Inconel: Definition, How It Is Made, Its Uses, and Its Types

Inconel alloys are high-performance nickel/chromium superalloys developed for high temperatures, corrosive environments, and oxidation resistance. They are used in the harshest environments such as in chemical processing reaction chambers/valves, polymer granule manufacture, military applications, gas turbine components, oil fields, and high-grade marine applications.

The original Inconel alloy was coded 600 and consists of >72% nickel, 14–17% chromium, 6–10% iron, and ~1% manganese, plus traces of copper, silicon, carbon, and sulfur. Constituents vary in proportions, and other alloys contain molybdenum, niobium, tantalum, cobalt, aluminum, titanium, lead, and boron at various levels.

All of the Inconel alloys are of high strength. Inconel 718, one of the higher-strength superalloys, delivers around 1,375 MPa of tensile strength and yield strength of around 1,100 MPa. Precipitation-hardened stainless steels can achieve tensile strengths of 850 MPa to 1,700 MPa and yield strengths of 520 MPa to over 1,500 MPa. However, precipitation-hardened stainless steels degrade fairly rapidly in strength above 650 °C, whereas Inconel 718 generally maintains its strength to over 1,000 °C.

Gas turbine blades.

Combustion chambers.

Chemical rocket combustion chambers.

High-pressure chemical reaction chambers.

Pollutant separation in harsh environments.

Oil field extraction processing systems.

1. Inconel 600

This is the original alloy. It is heat- and corrosion-resistant, noted for elevated temperature corrosion/oxidation resistance, good hot and cold processing characteristics, good low-temperature strength, and both hot/cold fatigue properties. It offers high strength under 650 °C, and it's suitable for chemical-plant components.

2. Inconel 601

This alloy has excellent oxidation resistance at elevated temperatures and high resistance to carburization. It is good for high-temperature organic-chemical plants. It resists oxidation from sulfur atmospheres and has good mechanical properties at room temperature and high temperature. It also has great stress corrosion crack resistance because of the low carbon content and fine grain structure. It has particularly good creep strength at elevated temperatures (above 500 °C, at which other alloys can fail). 601GC is the grain-controlled version of the alloy, offering some enhancement of properties over 601.

3. Inconel 625

This alloy offers better corrosion resistance than 600 and 601. It has better performance in retaining strength and resisting creep from low temperatures up to a maximum service temperature of 980 °C

4. Inconel 718

This alloy is suited to precipitation hardening. It offers good strength and toughness up to 650 °C and good general corrosion/oxidation resistance at all service temperatures. It is formulated for solution treatment or precipitation hardening.

5. Inconel X-750

This alloy offers elevated corrosion resistance and oxidation resistance, as well as good strength below 980 °C. In addition, Inconel X-750 has good low-temperature performance and molding performance. It is mainly used for aerospace and industrial gas turbine components.

6. Inconel 690

This alloy offers greater resistance to chloride corrosion (e.g., salt water), high temperature/pressure water stress corrosion, strong oxidation environments, and fluorinated atmosphere attack. It is mainly applied in nitric acid environments, steam generators, and nuclear waste treatment plants.

7. Inconel 792

This alloy has an elevated aluminum content which improves the high-temperature corrosion resistance. It is best suited to gas turbine blades and combustion chamber parts for chemical rockets.

All Inconel alloys are rated as offering good resistance to corrosion in normal environments and at moderately elevated temperatures. Some offer more extreme corrosion resistance properties in terms of fluorinated atmospheres, nitric acid, high-temperature combustion environments, etc.

While Inconel is not an easy group of materials to machine, with good tooling and cooling, great results and moderate cut depths and feeds can be maintained. Listed below are some helpful tips to follow:

Regularly check for signs of tool wear, as the material is hard on cutting tips.

Machine in the solutionized state for best results.

Always use cermet tips for continuous cutting.

Don't stop/start when drilling, but continue the feed for smooth chip flow.

Use four- and five-flute variable helix, high-performance milling cutters.

Inconel is hard to weld, as there is a strong tendency to crack at the welds. The use of TIG and Inconel 625 filler rods gives the best results when executed by automated welding equipment.

Inconel pricing is very dependent on the alloy type and the form in which it is delivered. As a rule, a tube is more expensive than a plate or bar. Solutionized is the normal delivery state, and prices start at around $40 per kg of stock product.

Two basic forms of heat treatment are used on Inconel alloys. These are:

Solution Annealing: At 925–980 °C followed by quenching in water.

Precipitation Hardening: At 720 °C for 8 hours, then slowly cool to 620 °C. Hold there for 18 hours of aging, then open to the atmosphere at room temperature for natural cooling.

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