Nickel Tube Bending, Tube-to-Tubesheet Expanding and Welding: The Fabrication and WPS Evidence Layer

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Nickel Tube Bending, Tube-to-Tubesheet Expanding and Welding: The Fabrication and WPS Evidence Layer

A straight nickel tube that passes the hydrostatic test on ASTM B161, B163, B622 or B444 has proved one thing: the straight length held pressure without leaking. It has not proved the tube can take a cold bend, be roller-expanded into a tubesheet and be welded with a clean root. Those are three separate operations with three separate acceptance records, and a mill test run at a pressure computed from the minimum wall says nothing about the wall after bending. Our seamless pipe specification guide, our Nickel 200 B161 pipe stock page and our hydrostatic test water guide settle which grade and which standard to order. This article starts where those pages stop: after the material reaches the fabrication shop. There is no single agreed bend radius, thinning percentage or joint sequence, so we publish the conflicts, six verification actions, eight rejection triggers and one 14-line method statement.

A material certificate describes what the metal is; a method statement describes what the shop did to it. Our nickel tube selection page, our pipe weight calculator and our nickel alloy bar specification page are specification tools, not fabrication records. We are a Baoji stockholder, not the author of your WPS, so where a number depends on an edition or an alloy family we give the verification action instead of an invented figure. Confirm every value against the edition in the purchase order.

At a Glance: Eight Decisions That Separate a Clean Bend from a Rejected Bundle

Rows three and four hide most of the rework. Our pure nickel B161 caustic pipe page gives the service context and our EN 10204 3.1 certificate page the form of a traceable record. A stockholder can certify a heat number; it cannot certify a bend it did not make.

The Hydrostatic Pass Trap: A B161/B163/B622/B444 Tube That Holds Water Can Still Fail a Bend

The four standards are not interchangeable, but each is a specification for a straight length. ASTM B161 covers Nickel Seamless Pipe and tube, B163 covers condenser and heat-exchanger tubes, B622 covers nickel and nickel-cobalt alloy pipe and tube, and B444 covers nickel-chromium-molybdenum-columbium alloy pipe and tube such as UNS N06625. Each calls for a hydrostatic test, or an electric test where the order permits, on the straight tube. B163 is described as limiting condenser tube outside diameter to about 3 in or 76.2 mm and offering U-bend and stress-relieved conditions as supplementary requirements. Confirm scope against the edition in your order.

ASTM B444 is explicit. The published test pressure is P equals 2St/D, where S is allowable fibre stress, t is minimum wall and D is outside diameter, with a stated maximum. Grade 1 annealed and Grade 2 solution-annealed material carry different allowable stresses. Each piece in each lot is tested and a leaking tube is rejected. None of that evaluates a cold bend, a rolled joint or a weld. The test uses straight-tube minimum wall; bending thins the extrados below that wall in a place the straight test never pressurises.

A hydrostatic pass is a leak-tightness result on a straight tube, not a bend, joint or weld procedure qualification. Keep the three records separate.

Our nickel tubes range lists the straight product, our WPS and NDT qualification guide shows a weld procedure is qualified separately, and our Ni200 versus Ni201 caustic guide shows a grade choice is a materials decision. Treating a passing hydrostatic certificate as permission to bend is the common failure. It is permission to receive.

When Cold-Bend Minimum Radius Numbers DO Disagree: Publish the Conflict, Not One Value

Ask six sources for the minimum cold-bend radius and you get a spread. Supplier pages quote about 2D, or 2 times outside diameter, for standard cold-drawn tubing, recommend 3D or 5D for high-pressure service, and quote about 1.5D for U-bend tubing. Some guidance ties the radius to an allowable forming strain, expressed as 100 times tube outside radius over bend centreline radius, rather than a fixed multiple of D. They disagree because they assume different service conditions, wall factors and acceptance criteria.

Thinning limits disagree too. Shops quote 10 percent or 12.5 percent, the latter a familiar mill wall tolerance. U-bend descriptions quote ovality near 10 percent of outside diameter. ASME B31.3 is reported as publishing no percentage limit; its requirement at para 304.2.1 is a minimum required wall for the finished bend, computed from design pressure and allowable stress. Flattening limits for internal and external pressure also differ. Presenting one figure as correct would be false precision, so we do not.

Our what a B363 fitting grade hides makes the parallel point for a different family, and our titanium tube receiving inspection page shows the straight-tube evidence at the gate. Name the edition, the method, the measurement points and who signs, then let the agreed criterion decide. Chapter ten gives six actions that settle the numbers.

Where the Wall Goes: Extrados Thinning, Ovality and a Measurement Map for the Bend

Cold bending stretches the outer arc and compresses the inner arc. The extrados thins, the intrados thickens, and the round section tends to go oval. A published geometric relation expresses predicted extrados wall as nominal wall times bend centreline radius over that radius plus half the outside diameter minus half the wall. It tracks measurement closely, but it is still a prediction, because mill tolerance, bending method and mandrel practice all shift the result.

The measurement map turns a bend into evidence. Mark the apex, the tangent start and stop, and stations at the quarter points of the arc. At each station record outside diameter in two directions for ovality, and wall thickness at the extrados, intrados and both neutral axes with a calibrated ultrasonic gauge. Log instrument identity, calibration date and operator. Express ovality as maximum minus minimum outside diameter over nominal outside diameter, and thinning as measured extrados wall against the required post-bend minimum. Attach the map to the heat number.

A bend map is a set of measured stations, not one apex reading. Agree the station count, the gauge, the formula and the criterion before the first tube is bent.

Our pure nickel caustic tube page and our N6 strong alkali tube page describe the service where a thin extrados becomes a corrosion problem, and our what B16.5 hides about flanges shows a dimensional standard leaves fit-up to the drawing. Where figures conflict, the map is the tie breaker. Confirm the criterion against the drawing and the code edition in the purchase order.

The Expand First or Weld First Question: Tube-to-Tubesheet Sequence and the Crevice

A tube-to-tubesheet joint can be expanded only, welded only, or both. Industry descriptions separate a light expansion, which closes the tube-to-hole gap without carrying pull-out load, from a strength expansion, and a seal weld, which provides tightness, from a strength weld, which carries load. Combined joints damp bundle vibration and reduce crevice corrosion. Published routes are strength expansion plus seal weld, strength weld plus light expansion, or strength weld plus strength expansion. Confirm which one your equipment drawing names.

The sequence has no single agreed rule. The common recommendation is weld first then expand, because roller-expanding lubricant wicks into the crevice, is hard to remove, and evolves gas that causes weld porosity. Expanding first also traps gas that expands during welding. Welding first demands tight control of tube outside diameter and hole fit: a large gap forces expansion that can crack the weld or break the joint loose. Hydraulic expansion is reported as giving more uniform light expansion than manual rolling. These are trade-offs, not one correct order.

There is no universal expand-then-weld or weld-then-expand rule. The order belongs in the qualified procedure, tied to the lubricant, the clearance and the weld type. Record what was actually done.

Our nickel heat exchanger selection page gives the equipment context, our Monel, Inconel and Hastelloy guide separates alloy families whose tubesheets differ, and our equipment leak repair guide shows the cost of an undocumented joint. Ask for expansion depth, rolling torque or hydraulic pressure, the cleaning step, and the signed sequence record. Confirm the joint type against the drawing.

After the Bend: Solution Anneal, Stress Relief, Root Purge and Filler Matching by Alloy Family

Post-bend heat treatment: solution anneal or stress relief

Cold bending leaves residual stress and cold work that can drive stress corrosion cracking. Published U-bend practice describes stress relief or solution anneal over the bend plus a length of each straight leg, and quotes a solution-anneal window for stainless U-bends. Nickel alloys are not stainless: the cycle depends on the alloy family. ASTM B444 gives Grade 1 annealed and Grade 2 solution-annealed conditions different minimum temperatures. We will not print one temperature for all nickel tubes. Confirm cycle, coverage, atmosphere and quench against the alloy datasheet and the WPS, and record the furnace chart.

Root-side back purging and oxygen control

Nickel alloys form tenacious oxides, and an unpurged GTAW root can be oxidised or sugared inside. Welding guidance describes argon back purging as standard on pipe and enclosed joints, continued until oxygen falls to a low threshold for critical service. Different pages quote different thresholds and times, so we give the action: fit a purge dam, meter the argon, monitor oxygen at the outlet, log the reading and duration, and hold the purge through the hot passes. Confirm the criterion against the WPS. Our GTAW shielding guide explains the same root-side rule for titanium.

Filler metal follows the alloy family, not the mill grade

A matching filler is not the same chemistry as the base metal. AWS A5.14 classifies bare nickel alloy rods by family: ERNi-1 for commercially pure nickel UNS N02200 and N02201, ERNiCu-7 for nickel-copper UNS N04400, ERNiCr-3 for nickel-chromium-iron UNS N06600, ERNiCrMo-3 for alloy 625 type, ERNiCrMo-4 for C-276 type and ERNiCrMo-10 for C-22 or 622 type. The A5.14 annex states ERNi-1 carries a titanium addition to control porosity, which is why filler composition does not equal the wrought grade. Name the AWS classification, the WPS and the certification in the purchase order. Our Nickel 201 tubing page and our corrosion-resistant tube stock page are straight-tube references; the filler decision is made at the weld.

Where the Standard Stops: B163 Condenser Tube and B161 Process Pipe Are Not One Acceptance Regime

A purchasing error that reaches the shop is to accept a B163 condenser tube with a B161 process-pipe checklist, or the reverse. B163 is described as covering seamless nickel and nickel-alloy condenser and heat-exchanger tubes, with tighter outside-diameter and wall tolerances for tubesheet fit, and supplementary requirements for U-bends, flaring tests, grain size, and annealed or stress-relieved condition. A scope summary caps condenser tube outside diameter near 3 in or 76.2 mm and allows wall as average or minimum. B161 is a Nickel Seamless Pipe and tube specification for process piping. Confirm exact limits against the edition in your order.

The difference surfaces in fabrication. A condenser tube for rolling into a tubesheet is judged on outside diameter, wall, flaring behaviour and, for U-tubes, bend geometry and post-bend heat treatment. A process pipe is judged on dimensions, flattening, hydrostatic or electric test, and the field weld procedure. A hydrostatic pass is common to both, but it is the start of acceptance, not the end. Applying one checklist to the other loses either the tubesheet fit-up criteria or the process-pipe weld criteria.

The standard gets the tube to the shop. It does not tell the shop how to bend, expand or weld it. B163 and B161 are two purchase specifications that become two different method statements.

Our Hastelloy C276 versus C22 grade guide shows a grade choice inside the Ni-Cr-Mo family, and our export FAQ for titanium and nickel covers the documents that travel with an export order. Write standard designation, UNS number, condition and supplementary requirements into the purchase order, then name the governing standard on the first line of the method statement. Where a supplier page and the standard text disagree, the named edition wins.

Limitations and Failure Conditions: Eight Fabrication and Welding Rejection Triggers

Trigger 1 - Cold bend below the qualified radius with no forming-strain record

Check the bend radius against the approved procedure and record forming strain as tube outside radius over measured centreline radius. Judge against the qualified procedure and the purchase order edition, not a rule of thumb. A bend tighter than the qualified value without a strain calculation is a hold. Our forming-limits article makes the parallel point. In a dispute, photograph the bend with a radius gauge and request the qualification record.

Trigger 2 - Extrados thinning or ovality beyond the agreed post-bend criterion

Measure extrados wall at the documented stations with a calibrated ultrasonic gauge and ovality in two directions. Judge the wall against the post-bend minimum from the governing clause and ovality against the drawing load case. A calculated value is not a measurement. Our tube dimensional inspection page shows the discipline. In a dispute, re-map the bend with the purchaser witness present.

Trigger 3 - Post-bend heat treatment skipped, or the wrong cycle for the alloy family

Read the furnace chart and confirm cycle, coverage, atmosphere and quench against the alloy datasheet and the qualified procedure. A solution-anneal window for stainless U-bends does not transfer to a nickel alloy. Our alloy tube stock page separates the families. In a dispute, quarantine the bundle and require a documented cycle review.

Trigger 4 - Tube expanded before welding with lubricant left, or welded then over-expanded

Review the signed record for the sequence used, the cleaning step, expansion depth, and rolling torque or hydraulic pressure. Judge against the qualified joint procedure. Lubricant residue drives weld porosity; excess gap drives over-expansion damage. Our heat exchanger selection page gives the context. In a dispute, require the joint qualification record and fit-up measurements.

Trigger 5 - Root-side oxidation or sugaring from a missing or late argon back purge

Examine the weld root by borescope and review the purge log for gas, flow, oxygen reading, duration and the stop point. Judge against the WPS and the cleanliness clause. An oxidised or sugared root is a reject, not a cosmetic note. Our GTAW shielding guide describes the equivalent rule for titanium. In a dispute, cut a sample joint.

Trigger 6 - Filler metal that does not match the alloy family, or treated as the base grade

Read the filler lot certificate and confirm the AWS A5.14 classification against the base metal UNS number and the WPS. ERNi-1, ERNiCu-7, ERNiCr-3, ERNiCrMo-3, ERNiCrMo-4 and ERNiCrMo-10 serve different families. Our Monel, Inconel and Hastelloy guide separates those families. In a dispute, rebuild the lot-to-weld map.

Trigger 7 - A B161 acceptance record applied to a B163 condenser tube, or the reverse

Confirm the method statement names the governing standard and the checklist matches that product: tubesheet fit-up, flaring and U-bend criteria for B163, or process-pipe dimensions and field weld criteria for B161. Our seamless pipe specification guide lists the standards side by side. In a dispute, rebuild the record against the correct standard.

Trigger 8 - Heat number, WPS or bend-map traceability lost after cutting and bending

Reconcile every bent tube and weld against the heat number, the WPS and the bend map, photographing identity before and after cutting. A bent tube whose heat number cannot be rebuilt is a reject regardless of a passing hydrostatic result. Our EN 10204 3.1 certificate page shows what the certificate proves. In a dispute, restore the heat-to-piece map.

The 14-Line Nickel Tube Fabrication and Welding Method Statement / ITP Checklist

Line six is often answered with a calculated value instead of a measured wall, line ten with an assumed sequence, line twelve with a trade name, and line fourteen with no heat-to-piece map. Our seamless pipe specification page and our certificate page separate material evidence from fabrication work. Our export documentation FAQ covers the paperwork, and our WPS qualification guide and leak repair guide are the references downstream. A checklist without a signature column is a note, not an inspection and test plan.

Six Verification Actions When Radius, Thinning Limit and Expanding Sequence Conflict

1. Require every acceptance value to carry its document, revision and clause. Name the ASTM edition, the code edition and the UNS number before quoting any radius or limit. Our B161 process pipe page is a product reference, not the governing clause.
2. State the sample condition and measuring method: cold-drawn, annealed or solution annealed; straight length or finished bend; which calibrated gauge and formula. Our tube receiving inspection page gives the baseline.
3. Publish the bend measurement map before bending: station count, thinning points, ovality stations, gauge, calibration and criterion. A model is a prediction; the signed map is the record. Our forming-limits article applies the same discipline.
4. Name who reviews the conflict: buyer quality, fabricator engineer and, where the contract calls for it, a third-party surveyor, with authority to accept or hold. Our WPS qualification guide shows qualification is a signed decision.
5. Keep records independent with their own dates: WPS and PQR, furnace chart, purge log, expanding pressure or torque log, filler lot certificate and signed ITP. Our certificate page covers what the mill record proves.
6. Use trigger-based re-verification: any re-bend, re-cut, heat mismatch, filler change or opened hold forces reassessment. Our leak repair guide and strong alkali service page show the downstream cost.

Most of these cost nothing at planning and a great deal after a bundle is bent and welded into a tubesheet. Ask for the purchase order clause, the bend map, the furnace chart, the purge log and the signed ITP for the same heat. We are a stockholder, not the author of your procedure; where figures come from supplier pages we label them, and where they disagree we give the disagreement and the action. Our caustic service tube page is a product reference, not the fabrication record.

Summary: Choose Cold Bending if, Escalate if, Choose a Different Route if

Choose cold bending with a qualified bend procedure if:

• The purchase order names the ASTM edition, UNS number and tube condition, and the bend procedure is qualified for that temper.
• A bend map fixes station count, gauge, calibration and post-bend minimum wall before the first tube is bent.
• The heat-treatment cycle, purge log, joint sequence and filler AWS classification are written into the WPS.
• The 14-line method statement is signed per line and the heat-to-piece map is restored after cutting.

Escalate to a fabrication hold or a procedure review if:

• A passing mill hydrostatic certificate is the only bend or weld acceptance evidence.
• Measured extrados wall sits below the required post-bend wall, or ovality exceeds the drawing criterion.
• The heat treatment, purge log, expanding sequence or filler classification is missing from the file.
• A B161 checklist was applied to a B163 condenser tube, or the heat number was lost after bending.
• Sources quote different radii or thinning limits and no one has named the governing edition.

Choose a different fabrication route if:

• The design calls for a tighter radius than the qualified limit, so a hot bend or a qualified fitting is cleaner. Our what a dimensional standard hides shows a fitting is not just a bent tube.
• The service is a condenser or heat exchanger; settle the joint type and U-bend requirement under B163 first.
• The alloy family and filler are not fixed; settle the grade from our Monel, Inconel and Hastelloy guide and the filler from the WPS.
• The buyer cannot produce a bend map, a sampling plan or a signed ITP. Set up the shop records first.

The whole subject reduces to one sentence: a hydrostatic pass qualifies a straight tube, while bending, expanding and welding need their own measured, signed records. When you enquire, send the grade, the ASTM edition, the outside diameter and wall, the bend radius, the joint sequence and the governing standard. We are a Baoji China Titanium Valley stockholder with roughly 30 tonnes of nickel material in stock, alongside our titanium ranges. We cut to non-standard and small-batch sizes, work from customer drawings, read and certify to ASTM, ASME and GB standards, export-pack in house, and dispatch confirmed in-stock orders the same day. Send us the tube schedule and bend drawing, and we will tell you what we can ship and which decisions belong with your team. Request a quote →

Frequently Asked Questions

Does a hydrostatic test pass prove a nickel tube can be cold bent?

No. A hydrostatic test on a straight B161, B163, B622 or B444 tube proves the straight length held pressure without leaking, using the supplied minimum wall. It does not evaluate forming strain, the thinner extrados wall, ovality or residual stress. Bending, tube-to-tubesheet expanding and welding are separate operations that each need their own qualified procedure and acceptance record. Keep the mill certificate as receipt evidence and judge the bend from a measured bend map.

What is the minimum cold-bend radius for ASTM B161 B163 B622 B444 tube?

There is no single agreed value. Supplier pages quote about 2D for standard cold-drawn tubing, recommend 3D or 5D for high-pressure service, and quote about 1.5D for U-bend tubing. Some guidance ties the radius to an allowable forming strain rather than a fixed multiple. We will not print one radius as correct. Name the standard edition, UNS number, tube condition and qualified bend procedure in the purchase order, and agree the criterion before bending.

How much wall thinning is allowed on the extrados of a bend?

Published figures disagree. Shops quote 10 percent or 12.5 percent maximum thinning, a geometric model gives higher values at tight radii, and ASME B31.3 is reported as stating a minimum required post-bend wall rather than a percentage at para 304.2.1. Confirm the governing code edition and design pressure, then measure the extrados wall at the documented bend stations against that required wall. A calculated value is a prediction, not the acceptance measurement.

Should a tube be expanded before or welded to the tubesheet first?

There is no universal rule. The common recommendation is to weld first and then expand, because roller-expanding lubricant can wick into the crevice and cause weld porosity. Welding first demands tight control of tube outside diameter and hole fit, since a large gap forces over-expansion that can crack the weld or break the joint loose. Light expansion first closes the crevice but can trap gas during welding. Name the sequence, expansion depth and cleaning step in the qualified joint procedure.

Is a post-bend solution anneal always required, or is stress relief enough?

It depends on the alloy family and the tube condition. Cold bending leaves residual stress and cold work that can reduce corrosion resistance, and published U-bend practice describes either a stress relief or a solution anneal over the bend and part of each straight leg. ASTM B444 describes Grade 1 annealed and Grade 2 solution-annealed conditions at different minimum temperatures, and a pure nickel grade does not take the cycle of a Ni-Cr-Mo alloy. Confirm the cycle against the alloy datasheet and the WPS, and record the furnace chart.

Which filler metal matches a given nickel alloy tube, and is it the same grade?

A matching filler is not the same chemistry as the base metal. AWS A5.14 classifies fillers by family: ERNi-1 for commercially pure nickel, ERNiCu-7 for nickel-copper, ERNiCr-3 for nickel-chromium-iron, ERNiCrMo-3 for alloy 625 type, ERNiCrMo-4 for C-276 type and ERNiCrMo-10 for C-22 or 622 type. These fillers carry deoxidiser additions such as titanium, so their composition does not equal the wrought grade. Name the AWS classification, the WPS and the required certification rather than a trade name.

Is B163 condenser tube accepted the same way as B161 process pipe?

No. B163 covers condenser and heat-exchanger tubes with tighter outside-diameter and wall tolerances for tubesheet fit, plus supplementary requirements for U-bends, flaring and grain size. B161 is a Nickel Seamless Pipe and tube specification for process piping. A hydrostatic pass is common to both, but applying one checklist to the other loses either the tubesheet fit-up criteria or the process-pipe weld criteria. Name the governing standard on the first line of the method statement.