ASTM B444 Alloy 625 Seamless U-Bend Tube: Guide
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What Is ASTM B444 Alloy 625 Seamless U-Bend Tube?
ASTM B444 is the standard specification for nickel-chromium-molybdenum-niobium alloy pipe and tube of grade UNS N06625, commonly known as Alloy 625. The ASME code edition is ASME SB444. Alloy 625 is a high-performance nickel-base alloy strengthened by the addition of molybdenum and niobium, which gives it an outstanding combination of high-temperature strength, toughness and corrosion resistance in both oxidising and reducing media. A U-bend tube is a seamless tube formed into a U shape with both ends in the same tube sheet, which is the preferred configuration for heat exchangers that must handle large temperature differences and aggressive fluids.
The specification covers two grades, Grade 1 and Grade 2, which differ in heat treatment. Grade 1 is annealed, while Grade 2 is solution annealed at a higher temperature and rapidly cooled to optimise corrosion resistance and ductility.
Chemical Composition
The composition limits for UNS N06625 pipe and tube to ASTM B444 are listed below.
| Element | Composition, % |
|---|---|
| Carbon, max | 0.10 |
| Manganese, max | 0.50 |
| Phosphorus, max | 0.015 |
| Sulfur, max | 0.015 |
| Silicon, max | 0.50 |
| Chromium | 20.0 - 23.0 |
| Molybdenum | 8.0 - 10.0 |
| Niobium + Tantalum | 3.15 - 4.15 |
| Iron, max | 5.0 |
| Aluminium, max | 0.40 |
| Titanium, max | 0.40 |
| Cobalt, max | 1.0 |
| Copper, max | 0.50 |
| Nickel | 58.0 min |
Mechanical Properties
Representative tensile properties of annealed material, measured at elevated temperatures, are shown below. These values are typical data for design guidance rather than specified limits.
| Temperature, F | Tensile, psi | 0.2% Yield, psi | Elongation in 2 in, % |
|---|---|---|---|
| 70 | 144,000 | 84,000 | 44 |
| 400 | 134,000 | 66,000 | 45 |
| 800 | 131,500 | 61,000 | - |
| 1200 | 119,000 | 60,000 | 34 |
| 1600 | 40,000 | 39,000 | 117 |
The alloy retains useful strength up to about 980 C and can be used from cryogenic temperatures upward, which makes it one of the most versatile nickel-base alloys for heat exchanger service.
Sizes and Product Forms
Seamless pipe and tube: outside diameter 4 to 219 mm, wall thickness 0.5 to 20 mm.
Welded pipe: 5.0 to 1219.2 mm outside diameter.
Schedules: SCH5, SCH10, SCH10S, SCH20, SCH30, SCH40, SCH40S, STD, SCH60, SCH80, XS, SCH120, SCH140, SCH160, XXS.
Forms: U-shape, round, square, rectangular, hollow, coiled and straight tubes.
Finishes: annealed and pickled, bright annealed, polished, No.1, No.4, 2B, BA, HL and mirror finish.
Typical Applications
Organic chemical processes containing chlorides, especially with acidic chloride catalysts.
Cooking and bleaching tanks in the pulp and paper industry.
Flue gas desulfurisation systems: absorption towers, reheaters, flue gas inlet baffles, wet fans, agitators and ducts.
Equipment and components for sour gas environments.
Acetic acid and acetic anhydride reaction generators, and sulfuric acid condensers.
Fabrication and Selection Points
Select Grade 2 where maximum corrosion resistance is required after forming, since the higher solution annealing temperature puts all alloying elements into solution. Welding of Alloy 625 is carried out with matching filler metal, typically AWS A5.14 ERNiCrMo-3. When U-bends are formed, the bend area should be checked for surface cracking and wall thinning, and stress relief or solution annealing should be applied if the forming reduction is severe. All tubes should be supplied with mill certification covering the heat number, chemical analysis and mechanical test results.
Frequently Asked Questions
What is the difference between Grade 1 and Grade 2 of ASTM B444?
Both grades cover UNS N06625, but they differ in heat treatment: Grade 1 is annealed, while Grade 2 is solution annealed at a higher temperature followed by rapid cooling, which gives optimum corrosion resistance and ductility for the most demanding services.
Is Alloy 625 the same as UNS N06625?
Yes. Alloy 625 is the common trade description of the nickel-chromium-molybdenum alloy designated UNS N06625 under the unified numbering system.
Why are U-bend tubes used instead of straight tubes in heat exchangers?
U-bend tubes allow both ends to be rolled or welded into the same tube sheet, which removes the second tube sheet, allows free thermal expansion, reduces shell-side leakage paths and makes the exchanger more compact.
What filler metal is used for welding Alloy 625?
Matching filler metal of class ERNiCrMo-3 to AWS A5.14, or the equivalent EN ISO 18274 designation, is used so that the weld deposit matches the corrosion resistance and strength of the base metal.
What is the maximum service temperature of Alloy 625?
The alloy has good oxidation resistance up to about 980 C and retains useful mechanical strength at high temperature, but for creep-limited pressure-retaining design the allowable stress must be checked against the applicable code for the intended life.
What tests are performed on B444 tubes?
The standard requires chemical analysis, tensile testing, flattening and flaring tests, and hydrostatic or non-destructive electric testing, with results documented on the mill certificate.
