Composition and Metallurgical Character of Alloy 600
Alloy 600 is a solid-solution strengthened nickel-chromium-iron material classified as UNS N06600. Under ASTM B168 for plate, sheet and strip, ASTM B166 for rod and bar, and ASTM B167 for seamless pipe and tube, the specified chemistry is nickel 72.0 % minimum, chromium 14.0-17.0 %, iron 6.0-10.0 %, carbon 0.15 % maximum, manganese 1.0 % maximum, silicon 0.5 % maximum, copper 0.5 % maximum and sulphur 0.015 % maximum. The alloy contains no molybdenum and is not hardened by precipitation; its matrix stays austenitic and structurally stable from cryogenic service up to roughly 1090 °C (2000 °F), which is the origin of its reputation as a general-purpose high-temperature material.
| Element | Specified range (ASTM B168, UNS N06600) |
|---|---|
| Nickel | 72.0 % min |
| Chromium | 14.0-17.0 % |
| Iron | 6.0-10.0 % |
| Carbon | 0.15 % max |
| Manganese | 1.0 % max |
| Silicon | 0.5 % max |
| Copper | 0.5 % max |
| Sulphur | 0.015 % max |
Mechanical Properties Across the Temperature Range
ASTM B168 places the minimum room-temperature properties of annealed N06600 sheet and plate at 550 MPa (80 ksi) tensile strength, 240 MPa (35 ksi) yield strength and 30 % elongation. Those values are modest compared with age-hardenable grades, so alloy 600 is selected for its environmental resistance rather than for load-bearing efficiency. Its advantage is retention: the alloy keeps usable strength and ductility at temperatures where carbon steels and many stainless grades creep or scale, and it remains tough at liquid-gas temperatures because it has no ductile-to-brittle transition in the normal engineering sense.
The 14-17 % chromium addition forms a tenacious, self-healing oxide film. In air and in combustion atmospheres the film resists scaling to about 1090 °C, and cyclic oxidation performance is better than that of straight nickel. Sulphur-bearing or highly reducing atmospheres degrade the film and should be evaluated case by case before the alloy is specified.
Corrosion Behaviour in Chloride and Caustic Media
The single most quoted property of UNS N06600 is resistance to chloride-induced stress corrosion cracking. In magnesium chloride and sodium chloride test environments the alloy performs far better than 300-series austenitic stainless steels, and it is widely used where chlorides are present together with stress and elevated temperature. It also tolerates high-purity water, caustic alkalies, and many organic and inorganic acids in neutral or mildly reducing conditions.
The limitation is localised attack. Because no molybdenum is present, resistance to pitting and crevice corrosion in wet chloride service is only moderate, and strongly oxidising acid media attack the alloy quickly. Where chlorides are combined with oxidising conditions, molybdenum-bearing grades such as UNS N10276 are the correct choice and should not be substituted by alloy 600 on the basis of the stress corrosion data alone.
Fabrication, Welding and Heat Treatment Practice
Alloy 600 cold forms readily, although it work-hardens faster than austenitic stainless steel, so intermediate anneals are needed on severe draws. Stress-relief treatments are usually carried out in the 870-980 °C range, while mill anneal schedules for maximum corrosion resistance run above 1040 °C followed by rapid cooling. Machining is comparable to austenitic stainless steel: positive rake geometry, rigid set-ups and generous coolant supply.
Welding is done by GTAW, GMAW, SMAW or SAW with matching nickel-base consumables. ERNiCr-3 bare wire under AWS A5.14 and ENiCrFe-2 or ENiCrFe-3 covered electrodes under AWS A5.11 are the standard choices. No preheat is required, interpass temperature is normally kept below 150 °C, and heat input is controlled to limit hot cracking in the weld pool. Weld surfaces must be free of sulphur, lead and copper contamination, since nickel-base deposits are sensitive to these elements.
Typical Service Applications
Furnace hardware accounts for a large share of consumption: muffles, retorts, radiant tubes, thermowells, heat-treating baskets and fixtures. Chemical processing uses the alloy for caustic evaporators, chlorinated organic synthesis equipment and fatty acid handling. Power and nuclear plants use it in high-purity water circuits, while aerospace and electrical applications exploit its oxidation resistance for spark-electrode bodies, sheathed heating elements and components that must survive repeated thermal cycling without scaling.
Frequently Asked Questions
Q: What is the maximum continuous service temperature of alloy 600?
A: Continuous service in air is normally limited to about 1090 °C (2000 °F), where the chromium-rich oxide remains protective. Above that level scaling accelerates rapidly and a higher-chromium or oxide-dispersion material is required.
Q: Does alloy 600 resist chloride stress corrosion cracking in all conditions?
A: No. It is markedly better than 300-series stainless steel in chloride-bearing environments, but concentrated hot chlorides combined with high stress can still cause cracking, and wet chloride pitting resistance is limited because the alloy contains no molybdenum.
Q: Which filler metal is used for welding UNS N06600?
A: ERNiCr-3 wire to AWS A5.14 for GTAW and GMAW, or ENiCrFe-2 and ENiCrFe-3 covered electrodes to AWS A5.11 for SMAW. Matching-composition consumables are not used because they are more crack sensitive than the nickel-base fillers.
Q: Is alloy 600 magnetic?
A: The fully austenitic structure is essentially non-magnetic at room temperature. Cold work can raise permeability slightly, which matters in instrument housings and magnetically sensitive assemblies.
Q: How does alloy 600 differ from alloy 601 and alloy 625?
A: Alloy 601 (UNS N06601) carries 21.0-25.0 % chromium plus aluminium for higher oxidation resistance. Alloy 625 (UNS N06625) adds 8.0-10.0 % molybdenum and niobium for far better pitting and crevice resistance. Alloy 600 sits between them as the lower-cost, chloride-cracking-resistant option.
Q: What hardness can be expected in the annealed condition?
A: Annealed product typically falls in the 120-190 HBW range depending on product form and section size. Hardness is not a specified acceptance criterion under ASTM B168; tensile testing to ASTM E8/E8M governs.
