ASTM B444 is a specific standard specification for cold-worked seamless tubing made of a nickel-chromium-molybdenum-niobium alloy (UNS N06625). This material is known globally as Inconel 625. Unlike multiple alloy specifications such as ASTM B622, B444 covers only one alloy: Inconel 625. This focus on a single alloy implies stricter quality requirements, specific heat treatment processes, and mechanical property standards tailored to the unique combination of high strength, corrosion resistance, and fatigue life of Inconel 625.
The specification defines the chemical composition, two different heat treatment grades (Grade 1 and Grade 2), mechanical properties, dimensional tolerances, hydrostatic testing, and inspection procedures. ASTM B444 covers both tubing (dimensions are classified by nominal size and diameter as specified in ANSI B36.19) and tubing (dimensions are classified by outer diameter and wall thickness).
Inconel 625 tubing conforming to ASTM B444 standards is widely used in chemical processing, offshore oil and gas, marine exhaust systems, aerospace components, nuclear fuel reprocessing, flue gas desulfurization (FGD), and subsea pipelines. This guide covers all aspects of the B444 standard (including two grades, chemical composition, mechanical properties, tolerances, heat treatment, testing, and practical application issues) to help engineers and purchasing personnel order the right product for their application needs.
Key Differences Between Grade 1 and Grade 2
1. Grade 1 (Annealed)
Meaning: The material is annealed at a relatively low temperature (typically above 1600°F / 871°C).
Characteristics: Finer grains, higher mechanical strength (higher tensile and yield strength).
Main Applications: High-pressure, low-temperature/room-temperature corrosive environments.
Typical Applications: The Brazilian deep-sea oil and gas project you mentioned, subsea manifolds, high-pressure instrument lines, chemical reagent injection lines.
2. Grade 2 (Solution Annealed)
Meaning: The material is solution-treated at a higher temperature (typically above 2000°F / 1093°C) to achieve complete carbide solution.
Characteristics: Coarser grains, slightly lower strength than Grade 1, but with excellent creep resistance and fracture toughness.
Main focus: Extreme high-temperature environments (typically referring to operating conditions exceeding 1100°F / 593°C).
Typical applications: Aero-engine parts, industrial furnace components, waste incinerators, petrochemical pyrolysis units.
Chemical Composition of ASTM B444
The table below lists the complete composition limits specified in ASTM B444.
| Element | Composition, % | Role in Alloy |
| Nickel (Ni) | 58.0 min | Matrix element, corrosion resistance base |
| Chromium (Cr) | 20.0–23.0 | Oxidation resistance, passive film former |
| Molybdenum (Mo) | 8.0–10.0 | Pitting/crevice resistance, reducing acid resistance |
| Niobium + Tantalum (Cb+Ta) | 3.15–4.15 | Solid-solution strengthening, stabilises against sensitisation |
| Iron (Fe) | 5.0 max | Residual/cost element |
| Carbon (C) | 0.10 max | Kept low to minimise carbide precipitation |
| Manganese (Mn) | 0.50 max | Deoxidiser, sulfur control |
| Silicon (Si) | 0.50 max | Deoxidiser |
| Phosphorus (P) | 0.015 max | Impurity limit |
| Sulfur (S) | 0.015 max | Impurity limit, hot cracking control |
| Cobalt (Co) | 1.0 max | Residual (if determined) |
| Aluminium (Al) | 0.40 max | Minor strengthening, oxidation support |
| Titanium (Ti) | 0.40 max | Minor carbide stabiliser |
Mechanical Properties of ASTM B444
| Property | Grade 1 (Annealed) | Grade 2 (Solution Annealed) | Test Method |
| Tensile Strength, min | 120,000 psi (827 MPa) | 100,000 psi (690 MPa) | ASTM E 8 |
| Yield Strength (0.2% offset), min | 60,000 psi (414 MPa) | 40,000 psi (276 MPa) | ASTM E 8 |
| Elongation in 2 in. or 4D, min | 30% | 30% | ASTM E 8 |
| Hydrostatic Fiber Stress (S) | 30,000 psi (207 MPa) | 25,000 psi (172 MPa) | P = 2St/D |
| Hydrostatic Test Pressure, max | 1,000 psi (6.9 MPa) | 1,000 psi (6.9 MPa) | - |
ASTM B444 Tolerances
The dimensional tolerances of the ASTM B444 standard cover the outer diameter, wall thickness, and cut length of cold-worked seamless tubing. Because B444 only covers cold-worked products, its tolerances are more stringent than those for hot-worked nickel alloy tubing.
| STANDARD | OD (D) | TOLERANCE (MM) | THICKNESS (S) | TOLERANCE | LENGTH | TOLERANCE |
| MM | MM | |||||
| B444 (Cold-worked Pipe and Tube) |
10≤D<16 | ±0.13 | - | ±15% | OD<50.8 : +3.2/-0 OD≥50.8 : +4.8/-0 Details refer to B829 |
|
| 16≤D≤38 | ±0.19 | ±10% | ||||
| 38 | ±0.25 | |||||
| 76 | ±0.38 | ±12.5% | ||||
| 114 | ±0.51 | |||||
| 152 | ±0.64 | |||||
| 168≤D<219 | ±0.79 | ±15%/-12.5% | ||||
| 219 | +1.57/-0.79 | |||||
| 356 | +3.18/-0.79 | |||||
ASTM B444 Grade 1 vs Grade 2
The two-tier system in ASTM B444 is one of the most important distinguishing criteria in the procurement of nickel alloy pipes. Choosing the wrong tier can lead to premature failure or unnecessary costs.
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 all the key parameters for the two levels.
| Parameter | Grade 1 (Annealed) |
Grade 2 (Solution Annealed) |
| Heat Treatment | Annealed at 1600°F (871°C) minimum | Solution annealed at 2000°F (1093°C) minimum |
| Stabilisation | Not required | Optional: 1800°F (982°C) min. for sensitisation resistance |
| Tensile Strength, min | 120,000 psi (827 MPa) | 100,000 psi (690 MPa) |
| Yield Strength, min | 60,000 psi (414 MPa) | 40,000 psi (276 MPa) |
| Elongation, min | 30% | 30% |
| Fiber Stress (Hydro Test) | 30,000 psi (207 MPa) | 25,000 psi (172 MPa) |
| Max Service Temperature | 1100°F (593°C) | Above 1100°F (593°C) |
| Primary Application | Corrosion resistance, subsea, marine | Creep/rupture resistance, furnace, nuclear |
| Default If Not Specified | Yes (Grade 1 ships) | - (Must be specified) |
| Grain Size | Finer (ASTM 5–8) | Coarser (ASTM 2–5) |
When to Use Grade 1 Alloys
Grade 1 alloys are ideal when operating temperatures are below 1100°F (593°C) and the primary requirements are corrosion resistance and high mechanical strength. The lower annealing temperature (minimum 1600°F / 871°C) results in the precipitation of fine carbides in the matrix, strengthening the alloy. This makes Grade 1 alloys 20% stronger in tensile strength and 50% stronger in yield strength than Grade 2 alloys.
Common applications for Grade 1 alloys include subsea oil pipelines, seawater cooling systems, acid gas production pipelines, marine exhaust bellows, and chemical process pipelines exposed to corrosive acid environments at temperatures below 600°F (315°C).
When to Use Grade 2 Alloys
Grade 2 alloys are required when operating temperatures exceed 1100°F (593°C) and long-term creep resistance or thermal stability is critical. Solution annealing at a minimum temperature of 2000°F (1093°C) dissolves all carbide and intermetallic compound precipitates, forming a pure solid solution matrix. This coarse-grained structure exhibits a slower creep rate at high temperatures compared to fine-grained Class 1 materials.
An optional stabilization treatment at a minimum temperature of 1800°F (982°C) is suitable for nuclear energy and critical chemical applications requiring materials to resist intergranular corrosion (sensitization) after prolonged thermal exposure. Common applications for Class 2 materials include gas turbine exhaust pipes, nuclear fuel reprocessing equipment, reformers, and furnace components.
ASTM B444 Heat Treatment Requirements
Heat treatment is crucial in distinguishing between Grade 1 and Grade 2 materials. It determines the balance between grain size, precipitate distribution, and room temperature strength and high-temperature creep properties. Proper selection of the heat treatment process during the procurement phase can avoid costly material replacements.
Grade 1: Annealing
The minimum annealing temperature for Grade 1 materials is 1600°F (871°C). This temperature is sufficient to recrystallize the cold-worked structure, but low enough to retain fine niobium carbide (NbC) and carbonitride precipitates within the grains. These precipitates act as grain boundary pinning agents, keeping the grain size small (typically ASTM 5-8). The resulting microstructure exhibits high room temperature strength (tensile strength 120 ksi, yield strength 60 ksi) and good fatigue properties.
Grade 2: Solution Annealing
Grade 2 materials are solution annealed at a minimum temperature of 2000°F (1093°C). At this temperature, all carbides, carbonitrides, and any Ni₂(Cr, Mo) type Laves phases dissolve in the nickel matrix. Grain size increases to ASTM grade 2-5. This clean, coarse-grained structure exhibits slower creep at temperatures above 1100°F (593°C) because there are fewer and longer grain boundaries that constitute the primary creep failure path.
Following solution annealing, a stabilization annealing at a minimum temperature of 1800°F (982°C) is optional. This controlled aging process re-precipitates niobium carbide at grain boundaries, forming a semi-continuous network that resists intergranular corrosion after prolonged thermal exposure. This stabilized state has specific requirements in nuclear fuel reprocessing and critical chemical applications.
Available Conditions and Finishes
ASTM B444 specifies the standard condition and surface finish for Inconel 625 tubing. The surface finish depends on whether the product is cold-worked and the outer diameter is ground (bright) or cold-drawn without grinding (pickled/matte).
| Condition | Process | Finish / Surface |
| Cold-drawn, Annealed/Sol Ann, Ground OD | Cold-drawn, heat treated, OD ground | Bright OD; ID depends on atmosphere (bright if annealed in protective atm) |
| Cold-drawn, Annealed/Sol Ann, Pickled | Cold-drawn, heat treated, acid pickled | Dull, matte (pickled) OD and ID surfaces |
| Size range (Ground OD) | - | 1/2 to 4 in. (12.7–102 mm) OD, normal & heavy wall, all NPS |
| Size range (Pickled) | - | 1/2 to 6-5/8 in. (12.7–168 mm) OD, normal & heavy wall, all NPS |
Hydrostatic Testing Requirements
Any pipe or tubing with an outer diameter of 1/8 inch (3 mm) or larger and a wall thickness of 0.015 inch (0.38 mm) or larger must pass a hydrostatic test. The test pressure is calculated using the following formula:
P = 2St / D
Where P = hydrostatic test pressure (psi), S = allowable fiber stress (psi), t = minimum wall thickness (in.), and D = outer diameter (in.).
The maximum test pressure is 1,000 psi (6.9 MPa). Any pipe or fitting that leaks during the test is unacceptable. Allowable fiber stress values are as follows:
| Grade | Fiber Stress, psi | Fiber Stress, MPa |
| Grade 1 (Annealed) | 30,000 | 207 |
| Grade 2 (Solution Annealed) | 25,000 | 172 |
The manufacturer and purchaser may agree to test at 1.5 times the calculated pressure. Hot-forming quality pipe and tube is exempt from tensile and hydrostatic requirements. It is supplied for chemical and surface inspection only.
Testing and Inspection Requirements
ASTM B444 requires chemical composition analysis, tensile testing, and hydrostatic testing for each batch of products. General requirements for sampling, batch definition, specimen preparation, test methods, and product marking are specified by ASTM B829.
Chemical Composition Analysis
One test is performed per batch (per furnace). The test method follows ASTM B829. If product analysis is requested by the buyer, the analysis results must meet the composition limits and inspection tolerances specified in B829.
Tensile Testing
One tensile test is performed per batch of products. The test method follows ASTM E 8. Specimens are taken from the finished product. All tubular materials should be tested in intact tubular form whenever possible. If intact tubular testing is not possible, longitudinal strip or circular specimens are used.
Product Marking
Each product must be marked with the following information: alloy name or UNS number (N06625), ASTM standard (B444), grade (1 or 2), condition (annealed or solution annealed), furnace number, and dimensions. Each bundle or shipping container must also be labeled with its specification number, alloy grade, condition, dimensions, gross weight/tare weight/net weight, and shipper/consignee details.
About Gnee Steel
Gnee Steel is a professional manufacturer of nickel-based alloys, including Nickel 201, Nickel 202, Hastelloy C-276, Hastelloy C-22, Hastelloy B, Hastelloy C-4, Inconel 600, Inconel 625, Inconel 718, Inconel X-750, Incoloy 800, Incoloy 800H/HT, Incoloy 825, Monel 400, Monel K500, and other high-temperature alloy materials. We specialize in the production and sales of alloy materials. Gnee Steel's products are widely used in aerospace, chemical, power generation, automotive, nuclear energy, and other fields, and we can provide customized alloy material solutions according to customer needs. For inquiries about alloy material prices or customized alloy material solutions, please feel free to contact us via email at ss@gneemetal.com for a quote.
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Frequently Asked Questions
Q: What is the difference between ASTM B444 Grade 1 and Grade 2?
A: Grade 1 steel is annealed at a minimum of 1600°F (871°C) to achieve higher room temperature strength (tensile strength 120 ksi). Grade 2 steel is solution annealed at a minimum of 2000°F (1093°C) to achieve superior creep resistance and fracture resistance above 1100°F (593°C). If no grade is specified, Grade 1 steel is shipped by default. Both have the same chemical composition, differing only in heat treatment processes and final microstructure.
Q: What is the difference between ASTM B444 and ASTM B622?
A: B444 is a single alloy specification, covering only Inconel 625 (UNS N06625); B622 is a multi-alloy specification, covering more than 30 nickel-based and nickel-cobalt alloys (e.g., Hastelloy C-276, B-2, G-30, etc.). Inconel 625 is not a B622 standard. If you require Inconel 625 seamless steel pipe, always choose the B444 standard. If you require other C-series alloys, choose the B622 standard.
Q: Can Inconel 625 withstand hydrochloric acid?
A: Yes, Inconel 625 can withstand hydrochloric acid concentrations up to approximately 20% at room temperature and dilute hydrochloric acid at high temperatures. However, it is not the optimal choice for high-temperature concentrated hydrochloric acid. For hydrochloric acid of various concentrations, nickel-molybdenum alloys (ASTM B622 standard) are recommended because they contain 28% molybdenum and virtually no chromium.
Q: What is the maximum operating temperature of Inconel 625?
A: Class 1 Inconel 625 has a rated continuous operating temperature up to 1100°F (593°C). Class 2 Inconel 625 has a rated operating temperature up to approximately 1800°F (982°C), where creep and oxidation resistance are primary considerations. Above 1200°F (649°C), the strength of Inconel 625 decreases, but its chromium oxide coating prevents oxidation. For load-bearing applications above 1500°F (815°C), ASTM B167 is recommended, as it offers superior cyclic oxidation resistance due to its aluminum content.
Q: Are ASTM B444 and ASME SB-444 the same?
A: ASME SB-444 is the same specification used in Part B (Non-ferrous Metals) of the ASME Boiler and Pressure Vessel Code. Materials manufactured according to ASTM B444 can bear the SB-444 designation if they meet all ASME requirements. This is mandatory for pressure vessels, heat exchangers, and piping with standard stamps in the power generation and process industries.
Q: How does Inconel 625 compare to Hastelloy C-276 in chemical applications?
A: Because Hastelloy C-276 contains 16% molybdenum, while Inconel 625 contains 9%, it exhibits superior resistance to reducing acids (H₂SO₄, HCl). Inconel 625 also boasts higher mechanical strength (tensile strength of 120 ksi compared to 100 ksi for C-276) and, due to its niobium content, better resistance to chloride pitting at high temperatures. C-276 is suitable for highly corrosive acidic environments, particularly for applications requiring only corrosion resistance. Inconel 625 is suitable for applications demanding both corrosion resistance and high strength, especially in acidic gas, subsea, and high-pressure chemical environments.
Q: Can ASTM B444 be used for seawater pipelines?
A: Yes, Inconel 625 is one of the top materials for seawater pipelines. Its pitting resistance equivalent (PREN = %Cr + 3.3 × %Mo + 16 × %N) exceeds 50, while 316L stainless steel is approximately 25 and super duplex steel is approximately 42. Inconel 625 resists crevice corrosion, corrosion caused by biofouling, and erosion corrosion in flowing seawater with velocities exceeding 100 feet per second (30 meters per second). It is specified for use in seawater lift pipes, fire-fighting water systems, and marine exhaust manifolds on offshore platforms and naval vessels.
