UNS N04400, widely known as Monel 400, is a single-phase nickel-copper alloy containing roughly two thirds nickel and one third copper. It is ordered as seamless pipe and tube under ASTM B165, a specification that fixes heat analysis, tensile requirements, dimensional tolerances and the test regime applied to each lot. For natural gas gathering and transmission lines the grade is selected because it keeps useful strength from cryogenic temperature up to about 480 °C while resisting chlorides, seawater, hydrofluoric acid and caustic solutions that attack carbon steel and many stainless grades.
What ASTM B165 Covers
ASTM B165 applies to cold-worked seamless pipe and tube in the annealed temper. Pipe is ordered by nominal pipe size and schedule, tube by outside diameter and wall thickness. The standard controls heat analysis, tensile properties, permissible variations in outside diameter, wall thickness and length, plus either a hydrostatic test or a nondestructive electric examination. Material released to this specification is normally supplied in the annealed condition; a cold-drawn temper may be ordered where higher strength is needed, in which case the tensile requirements are agreed between purchaser and producer.
Chemical Composition
| Element | Requirement, % |
|---|---|
| Nickel, Ni | 63.0 min |
| Copper, Cu | 28.0-34.0 |
| Iron, Fe | 2.50 max |
| Manganese, Mn | 2.00 max |
| Carbon, C | 0.30 max |
| Silicon, Si | 0.50 max |
| Sulfur, S | 0.024 max |
The copper addition is what separates N04400 from the nickel-chromium grades. Copper raises resistance to reducing acids and to seawater while keeping the alloy fully austenitic and non-magnetic at ambient temperature. Because no chromium is present, the grade is not intended for strongly oxidising media such as nitric acid or hot hypochlorite.
Mechanical Behaviour
Annealed N04400 pipe develops a typical tensile strength near 550 MPa, a 0.2 % offset yield strength near 240 MPa and elongation above 35 % in the longitudinal direction. Cold drawing raises yield strength substantially at the cost of ductility, which is why cold-worked tube is normally specified only where the design pressure or the mechanical loading justifies it. Impact toughness at -196 °C remains high because the alloy has no ductile-to-brittle transition in the normal engineering sense, a property that matters in liquefied natural gas and low-temperature gas processing lines.
Why N04400 Suits Natural Gas Service
Sour and wet gas streams carry chlorides, carbon dioxide and traces of hydrogen sulfide. N04400 resists chloride stress corrosion cracking under these conditions, tolerates high-velocity seawater and brine, and shows negligible attack in caustic and amine systems used for gas sweetening. It also resists erosion-corrosion better than copper-free austenitic stainless steels at comparable flow rates. Where the gas is dry and the design temperature stays below 150 °C, the specification is sometimes extended to allow the lower alloyed alternative, but the corrosion allowance then has to be calculated on a case-by-case basis.
Dimensions, Temper and Delivery Condition
Seamless pipe is produced by hot piercing followed by cold drawing or cold pilgering, then annealed and pickled or bright annealed. Wall thickness tolerance is expressed as a percentage of the ordered wall, and pipe ends are supplied plain, bevelled or threaded according to the order. Tube is frequently supplied in straight lengths up to about 12 m, in the annealed and pickled condition, with hardness limited to a level agreed at enquiry. Random lengths, fixed lengths and the acceptance test requirements should be stated in the purchase order because ASTM B165 permits several options.
Inspection and Traceability
Each lot should be identified by heat number, and the mill test certificate should report heat analysis, tensile results and the results of the hydrostatic or electric test. Dimensional verification covers outside diameter, wall thickness, length and straightness. Where the pipe will be welded or expanded into a tubesheet, supplementary ultrasonic examination is commonly requested to confirm wall integrity. Intergranular corrosion testing is not applicable to this alloy, but hardness checks on cold-drawn product are useful for confirming the delivered temper.
Frequently Asked Questions
Q: Which ASTM standards apply to N04400 in other product forms?
A: ASTM B127 covers plate, sheet and strip, ASTM B163 covers seamless tube, ASTM B164 covers rod, bar and wire, and ASTM B165 covers seamless pipe and tube. For welded pipe the welded-tube specification is used instead.
Q: Can N04400 be used above 480 °C?
A: No. Above roughly 480 °C the alloy suffers from graphitisation and grain-boundary attack in sulfur-bearing environments, so nickel-chromium grades are normally substituted at higher temperatures.
Q: How does N04400 compare with 316L stainless steel in seawater?
A: N04400 has no molybdenum addition but is far more resistant to chloride pitting and stress corrosion cracking than 316L; the practical limitation is cost and the lower allowable stress at elevated temperature.
Q: Is a hydrostatic test always required?
A: No. ASTM B165 allows either the hydrostatic test or a nondestructive electric examination, and the purchaser should state which option is required when the pipe is ordered.
Q: What is the maximum practical pipe size in seamless N04400?
A: Cold-worked seamless pipe is routinely produced up to about NPS 8 in schedule ranges common to process piping; larger sizes move into welded construction or are supplied to a plate-based specification instead.
Q: Does cold working affect corrosion resistance?
A: Heavy cold reduction raises residual stress and can lower resistance in severe chloride environments, so cold-worked pipe is usually given an anneal before it is placed in wet sour service.
