities, where the parts will be bent or formed after purchase, and where the temperature is high but the mechanical load is moderate. Choose Inconel 718 bar where high strength must be retained at temperature, where weight saving through a smaller section matters, or where fatigue and stress rupture life are the governing design criteria. Where a fastener application sits between the two, the deciding questions are usually the required preload, the service temperature and whether the joint will be re-tightened in service.
Frequently Asked Questions About Inconel 600 and 718 Round Bars
Q: Which grade has higher strength, Inconel 600 or Inconel 718?
A: Inconel 718 is substantially stronger in the aged condition, with yield strength roughly four times that of annealed Inconel 600. The gap narrows in the solution treated condition, but 718 is almost always specified where high strength is the main requirement.
Q: Can Inconel 718 bar be machined before ageing?
A: Yes, and this is the usual practice. Machining in the solution treated condition is easier and leaves less residual stress, and the part is then aged to develop its final strength. Allowance must be made for the small dimensional change that occurs during ageing.
Q: Are the two grades interchangeable in high-temperature bolting?
A: Not automatically. The bolt must be checked for allowable stress at temperature, for differential expansion against the flange, and for relaxation over the design life. Inconel 718 is normally the choice for hot, highly loaded joints, while 600 is used where ductility and thermal cycling tolerance matter more.
Q: What bar specifications should appear on the mill certificate?
A: Inconel 600 bar is normally certified to ASTM B166 or ASME SB166, and Inconel 718 bar to ASTM B637 or ASME SB637. Aerospace applications frequently add an AMS specification that imposes extra controls on grain size, cleanliness and heat treatment.
Q: Which grade resists chloride stress corrosion cracking better?
A: Inconel 600 is highly resistant because of its high nickel content and is widely used where chlorides and hot water are present. Inconel 718 also resists stress corrosion cracking in many chloride environments, but its selection is usually driven by strength or fatigue requirements rather than by that property alone.
Q: Is Inconel 718 more expensive than Inconel 600 bar?
A: Yes, generally by a wide margin. Inconel 718 contains more alloying additions, requires a controlled solution treat and age cycle, and is harder to roll and machine. The higher price should be weighed against the smaller section or longer life the strength advantage makes possible.
