ASTM B161 Nickel 200/201 Seamless Pipe & Tube Standard

What is ASTM B161?

ASTM B161 is the standard specification for nickel seamless pipe and tube. It falls under ASTM Committee B02 on Nonferrous Metals and Alloys, Subcommittee B02.07 on Refined Nickel and Cobalt. The specification references ASTM B829 (General Requirements for Nickel and Nickel Alloy Seamless Pipe and Tube) for supplementary requirements like marking, packaging, and inspection.

Only 2 alloys fall under B161. Both are commercially pure nickel with 99.0% Ni minimum. The critical difference between them is carbon content. Nickel 200 allows up to 0.15% carbon; Nickel 201 caps it at just 0.02%. That single variable changes how each alloy behaves at elevated temperatures, which determines where each grade gets specified.

B161 covers both pipe (ordered by standard pipe size and schedule per ANSI B36.10/B36.19) and tube (ordered by OD and wall thickness or ID and wall thickness). Both product forms are produced by the seamless cold-working process, though hot-worked material can be supplied if the buyer and manufacturer agree on properties.

Heat Treatment and Temper Conditions

B161 material ships in one of two conditions: annealed or stress-relieved. Unlike austenitic stainless steels or duplex grades, commercially pure nickel does not require rapid quenching after annealing. The single-phase face-centred cubic (FCC) structure of nickel means there are no detrimental intermetallic phases to suppress through fast cooling.

Annealing is performed at 1300–1600°F (700–870°C) in a protective atmosphere (hydrogen, dissociated ammonia, or vacuum) to prevent surface oxidation. The anneal recrystallises the cold-worked grain structure, restoring full ductility. Stress-relieving uses a lower temperature range, typically 525–900°F (275–480°C) to reduce residual stresses while keeping most of the cold-work strength intact.

Annealed material is the default for forming-intensive work: bending, flanging, U-tube fabrication, and expansion. Stress-relieved material is specified when a higher allowable stress is needed in straight pipe runs and pressure vessels, where forming is minimal.

Corrosion Resistance of Nickel 200/201

Commercially pure nickel is preferred in strongly alkaline environments, particularly caustic soda service, where it significantly outperforms stainless steels and many nickel alloys.

• Caustic soda (NaOH): Nickel 200/201 handles all concentrations of NaOH at temperatures up to 600°F (315°C) for Nickel 200 and well above that for Nickel 201. Corrosion rates stay below 1 mpy (mil per year) in most caustic service. Stainless steels suffer rapid caustic cracking under the same conditions.
• Dry halogens: Nickel resists dry chlorine, hydrogen chloride, and hydrogen fluoride gas. It forms a thin, protective nickel halide film. The “dry” means that when moisture is present, the protective halide film breaks down and corrosion increases.
• Reducing acids: Nickel has moderate resistance to de-aerated sulfuric acid, especially at lower concentrations. It is not the best choice for oxidising acids like nitric acid.
• High-purity water and food-grade media: Nickel does not impart taste or colour to process fluids. It sees heavy use in food and pharmaceutical plants, particularly for handling synthetic fibres, alkalis, and de-ionised water systems.

Testing and Inspection Requirements

ASTM B161 testing is governed by ASTM B829 (General Requirements for Nickel and Nickel Alloy Seamless Pipe and Tube). Each lot must pass chemical verification, mechanical testing, and a hydrostatic or nondestructive test.

Tension Test

One specimen per lot. The test confirms tensile strength, yield strength (0.2% offset), and elongation. Specimens are taken longitudinally from the pipe or tube wall. For small-diameter or light-wall tubes (converter sizes), the mechanical values specified by B161 apply.

Hydrostatic Test

Every pipe and tube must be hydrostatically tested unless a nondestructive electric test is substituted. The test pressure is calculated using P = 2St/D, where S is the allowable fibre stress (per B829), t is the wall thickness, and D is the OD. Any pipe or tube that leaks during the test is rejected, in accordance with ASTM B829 provisions.

Nondestructive Electric Test (NDE)

When agreed between buyer and manufacturer, eddy-current testing per ASTM E571 can replace the hydrostatic test. The calibration reference standard uses a drilled hole not larger than 0.031 in. (0.79 mm) in diameter for specific wall thickness ranges. Tubes producing signals equal to or greater than the calibration defect are rejected, unless the indication can be explored and found harmless.

Flattening and Flare Tests (If Specified)

These tests are not mandatory under B161 but can be added as supplementary requirements in the purchase order. When required, the flattening distance is not typically expressed as a multiple of wall thickness; it's usually expressed as a function of OD and wall or defined as a specific H value (the distance between plates) without cracking. The flare test expands the tube end over a tapered mandrel to verify ductility.