Why C-276 for Aerospace Heat Exchangers?
nickel-molybdenum-chromium alloy UNS N10276, designated UNS N10276, is a nickel-molybdenum-chromium superalloy with a tungsten addition and one of the most versatile corrosion-resistant alloys available. For aerospace engineering heat exchangers it is preferred because it combines exceptional resistance to a wide range of corrosive media with outstanding high-temperature stability and excellent fabricability, in an application where failure is not an option.
The alloy maintains its mechanical properties from cryogenic temperatures to 540°C (1000°F), forms a protective chromium oxide scale in oxidizing service, welds without post-weld heat treatment, and withstands the thermal cycling of aerospace operation. Typical heat exchanger applications include fuel and oil coolers exposed to jet fuel and hydraulic fluids, environmental control system (ECS) units handling temperature extremes and corrosive condensates, engine bleed air heat exchangers facing hot oxidizing combustion gases, hydraulic system coolers resisting phosphate ester fluids, cryogenic exchangers at liquid hydrogen and oxygen temperatures, and exhaust gas recirculation units subject to sulfuric acid condensation.
Composition, Standards, and Mechanical Requirements
The balanced composition of C-276 delivers its unique properties. Molybdenum at 15.0-17.0% provides exceptional resistance to pitting, crevice corrosion, and reducing environments; chromium at 14.5-16.5% gives oxidation resistance; tungsten at 3.0-4.5% enhances pitting resistance and high-temperature strength; iron at 4.0-7.0% adds solid-solution strengthening; and carbon is held to 0.010% maximum to prevent carbide precipitation and intergranular corrosion.
| Element | Composition (UNS N10276) |
|---|---|
| Nickel (Ni) | Balance, minimum 57% |
| Molybdenum (Mo) | 15.0-17.0% |
| Chromium (Cr) | 14.5-16.5% |
| Iron (Fe) | 4.0-7.0% |
| Tungsten (W) | 3.0-4.5% |
| Carbon (C) | 0.010% max |
| Silicon (Si) | 0.08% max |
| Manganese (Mn) | 1.0% max |
| Sulfur (S) | 0.030% max |
| Phosphorus (P) | 0.040% max |
Plate is governed by ASTM B575, the standard specification for low-carbon nickel-chromium-molybdenum alloy plate, sheet, and strip, with ASME SB575 for pressure vessel and heat exchanger construction and AMS 5504 for aerospace-grade sheet and plate. In the solution-annealed condition, ASTM B575 requires a minimum tensile strength of 100 ksi (690 MPa), minimum yield strength of 41 ksi (283 MPa), and minimum elongation of 40%, with hardness typically 90-100 HRB. Solution annealing is performed at 1120-1200°C (2050-2200°F) followed by rapid cooling by water quench or rapid air cool to dissolve carbides and intermetallic phases. AMS 5504 adds vacuum induction melting (VIM) or consumable electrode remelting (VAR), strict surface finish requirements, ultrasonic or eddy current examination, full heat-number traceability, and aerospace-grade certification documentation.
Thermal Properties and Design Considerations
C-276 offers a thermal conductivity of 10.0-11.5 W/m·K from 20°C to 400°C, a specific heat capacity of 410-460 J/kg·K, a coefficient of thermal expansion of 11.2-13.2 x 10-6/°C, a melting range of 1325-1370°C, and a maximum service temperature of 540°C (1000°F) continuous or 815°C (1500°F) intermittent. Conductivity is lower than copper but comparable to stainless steels; the CTE is compatible with other austenitic alloys, allowing bimetallic joints with 316 stainless steel and excellent matching with nickel alloy UNS N06625.
For heat exchanger design, the high corrosion resistance allows reduced wall thickness that enhances heat transfer, the smooth passive surface reduces fouling, small-diameter tubes of 6-25 mm suit compact exchangers, and good weldability permits reliable fin attachment by welding or brazing. The alloy supports plate-fin, shell-and-tube, printed circuit, compact, and regenerative heat exchanger concepts.
Fabrication and Welding
C-276 exhibits excellent weldability, a key advantage in heat exchanger fabrication. GTAW (TIG) is preferred, with GMAW for thicker sections and plasma arc for precision work. The standard filler is ERNiCrMo-4, matching the C-276 composition; ERNiCrMo-10 (Alloy C-22 type) is an alternative for specific applications, while ERNiCrMo-3 (Alloy 625) is not recommended because of lower corrosion resistance. Argon or argon-helium shielding with back purging is essential, heat input is controlled to minimize distortion and grain growth, interpass temperature is maintained below 150°C (300°F), and neither preheating nor post-weld heat treatment is required. The absence of PWHT is a critical advantage: the ultra-low carbon content prevents carbide precipitation and the controlled chemistry maintains corrosion resistance in the as-welded condition, simplifying large assemblies and reducing cost and lead time.
Forming follows standard practice: excellent cold formability in the solution-annealed condition with minimum bend radii of 2-4 times thickness and allowance for moderate springback; hot forming at 950-1150°C (1740-2100°F) requires subsequent solution annealing; and intermediate annealing at 1120-1200°C with rapid cooling is required after significant cold work. Machining uses carbide tooling of C-2 or C-3 grade at 80-120 SFM for roughing and 100-150 SFM for finishing, aggressive feeds of 0.005-0.015 in/rev to cut below the work-hardened layer, and flood coolant; light cuts should be avoided to maintain constant engagement.
Quality Assurance, Testing, and Procurement
Aerospace procurement starts with documentation: mill test reports with heat number, chemical analysis, mechanical properties, and heat treatment; heat treatment records; product marking; and full traceability from melt to finished product. Chemical composition is verified by heat analysis plus positive material identification, with carbon, molybdenum, and tungsten receiving special attention. Mechanical testing per heat or lot includes room-temperature tensile at 690 MPa minimum UTS and 283 MPa minimum YS, 40% minimum elongation, hardness, and bend testing for sheet products. Corrosion testing may include ASTM G28 for intergranular attack and ASTM G48 for pitting resistance, plus custom simulated-service tests with aerospace fluids.
Non-destructive examination includes ultrasonic testing for plate over a certain thickness, eddy current testing for sheet and thin plate, liquid penetrant testing of critical areas, and visual examination of all products. Weld inspection adds radiographic testing, hydrostatic testing, and helium leak testing where leak-tight integrity is required. Suppliers should hold AS9100 aerospace quality management certification and ISO 17025 testing laboratory accreditation, with NDE performed by certified personnel. Receiving inspection verifies markings against the purchase order, reviews MTR conformance to AMS 5504 and ASTM B575, confirms melting process documentation, performs PMI, and checks surface condition, dimensions, flatness, and packaging. Storage must keep material clean, segregated by heat number, and protected from carbon steel contamination.
Frequently Asked Questions
Why is C-276 preferred over stainless steel 316 for aerospace heat exchangers? 316 stainless steel has poor pitting and reducing-acid resistance and limited high-temperature strength, while C-276 offers excellent pitting resistance, reducing-acid resistance, oxidation resistance, and high-temperature strength, with only moderate thermal conductivity trade-offs.
What standards govern C-276 plate? ASTM B575 covers plate, sheet, and strip; ASME SB575 applies to pressure vessel and heat exchanger construction; and AMS 5504 is the aerospace material specification with stricter quality controls, VIM or VAR melting, and full traceability.
Does C-276 require post-weld heat treatment? No. The ultra-low carbon content prevents carbide precipitation and the controlled chemistry maintains corrosion resistance in the as-welded condition, which simplifies fabrication of large heat exchanger assemblies and reduces cost and lead time.
What filler metal is used for welding C-276? ERNiCrMo-4, matching the C-276 composition, is the standard filler. ERNiCrMo-10 is an alternative for specific applications; ERNiCrMo-3 (Alloy 625) is not recommended because of its lower corrosion resistance.
What is the maximum service temperature of C-276? The article cites 540°C (1000°F) continuous and 815°C (1500°F) intermittent service, which suits most aerospace heat exchanger applications. The exact limit should be confirmed against the governing specification for the specific duty.
What inspection is required for aerospace heat exchangers? Typical requirements include liquid penetrant testing of welds and critical areas, radiographic testing of pressure-containing welds, hydrostatic pressure verification, helium leak testing for leak-tight assemblies, and visual inspection of surface condition and weld profile.
