Two Grades From the Same Family
Alloy 600 and alloy 690 are both solid-solution nickel-chromium-iron alloys, and at a glance their datasheets look similar: both are austenitic, both are non-magnetic, both resist oxidation and both are used for tubing and heat-exchanger components. The difference is one element. Alloy 690 was developed as a higher-chromium version of alloy 600, and the chromium content roughly doubles, from 14.0-17.0% in UNS N06600 to 27.0-31.0% in UNS N06690 per ASTM B168.
Composition Comparison
The table below quotes the chemical requirements of ASTM B168 for plate, sheet and strip. Bar and rod follow ASTM B166, and seamless tube follows ASTM B163 and ASTM B167.
| Element (%) | Alloy 600, UNS N06600 | Alloy 690, UNS N06690 |
|---|---|---|
| Nickel | 72.0 min | 58.0 min |
| Chromium | 14.0-17.0 | 27.0-31.0 |
| Iron | 6.0-10.0 | 7.0-11.0 |
| Carbon | 0.15 max | 0.05 max |
| Manganese | 1.00 max | 0.50 max |
| Silicon | 0.50 max | 0.50 max |
| Sulfur | 0.015 max | 0.015 max |
| Copper | 0.50 max | 0.50 max |
What the Chromium Increase Changes
The extra chromium stabilises a more protective and more adherent oxide film, and it changes the alloy's response to several specific degradation modes.
Stress corrosion cracking. Alloy 600 is susceptible to caustic SCC above roughly 300 degrees Celsius in concentrated sodium hydroxide, and to primary water stress corrosion cracking in high-purity water at elevated temperature. Alloy 690 is markedly more resistant in both environments, which is why it displaced 600 for nuclear steam generator tube bundles from the 1990s onwards.
High-temperature sulfidation. In sulfur-bearing reducing gases, the higher chromium content gives alloy 690 a longer life than alloy 600 at the same metal temperature.
Oxidation and carburization. Both alloys resist oxidation in air to roughly 1090 degrees Celsius, but 690 tolerates cyclic conditions and carburizing atmospheres better because the chromium reservoir feeding the protective film is twice as large.
Strength at temperature. Alloy 600 has the higher room-temperature tensile strength of the two in the annealed condition, and it retains useful creep strength to about 700 degrees Celsius. Alloy 690 is softer in the annealed state and derives its high-temperature strength from a thermal treatment that precipitates chromium carbides at grain boundaries, the treatment known as thermally treated condition for tubing.
Specifications in Real Purchase Orders
A purchase order for alloy 600 sheet normally references ASTM B168, with ASME SB-168 when the part is stamped. Tube for heat exchangers references ASTM B163 for seamless and ASTM B167 for the welded and drawn product. For alloy 690 the same documents apply, and tube is commonly purchased to ASTM B163 or B167 with a requirement for the thermally treated condition and an intergranular corrosion test. Forgings of either grade are ordered to ASTM B564. Where nuclear quality is required, the applicable ASME code case and an accredited quality programme must be agreed before the billet is released.
Applications and Selection Logic
Alloy 600 remains the economical choice for moderate-temperature furnace hardware, thermocouple protection sheaths, nitric-hydrofluoric pickling equipment in its low-carbon variant, and chloride-free high-temperature process components. Alloy 690 is selected when caustic or high-purity water cracking risk justifies the higher alloy cost: steam generator tubing, nuclear waste vitrification components, sulfuric acid coolers, and equipment handling sulfur-bearing gas at high temperature. Where both could work, the decision is normally driven by the specific corrodent and by the temperature at the metal wall rather than by the bulk process temperature.
Frequently Asked Questions
Q: Are alloy 600 and alloy 690 interchangeable in fabrication?
A: Not directly. Alloy 690 has lower thermal conductivity and a slightly higher coefficient of expansion, and it work-hardens differently. Machining parameters, welding procedures and forming allowances need to be re-qualified when switching from one to the other.
Q: Can alloy 600 and 690 be welded to each other?
A: Yes. Both are austenitic nickel alloys, and matching or higher-alloy filler can be used. Dissimilar joints are common, for example when a 690 tube is welded to a 600 tubesheet, and the procedure should be qualified with the actual filler and thickness combination.
Q: Which grade has better resistance to chloride pitting?
A: Alloy 690, because its chromium content is nearly twice that of alloy 600. Neither grade contains molybdenum, so both are still limited in strong chloride environments compared with molybdenum-bearing grades.
Q: What heat treatment is used for alloy 690 tubing?
A: Tubing is supplied in the thermally treated condition, which involves a controlled anneal that precipitates intergranular chromium carbides without forming a continuous chromium-depleted zone. This condition improves resistance to intergranular attack and to stress corrosion cracking.
Q: Does alloy 600 become brittle after long exposure at high temperature?
A: Alloy 600 retains ductility better than many nickel alloys at temperature, but prolonged exposure in the 600 to 700 degrees Celsius range can reduce impact toughness in heavy sections. Alloy 690 is less prone to this because of its finer carbide distribution.
Q: Which grade is more expensive?
A: Alloy 690 carries a higher price per kilogram because of its lower nickel content and higher chromium addition, and finished tube cost is further influenced by the thermal treatment and testing the purchaser specifies.
