Grade Identity and Standard Basis
A381108 is the designation used on this site for the nickel-chromium-iron alloy pipe supplied to nuclear code requirements for steam generator and primary-circuit components, where the material is written as NC30Fe. In the ASTM and ASME system the same family is the nickel-chromium-iron alloy 690, UNS N06690, covered by the product specification for seamless and welded nickel-chromium-iron alloy tube and by the matching specification for rod and bar.
The alloy contains nominally 60 % nickel with about 30 % chromium and a controlled iron content. That high chromium level is the source of its resistance to stress corrosion cracking in high-temperature water and to attack in caustic and oxidising environments, and it is the reason high-chromium nickel alloys replaced lower-chromium grades in nuclear steam generator tubing and in primary circuit penetrations. The grade is supplied as seamless tube, welded tube and bar, and it is used where a long design life at temperature must be combined with a very low probability of cracking.
Chemical Composition
| Element | Requirement, % |
|---|---|
| Nickel, min | 58.00 |
| Chromium | 27.00 - 31.00 |
| Iron | 7.00 - 11.00 |
| Carbon, max | 0.05 |
| Manganese, max | 0.50 |
| Silicon, max | 0.50 |
| Copper, max | 0.50 |
| Sulfur, max | 0.015 |
The chromium range is narrow on purpose, because both higher and lower chromium levels change the precipitation behaviour of the alloy during long exposure at temperature. Carbon and sulfur are kept low to limit the formation of grain boundary precipitates that would reduce ductility in service. Nuclear purchase specifications add residual element controls and require chemical analysis to be reported for every heat, together with documentation of the melting practice and the heat treatment applied to the finished product.
Mechanical Properties and High-Temperature Behaviour
| Property, annealed condition | Requirement |
|---|---|
| Tensile strength, min | 586 MPa |
| Yield strength, min | 241 MPa |
| Elongation, min | 30 % |
| Typical service range | Elevated-temperature service up to about 700 degrees Celsius depending on the design code |
The alloy has good creep and stress rupture strength in the range where steam generator and primary circuit components operate, and it retains useful ductility after very long exposure at temperature. Its thermal expansion is lower than that of the austenitic stainless steels, which reduces the thermal fatigue penalty at transition joints between dissimilar materials. Tensile and hardness tests are performed on the finished product, and the results are reported per heat and per lot.
Corrosion Resistance and Nuclear Service Behaviour
Three properties drive the selection of this alloy. First, resistance to stress corrosion cracking in high-temperature water: the combination of high nickel and high chromium produces a surface oxide that is far more stable than that formed on lower-chromium alloys. Second, resistance to caustic environments, where the alloy withstands concentrated alkaline conditions that readily crack stainless steels. Third, resistance to oxidising conditions and to intergranular attack, which is verified by the corrosion test applied to nickel-chromium-iron alloys using a ferric sulfate and sulfuric acid solution.
Because the material is used in safety-related systems, the quality of the finished surface and of every weld is treated as a corrosion variable rather than as a cosmetic matter. Embedded iron, heat tint, grinding marks and incomplete cleaning all create local chemistries that can initiate attack, so fabrication procedures for this alloy include explicit controls on tooling, cleaning and surface restoration. Documentation of the full manufacturing route is normally required so that the corrosion performance of the delivered component can be traced back to the process steps that produced it.
Welding, Heat Treatment and Inspection
Welding is carried out by gas tungsten arc welding with a matching nickel-chromium-iron filler metal, using controlled heat input and dilution so that the weld metal composition remains within specification. Preheating is not required and post-weld heat treatment is not normally applied, because the alloy is designed to be used in the solution-annealed condition and because reheating can precipitate grain boundary phases. Where mill products are solution annealed, the treatment is carried out in a controlled atmosphere and followed by a rapid quench, and the hardness and mechanical test results are reported with the heat treatment record.
Inspection normally includes dimensional verification, hydrostatic testing of tube, eddy current or ultrasonic examination, radiographic examination of welded seams, dye penetrant examination of finished surfaces, and positive material identification of each component. For steam generator tube in particular, dimensional tolerances, surface finish and the eddy current test results are the measurements that determine acceptance.
Applications
Steam generator tubing in pressurised water reactors
Primary circuit penetrations, nozzles and safe ends
Pressuriser heater sheaths and vessel internals
Control rod drive mechanism components
High-temperature chemical process equipment and furnace internals outside the nuclear sector
FAQ
Q: What does NC30Fe mean?
It is the nuclear code designation for this nickel-chromium-iron alloy, indicating a nickel-base material with approximately 30 % chromium and a controlled iron content and corresponding to the alloy 690 family, UNS N06690.
Q: Why is this alloy used instead of stainless steel in steam generators?
Its high chromium content gives markedly better resistance to stress corrosion cracking in high-temperature water and to caustic attack, which is essential for a component that must remain leaktight for decades.
Q: Does the alloy require post-weld heat treatment?
No. It is used in the solution-annealed condition, and welding procedures are qualified on the basis of controlled heat input and correct filler metal selection rather than thermal treatment after welding.
Q: How is corrosion resistance demonstrated?
By strict chemical control of the heat, by corrosion testing in a ferric sulfate and sulfuric acid solution for the finished product or weld area, and by documented cleanliness and surface conditioning during fabrication.
Q: Is the material magnetic?
It is a nickel-base austenitic alloy with very low magnetic permeability, which matters in applications where magnetic tools and instruments are used around the component.
Q: How is welding quality controlled?
Through procedure qualification with the matching filler metal, control of heat input and dilution, and inspection by radiographic, eddy current or dye penetrant methods as directed by the purchase order.
