ASTM B444 Inconel 625 (UNS N06625) Seamless Pipe & Tube Standard

ASTM B444 Grade 1 & Grade 2

The two-grade system in ASTM B444 is one of the most important distinctions in nickel alloy pipe procurement. Specifying the wrong grade can lead to premature failure or unnecessary cost.

Grade 1 is annealed and has higher room-temperature strength. It is the right choice when the operating temperature stays below 1100°F (593°C), and the primary requirement is corrosion resistance combined with high mechanical strength. This grade is mainly found in subsea flowlines, seawater cooling systems, sour gas production tubing, marine exhaust bellows, and chemical process piping exposed to aggressive acids below 600°F (315°C).

Grade 2 is solution annealed and has superior elevated-temperature creep and rupture life. It is required when the service temperature exceeds 1100°F (593°C) and long-term creep resistance or thermal stability matters. You will find this grade in gas turbine exhaust ducting, nuclear fuel reprocessing equipment, reformer tubing, and furnace components.

The table below compares the two grades across all key parameters.

Chemical Composition of ASTM B444

ASTM B444 defines a single chemical composition for UNS N06625, applicable to both Grade 1 and Grade 2. The chemical composition is identical, with the only difference being the heat treatment and the resulting microstructure. Inconel 625 is a solid-solution-strengthened alloy with significant additions of chromium (20-23%), molybdenum (8-10%), and niobium/columbium (3.15-4.15%).

The table below shows the full composition limits per ASTM B444.

Note: Nickel is calculated arithmetically by difference. One chemical analysis test per lot (one heat) is required. Product (check) analysis variations follow ASTM B829. The niobium-tantalum content (3.15-4.15%) is the signature element of Inconel 625. It provides the solid-solution strengthening that differentiates 625 from the Hastelloy C-family alloys, which rely more heavily on molybdenum and tungsten.

Mechanical Properties of ASTM B444

The mechanical properties of ASTM B444 depend entirely on the grade (heat treatment). Grade 1’s fine-grained annealed microstructure produces 120,000 psi (827 MPa) minimum tensile and 60,000 psi (414 MPa) minimum yield. This is among the highest room-temperature strengths of any solid-solution-strengthened nickel alloy. Grade 2’s coarser solution-annealed structure drops to 100,000 psi (690 MPa) tensile and 40,000 psi (276 MPa) yield but gains the elevated-temperature creep life needed for service above 1100°F (593°C).

Note: The minimum tensile strength for Grade 1 applies only to tubing in straight lengths. Hot-forming quality pipe and tube (for bending, etc.) is furnished to chemical requirements and surface inspection only. This means no tensile properties are guaranteed. For small-diameter converter sizes (≤1-1/4 in. / 31.8 mm OD), see the specification for temper-specific properties.

ASTM B444 Tolerances

ASTM B444 cold-worked pipes and tubes follow specific dimensional tolerances to ensure accuracy and quality. The outside diameter tolerance ranges from ±0.13 mm for 10–16 mm OD to +3.18 / −0.79 mm for 356–610 mm OD, depending on the pipe size. Wall thickness tolerance generally varies between ±10% and ±15%, while OD limits for sizes below 50.8 mm are +3.2/−0 mm, and for 50.8 mm and above are +4.8/−0 mm, as referenced in ASTM B829.

Heat Treatment Requirements for ASTM B444

The heat treatment is the defining difference between Grade 1 and Grade 2. It determines grain size, precipitate distribution, and the balance between room-temperature strength and elevated-temperature creep resistance. Getting this right at the purchase order stage avoids costly material replacements.

Grade 1: Annealing

Grade 1 material is annealed at a minimum temperature of 1600°F (871°C). This temperature is high enough to recrystallise the cold-worked structure but low enough to retain fine NbC precipitates that impede dislocation movement and restrict grain growth during annealing, maintaining a fine grain size. These precipitates act as grain-boundary pinning agents, keeping the grain size fine (typically ASTM 5-8). The result is a microstructure with high room-temperature strength (120 ksi tensile, 60 ksi yield) and good fatigue resistance.

Grade 2: Solution Annealing

Grade 2 material is solution annealed at a minimum temperature of 2000°F (1093°C). At this temperature, all carbides, carbonitrides, and any Ni2(Nb, Mo)-type Laves phase dissolve into the nickel matrix. The grain size grows to ASTM 2-5. This clean, coarse-grained structure creeps more slowly at temperatures above 1100°F (593°C) because grain boundaries, which are the primary creep failure path, are fewer and longer.

An optional stabilisation anneal at 1800°F (982°C) minimum can follow the solution anneal. This controlled ageing reprecipitates niobium carbides at grain boundaries in a semi-continuous network that resists intergranular corrosion after long-term thermal exposure. This stabilised condition is specified in nuclear fuel reprocessing and critical chemical service.

Available Conditions and Finishes

ASTM B444 describes the standard conditions and finishes available for Inconel 625 pipe and tube. The finish depends on whether the product is cold-worked with ground OD (bright) or cold-drawn without grinding (pickled/matte).