and Primary Applications
nickel-chromium-iron-molybdenum alloy UNS N06002 (UNS N06002) is a nickel-chromium-iron-molybdenum alloy engineered for extreme high-temperature service. It occupies a practical niche between ordinary stainless steels and advanced precipitation-hardened superalloys, offering an optimized balance of creep strength, oxidation resistance, and fabricability for sustained use in the 1800°F to 2200°F (980°C to 1200°C) range.
Unlike nickel alloy UNS N07718, which is strengthened by gamma double-prime precipitation and loses that strengthening above roughly 1300°F (700°C), nickel-chromium-iron-molybdenum alloy UNS N06002 is solid-solution strengthened and maintains useful creep-rupture strength and oxidation resistance at much higher temperatures while remaining readily weldable. Compared with nickel-iron-chromium alloy UNS N08810, its higher chromium content combined with cobalt and tungsten additions forms a more stable protective oxide scale above about 2000°F.
Pipe and tube in nickel-chromium-iron-molybdenum alloy UNS N06002 find primary application in industrial gas turbines and aerospace auxiliary systems, including combustion liners, transition ducts, afterburner components, and exhaust ducting. High-temperature process piping in pyrolysis, reforming, and heat treatment furnaces is another major use where service exceeds the capability of 800H and the environment is highly oxidizing.
Chemical Composition and High-Temperature Properties
The alloy contains approximately 22% chromium and 9% molybdenum, with about 1.5% cobalt and 0.6% tungsten in a nickel base. Chromium forms a stable chromium oxide scale, while molybdenum, cobalt, and tungsten provide solid-solution strengthening at temperature. Tight control of carbon in the 0.05-0.15% range supports carbide strengthening, and boron at about 0.005% enhances grain boundary strength and creep life.
Under thermal cycling, several metallurgical factors govern performance. High chromium content with a small lanthanum addition helps form an adherent, spallation-resistant oxide scale, limiting cyclic loss of wall thickness that could otherwise become crack initiation sites. Creep resistance prevents gradual distortion, and retained ductility after long exposure allows the material to accommodate thermal strain without brittle failure. The coefficient of thermal expansion is higher than ferritic steels and generally lower than austenitic stainless steels such as 304H, so piping systems must include flexible bellows, expansion joints, or sliding supports to manage differential growth and avoid low-cycle fatigue cracking.
Standards and Supplementary Requirements
Wrought plate, sheet, and strip are covered by ASTM B435, while welded pipe and tube of nickel alloys including UNS N06002 are covered by ASTM B619. For high-temperature creep service, these base standards are only a starting point, and performance-based supplementary requirements should be written into the procurement specification.
Essential supplementary requirements include: a coarse grain size of ASTM 5 or coarser after solution annealing to improve creep-rupture strength; certified stress-rupture test data from the heat lot per ASTM E139, for example stress for rupture in 1000 hours at 1800°F (980°C); cyclic oxidation test data for critical service to verify scale adherence and spallation resistance; tight control of carbon and boron; 100% radiographic and liquid penetrant examination of all weld seams on welded pipe; and source inspection by an approved third party to witness testing and review mill certifications.
Welding and Fabrication Considerations
nickel-chromium-iron-molybdenum alloy UNS N06002 is more weldable than precipitation-hardened alloys, but disciplined procedures are required to avoid weld hot cracking and strain-age cracking in the heat-affected zone. All surfaces must be cleaned of oil, grease, paint, and marker residues that could introduce sulfur, phosphorus, or lead. Generous root openings and groove angles compensate for the lower fluidity of the weld metal.
Gas tungsten arc welding (GTAW) is preferred for root and hot passes because of precise heat input control, with SMAW or GMAW acceptable for fill passes. Matching filler metals are ERNiCrMo-2 (AWS A5.14) or ENiCrCoMo-1 (AWS A5.11), and a filler with slightly higher chromium may be chosen where maximum high-temperature oxidation resistance is needed. Technique matters: stringer beads, low heat input, controlled interpass temperature, and 100% argon backing gas to prevent oxidation of the root bead are standard practice.
For stress relief, an anneal near 1800°F (980°C) is recommended for thick sections or highly restrained joints to reduce residual stresses that could contribute to relaxation cracking in service. Full solution annealing near 2150°F (1175°C) is not typically required unless the weldment has been heavily cold worked.
Inspection and Life Assessment
The dominant long-term degradation mechanisms for nickel-chromium-iron-molybdenum alloy UNS N06002 pipe are creep and stress rupture, thermal fatigue, oxidation with scale spallation, and microstructural instability such as sigma phase or mu phase formation after very long exposure. Inspection programs therefore combine dimensional surveys to detect bulging or ovality, ultrasonic testing to measure remaining wall thickness and find internal voids, replication metallography to reveal grain boundary cavitation and microcracking in the field, and hardness testing to flag over-aging or phase transformation. Remnant life can be estimated with models based on the Larson-Miller parameter, fed by operating history and field inspection data.
Frequently Asked Questions
What is the difference between nickel-chromium-iron-molybdenum alloy UNS N06002 and nickel alloy UNS N07718? nickel alloy UNS N07718 is precipitation hardened and has superior tensile and yield strength up to about 1300°F (700°C), but it loses strength above that temperature and is prone to strain-age cracking when welded. nickel-chromium-iron-molybdenum alloy UNS N06002 is solid-solution strengthened and retains useful creep strength and oxidation resistance up to about 2200°F while being readily weldable.
Which standards cover nickel-chromium-iron-molybdenum alloy UNS N06002 pipe? ASTM B435 covers plate, sheet, and strip, and ASTM B619 covers welded pipe and tube of nickel alloys including UNS N06002. Pipe is often fabricated from plate conforming to B435 and welded to B619 requirements.
Why is a coarse grain size specified for high-temperature creep service? uniform coarse grain size of ASTM 5 or coarser improves creep-rupture strength, because fewer grain boundaries reduce diffusion-assisted creep mechanisms. The specification should state that material is solution annealed to achieve this grain size.
What filler metal is used to weld nickel-chromium-iron-molybdenum alloy UNS N06002 pipe? The matching filler metal is ERNiCrMo-2 (AWS A5.14), with ENiCrCoMo-1 (AWS A5.11) as an alternative. A filler with slightly higher chromium content may be selected when maximum high-temperature oxidation resistance is required.
What is the recommended stress relief treatment for welded nickel-chromium-iron-molybdenum alloy UNS N06002? For thick sections or highly restrained joints, a stress relief anneal near 1800°F (980°C) is recommended to reduce residual stresses. Full solution annealing near 2150°F (1175°C) is only required if the weldment has been significantly cold worked.
How is remnant creep life of nickel-chromium-iron-molybdenum alloy UNS N06002 pipe assessed? Engineers combine operating history with inspection data, including ultrasonic wall-thickness measurement, replication metallography to detect grain boundary cavitation, and hardness testing, then apply models such as the Larson-Miller parameter to estimate remaining creep life.
