Inconel 690 is a high-chromium nickel-iron austenitic superalloy specifically engineered for exceptional resistance to corrosion-particularly stress corrosion cracking (SCC)-in aggressive, high-temperature environments. It builds on the performance of earlier Inconel alloys (e.g., Inconel 600) but features a significantly higher chromium content, making it the material of choice for applications where corrosion resistance is critical, such as nuclear power systems.
Key defining traits of Inconel 690 include:
Superior corrosion resistance: Its high chromium content (over 27%) enables it to form a dense, stable chromium oxide (Cr₂O₃) passive film on the surface. This film prevents penetration by corrosive media like water, steam, and mineral acids, and strongly resists SCC in high-temperature water environments (a major risk for nuclear components).
High-temperature stability: It maintains mechanical strength and oxidation resistance up to approximately 1093°C (2000°F), with reliable performance in both oxidizing and moderately reducing atmospheres.
Austenitic microstructure advantages: Like Inconel 600, it has an austenitic structure (face-centered cubic) that provides excellent ductility, toughness, and fabricability. It can be easily welded, forged, rolled, or machined without requiring complex precipitation-hardening heat treatments.
Specialized application focus: It is primarily used in nuclear power plants (e.g., steam generator tubes, reactor vessel internals), as well as in chemical processing equipment and high-temperature heat exchangers-where resistance to corrosion and thermal cycling is non-negotiable.
The chemical composition of Inconel 690 is tightly controlled to prioritize corrosion resistance, especially against SCC, while maintaining mechanical integrity. Below is the typical composition (by weight percentage, wt%) per key industry standards (e.g., ASTM B168, AMS 5541, ASME SB-168):
The hardness of Inconel 690 is primarily determined by its heat treatment state, as it is an austenitic alloy (not precipitation-hardened) and relies on cold work or annealing to adjust mechanical properties. Unlike Inconel X750, it cannot be strengthened via aging heat treatments. Below are typical hardness values for its most common heat-treated conditions, measured using the Rockwell B (HRB), Rockwell C (HRC), or Brinell (HB) scales (per ASTM/AMS standards):
Annealing is the standard heat treatment for Inconel 690, involving heating to 1065–1120°C (1950–2050°F) followed by rapid cooling (water quenching) or controlled cooling. This state optimizes corrosion resistance and ductility, making it suitable for most critical applications (e.g., nuclear steam generator tubes).
Rockwell B (HRB): ~70 – 80
Brinell (HB): ~170 – 200
Rockwell C (HRC): ~<20 (too soft to measure accurately with the HRC scale)
Cold working (e.g., cold rolling, cold drawing, or cold extrusion) strains the austenitic microstructure, increasing hardness and strength at the cost of reduced ductility. This state is used for applications requiring higher mechanical performance (e.g., high-pressure fittings) where corrosion resistance can be slightly compromised.
Hardness values scale with the degree of cold work (e.g., 20%, 40%, or 60% reduction in thickness):
Light cold work (20–30% reduction): Rockwell B (HRB) ~80 – 85; Brinell (HB) ~200 – 230
Moderate cold work (30–50% reduction): Rockwell B (HRB) ~85 – 90; Brinell (HB) ~230 – 260
Heavy cold work (>50% reduction): Rockwell B (HRB) ~90 – 95; Brinell (HB) ~260 – 300; Rockwell C (HRC) ~20 – 25
A stress-relief heat treatment (typically 700–900°C / 1290–1650°F for 1–4 hours) is sometimes applied after cold working or welding to reduce residual stresses (which can trigger SCC). This process slightly softens the alloy compared to the cold-worked state:
Rockwell B (HRB): ~75 – 85 (depending on prior cold work)
Brinell (HB): ~180 – 230
