Inconel 600

What is Inconel 600?

Inconel 600 is a nickel-chromium-iron solid solution alloy. Its designations are UNS N06600 and material number 2.4816. It has a nickel base and is used from cryogenic temperatures to above 1095 degrees Celsius. Its principal properties are resistance to heat and corrosion.

The alloy originator specifies nickel plus cobalt at 72.0 percent minimum, chromium at 14.0 to 17.0 percent and iron at 6.0 to 10.0 percent. The maximum contents are 0.15 percent carbon, 1.0 percent manganese, 0.5 percent silicon, 0.5 percent copper and 0.015 percent sulphur.

The high nickel content makes the alloy virtually immune to chloride ion stress corrosion cracking. This form of failure affects austenitic stainless steel in hot chloride service. Chromium provides resistance to sulphur compounds and oxidising conditions at high temperature. Inconel 600 is not precipitation hardenable. It has no ageing treatment, and cold work is the only means of increasing strength above the annealed level.

Density is 8.47 grams per cubic centimetre. The melting range is 1354 to 1413 degrees Celsius.

Laxcon Steels lists Inconel 600 in its grades reference as a nickel alloy grade. The same steel is written Alloy 600, N06600 and 2.4816.

What is the chemical composition of Inconel 600?

Nominal contents in weight percent are nickel 75.5 typical, chromium 15.5 typical, carbon 0.04 typical, copper 0.1 typical and iron 8.0 typical.

What is Inconel 600 equivalent to in other standards?

Equivalence means nearest counterpart, not identity: each standards body sets its own composition window, so check the limits of the standard actually named on the order before substituting.

Corrosion and temperature behaviour

The tendency of an austenitic alloy to develop transgranular cracking in chloride solutions decreases as nickel content increases. Inconel 600 contains at least 72 percent nickel and is virtually immune to chloride ion stress corrosion cracking. This property led to its use instead of stainless steel in hot chloride process streams and nuclear service. It also has excellent resistance to corrosion by high-purity water. Reactor water systems have shown no indication of chloride cracking.

Inconel 600 is used from cryogenic temperatures to above 1095 degrees Celsius. It retains useful strength and oxidation resistance beyond the temperature range of steel. High-temperature applications include retorts, muffles, roller hearths, heat treating baskets and trays. Chemical plant applications include fatty acid stills and evaporator equipment.

The alloy is subject to stress corrosion cracking in high-temperature, high-strength caustic alkalies. Material for this service should be fully stress relieved before use. The stated treatments are 900 degrees Celsius for one hour or 790 degrees Celsius for four hours. Operating stresses should remain at a minimum. The same precaution applies to contact with mercury at elevated temperature.

Mechanical properties

Inconel 600 has no hardening heat treatment. Its mechanical properties depend on the amount of cold work in the product and on subsequent annealing. The alloy originator publishes typical ranges rather than specification minimums.

Typical mechanical property ranges for rod and bar, from the alloy originator's technical bulletin. Typical values, not specification minimums.

Annealed material has yield strengths of about 170 to 345 MPa and elongations of 55 to 35 percent. These properties permit forming and joining. Heavy cold work can produce tensile strength as high as 1517 MPa. This increase occurs with a corresponding loss of ductility.

Specifications and product forms

ASTM B166 and ASME SB166 cover rod, bar, wire and forging stock. ASTM B564 and ASME SB564 cover forgings. AMS 5665 and 5687 apply in the aerospace system. ASTM B168 and ASME SB168 cover plate, sheet and strip. ASTM B167 and B163 cover pipe and tube. The alloy is approved under Sections I, III and VIII of the ASME Boiler and Pressure Vessel Code. The nuclear grade produced to tighter requirements is designated 600T.

Processing

Inconel 600 can be formed and welded. It cannot be hardened by heat treatment, so welding does not require a post-weld ageing cycle. The alloy work hardens during processing. Forming operations therefore require staged work with intermediate annealing.

Machining uses the practices applied to high-nickel alloys. These include rigid setups, positive rake, slow speeds, heavy feeds and continuous coolant. The tool must cut below the work-hardened layer instead of rubbing against it. Tool dwell hardens the surface that is subsequently cut.

Related nickel-chromium alloys

Inconel 601 contains about 23 percent chromium and includes aluminium. The aluminium forms a tightly adherent oxide and provides greater oxidation resistance than Inconel 600. Inconel 690 contains 27 to 31 percent chromium. This chromium level supports its use in caustic and high-temperature water service where Inconel 600 is vulnerable.

Inconel 625 contains about 9 percent molybdenum and 3.15 to 4.15 percent niobium plus tantalum. These additions provide strength and resistance to pitting and crevice corrosion that Inconel 600 does not have.

Applications

Chemical process applications include heaters, stills, bubble towers and condensers for fatty acid processing. They also include evaporator tubes, tube sheets and flaking trays in sodium sulphide manufacture, and equipment that handles abietic acid in paper pulp production.

Heat treatment plant applications include retorts, muffles, roller hearths, furnace components, baskets and trays. Aerospace uses include high-temperature engine and airframe parts, lockwire, exhaust liners and turbine seals. The nuclear industry uses the alloy as a standard material of construction for reactor components.

Where Inconel 600 is not the right choice

Inconel 600 is unsuitable for strongly oxidising acids. Its chromium content of 14 to 17 percent is the lowest among the common nickel-chromium alloys. Nitric acid and similar oxidising conditions require a higher-chromium alloy such as Inconel 690 or, in many cases, stainless steel.

Inconel 600 is unsuitable for hot, concentrated caustic service under stress. The alloy originator records stress corrosion cracking in this environment and requires full stress relief before service. The same restriction applies to contact with mercury at elevated temperature.

Inconel 600 is unsuitable where pitting and crevice corrosion in stagnant chlorides control material selection. The alloy has no deliberate molybdenum addition. Molybdenum provides resistance to this local attack. Seawater and similar service therefore require Inconel 625 or a high-molybdenum stainless steel.

Inconel 600 is also unsuitable where stainless steel provides adequate performance. Nickel alloys cost several times as much, machine slowly and are harder to procure. Their use is justified when temperature exceeds the capability of steel, when hot chlorides cause cracking in an austenitic grade, or when the chemistry exceeds the capability of the passive film. Below these conditions, a duplex grade or a six percent molybdenum austenitic stainless steel usually performs the same function.

Other nickel alloy grades