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Views: 0 Author: Site Editor Publish Time: 2026-10-20 Origin: Site
Inconel 690 — also designated Alloy 690, UNS N06690 and Werkstoff No. 2.4642 — is a solid-solution-strengthened nickel-chromium-iron alloy containing 27–31% chromium, roughly double the chromium level of Alloy 600. That single compositional change, combined with controlled thermal treatment, gives the alloy markedly improved resistance to stress corrosion cracking in high-temperature water. Its density is approximately 8.19 g/cm³ and its melting range is approximately 1343–1377 °C. The alloy is best known as the reference tubing material for pressurized water reactor (PWR) steam generators, and it is also used in nitric acid and selected mixed-acid chemical service.
Item | Value |
|---|---|
Common designations | Inconel 690, Alloy 690, UNS N06690, W. Nr. 2.4642, ISO NW6690 |
Material family | Nickel-chromium-iron, austenitic, solid-solution strengthened |
Chromium content | 27.0–31.0% (nuclear amendments: 28–31%) |
Density | ≈ 8.19 g/cm³ (0.296 lb/in³) |
Melting range | ≈ 1343–1377 °C (2450–2510 °F) |
Crystal structure | Face-centered cubic (FCC) austenite |
Primary applications | PWR steam generator tubing, nuclear components, nitric and mixed-acid chemical equipment |
The table above is a quick reference for engineers and buyers who need the essential figures before reading further. Three points frame everything that follows. First, the alloy's identity as a solid-solution nickel-chromium-iron grade explains both its corrosion behavior and its fabrication characteristics — it is not precipitation hardened, and it is normally used in the annealed condition. Second, the chromium range is the single most important compositional variable, and nuclear specifications narrow it further while adding a cobalt restriction. Third, the property values quoted here are published reference data for annealed product; they are not acceptance limits, which come from the governing product standard and the purchase specification.
Inconel 690 is the trademarked designation of Special Metals Corporation; Alloy 690 is the generic industry name used by other mills and by most procurement documents. The same material appears in European specifications as Werkstoff No. 2.4642 and in ISO nomenclature as NW6690. Seamless tube and pipe are commonly supplied to ASTM B163 / ASME SB163 and ASTM B167 / ASME SB167, plate, sheet and strip to ASTM B168 / ASME SB168, and rod, bar and forgings to ASTM B166 / ASME SB166 and ASTM B564 / ASME SB564. Nuclear steam generator tubing is frequently procured under ASME Code Case N-525 together with project-specific supplementary requirements.
Because the applicable standard changes the acceptance criteria — not the nominal grade — a purchase specification should always state product form, dimension, temper, standard revision and any nuclear supplement.
The alloy was developed in response to a specific engineering problem. Early PWR steam generators used Alloy 600 tubing, which contains approximately 14–17% chromium. As operating experience accumulated through the 1970s and 1980s, a proportion of Alloy 600 tubes exhibited stress corrosion cracking and related degradation in secondary-side water chemistry, under the combined influence of residual stress, sludge and deposit conditions, and manufacturing variables.
The industry response was threefold: revise water chemistry, revise steam generator design, and raise the chromium content of the tubing alloy. Alloy 690 raised chromium from roughly 14–17% to 27–31%. The higher chromium level promotes a more protective chromium-rich surface oxide and, with the correct thermal treatment, a grain-boundary microstructure that is substantially more tolerant of the environments that caused cracking in Alloy 600.
U.S. Nuclear Regulatory Commission operating-experience documentation records that thermally treated Alloy 690 tubing became the standard replacement choice for recirculating steam generators from the late 1980s onward, and that its service performance has been superior to that of the earlier Alloy 600 tubing it replaced. That improvement is attributable to the alloy chemistry, the thermal treatment, and concurrent design and chemistry improvements acting together — not to the alloy alone.
Alloy 690 is best described as a high-chromium nickel alloy optimized for high-temperature water environments and the associated stress corrosion cracking risk. It is not an immune material. No engineering alloy eliminates every corrosion or cracking mechanism under every combination of chemistry, stress and temperature, and marketing language that implies otherwise is both technically indefensible and commercially counterproductive with qualified buyers.
Outside nuclear service, the alloy is used in nitric acid and nitric/hydrofluoric acid media and in other oxidizing environments where its chromium content is advantageous. Suitability in each case still depends on the actual medium composition, concentration, temperature, flow velocity and stress state.
Corrosion resistance in Alloy 690 derives from the interaction of a high chromium level with a nickel-rich austenitic matrix. Carbon content, thermal treatment and grain-boundary structure then determine how much of that potential is realised in service.
Element | Typical range (wt%) | Function |
|---|---|---|
Ni | 58.0 min | Austenitic matrix; resistance to chloride and caustic stress corrosion cracking |
Cr | 27.0–31.0 | Primary passivating element; resistance to oxidizing media and high-temperature oxidation |
Fe | 7.0–11.0 | Matrix balance; cost and phase stability |
C | 0.05 max | Controls grain-boundary carbide precipitation and sensitisation risk |
Mn | 0.50 max | Deoxidation and melting control |
Si | 0.50 max | Deoxidation and melting control |
S | 0.015 max | Residual impurity control |
Cu | 0.50 max | Specified in general material specifications |
P | Per applicable specification | Not fixed by a single universal limit; verify against the governing standard |
Note on phosphorus: the widely quoted general composition table for Alloy 690 does not include a phosphorus limit, so a value should not be presented as a universal requirement. Some product specifications do state one — for example, P ≤ 0.020% in certain tube mill datasheets — which is precisely why the governing specification, not a generic table, must be cited in a purchase order.
For nuclear applications, the same material is subject to tighter amendments, typically Cr 28–31%, C ≤ 0.04% and Co ≤ 0.10%. Cobalt is restricted because it activates under neutron flux and contributes to radiation dose. Nuclear-grade material must therefore be ordered and certified against the project technical specification, not merely against the generic grade name.
At 27–31%, chromium is far above the level at which a continuous, self-healing chromium-rich oxide can form and remain stable in oxidizing conditions. This is the origin of the alloy's performance in nitric acid, in nitric/hydrofluoric mixtures and in high-temperature oxidizing gases.
Chromium content alone, however, does not predict performance in every medium. Actual behavior also depends on the redox character of the solution, temperature, impurities, surface finish, stress level and thermal treatment condition. Two alloys with similar chromium contents can behave very differently in a reducing acid.
Nickel forms the continuous austenitic matrix and is largely responsible for the alloy's resistance to chloride-induced stress corrosion cracking and to caustic environments — the classic weaknesses of high-chromium stainless steels. The face-centered cubic structure also provides ductility and toughness at both ambient and cryogenic temperatures.
The nickel matrix should not be read as universal immunity. In hydrochloric acid and in strongly reducing sulfuric acid, nickel-chromium alloys as a class are limited, and selection should be based on comparative corrosion data for the specific concentration and temperature rather than on the grade name.
When carbon combines with chromium to form grain-boundary carbides, the adjacent matrix can become locally chromium-depleted. Under certain thermal exposure conditions this reduces local corrosion resistance and can create a preferential path for intergranular attack.
Carbon is therefore held to 0.05% maximum, and to 0.04% maximum for nuclear service. Low carbon is a necessary but not sufficient condition: it must be combined with the specified thermal treatment and the resulting grain-boundary carbide distribution, which is why thermal treatment is a qualification item in its own right rather than a process detail.
Inconel 690 is a solid-solution alloy and is not hardened by precipitation treatment. Its design logic is not to maximise room-temperature strength through heavy precipitation, but to combine composition, grain-boundary structure and thermal treatment so that the material performs in the intended environment. Its microstructure normally contains M₂₃C₆ carbides, together with titanium nitrides and carbonitrides; no embrittling intermetallic phases such as sigma phase have been identified in the alloy, which underpins its metallurgical stability during long high-temperature exposure.
In suitable environments, chromium supports the formation of a chromium-rich oxide film that separates the metal from the corrosive medium and reduces dissolution and corrosion rates. This is the mechanism behind the alloy's low corrosion rates in nitric acid across a wide concentration range.
The composition, thickness and stability of that film vary with temperature, water chemistry and electrochemical potential. It is therefore inaccurate to describe the surface as a perfectly dense Cr₂O₃ layer that can never break down locally; film breakdown and localised attack remain possible where conditions are aggressive enough.
For nuclear tubing, thermal treatment does more than adjust mechanical properties. It determines the morphology and distribution of grain-boundary carbides, and therefore the material's sensitivity to environment-assisted cracking. A continuous, properly distributed carbide network can modify crack initiation and propagation behavior at the grain boundaries.
The service record of thermally treated Alloy 690 in steam generators illustrates the point: performance is not a function of nominal chemistry alone. The microstructure produced by composition plus thermal treatment is an explicit design variable, and it should be specified, verified and documented accordingly.
Cold drawing, bending, local deformation, welding and other fabrication steps alter both the stress state and the microstructure of the finished component. NRC-sponsored research has specifically examined how cold work level and grain-boundary carbide morphology affect the stress corrosion cracking behavior of Alloy 690 in simulated PWR primary water.
The practical consequence for procurement and manufacturing is that chemistry control is only one of several gates. Tube reduction ratio, straightening, stress relief, dimensional accuracy, surface condition and non-destructive examination all belong in the technical specification for nuclear-grade tubing, and all should be evidenced in the quality documentation.
Element | Content (wt%) | Element | Content (wt%) |
|---|---|---|---|
Ni | 58.0 min | Cr | 27.0–31.0 |
Fe | 7.0–11.0 | C | 0.05 max |
Mn | 0.50 max | Si | 0.50 max |
S | 0.015 max | Cu | 0.50 max |
This table is a general composition reference and does not represent the complete acceptance conditions for nuclear-grade product. Phosphorus and nuclear supplementary limits must be verified against the governing standard and the purchase specification.
Property | Reference value |
|---|---|
Density | ≈ 8.19 g/cm³ (0.296 lb/in³) |
Melting range | ≈ 1343–1377 °C (2450–2510 °F) |
Crystal structure | Face-centered cubic (FCC), austenitic |
Young's modulus (room temperature) | ≈ 211 GPa (30.6 × 10³ ksi) |
Poisson's ratio | 0.289 |
Specific heat | ≈ 450 J/kg·°C |
Thermal conductivity (100 °C) | ≈ 13.5 W/m·°C |
Electrical resistivity | ≈ 1.148 μΩ·m |
Magnetic permeability (at 200 Oe) | ≈ 1.001 |
The permeability figure of approximately 1.001 indicates that annealed Alloy 690 is effectively non-magnetic. Because cold work can raise the magnetic response, technical documents should describe the annealed condition as "essentially non-magnetic" with the measured value stated, rather than asserting absolute non-magnetic behavior under all conditions.
Mechanical properties vary with product form, size and temper. The values below are room-temperature tensile results for annealed product taken from published mill data, and are presented to show the practical range rather than to define minimum acceptance values for every product.
Product form and size | 0.2% yield strength Rp0.2 | Tensile strength Rm | Elongation |
|---|---|---|---|
Cold-drawn tube, 12.7 mm OD × 1.27 mm wall | ≈ 461 MPa | ≈ 758 MPa | ≈ 39% |
Cold-drawn tube, 19.0 mm OD × 1.65 mm wall | ≈ 379 MPa | ≈ 700 MPa | ≈ 46% |
Hot-rolled rod, ≈ 51 mm diameter | ≈ 334 MPa | ≈ 690 MPa | ≈ 50% |
Hot-rolled flat, ≈ 13 × 51 mm | ≈ 352 MPa | ≈ 703 MPa | ≈ 46% |
Two points deserve emphasis for buyers. First, the spread between a thin-wall cold-drawn tube and a hot-rolled bar is substantial — around 130 MPa in yield strength — so a single "typical" strength figure is misleading. Second, mill datasheets for annealed tube typically publish minimum values in the order of Rm 590 MPa, Rp0.2 240 MPa and elongation ≥ 30%, which serve as acceptance floors, not as expected performance. Figures such as "Rm ≥ 550 MPa" that circulate online without stating a standard, product form or specimen size should not be used in a specification.
Alloy 690 in the annealed condition is a strong, ductile alloy, but strength is a low bar for steam generator tubing. The qualification package for nuclear-grade tube typically addresses:
Chemical composition plus nuclear supplementary limits, including cobalt;
Thermal treatment condition and the resulting grain-boundary carbide distribution;
Tube outside diameter, wall thickness and dimensional tolerances;
Surface finish and freedom from defects;
Non-destructive examination — typically eddy current and ultrasonic testing — to the specified acceptance criteria;
Mechanical properties, grain size and, where required, corrosion or metallographic checks;
Heat and lot traceability, mill certificates and a nuclear quality assurance program.
These requirements follow from the applicable product standard and the project's procurement specification. Industrial-grade Alloy 690 that meets the basic composition limits does not thereby satisfy nuclear steam generator tube requirements.
Grade | Typical composition | Principal advantage | Typical application |
|---|---|---|---|
Inconel 690 | Ni-Cr-Fe, Cr 27–31% | Resistance to high-temperature water corrosion and stress corrosion cracking | PWR steam generator tubing, nitric acid equipment |
Inconel 600 | Ni-Cr-Fe, Cr 14–17% | General corrosion resistance, high-temperature service | Existing equipment, general industrial and thermal applications |
Incoloy 800 | Fe-Ni-Cr, Cr typically 19–23% | High-temperature oxidation resistance and structural stability | Furnace equipment, heat exchangers, high-temperature components |
Inconel 625 | Ni-Cr-Mo-Nb | Combined corrosion resistance, strength and localised corrosion resistance | Marine, chemical processing, high-temperature service |
Inconel 686 | Ni-Cr-Mo, high Cr and Mo | Resistance to aggressive chemical media | Chemical processing, flue gas desulfurisation |
PWR steam generator tubing. Alloy 690 remains the mature reference choice. Selection should focus on the applicable nuclear standard, thermal treatment condition and the full qualification package rather than on the grade name alone.
Retubing or upgrading existing Alloy 600 equipment. Alloy 690 is a credible candidate, but structural and dimensional compatibility, welding or expansion procedures, heat treatment, examination method and code compliance must each be re-evaluated, and procedures may require requalification.
High-temperature oxidation service. Alloy 800-series grades may be equally or more appropriate depending on temperature, atmosphere and mechanical loading, and should be compared on operating data.
Reducing or complex mixed acids. Do not select on "high chromium" or "high molybdenum" labels. Compare corrosion data for the actual concentration, temperature, impurity level and flow condition.
One frequent misconception is worth correcting: Alloy 690 is not excluded from nitric/hydrofluoric acid service. Published technical data list specific nitric/hydrofluoric mixtures among its applications, with measured corrosion rates of roughly 0.15–0.25 mm/a (6–10 mpy) in 10–20% nitric / 2–3% hydrofluoric acid at 60 °C. Applicability remains condition-specific, but it should not be reduced to the rule that any hydrofluoric content disqualifies the alloy.
Steam generator tubing transfers heat from the primary coolant to the secondary side while maintaining the pressure boundary between the two. Tube integrity therefore bears directly on plant safety and on equipment availability, and the material is qualified accordingly.
Thermally treated Alloy 690 tube is the established replacement material for recirculating steam generators. NRC operating experience indicates better long-term performance than the earlier Alloy 600 tubing, but the equipment still requires in-service inspection and integrity management under the applicable codes and plant programs. Material selection reduces risk; it does not remove the need for monitoring.
The alloy performs well in oxidizing acid service. In nitric acid it corrodes at rates below about 1 mpy (0.03 mm/a) in concentrations up to 70% at room temperature and at 80 °C, and it is used for tail-gas reheaters in nitric acid plants and for heating coils and tanks handling nitric/hydrofluoric pickling and nuclear fuel reprocessing solutions. It is also highly resistant to phosphoric acid up to about 85% at temperatures up to 80 °C.
Mixed-acid behavior is more sensitive to operating detail. Concentration, temperature, impurities and redox conditions can shift corrosion rates considerably, and a qualification test using the actual process liquor is advisable before committing to a material.
Beyond tubing, the alloy is used for baffles, tube sheets and internal hardware in nuclear steam supply systems, and for components in high-temperature water or oxidizing gas environments with comparable demands. Suitability depends on the medium, temperature, stress state, fabrication route and design code; a nuclear reference application is evidence of capability, not a general licence to substitute the alloy into unrelated duties.
Chromium content. Alloy 690 contains approximately 27–31% chromium; Alloy 600 typically contains 14–17%. The higher chromium level is the principal reason for the improved corrosion and stress corrosion cracking resistance of Alloy 690 in high-temperature water. Actual performance also depends on thermal treatment, grain-boundary structure, residual stress, water chemistry and equipment design. Alloy 690 is properly described as a significant upgrade for steam generator tubing, not as a material that eliminates cracking risk.
No. Grade name equivalence is not qualification. Nuclear steam generator tubing must satisfy the project material specification, nuclear quality assurance requirements and procurement conditions, which may cover composition including cobalt, thermal treatment, grain size, mechanical properties, dimensional tolerances, non-destructive examination, documentation and lot traceability. Whether material is acceptable for nuclear duty is determined by the governing code and project acceptance requirements, not by a supplier's grade label.
Alloy 690 is a candidate for part of that scope, but the original fabrication and installation procedures cannot simply be carried over. Structural and dimensional compatibility, welding or tube expansion methods, heat treatment, examination techniques, design code and service conditions all need reassessment, and the affected procedures may need requalification.
Not in general terms. The alloy is effective in many oxidizing media, but in hydrochloric and sulfuric acid its behavior varies strongly with concentration, temperature, impurities and redox conditions. Selection should rest on comparative corrosion data for the specific duty rather than on the presence or absence of molybdenum.
Thin-wall steam generator tubing must meet composition and mechanical property requirements plus tighter controls on dimensions, surface quality, thermal treatment, non-destructive examination, traceability and quality assurance. These requirements exist to satisfy the design and acceptance criteria of the specific nuclear component. Their scope is defined by the project technical specification and applicable code, and it would be inaccurate to claim that all industrial Alloy 690 tube must meet identical conditions.
No. Operating experience shows that thermally treated Alloy 690 tubing has performed better than earlier Alloy 600 tubing in steam generator service, but that record does not establish immunity across all material conditions, media and stress states. Cold work level, local residual stress, thermal treatment condition and service environment remain significant variables, and nuclear equipment is subject to inspection, monitoring and integrity assessment under applicable requirements.
Inconel 690 (Alloy 690, UNS N06690) is a representative high-chromium nickel alloy. Its 27–31% chromium content, austenitic nickel matrix and thermally controlled grain-boundary microstructure together provide the basis for its performance in high-temperature water and oxidizing chemical environments.
In PWR steam generator tubing the alloy is an established engineering choice; in nitric acid and selected mixed-acid service it has well-documented application value. For procurement, fabrication and design, selection should be confirmed against chemical composition, product form, thermal treatment condition, mechanical properties, the actual corrosive environment and the governing standard. For nuclear applications in particular, the project's nuclear technical specification and quality assurance requirements are the decisive criteria — not the alloy grade name and not any single property value.
For buyers, the practical conclusion is straightforward. Specify the standard, the product form, the temper and, where relevant, the nuclear supplement; require certified composition and mechanical data for the actual lot; and keep typical published values clearly separated from contractual acceptance limits in every quotation and technical submittal. That discipline protects both the supplier and the purchaser, and it is what distinguishes a defensible technical page from a generic grade description.
