Tubesheet Materials and Tube-to-Tubesheet Joints

The tubesheet of a heat exchanger is selected by the more corrosive of the two fluids it separates, not by the tube material, so a Hastelloy C-276 (UNS N10276) tubesheet in a 316L shell and a carbon steel tubesheet behind an Inconel 625 (UNS N06625) bundle are both normal designs. Two decisions follow from that, and they are usually made together: which material the tubesheet is made from, and whether the tube-to-tubesheet joint is rolled, welded or both. Getting the second decision wrong is more expensive than getting the first wrong, because a joint failure is a leak into a process stream rather than a wall that can be measured.

This guide covers the material side — solid alloy, clad plate and weld overlay — and the joint side — expansion, seal welding and strength welding — together with the standards and tests that prove both. For the tube itself, see Which Alloy for Heat Exchanger Tubes?.

What decides the tubesheet alloy?

Five inputs drive the choice, and they are not always satisfied by the same answer:

• 1. **The corrosive medium on each side.** A tubesheet is a single plate, so it must survive the tube-side fluid, the shell-side fluid and any leakage between them. Where the two sides differ, the tubesheet takes the more aggressive of the two.
• 2. **Cladding or overlay feasibility.** If the aggressive side is a single wetted face, the plate can be carbon steel or low-alloy steel with a corrosion-resistant barrier, which changes the cost by a large factor.
• 3. **The galvanic couple with the tube.** A nickel alloy tube in a carbon steel tubesheet with a conductive electrolyte between them creates a couple in which the steel becomes the anode. Where the tubesheet cannot be coated, the material has to be close enough to the tube in the galvanic series that the couple is harmless.
• 4. **Mechanical requirements.** Tubesheets are thick, drilled plates carrying the full pressure load and the tube loads. Yield strength and through-thickness properties matter, and some of the highest-corrosion-resistance alloys are also the weakest.
• 5. **Fabrication.** Deep drilling, groove cutting, weld overlay and expansion all have to be possible in the chosen material. Alloy 625 overlays readily; some high-molybdenum alloys are much less forgiving.

Where the exchanger is in sour service, the tubesheet and the joint are both inside the scope of NACE MR0175, which caps hardness rather than specifying an alloy. For the limits themselves, see NACE MR0175 Alloys for Sour Service.

Which standard covers tubesheet plate and forgings?

Two selection notes are worth carrying into the enquiry. First, a forged tubesheet is not just a plate cut to a disc: forging gives a better through-thickness structure and is preferred for thick tubesheets where the plate would need heavy machining, and for any tubesheet that will be welded into a closure. Second, the general requirements standard — ASTM B906 for nickel alloys — carries the tolerances, the flatness limits and the marking requirements, and it is the document that decides whether a plate is delivered in a condition that can actually be machined flat.

When is a clad or weld-overlaid tubesheet better than solid alloy?

Above roughly 1.5 m in diameter the plate mass makes solid alloy uneconomic, and the design moves to a composite. Three routes are in common use:

Weld overlay is often the better answer on a large forged tubesheet because the overlay can be applied after the forging is made, and because the overlay can be restricted to the wetted face and the tube-hole bores. The two failure modes to specify against are **dilution**, where the first layer of overlay is diluted by the base metal and no longer meets the chemistry of the alloy it claims to be, and **iron contamination** at the finished surface, which shows up as rusting in service and is the start of a pit.

For both methods the acceptance test is the same: a chemical analysis of the finished surface, a ferrite or iron-contamination check, and for clad plate a shear or bend test of the bond. A clad plate or overlay is not accepted on the certificate of the input materials.

How do you choose between rolling and welding the tube joint?

The joint types are ranked by integrity and by cost, and the choice should be driven by what a leak would do:

Three rules cover most cases. **Rolled only** is acceptable where a leak between the circuits is tolerable and the tube and tubesheet are of similar material. **Seal welded** adds a barrier against leakage without making the weld a structural element, and is the usual answer where the two circuits must not mix. **Strength welded and expanded** is used where the joint is part of the pressure-containing boundary, and there the expansion step is there only to eliminate the crevice between tube and hole, not to carry load.

The material consequence is that a strength-welded joint needs a weldable tube and tubesheet pair, and weldability varies a lot across the alloy families. Where the tube is a high-molybdenum alloy with limited weldability, the joint is often designed as expanded-and-seal-welded with a compatible filler metal rather than as a full strength weld.

What tests qualify the tube-to-tubesheet joint?

The pull-out test is the one that is most often omitted and most often needed. It is run on a mock-up made from the production tube and tubesheet materials with the production hole geometry and expansion procedure, and it produces a load that can be compared with the design axial load. Where a joint is critical, the mock-up is also sectioned to confirm the expansion percentage and the groove fill.

What should a tubesheet enquiry contain?

• **Both fluids** — tube side and shell side — with chloride content, pH, temperature and any sour service designation.
• **Material and standard for each wetted face**, and whether the tubesheet is solid, roll-bonded clad or weld overlaid.
• **Dimensions**: outside diameter, thickness, tube-hole pattern, hole diameter and tolerance, pitch, groove geometry, and whether the holes are to be reamed.
• **Bore surface requirement** for the corrosion-resistant barrier, if overlaid.
• **Joint type** and whether a mock-up pull-out test is required.
• **Heat treatment condition** and any hardness limit, particularly for sour service.
• **NDE requirements**: ultrasonic test of the plate or forging, per ASTM A388 or the applicable standard, plus surface examination of the finished faces.
• **Certification**: EN 10204 3.1 as standard, with third-party inspection available on request.

Tubesheets and tube joints from Hangbo Alloy

Hangbo Alloy supplies tubesheet material and finished tubesheets for heat exchangers, condensers and pressure vessels: nickel alloy plate and forgings in Inconel 600, 601, 617, 625, 690 and 718, Incoloy 800, 800H, 825 and 926, Hastelloy C-276, C-22 and C-2000, Monel 400 and K-500, and Nickel 200 and 201, together with 2205, 2507 and 904L stainless, carbon and low-alloy steel, and titanium Grade 2 and Grade 7 for clad and overlay construction.

Tubesheets are supplied drilled to drawing or as plate and forgings for in-house machining, with ultrasonic testing and dimensional records as standard, weld overlay to the required alloy with surface chemistry and iron contamination reports, and EN 10204 3.1 certification with third-party inspection available on request.

Send us the two fluids, the design temperature and pressure, the tube-hole pattern and the joint type you intend to use, and we will confirm the material arrangement, the barrier method and the test package before the order is placed.

Continue reading

• How to Weld Hastelloy C276 Without Losing Corrosion ResistancePractical welding guidance for Hastelloy C276 — filler metal selection, heat input limits, interpass temperatu
• Inconel 718 Heat Treatment: How to Reach Maximum StrengthHow to solution treat and age Inconel 718 to AMS 5662 and AMS 5663 — temperatures, soak times, cooling rates,
• How Are Nickel Alloy Flanges Made? Forging, Ring Rolling and StandardsA nickel alloy flange is pinned down by three specifications at once — the alloy, the forging standard such as
• Forging and Hot Working Temperatures for Nickel AlloysNickel alloys are forged in a narrow window — roughly 120 to 250 C wide — and the correct range is different f
• How to Machine Inconel and Hastelloy: Speeds, Feeds and ToolingPractical machining data for Inconel 718, 625 and Hastelloy C-276 — cutting speeds, feed rates, tool grades, c
• All 8 Welding, Forming & Heat Treatment guides

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