Which Alloy for Heat Exchanger Tubes?

A heat exchanger tube is selected from the medium on the tube side, the metal temperature of the tube wall and the tube standard that governs its dimensions and testing — Inconel 625 (UNS N06625) and Hastelloy C-276 (UNS N10276) for chloride and acid duty, Incoloy 800 and 825 (UNS N08800 and N08825) for high-temperature and mixed-acid service, titanium Grades 2, 7 and 12 (UNS R50400, R52400 and R53400) for seawater, and 316L or duplex stainless for benign cooling water. A 316L tube and an Inconel 625 tube may both be described as "seamless heat exchanger tube", but they are ordered under different ASTM specifications with different tolerances, different inspection requirements and different price levels, and they cannot be substituted for one another without re-checking the corrosion allowance.

This guide sets out which standard applies to which alloy, how to match the tube to the medium, and what a complete tube line item on an enquiry actually needs to contain. For the welded-versus-seamless question in general piping, see Seamless or Welded Tube?.

Which standard covers which heat exchanger tube alloy?

The tube standard is decided by the alloy family and by whether the tube is seamless or welded. Getting this wrong is the most common cause of a rejected mill certificate, because the mechanical property limits and the permissible tolerances differ between the specifications:

Two practical consequences follow from that table. First, **a welded tube is not automatically cheaper than a seamless tube** — for the C-family alloys the welded product (B626) carries a lower price but also a lower permissible design stress in most pressure-vessel codes, so the wall thickness has to be recalculated before the saving is real. Second, **titanium tubes are ordered under one standard for both product forms**, which is why the testing clause — eddy current for all, hydrostatic at the buyer's option — needs to be written explicitly rather than assumed.

For tube sheets, channels and covers in the same exchanger, the corresponding product standards are different again: plate and sheet in the 600 and 800 families come under ASTM B168, the C-family under ASTM B575 for low-carbon grades, 625 plate under ASTM B443, and forgings under ASTM B564.

How do you match the tube alloy to the cooling medium?

Selection is driven by the tube-side medium, the external medium (including cooling water quality and whether it is once-through or closed circuit), and the tube wall temperature. The table below covers the cases that account for most real orders:

The single most valuable check on this table is the **tube wall metal temperature**, not the process inlet temperature. In a high-temperature exchanger the tube wall runs hotter than the bulk fluid on the cold side, and if that wall crosses roughly 60 °C in a chloride-bearing water the SCC risk changes from theoretical to immediate. Ask the process engineer for the design metal temperature and calculate the tube wall condition from it before locking the material.

When is a nickel alloy tube worth the extra cost over stainless?

The honest answer is: when the failure mode you are avoiding costs more than the alloy. There are four cases where this normally holds.

• 1. **Chloride stress corrosion cracking.** Austenitic stainless steels crack in the presence of chlorides and tensile stress above roughly 60 °C. The failure is not a gradual wall loss — it is a through-wall crack, and it typically appears on the cold side of the tube where the chlorides concentrate. Inconel 625, Inconel 690, Incoloy 800 and duplex 2205 all resist this. This is the most common reason a nickel alloy tube is specified.
• 2. **Pitting and crevice corrosion under deposits.** Once-through seawater and dirty river water leave deposits that create oxygen-depleted crevices. Titanium and 2507 super duplex resist this; 316L does not.
• 3. **Corrosion at temperature where the acid is reducing.** The C-family (C-276, C-22) is the only common tube material that handles both oxidising and reducing acids in the same exchanger, which is why it dominates phosphoric acid and mixed-acid duties.
• 4. **High-temperature strength.** Above roughly 550 °C, 316L loses the creep strength the design assumed. Inconel 625, Incoloy 800HT and Hastelloy X keep usable strength to 700–900 °C depending on load.

The trade-off is heat transfer. Copper alloys conduct heat at 100–400 W/m·K, carbon and stainless steels at roughly 16 W/m·K, and nickel alloys at roughly 10–15 W/m·K, depending on grade and temperature:

Two consequences follow. A nickel alloy tube bundle has to be larger than the copper alloy bundle it replaces for the same duty, and the tube wall should be kept as thin as the corrosion allowance and the pressure duty permit — every 0.1 mm of wall is a measurable loss of duty. This is also why the standard wall thickness for a nickel alloy condenser tube sits between about 0.9 mm and 2.1 mm rather than at the heavier schedule wall used for process piping of the same diameter.

What should a complete tube line item contain?

A tube enquiry that cannot be quoted without a follow-up question is an enquiry that loses a week. The line item needs:

• **Alloy and UNS number**, plus the trade name if one is in common use, e.g. "Inconel 625, UNS N06625".
• **Product standard and edition**, e.g. ASTM B163, and whether seamless or welded.
• **Dimensions**: outside diameter, wall thickness or BWG gauge, and length. State whether you want random lengths, a fixed length, or tube cut to a U-bend leg length.
• **Dimension basis**: outside diameter and wall, or outside diameter and inside diameter. For condenser tubes, outside diameter and wall is standard; for some heat exchanger duties the required inside diameter is fixed by the flow calculation and the wall must be derived from it.
• **Temper condition**: annealed (soft), as-drawn, or annealed and pickled. For U-bent tubes the U-bend area must be stress relieved after bending.
• **Form**: straight, U-bent, finned, or with a welded end fitting.
• **Testing**: eddy current test is normally applied to every tube for heat exchanger service; hydrostatic test, flattening test and flare test are applied per the product standard or at the buyer's option.
• **Certification**: EN 10204 3.1 as standard, with third-party inspection available on request.
• **Ends**: plain, bevelled, or with a specified end preparation.

What does the test package for a condenser tube order look like?

For high-chloride or acid service, the pitting resistance tests in ASTM G48 give a comparative number — critical pitting temperature — that can be written into the specification instead of leaving the resistance claim to the grade name. This is worth doing where two alloys are close on price and you need a defensible basis for the choice.

Heat exchanger tubes from Hangbo Alloy

Hangbo Alloy supplies seamless and welded heat exchanger and condenser tube in Inconel 600, 601, 617, 625, 690 and 718, Incoloy 800, 800H, 800HT, 825 and 926, Hastelloy C-276, C-22 and C-2000, Monel 400 and K-500, Nickel 200 and 201, and titanium Grades 2, 7 and 12 — with the matching tube sheet plate and forgings supplied from the same plant so the material certificate set is consistent across the exchanger.

Tube is supplied solution annealed and pickled, straight or U-bent, with eddy current testing and full dimensional records as standard, and EN 10204 3.1 certification with third-party inspection available on request.

Send us the tube-side medium, the design metal temperature, the standard you are working to and the dimensions, and we will confirm the alloy, the tolerances and the test package before the order is placed.

Continue reading

• How to Specify Nickel Alloy Plate and Sheet: Standard, Condition and FinishA nickel alloy plate order is not complete until it names the product standard, the rolling condition, the thi
• How to Specify Nickel Alloy Round Bars on a Purchase OrderWhat a complete Inconel, Hastelloy or Monel round bar order must state — grade, UNS, product standard, conditi
• Inconel 718 Round Bar (φ2–600 mm): Composition, Mechanical Properties & SupplierInconel 718 round bar (N07718) supplier: composition, aged properties, sizes and tolerances. Factory price fro
• Inconel 625 Round Bar (φ2–600 mm): Composition, Mechanical Properties & SupplierInconel 625 round bar (N06625) supplier: composition, aged properties, sizes and tolerances. Factory price fro
• Hastelloy C-276 Plate & Sheet: Composition, Mechanical Properties & SupplierHastelloy C-276 plate (N10276) supplier: composition, aged properties, sizes and tolerances. Factory price fro
• All 11 Product Forms & Specifications guides

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