Alloy 600, designated UNS N06600, is a nickel-chromium-iron alloy that combines a high nickel content for resistance to stress corrosion cracking with chromium for oxidation resistance and high-temperature strength. The balanced chemistry makes it a standard choice for chemical processing equipment, heat treating furnaces, nuclear components and aerospace hardware. Because the alloy is supplied in several product forms, engineers must match the form to the duty cycle, the fabrication route and the commercially available sizes. This guide summarises each form and the criteria used to choose between them.
Chemical Composition and Designated Standards
Alloy 600 contains roughly 72 percent nickel, 14 to 17 percent chromium and 6 to 10 percent iron, with deliberate limits on carbon, sulfur and residual elements. The chemical limits below follow the standard specification for the grade.
| Element | Content, percent |
|---|---|
| Ni | 72.0 min |
| Cr | 14.0-17.0 |
| Fe | 6.0-10.0 |
| C | 0.15 max |
| Mn | 1.0 max |
| Si | 0.5 max |
| Cu | 0.5 max |
| S | 0.015 max |
In the annealed condition the grade typically meets a minimum tensile strength of 550 MPa, a minimum yield strength of 240 MPa and a minimum elongation of 30 percent. Ordering standards differ by form: ASTM B168 for plate, sheet and strip, ASTM B166 for bar and rod, ASTM B167 for seamless pipe, ASTM B163 for seamless heat-exchanger tube and ASTM B564 for forgings. Werkstoff 2.4816 is the European designation of the same alloy.
Plate, Sheet and Strip
Plate is produced by hot rolling or cold rolling, from thin sheet at 0.5 mm up to heavy plate beyond 50 mm. Its large surface area and uniform thickness make it the preferred form for pressure vessels, heat exchanger shells, furnace muffles and linings of chemical equipment. In high-temperature service the plate resists oxidation and carburisation, and the chromium oxide layer protects the surface when it cycles between oxidising and reducing conditions. Cold-rolled strip is used where tight thickness tolerance and a smooth surface finish are required, for example in flexible components and gaskets.
Bar, Rod, Wire and Forgings
Bar is available in hot-rolled, cold-drawn and forged conditions, with diameters from a few millimetres to several hundred millimetres. Hot-rolled bar is economical for general machining of shafts, bolts and valve bodies, while cold-drawn bar holds tighter dimensional accuracy and better surface finish for precision parts.
Wire is cold drawn, usually below 6 mm diameter, and offers excellent flexibility and fatigue resistance. It is used for springs, braiding, thermocouple protection and heating elements, where service temperatures in air may reach about 1100 degrees C in well designed assemblies.
Forgings are formed by hot deformation and consequently have a dense, uniform grain structure with higher mechanical properties than cast or simply machined parts. They are selected for turbine discs, tube sheets, flanges and other components that carry complex stresses. Ultrasonic testing is normally specified for large forged parts.
Seamless and Welded Tube and Pipe
Seamless tube is the first choice for heat exchanger bundles, boiler tubing and chemical fluid transfer because it contains no weld seam and therefore no heat-affected zone that could act as a preferential corrosion path. It performs best under high pressure and in aggressive media such as chloride-bearing process streams. Welded tube is more economical for moderate pressure duty, provided the weld parameters are qualified and the weld zone is properly annealed and inspected. For high-temperature furnace parts, both forms are used where a balance between cost and reliability is required.
Selection Criteria and Application Notes
| Selection factor | What to check | Preferred form |
|---|---|---|
| Working temperature | Continuous and peak temperature, thermal cycling | Plate and tube for oxidising service |
| Corrosion severity | Acid or alkali concentration, chloride level, flow velocity | Seamless tube, plate lining, forgings |
| Mechanical load | Tensile, fatigue and creep duty | Forgings and cold-drawn bar |
| Fabrication route | Welding, machining, forming capability | Plate and bar for welded structures |
| Cost and lead time | Availability of the required size and form | Welded tube where duty allows |
Typical applications illustrate how the criteria combine. In chlor-alkali plants, Alloy 600 plate is used as cell lining because it withstands chloride ions and hot alkali solutions. In nuclear steam generators, seamless tubing is selected for its resistance to stress corrosion cracking in high-purity, high-temperature water. In aerospace engines, sheet and wire are used for exhaust ducting, nozzle parts and shielding braid, where oxidation resistance and electrical behaviour both matter. In heat treatment plants, furnace rollers, retorts and fixtures are made from plate and tube because the alloy keeps its shape during long exposure at high temperature.
Frequently Asked Questions
Q: Why is Alloy 600 chosen instead of a standard austenitic stainless steel?
Because it offers much higher resistance to chloride stress corrosion cracking and better high-temperature oxidation resistance, at a higher raw-material cost.
Q: When should seamless tube be preferred over welded tube?
When the tube will see high pressure, thermal cycling or aggressive chloride-bearing media, because a seamless wall removes the weld seam as a potential corrosion and failure path.
Q: What is the difference between hot-rolled and cold-drawn bar?
Hot-rolled bar is cheaper and adequate for general machining. Cold-drawn bar has tighter tolerance, a smoother finish and better straightness, which suits automatic machining and precision parts.
Q: Are forgings worth the higher price?
Yes for highly stressed parts. Forging refines the grain structure and aligns the flow lines with the load path, giving better fatigue and creep performance than part-from-plate machining.
Q: Can Alloy 600 be hardened by heat treatment?
No. It is a solid-solution alloy, so it is strengthened by cold working or by the choice of product form rather than by a hardening heat treatment.
Q: How should the correct form be selected at the design stage?
List the temperature, corrosion and load requirements first, then confirm the fabrication route and finally verify that the required size is available before the drawings are released.
