Longitudinal Seam Welding: Process and Best Practices

The tank finally rolls, the plates fit, and the first arc lights up twelve metres down the long seam of a storage vessel. Two hours later the seam looks textbook — and the shell is out of round by more than the code allows, because the weld pulled the plate while it was still free to move. Longitudinal seam welding punishes poor planning in a way girth welding rarely does: the seam runs the entire length of the component, so distortion compounds metre by metre and a single defect becomes a very large repair. This guide covers how longitudinal seams are actually made, which processes suit which job, how to prep and fit the joint, control heat input and residual stress, qualify the procedure, inspect the result, and troubleshoot the failures that show up most often.

What Longitudinal Seam Welding Is and Why It Deserves Its Own Playbook

Put simply, a longitudinal seam weld runs parallel to the axis of a pipe, tube, vessel shell or extruded section. A girth weld does the opposite: it circles the component like a belt. That single geometric difference changes almost everything downstream, from how the metal is fed and supported while welding, to how the finished joint gets inspected and what it is allowed to cost you in wall thickness.

Get the definition right first, because everything else in this guide builds on it.

The three seam families

Shop talk uses three terms, and they are worth keeping straight:

• Longitudinal (straight) seam: a butt weld running along the length of the component in a straight line. Most mill-made line pipe has one, sometimes two, of these.
• Spiral (helical) seam: the same basic idea as a longitudinal seam, but the plate edges are offset so the weld advances at an angle, wrapping around the cylinder like a spring. It is longer than a straight seam, which is both its advantage and its headache.
• Girth (circumferential) seam: a weld joining two pipe ends or two shell courses head to head, closing the component around its perimeter. Every length of pipe has girth welds at its ends.

So why do fabricators, pipe mills, boiler shops, vessel shops and pipeline contractors care so much about the distinction? Because each group meets the same term in a completely different context, and the fabrication route changes the rules that apply.

Where you will actually meet a longitudinal seam

The keyword maps onto at least five very different real jobs.

1. Mill-made line pipe. API 5L defines SAW pipe as a tubular product with one or two longitudinal seams, or one helical seam, made by the submerged arc welding process. SAWL pipe is the specific product with one or two longitudinal seams. API 5L also defines EW pipe as a tubular product with one longitudinal seam produced by low- or high-frequency electric welding, and LFW as the low-frequency variant with welding current below 70 kHz. See the API 5L product standard for the full definitions.
2. Shop-fabricated tank and vessel shells. Plate is rolled into a cylinder and the long seam is welded in the shop, usually flat with the shell rotating underneath the torch. These seams are often the most heavily inspected welds on the job.
3. Pressure vessel nozzles and longitudinal stiffeners. These are short, local longitudinal welds, but they sit in the highest-stress regions of a vessel and are subject to the same code rigour as the shell seam.
4. Ductwork and spiral-wound pipe. Spiral seams let a fabricator roll large-diameter duct from narrow strip and produce a continuous product that a straight seam cannot match economically.
5. Sheet-metal closures by resistance seam welding. Can ends, drums and closures are often joined by a continuous resistance seam rather than a fusion weld at all. AWS covers this family in its welding handbook on resistance spot and seam welding.

The commercial context: four codes, four rulebooks

Which code governs your longitudinal seam depends entirely on what the component does. ASME B31.3 governs process piping, ASME Section VIII governs pressure vessels, API 5L governs line pipe, and AWS D1.1 governs structural steel fabrication. Each treats the longitudinal seam differently, and each one hands you a different inspection burden.

The 2025 edition of the AWS structural welding code, AWS D1.1/D1.1M:2025 (approved 19 March 2025, its 25th edition), is organised into Clause 1 general provisions, Clause 4 design, Clause 5 prequalification, Clause 6 qualification, Clause 7 fabrication and Clause 8 inspection. The AWS D1 Committee page tracks revisions as they land.

Why the seam is treated differently: fatigue and joint efficiency

Here is the structural consequence that justifies giving this seam its own playbook. A longitudinal seam is a permanent, continuous line running through the pressure envelope or the primary load-carrying member of the component. It does not stop at a joint. It does not get a fresh inspection every time the plant turns around. That means it inherits consequences a girth weld on the same component may never see.

Fatigue categorisation. AWS D1.1 puts plain unwelded pipe in fatigue stress Category A, and pipe with a longitudinal seam in Category B. In cyclic service the welded pipe is penalised relative to the unwelded material, and a designer has to size the member accordingly. If you work in stainless or aluminium rather than carbon steel, the material-specific welding guidance is well worth cross-checking, for example our guides to aluminum flux core welding and aluminum MIG welding.

Joint efficiency derating. ASME B31.3 assigns a longitudinal weld joint quality factor, usually written Ej, that depends on how the pipe was made and what examination it received. Specifying a seamless or fully examined longitudinal seam versus a lesser route changes the allowable stress, which changes the required wall thickness, which changes the material cost. The seam type is a design decision, not a purchasing decision.

Inspection attention. API 5L Clause 8.5 requires that, in addition to routine radiographic requirements, the girth weld intersecting the longitudinal seam in SAWL pipe, or the helical seam in SAWH pipe, be radiographed. That intersection gets singled out for a reason. It is where a longitudinal defect and a circumferential defect meet, and it is the highest-stress point on the pipe.

That is the case for treating longitudinal seam welding as its own discipline. On a short girth weld, a mediocre pass at a single spot is a local problem. On a twelve-metre longitudinal seam, the same mediocre pass, repeated consistently along the whole length, becomes a systemic defect that is very expensive to find and very expensive to fix. The rest of this guide works through how those seams are made, how to choose a process, how to prep and fit them, and how to inspect and repair them.

How a Longitudinal Seam Is Made: From Plate to Finished Pipe

A longitudinal seam is not a single weld. It is the end result of a production sequence in which every step either sets up the next one or quietly ruins it. Most seam problems trace back to something that happened before the arc was ever struck.

Plate Preparation and Edge Milling

Mill plate arrives with edges as-rolled, sheared or flame-cut, or already prepared. For longitudinal seam welding the edges are normally milled to a controlled bevel before forming, so groove geometry stays consistent along the seam length instead of varying with the edge condition. A sheared edge that wanders in squareness produces a root gap that wanders too, and a wandering root gap means wandering penetration.

Preparation also means cleaning. Mill scale, rust, paint, galvanising, weld spatter, cutting oil and grease have to be ground or shot-blasted from the groove area, over a specified distance either side of the edge. On a seam that may run twelve metres, one pocket of grease is a defect waiting for the arc to find it. Plate heat numbers, consumable certificates and traceability records are collected here so the finished pipe can be traced back to the plate it came from.

Forming: Roll Forming, JCOE and UOE

Once prepared, flat plate becomes a tube. In continuous roll forming, progressive rolls bend the plate gradually into curvature, which suits high-volume production and spiral-wound pipe. The JCOE route works in three presses: the plate is pressed into a J, then a C, then a complete O. The UOE route presses the plate into a U, closes it into an O, then uses a hydraulic expander to expand the body against the tooling, refining roundness and wall geometry. Edges are also pre-bent to the pipe radius, which prevents flat spots adjacent to the longitudinal seam.

Tack Welding and Closing the Seam

The formed tube has a longitudinal gap that must be closed before welding. Tacks do that job, and they matter more than their appearance suggests: they pull the edges into a uniform root gap and hold the joint in position until the root pass. A tack that is too large, misplaced or made with the wrong consumable becomes a defect you grind out later, and unevenly spaced tacks let the plate spring back between them. Good tacks are small, regular and matched to the root pass that follows.

The Weld Sequence

The main weld is built in layers: an internal root pass, fill and cap passes, then an external cap pass. Between passes, slag is removed and the surface cleaned, because trapped slag becomes an inclusion once the next pass buries it. Where full penetration is required on a heavy wall, the root is back gouged from the inside before the final passes, and carbon arc gouging remains the standard way to cut that groove cleanly. After the last pass, grinding and dressing leave the seam flush or with controlled reinforcement.

Mill-reported submerged-arc longitudinal seam capability spans outer diameters from roughly 406 mm to 12,000 mm, wall thicknesses from 6 to 150 mm, product lengths from 1,500 to 28,000 mm and welding speeds of about 300 to 2,400 mm/min using one to five wires, all within the scope of API 5L line pipe. Shop-based systems are smaller, typically covering pipe diameters from 80 to 1,250 mm and flat sheet components up to about 4,100 mm long, and follow the same logical order on more compact equipment.

The Welding Head

Modern longitudinal seam heads are far more than a torch on a carriage. Electrode and wire straightening units, side-scanning cameras for non-contact or tactile seam tracking, and servo-driven wire feed are standard, because the head must hold a controlled standoff, angle and wire length along every millimetre of a long seam. A head that drifts in tracking varies heat input along the seam.

Sizing, Straightening, Testing and In-Line Inspection

After welding, the pipe goes through sizing and calibration to reach the specified diameter and ovality, then through a straightener to remove the curvature the welding heat introduced. Heavy sections may be heat treated, then hydrostatically tested, and in-line NDT units find and mark any indication before the pipe is cut to length or coiled. For structural hollow sections, acceptance and qualification rules come from AWS D1.1.

The Resistance Welding Route

Not every longitudinal seam is a fusion weld. In electric resistance welding, the prepared edges are brought to a converging V-shaped gap and heated by high-frequency current concentrated at the vertex, then upset together so the surfaces forge against each other. The result is a forge-welded seam with no filler metal, metallurgically different from a fusion weld and carrying a different defect profile.

The fundamentals, from joint design and welding schedules to surface preparation, weld quality, economics and safe practice, are set out in the AWS Welding Handbook Chapter WHC3.01 on Resistance Spot and Seam Welding. Process control is critical: one patent describes a thermal imaging camera aimed at the vertex of the gap to detect temperature fluctuations and the shift of the welding point, so the hot spot can be held at a fixed position.

Flat Position and the Rotating Workpiece

One practical reality shapes everything downstream: the longitudinal seam is welded with the axis of the workpiece horizontal and the pipe rotating beneath a stationary or travelling head, so the arc always works at the flat of the pipe. Access, fume extraction and fixture design differ from girth welding, where the torch moves around a standing joint. That is why OSHA 1910.252 ventilation and screening requirements must be planned around the rotating workpiece rather than borrowed from a girth welding procedure, and why the fume handler is positioned for a constant, changing arc rather than a fixed torch angle.

Choosing a Welding Process for Longitudinal Seams

On a girth weld, process choice is convenience. On a longitudinal seam it becomes a commitment: the same decision sets how fast you fill metres of groove, how much heat enters the plate, what the fume load looks like, and what the inspection contractor can find.

Submerged arc: the mill default

SAW dominates longitudinal seams in large-diameter, thick-walled pipe. The granular flux blankets the arc, shields the pool and contributes metal to the weld, allowing deep penetration, very high deposition rates and clean weld metal with minimal spatter. Multi-wire systems run one to five wires from separate heads, spreading arc and pool over a wider area. That limits heat input per unit length and improves the penetration profile an ultrasonic exam later judges. Commercial systems cover carbon steel, stainless steel, aluminium, duplex and nickel-based alloys for pipeline and container work.

One safety point needs clarity. 29 CFR 1910.252(a)(1)(i)(A) requires helmets or hand shields for all arc welding and arc cutting, and explicitly excludes submerged arc welding because no arc radiation is exposed. That exemption covers the arc only. Flux handling, slag breaking and fume control still apply, and flux must be kept dry, since moisture in it is one of the fastest routes to porosity.

Gas-shielded processes: GMAW and GTAW

For shop-fabricated spools, stainless and alloy work, GMAW and GTAW take over. There is no flux to handle and no slag to chip between passes, which matters when the seam is welded inside a pipe or the spool shifts between fits. The trade is an exposed pool, so gas type, flow, nozzle size and wind protection become part of the procedure. A nozzle sized for a 300 mm coupon will starve the ends of a long joint and produce porosity the coupon never showed. Our guide to shielding gas supply covers that side. On stainless, both processes generate hexavalent chromium fume, and that exposure is what drives local exhaust ventilation.

Very thick wall: narrow gap and electroslag

Past roughly 40 to 50 mm of wall, single-V and double-V grooves stop being economical. Narrow-gap variants, SAW or GMAW with a parallel-sided preparation, cut deposited metal sharply, so there is less heat, less distortion and a shorter cycle. Electroslag welding sits at the other end: a conductive granular slag is deposited first, the arc struck between electrode and work through the molten slag, and the seam made in one pass with the slag acting as heatsink and cap. It is fast and clean, but it needs a rigid setup and a joint prepared for vertical or horizontal-vertical placement, awkward on a round longitudinal seam.

Laser welding

Laser welding has moved from exotic to specified. API 5L includes LW as a defined process: LW pipe is a tubular product with one longitudinal seam produced by a laser beam using the keyhole technique, with or without preheating and with externally supplied shielding gas. The keyhole concentrates energy into a narrow, deep weld with a small heat-affected zone, which suits stainless and alloy tube and gives fast, repeatable seams with little filler metal. Our piece on alpha laser welding covers the systems and safety angle.

The resistance family: ERW and HFW

Electric resistance welding and high-frequency induction welding make the seam without melting it. Edges are brought to a V-shaped converging gap, heated by current, then upset together so the metal is forged into a metallurgically bonded joint. The result is a fast, filler-free seam for high-volume tube that behaves differently under test than a fusion weld, so NDE acceptance for ERW pipe is handled differently, as described in the AWS guidance on resistance spot and seam welding. Common routes are submerged arc (SAW/LSAW), electric resistance (ERW) and high-frequency (HFW); the choice is driven by diameter, wall thickness, throughput and design code.

Process	Material and thickness	Deposition rate	Distortion	Fume and access	Inspection and cost
SAW, multi-wire	Carbon steel, 6 to 40 mm and up	Very high	Moderate	No arc radiation; flux and slag handling	RT or UT standard; lowest cost per metre
GMAW / GTAW	Carbon, stainless, alloy, 1.5 to 25 mm	Medium	Moderate to high	Cr(VI) fume on stainless; extraction; root access	RT, UT or PT; moderate to high cost
Narrow gap / electroslag	Carbon and low alloy, 40 mm and up	High	Low for narrow gap	Slag handling; rigid prep, vertical access	UT preferred; specialist setup
Laser	Stainless and alloy tube	High travel speed	Very low	Extraction essential; shielding gas; little filler	RT or UT; high equipment cost
ERW / HFW	Carbon steel tube, high volume	Very high, no filler	Low, mill controlled	No arc fume; mill line only	UT standard; high capital, lowest unit cost

Mill versus shop: the same question, two answers

A mill making kilometres of API 5L line pipe and a maintenance shop welding a 2 m spool answer different questions. The mill values throughput, repeatability and in-line NDT, so SAW or a resistance route wins under fixtured, rotating mill conditions. The shop values access, flexibility and a procedure one qualified welder can repeat after a single setup, so GMAW or GTAW wins. Same seam, opposite answers, because the constraints differ.

Process choice comes last

Process selection is downstream of joint design and code requirements, not upstream. Groove angle, root face, backing arrangement and examination method are fixed first by the design code and joint detail, and only then is a process chosen to fill that groove. Codes such as ASME B31.3 for process piping and AWS D1.1 for structural steel limit which processes may be used in prequalified details and what qualification each demands, so much of the decision is made for you. Choosing the process first is how seams end up with a root gap no process can hold, or an examination method that cannot see what that process produced.

Joint Preparation, Fit-Up and Alignment: Where the Seam Is Won

A girth weld is short; a longitudinal seam is long. The same edge preparation, root gap and alignment must hold for every metre of a component that travels from fit-up station to radiographic bay unrechecked. Fit-up, more than the torch, decides whether the seam passes inspection or lands in the repair log.

Edge condition and bevel geometry

Start with the edge. AWS D1.1 is blunt about it: surfaces and edges to be welded must be smooth, uniform and free from fins, tears and cracks. That is why quality longitudinal work uses edge-milled plate, not flame-cut plate, whose irregular, oxidised surface repeats along the whole seam.

Groove shape follows wall thickness and access, and the workhorse is the single-V: for submerged-arc fillet-and-cap work, start at roughly a 60 degree included angle with a small root face. The angle lets the arc reach the root without fouling the sides; the small root face limits parent metal in the pool. Double-V grooves on very thick wall cut weld metal volume and distortion but need two-sided access and more passes. U-grooves suit the thickest sections, where machining costs less than filling a deep V.

Because the sides of a V converge, the groove closing rate sets how deep the root pass penetrates. A gap wider than the procedure assumes gives a cold root run and lack of fusion at the bottom; a narrower gap melts through. Constant travel speed plus constant root gap equals constant penetration, so root gap is a controlled dimension, not a shop preference.

Fit-up: tacking, gap and alignment

Tacking closes the joint before the main pass. Tacks should be small, evenly spaced and made with a procedure you would be happy to inspect: a badly shaped tack pulls distortion into the seam as it shrinks. On a long seam, spacing matters: sparse tacks let the plates move, dense tacks fight shrinkage and crack the parent metal.

AWS D1.1 requires butt-welded parts to be carefully aligned so offset does not exceed 10 percent of the thinner part’s thickness, and in no case more than 1/8 in. Where backing is used, the gap between faying surfaces must not exceed 1/16 in. Correcting misalignment must not draw the parts into a slope exceeding 1/2 in in each 12 in of length, offset measured on the centreline. If tolerances cannot be met, the code allows end preparations built up by welding or re-prepared by machining or grinding. Clause 5 of D1.1 covers joint dimension tolerances, tubular girth weld alignment and groove dimensions.

Fit-up parameter	Typical limit	Why it matters on a long seam
Offset between parts	10% of thinner part, max 1/8 in	Repeats full length, uneven fusion toe
Faying-surface gap with backing	1/16 in max	Excess gap escapes as melt-through
Slope when drawing into alignment	1/2 in per 12 in	Forces bending, adds residual stress
Root opening, SAW single-V	Per WPS, 1/8 to 3/16 in	Governs penetration and burn-through
Root face	Commonly 1/8 in (3 mm)	Too large, lack of fusion; too small, melt-through

Backing: removable and permanent

Backing does two jobs: it supports the molten root and controls root reinforcement. Removable backing (copper, ceramic or mild steel bar) is placed under the joint and ground or chipped away after welding. Permanent backing (a strip, plate or a length of pipe inside) stays in the component, joining the pressure envelope, so pressure-retaining codes often prohibit it or restrict material and procedure. Check your code first.

Backing is not free. Too rigid, it restrains the plates as the seam shrinks or chills the root so the root run bonds poorly. Removable backing must be smooth, clean, free from rust and oil, and flat enough to stop the root dropping out. Where the root has to be removed later, air arc gouging is standard, and the gouged surface is ground smooth before rewelding.

Cleanliness

Mill scale, slag, paint, galvanising, grease and cutting oil must be cleaned from the groove within the distance the procedure specifies, commonly an inch or two either side. Out-of-square cuts get ground out rather than welded over. A longitudinal seam is unforgiving here: contamination running with the seam runs its full length, so one drop of oil trapped in the root can become twenty metres of porosity. In a girth weld that is a local defect; here it is a production run.

Access and out-of-position working

Ask who is going to see the root. Reaching the inside root of a 1.2 m diameter pipe means climbing into a dark, hot, confined space, working overhead with your head metres from the arc. That is an ergonomic and safety cost, and the strongest argument for welding inside out: root and inner fill from inside, fill and cap from outside, so the root is inspected before it is buried.

Long seams are welded in the flat position with the component rotating beneath a stationary or travelling head, which removes vertical-up positional penalties but makes every dimensional error permanent along the whole length. Tacks and root work still happen in awkward positions, where a guide to all-position welding rod earns its place. Work inside a pipe or vessel must be planned as confined space work under OSHA’s welding guidance, with ventilation set before the arc is struck.

Heat Input, Interpass Temperature and the Fight Against Distortion

Most welding defects announce themselves loudly. Porosity spits out, undercut leaves a visible groove, a crack shows up on the film. Distortion is quieter. It creeps up over metres, and by the time anyone measures the out-of-roundness of a tank shell, the weld itself was fine and the shape is not. Long seams are where heat control stops being a nice-to-have and becomes the whole job.

Heat Input Is a Control Metric, Not a Measurement

Heat input expresses how much energy you are pouring into the metal per unit length of weld. The arithmetic is simple:

Heat input (kJ/mm) = (voltage x amperage x 60) / (travel speed in mm/min x 1000)

What matters on a long seam is the denominator. A single pass on a short joint can absorb a heat input spike and nobody notices. On a longitudinal seam, the same pass multiplied by twelve metres of seam becomes a thermal load the plate cannot absorb quietly. So the discipline is simple: hold travel speed steady, hold wire feed steady, and hold the arc length steady. Constant travel speed with a consistent wire feed is what keeps heat input per unit length constant, and that constancy is what keeps the bead profile, the fusion pattern and the shrink along the whole seam identical.

Multi-wire submerged-arc configurations help here. Spreading the arc and the pool across a wider area limits heat input per unit length and improves control of the penetration profile on thick-wall longitudinal seams, where a single arc would dig in too fast.

Dilution, Bead Profile and Why Slag Forms

Heat input drives the weld chemistry, not just the temperature. Raise it and the base metal melts into the pool in greater proportion, which is dilution. Too much dilution pulls base metal chemistry into the weld and can leave the deposit below the strength, toughness or corrosion resistance the filler was chosen for. Too little heat input means poor fusion and a cold lap risk.

The same knob sets bead shape. Low heat input gives a narrow, tall, poorly fused bead. High heat input flattens the bead, widens the penetration and floods the groove with molten metal, which is what pushes slag to the surface and makes clean interpass removal mandatory.

Preweld Heat: Two Different Reasons

Fabricators routinely confuse two separate justifications for preheat, and confusing them leads to either wasted energy or, worse, underheating a crack-sensitive joint.

• Hydrogen control. In thick carbon steel, preheat slows the cooling rate through the range where hydrogen can combine with susceptible microstructures to initiate cold cracking. This is the reason preheat exists for most high-restraint thick-section welds.
• Weldability and arc stability. On thinner or lightly restrained joints, preheat may be present purely to help the arc start, stabilise the pool, improve bead shape or manage the cooling of the coating and surface condition.

Getting this right is a specification problem, not a torch problem. Interpass limits and preheat requirements come from the Welding Procedure Specification, not from the welder’s judgement in the moment.

Interpass Temperature and the Heat-Affected Zone

Interpass temperature is the maximum temperature the weld metal and adjacent base metal are allowed to reach between passes. It is enforced because repeated reheating does damage: grain growth in the heat-affected zone, local hardness increases in hardenable steels, and a loss of toughness where prior passes have refined a structure and the next pass coarsens it again.

On a long seam the risk is not one hot pass, it is the accumulated thermal cycle. A seam welded continuously for two hours sees the same zone reheated again and again, and the code limits exist to stop that from becoming a metallurgical problem nobody measured.

How Long Seams Distort

There are three distortion mechanisms at work on a longitudinal seam, and they stack up as the seam lengthens.

1. Longitudinal shrink. The seam contracts along its length as the weld metal cools. Over many metres, that contraction pulls the shell shorter and distorts the ends.
2. Angular distortion. The V-groove closes as the fill and cap passes shrink. If the joint is not properly restrained, the plates rotate toward each other.
3. Bending of a single-edge-welded plate. When you weld one edge of a plate and leave the other free, the out-of-plane shrinkage on the welded side is unconstrained. The plate bends toward the weld. This is exactly why a longitudinal seam in a vessel shell or a tank course must either be welded in a balanced sequence or the shell must be fixtured against it.

The cumulative effect over metres is the killer. A quarter-millimetre of angular distortion at one end is invisible. Twelve metres of it is a shell that is out of round beyond what the code allows.

The Mitigation Toolkit

• Sequence welding, welding in short segments and back-stepping so heat is spread rather than concentrated at the leading end.
• Symmetrical welding inside and outside, alternating between the root side and the cap side to balance the shrink.
• Strongbacks and fixturing to hold alignment against the pull of the weld.
• Pre-cambering and pre-bending of the plate before welding so the finished shape is the result of planned compensation rather than luck.
• Controlling heat input and interpass, the two variables that shrink everything else follows from.

Mechanical restraint needs a caution. Restraining a seam reduces distortion but raises the residual stress in the joint, and restraint plus hydrogen plus a susceptible microstructure is the classic cold cracking combination. Restraint is a tool, not a free action.

Fabricators looking for the accepted approach on structural work find it in AWS D1.1 Clause 5, which sets explicit requirements for the control of distortion and shrinkage as well as for base metal preparation and assembly.

Induction Heating and Pre-Bend

In the pipe manufacturing flow, induction heating of the plate prior to forming and plate pre-bending are used together, and the reason is that both directly attack the ovality and flat-spot risk associated with a longitudinal seam. Induction heating makes the plate easier to form, so the shape comes out of the mill closer to true. Pre-bending removes the spring-back that would otherwise leave a flat spot alongside the seam and a slightly oval shell.

Residual Stress and Post-Weld Heat Treatment

Distortion is the visible symptom. Residual stress is the invisible one, and it lives along the seam as well as across it. Those residual stresses act as stress raisers, they can distort the component further once the assembly is released, and in service they interact with the primary stress to drive fatigue cracking that starts at the seam.

That is why post-weld heat treatment appears in the requirements for pressure vessels and heavy sections. PWHT relieves residual stress around the weld. It also relieves the residual stress that was holding the assembly in its welded shape, so dimensional change after PWHT must be expected and allowed for. Plan the fit-up around a final post-PWHT measurement, not the as-welded one.

For process piping and vessels, the requirements that govern heat input, preheat and interpass live in the code and the supporting procedure rather than in the shop’s habit. ASME B31.3 sets the framework, and the WPS translates it into the numbers your welder actually works to on the eleventh hour of a twelve-metre seam.

Codes, Welding Procedures and Getting the Welder Actually Qualified

A longitudinal seam is never just a weld. It is a weld attached to a stack of documents that a client, an inspector and sometimes a legal reviewer will read long after the arc went out. That stack is well defined, because the codes covering long seams were written around exactly the problems long seams cause: joint efficiency, fatigue, distortion and repair. Here it is, from the top down.

The compliance stack

1. The governing design code. Fixes materials, allowable stresses, the joint efficiency factor and how much examination the joint requires.
2. The qualification standard called up by that code. ASME codes point to Section IX, AWS D1.1 uses Clause 6, and API 5L carries its own mill qualification rules for line pipe.
3. The Welding Procedure Specification, or WPS. The welder’s instruction sheet: groove, filler metal, preheat, interpass temperature, heat input, travel speed, position and cleaning.
4. The Procedure Qualification Record, or PQR. The test coupons and test results that prove the WPS is sound.
5. The welder or welding operator performance qualification. Proof that the individual can make that kind of weld, in that position, with that process, on material within a stated range.

Governing code	Typical scope	Qualification reference
ASME BPVC Section VIII	Pressure vessels, tanks and shells	ASME Section IX
ASME B31.3	Process piping in industrial plants	ASME Section IX
ASME B31.1	Power piping in plants and boilers	ASME Section IX
API 5L	Line pipe for oil, gas and water	API 5L mill qualification
AWS D1.1/D1.1M	Structural steel fabrication and erection	AWS D1.1 Clause 6

Why B31.3 treats the longitudinal seam differently

ASME B31.3 governs the materials and components, design, fabrication, assembly, erection, examination, inspection and testing of piping for petroleum, chemical, pharmaceutical, hydrogen, textile, paper and pulp, power generation, semiconductor and cryogenic plants. It also does something few codes do so explicitly: clause 302.3.4 deals with the Longitudinal Weld Joint Quality Factor, and Table A-1B lists basic quality factors for longitudinal weld joints in pipes, tubes and fittings.

That factor is not academic. Los Alamos National Laboratory’s process piping guidance states that radiography is performed for longitudinal groove welds required to carry a weld joint factor of 0.90 or greater. The examination method you are obliged to use therefore feeds straight back into the design, because it can change the allowable stress and the wall thickness you need.

Structural steel: D1.1 and its revision cycle

For structural work the edition matters as much as the clause. AWS D1.1/D1.1M:2025 was approved on 19 March 2025 and revises the 2020 24th edition, which had itself superseded the 2015 23rd edition. That is roughly a five-year revision cycle engineers have to track. AWS also publishes errata for D1.1 in the Society News section of the Welding Journal and on the AWS website, so a printed book that looks current may already have been amended.

Know the structure before you pick up a torch. Clause 7 covers fabrication and erection requirements including material preparation and assembly, workmanship and weld repair. Clause 8 covers inspection, including radiographic and ultrasonic testing. If the job involves aluminium, sheet steel of 5 mm (3/16 in) or less, reinforcing steel, stainless, or strengthening and repair of existing structures, D1.1 is the wrong document: those live in D1.2, D1.3, D1.4, D1.6 and D1.7, as set out on the AWS D1 committee page.

Prequalified details versus qualified by test

Most codes let a WPS stand on one of two legs. A prequalified joint detail is one the code already blesses: use the specified groove, filler metal and preheat range at a thickness inside the listed range, and no separate PQR is needed. A qualified by test procedure is anything outside those limits and must be proved by welding coupons and testing them. On a longitudinal seam the temptation to stray from a prequalified groove is strong, because a straightforward single V is easier to fit and faster to fill than a narrow groove or a double V. Every such deviation converts a free procedure into one that needs coupons, testing and a full qualification package before the seam starts.

What the PQR actually proves

A PQR is not a general blessing of a weld. It records a tested range of essential variables: process and consumable type, joint type, groove, position, diameter, thickness, backing, preheat and interpass temperature, shielding and, for some processes, voltage, travel speed and heat input. What it proves is that welds made inside that range will meet the code’s acceptance criteria. Push outside the range and you have no qualification at all, however good the weld looks. The PQR does not cover the individual welder either; that is a separate performance qualification, usually granted across a range of positions and thicknesses. Other sectors run their own systems entirely, as our guide to aerospace welding standards and careers shows.

Where the codes bite hardest on a longitudinal seam

• Alignment and offset limits. Codes cap the misalignment permitted between parts and the slope allowed in bringing them into alignment. On a seam measured in metres, a small per-metre allowance accumulates into an out-of-shape component.
• Groove dimension tolerances. Root opening, root face, bevel angle and included angle all have permitted ranges. Root opening has to be held constant over the whole length, because the closing rate of the V governs penetration.
• Backing rules. Removable or permanent backing is permitted only under stated conditions, and the code specifies how much root penetration or melt-through is acceptable when backing is used.
• Inspection and repair provisions. Acceptance criteria apply per joint, and repairs must meet the same requirements as the original weld, be re-inspected, and be documented.

The version trap

Codes are revised on multi-year cycles and errata accumulate between printings. A WPS written against a superseded edition may not be valid against the current one, because the qualification standard itself may have changed its essential variables. Before accepting any procedure package, confirm the edition year of both the design code and the qualification standard, and check for errata and addenda. That five-minute check prevents the kind of finding that stops a vessel at the inspection hold point.

The documentation package a client will ask for

• Drawings showing the groove details and the seam orientation
• The WPS and its supporting PQR
• Welder and welding operator performance qualification records
• Filler metal and consumable certificates, with lot traceability
• Heat treatment records where post-weld heat treatment is required
• NDT reports, including any area of reduced coverage or repaired defect
• The repair log, showing what was found, removed, re-welded and re-examined

Inspection and NDT: Proving the Seam Is Sound

A long seam is a bigger target than a short one: a 600 mm girth weld offers a handful of places to hide a flaw, a 12-metre shell seam offers hundreds, and a mill line producing kilometres of longitudinal weld has effectively unlimited exposure. Inspection therefore favours methods that scale, in escalating order of capability and cost, which is also the order most codes expect you to work in.

Visual inspection: the first line of defence

Nothing replaces a welder or inspector with a torch, a bright light and a straightedge on the seam. Visual inspection covers reinforcement height and profile, undercut, cracks visible to the eye, and the condition of the weld toes and heels where cracks like to start. AWS D1.1 Clause 8 sets the model, splitting inspection into visual plus the nondestructive methods in its parts, including radiographic, ultrasonic and magnetic particle testing.

Acceptance criteria for a groove weld are unforgiving:

• Cracks are prohibited outright, in weld metal, fusion boundary or parent metal.
• Undercut is limited rather than banned, commonly 1/32 in (1 mm) on material 1/8 in (3 mm) and thicker, and not permitted on thinner material.
• Excess reinforcement is usually capped around 1/8 in (3 mm).
• A rough, irregular or poorly blended bead profile is rejectable, and on a long seam a wandering bead flags a wander in travel speed or wire feed.

Radiographic testing: the pressure-application benchmark

Radiography remains the reference method for pressure-retaining welds and the code benchmark for acceptance, because it images volumetric defects such as porosity, slag and lack of fusion and leaves a permanent record. Film and digital radiography differ in workflow only: the digital version swaps film for a detector and a licensed image, removing chemical processing, allowing re-shoots without consumables and making results searchable.

What radiography cannot do matters just as much. It struggles with tight or planar defects lying parallel to the beam, generally will not find a thin laminar in the parent plate, and its sensitivity shifts with material and reinforcement thickness.

The API 5L specification also allows alternative techniques for particular seam types, including ultrasonic inspection for laminar imperfections in the pipe body and along the coil or plate edges and weld seam of EW, SAW and COW pipe, which is why laminar detection is handled separately from volumetric detection.

Ultrasonic testing and the rise of phased array

Ultrasound is the method that scales to a long seam. Manual pulse-echo is a spot reading and operator-dependent; time-of-flight diffraction excels at locating and sizing tip and root flaws in thicker material. Phased array is preferred on long seams because the encoded data file is a permanent, re-playable record, so the seam can be imaged and reviewed later rather than judged in real time.

In a modern LSAW mill, automated ultrasonic testing runs in-line immediately after welding, with the scanner traversing the full weld seam. Off-line ultrasonic testing and radiography of pipe ends and weld seams follow later. One requirement catches crews out: the surface must be ground flush before ultrasonic testing, because a proud reinforcement profile decouples the probe and destroys sensitivity.

Surface and near-surface methods

Magnetic particle testing finds surface and near-surface cracks in ferromagnetic material fast, but only on ferromagnetic base metals. That limitation alone excludes it from the standard NDT battery for stainless, aluminium and other nonferrous longitudinal seams. For those, dye penetrant is the surface method of choice, while eddy current handles tube and small-bore product, where an internal probe sweeps a long length quickly. Aluminium seams need their own preparation, so read up on aluminium MIG welding practice before choosing a method.

The girth-to-longitudinal intersection

Two spots deserve attention: the longitudinal seam itself, and the girth weld crossing it. API 5L Clause 8.5 requires radiographic inspection of the girth weld where it intersects the longitudinal seam in SAWL pipe and the helical seam in SAWH pipe, on top of the standard radiographic requirements. The reason is geometric: the intersection combines the seam’s own imperfections with a discontinuity in the wall, and it is where bending, ovality and stress concentration pile up.

Making inspection workable on a real long seam

Coverage is the other hard truth. Reaching 100 percent radiography on a multi-kilometre seam is impractical on film and demanding even digitally, so mills run in-line automated UT as the primary volumetric screen and use radiography selectively. The shift is toward complementary methods, with data systems that tag every shot to a pipe, length and position.

Under a code rather than a mill specification, shop instructions mirror the code tables. The Los Alamos National Laboratory B31.3 guide, for example, points to Table A-1B of B31.3 for basic quality factors and directs weld examination through the LANL welding manuals, which is how a code requirement becomes a shop instruction.

Don’t forget the leak check. In the mill sequence every pipe is filled with water and pressurised for a specified period as a hydrostatic leak-tightness test before final visual inspection and packaging, catching the class of defect no surface method ever will.

Finally, plan inspection hold points before the arc is struck, and know how defects will be removed. Excavating a flaw to sound metal along a long seam usually means carbon arc gouging, then a re-weld at a preheat that may exceed the original, then re-inspection of the repair. Deciding that in advance, with the procedure already qualified, is far cheaper than improvising it in a tank bottom with the job on hold.

Common Defects in Longitudinal Seams: Causes, Prevention and Repair

A girth weld gives you one short, isolated trouble spot. A longitudinal seam gives you a problem measured in metres. Any imperfection that repeats at a steady rate along the seam turns into a yield calculation, a repair bill, or a rejected pipe, so experienced crews read a long seam as a pattern rather than a series of points.

Weld Metal Defects

The six classic weld metal defects all appear on longitudinal work, and the geometry makes each one worse.

Defect	What it looks like	Why it happens on a long seam	Prevention
Porosity	Round voids on radiography or a cut section	Oil, paint, rust, damp flux or wire spool, or a leaking gas line is dragged along the whole run instead of being trapped in one spot	Clean the groove a set distance ahead of the torch, keep flux dry and re-baked, check shielding gas flow and torch seals before striking the arc
Lack of fusion / cold lap	Dark unfused streak at the sidewall or toe	Travel speed too high, heat input too low, arc too wide or bead badly placed, so a low-heat parameter runs unnoticed for a whole coil	Confirm bead placement, rebalance speed and heat input, validate on a macro-etched coupon
Undercut	Groove cut into the toe, thinning the throat	Excessive travel speed, high current with too little weave or a wrong torch angle; speed drifts as the plate heats over distance	Cap travel speed, weave, hold a consistent torch angle, grind out to sound metal
Overlap / high reinforcement	Bead riding over the toe, lumpy convex profile	Travel speed too slow or heat input too high; reinforcement must stay uniform along the entire seam	Set travel speed from measured deposition, check crown regularly
Burn-through / melt-through	Hole or drip of molten metal on the root side	Excessive heat input, root gap too wide, absent or inadequate backing; a gap in the backing strip burns through right there	Keep backing continuous, control root opening and heat input, add a second pass on thin wall
Slag inclusion	Dark elongated stringer along the seam on radiography	Flux handling, weak interpass cleaning or mismatched wire and flux; slag runs the length of the seam, so trapped slag becomes a continuous stringer	Clean every pass, keep flux dry and uncontaminated, use matched consumables

Cracking: Hot and Cold

Hot cracking occurs in the weld metal and is driven by high restraint combined with high sulphur or low manganese content. A long seam carries far more restraint than a short one, so susceptibility rises along the whole seam rather than showing up in isolated spots. Keep sulphur low, match consumables to the base metal, hold dilution steady and preheat per the WPS.

Cold cracking is the delayed one, appearing in the heat affected zone hours after the arc goes out, and it is driven by hydrogen, a fast cooling rate and restraint. A long seam locks in restraint continuously, so it can pass a hot visual check and still fail NDT later. Low hydrogen consumables, correct preheat and interpass temperature, and a temper bead where the procedure calls for it are the standard controls.

Plate and Forming Defects

Laminations and delaminations are a plate and forming problem, not a welding one. They surface as stepped ultrasonic signals or a flaky layer lifting at the weld toe. On a JCOE line, plate edge condition drives much of the downstream seam trouble, which makes edge milling quality a preventive control rather than a production detail. Mills cite bevel angle and size, tube shape, weld zone cleanliness, welding materials, welding parameters, operating method and weld procedure as the levers for reducing these problems.

Edge cracking in the parent metal is the same story one stage earlier. Press and roll forming can crack the plate edge before the arc is struck, and welding over it turns those cracks into crack-like indications. Pre-bending to avoid flat spots next to the seam and inspecting the edge before forming prevent the problem.

Misalignment and offset get caught at fit-up. Structural work typically limits offset to 10 percent of the thickness of the thinner part, not exceeding 1/8 in, with parts drawn into alignment without exceeding a slope of 1/2 in in 12 in of length, per AWS D1.1 structural welding requirements. Out-of-roundness and wave bend are the dimensional fallout of seam shrinkage and are handled by straightening or rolling, not by welding.

Mill-side longitudinal seam defects also include flash, burn-through, wrong-side slag, arc blow, and reinforcement that is too high or too low. High pre-weld and outer seam reinforcement produces an excessive weld depth and an oversized bead, a defect in its own right rather than a cosmetic issue. Low internal back-weld reinforcement can generate cracks in the main internal seam.

Repairing a Longitudinal Seam

1. Map the indication. A defect located by an in-line inspection system at a known distance can be mapped, ground out and re-welded in place without scrapping the pipe, provided the affected length falls within the permitted repair limits of the governing code such as API 5L.
2. Excavate to sound metal. The discontinuity must be removed back to clean base metal. Gouge to a U or a double-V with generous radii, since a sharp excavation edge acts as a stress raiser and a crack initiation site. Carbon arc gouging is the usual tool, with dust and fume extraction running.
3. Re-weld to a qualified procedure. Preheat may need to exceed the original preheat because the remaining material is now more restrained. Root passes made from the inside are often done with a consumable electrode, since stick welding tolerates a dirty root and gives predictable fusion in awkward access.
4. Re-inspect the repair. The repaired area is subject to the same inspection requirements as the original weld, and adjacent material should be checked for cracking that the excavation may have exposed.
5. Document it. Repair welds on pressure-retaining seams must be qualified and documented, with the repair log, consumable certificates and heat treatment records attached to the package.

Longitudinal vs Girth vs Spiral Seams: Choosing the Right Seam for the Job

Ask a supplier for “6-inch schedule 40 ERW” and you get a price. Ask whether the longitudinal or the girth joint should carry the schedule and the real conversation starts. The seam is not a manufacturing footnote. It decides how the product was made, what it is allowed to carry, how hard it is to inspect, how it behaves under cyclic loading, and what it costs. Get the seam type wrong and you end up with unnecessary wall thickness, or a joint efficiency factor that does not exist in the code.

Three seam families cover almost everything in a mill, a shop or a pipeline: the straight longitudinal seam, the spiral (helical) seam, and the girth (circumferential) seam. Here is how they compare.

The comparison at a glance

Factor	Longitudinal (straight)	Spiral (helical)	Girth (circumferential)
How it is made	Mill-formed: submerged arc, resistance or laser weld	Same forming route, strip fed at an angle	Two finished shells or spool ends joined together
Material use and coil size	Limited by plate width and seam location	Wider coil consumed before the edge is reached	No effect on coil yield
Seam length and defect probability	One long seam per pipe length, sometimes two	Longer weld length than a straight seam	Shortest weld of the three
Stress orientation and fatigue	Parallel to the principal stress axis, the worst case	Helical angle shifts the weld off the worst axis	Near perpendicular to hoop stress, most favourable
Inspection burden	Mill-examined, awkward once installed	Longer to scan, slightly harder to cut into	Easiest field inspection, often 100% radiographed
Field welding	Rare; repair is the exception	Rare	The normal field weld
Dimensional tolerance	Forming step controls ovality and straightness	Same, with a helical geometry effect	No added form error if aligned before welding
Cost profile	Driven by plate conversion, welding and NDT	Often competitive thanks to better coil yield	Cheapest per joint, but every joint costs fit-up time
Typical use	Line pipe, process piping, vessel shells	Line pipe, spiral-wound flexible pipe	Spools, tank shells, heat exchangers, structural pipe

The design consequence: joint quality factors

Seam choice is a design decision, not a purchasing one, because the code pays attention to it. ASME B31.3 Table A-1B sets basic quality factors for longitudinal weld joints in pipes, tubes and fittings, and Clause 302.3.4 governs how the longitudinal weld joint quality factor is applied. The factor reflects the manufacturing method and the degree of examination, so a submerged-arc longitudinal seam with full radiography does not earn the same credit as a lesser-examined one.

The Los Alamos National Laboratory B31.3 guide identifies the Table A-1B basic quality factors and the ASME Section VIII UW-12 joint efficiencies as the guidance used to establish adequate wall thickness for welded components. Change the pipe manufacturing route and the allowable stress or the required wall thickness moves with it.

Fatigue orientation is the deciding factor

In a pipe spool, the principal stress is hoop stress, running around the circumference. A longitudinal seam lies parallel to the pipe axis, the least favourable orientation for cyclic loading. AWS D1.1 reflects this by placing pipe with a longitudinal seam in a lower fatigue stress category than plain unwelded pipe, treating the seam as a continuing fatigue detail rather than a local one. A girth weld crosses the stress direction, which is why it is far less likely to start a fatigue crack.

The spiral argument

A helical seam changes the geometry. The weld’s orientation relative to hoop stress is distributed rather than aligned with the axis, and a mill can consume a wider coil before running out of plate, so material yield improves. The trade-off is a longer seam, a harder inspection job and a seam that is awkward to cut into when a spool must be shortened. Spiral-wound flexible pipe, hundreds of helically wound steel wraps inside a polymer liner, takes this logic to its extreme for offshore and sour service.

When a longitudinal seam is unavoidable

A tank or vessel shell has to be rolled from plate, so a longitudinal seam is not optional. The same applies to a plate-built hull, where the keel seam is a continuous longitudinal joint and the joinery work is covered in our aluminum boat welding guide. For pipe, API 5L treats seam type as a product category in its own right: SAWL with one or two longitudinal submerged-arc seams, SAWH with a single helical seam, EW with one longitudinal electric welded seam, plus COW, COWL and continuously welded (CW) variants. Each carries its own qualification and inspection requirements. API 5L also requires the girth-to-longitudinal intersection to be radiographed, an acknowledgement that where two seam types meet is both a discontinuity and a high-stress region.

So buy the seam, not just the pipe. Decide whether fatigue, wall thickness, inspection access or field repairability is the controlling constraint, and let that choice set the product.

Best-Practice Workflow, Safety and a Final Pre-Weld Checklist

Everything upstream of the arc on a longitudinal seam comes down to one question: is the process under control for the whole length, not just where the torch happens to be? A long seam gives you more time to get it right and more distance to get it wrong.

The Pre-Weld Checklist

Run this list before the first arc and again at every restart. On a twelve-metre shell plate with rolling fit-up, “before we start” is a moving target, so treat it as hold points, not one event.

1. Confirm the code and its edition. ASME B31.3 for process piping, ASME Section VIII for vessels, API 5L for line pipe, AWS D1.1 for structural. Write the edition and addendum on the work package. The ASME B31.3 process piping code is revised on a multi-year cycle, so a superseded edition is a finding waiting to happen.
2. Confirm the specification and joint efficiency factor. The manufacturing route of the seam (longitudinal, spiral, girth) changes allowable joint efficiency and can change required wall thickness. Check before the plate is on the roll.
3. Verify material certificates and traceability. Mill cert, heat number and full chain of custody to the joint. A seam that cannot be traced cannot be repaired to a code-compliant record.
4. Inspect and machine the bevel against the drawings: edge, bevel angle, root face, root opening. Mill scale, paint and galvanising must be ground back sufficiently far from the groove.
5. Verify fit-up, root gap and offset. Offset commonly 10 percent of the thinner part’s thickness, not over 1/8 in; faying surface gap 1/16 in or less with backing; alignment not sloped more than 1/2 in in 12 in. AWS D1.1 and the ASME codes are specific about these.
6. Check the tacks: same consumable, same preheat, same cleanliness as the main weld, ground out or properly incorporated. A badly placed tack is a crack waiting for fatigue.
7. Confirm consumables, shielding gas and flux. Correct classification, dry storage, gas mix and flow, flux free of moisture from a damp floor. For stainless, gas purity matters as much as filler choice.
8. Confirm preheat and interpass temperature from the WPS, based on thickness and carbon equivalent, not on what the torch feels like. Verify with a calibrated indicator on the part.
9. Confirm the WPS and the welder’s qualification, covering position, diameter and thickness actually welded: flat position on 3 in plate is not 12 in pipe in the 2G position. The AWS Welding Handbook chapter on resistance spot and seam welding covers qualification logic for the resistance family and closes with safe practices for those processes, whose hazards differ from arc welding.
10. Set and monitor heat input and travel speed from the WPS, then watch them for the full length. Constant voltage and a wandering arc are not a heat control system. Log it.
11. Confirm ventilation and fume extraction before striking the arc, not after the first complaint.
12. Confirm fire watch and hot work permits: fire watch for the whole duration, extinguishers in place, clear escape route.
13. Plan the repair strategy and inspection hold points: who calls the repair, who re-inspects it, where work stops for NDT.

Safety on Long Seams

Safety slips on long seams because the work is spread out, the crew rotates through it and it feels routine. It is the same hazardous work stretched until the hazards are easy to walk past.

OSHA 1910.252 governs welding, cutting and brazing. Where welding is done in a space screened on all sides, 1910.252(c)(1)(ii) requires the screens to allow no serious restriction of ventilation and requires local exhaust or general ventilation to keep fumes, gases and dusts below the maximum allowable concentration in 1910.1000. A screen that blocks the plume is a container, not a solution.

The fume driving most ventilation work is hexavalent chromium. Cr(VI) forms when chromium in stainless steel, nonferrous alloys, chromate coatings or some consumables is converted during welding, and it is a carcinogen. OSHA’s permissible exposure limit is 5 micrograms per cubic metre as an 8-hour time-weighted average, enforced under 29 CFR 1910.1026 in general industry and 29 CFR 1926.1126 in construction. Stainless spools and 6xxx aluminium are worst; the guides to aluminum MIG welding and aluminum flux core welding cover fume control for those materials.

OSHA’s welding fact sheet adds controls: substitute a lower fume-generating process or consumable where practical, keep fume hoods and extractor guns close to the plume source rather than letting the arc run out in front of the hood, and never weld in a confined space without ventilation.

Welding inside a pipe or vessel is confined space work under 29 CFR 1910.146: attendant, atmospheric testing, retrieval system and rescue plan apply before the arc is struck, not after someone feels unwell. A properly prepped surface cuts fume and contamination but does not change the confined space classification.

One control is easy to overlook: 1910.252 requires the welder to mark hot metal or warn other workers after welding. On a long seam the hot zone can extend many metres and cool for hours. Mark it, tag it, barricade it.

Fire prevention follows NFPA Standard 51B, referenced in OSHA’s welding subpart: hot slag runs, molten spatter falls through grating, and gas cylinders share the footprint of the hot work. Keep the area clear and keep the fire watch.

Closing: The Problem Is Distance, Not the Arc

Look back at the quality problems in this guide and a pattern appears. The undercut was travel speed drifting over twenty metres. The porosity was a wet flux nobody checked. The out-of-roundness was shrink nobody measured. Almost everything on a longitudinal seam is a problem that went uncontrolled along a distance, not at a point. The best welder in the shop can lay a beautiful bead for two metres. The job is to lay that same bead two hundred times.

Frequently Asked Questions

What is longitudinal seam welding, and how does it differ from girth welding?

Longitudinal seam welding joins edges that run parallel to the axis of a pipe, tube, vessel or section, creating a seam along its length. Girth welding closes the component with a circumferential weld around the circumference. Longitudinal seams are usually made by a mill or a seam welder before the component is cut to length, while girth welds are typically field or shop joints made to assemble or extend the component.

Which welding process is best for longitudinal seams in large-diameter pipe?

Submerged-arc welding (SAW) is the industry standard for large-diameter, thick-wall carbon steel because the flux shields the pool, allows deep penetration, high deposition rates and multi-wire configurations, and produces weld metal with good toughness. For smaller diameters and thinner walls, electric resistance welding and high-frequency welding dominate because they are far faster on a continuous line. The governing code and the pipe specification usually make the decision for you.

What is the difference between LSAW, ERW and HFW pipe?

LSAW (submerged-arc longitudinal welded) pipe has one or two longitudinal seams made by submerged-arc welding; ERW (electric resistance welded) pipe has one longitudinal seam formed by resistance heating and upsetting the edges together; HFW (high-frequency welded) pipe is a subset of electric welded pipe made at high frequency. API 5L defines each as a separate product category with its own qualification and inspection rules.

What bevel angle and root opening should I use on a longitudinal seam?

For submerged-arc butt welds on carbon steel, a single-V groove of roughly 60 degrees with a small root face and a root opening of about 3 mm is a common starting point, but the correct values come from the WPS and the prequalified joint details in the governing code. The critical point is consistency: the root opening must be uniform along the entire seam, because the closing rate of the V directly sets penetration.

Why does my longitudinal seam make the component go out of round?

A long seam shrinks along its length as it cools, and the shrinkage is only restrained if the assembly is fixtured against it. On a single-edge-welded plate the out-of-plane shrinkage is unconstrained, so the shell pulls oval. The standard fixes are pre-bending or pre-cambering the plate, balanced and segmented welding sequences, strongbacks and fixturing, and controlling heat input and interpass temperature.

What NDT is required for longitudinal seams in API 5L pipe and ASME B31.3 piping?

API 5L permits radiographic, ultrasonic and other defined techniques, and additionally requires radiography of the girth weld where it intersects the longitudinal seam. ASME B31.3 ties the allowable design factor to a longitudinal weld joint quality factor, so the required examination follows the quality factor assigned to the manufacturing method and level of NDE; Los Alamos guidance notes that longitudinal groove welds needing a factor of 0.90 or greater receive radiography. Always confirm against the current edition of the code.

Can you make a longitudinal seam in an existing pipe?

It can be done, but many pipeline operators prohibit hot tapping of longitudinal seams because the existing weld’s toughness, geometry and inspection history are less predictable than those of a girth weld, and the runout of a hot tap into a longitudinal seam is a known stress raiser. Any hot tap on a longitudinal seam needs explicit approval from the operator and the design authority, with a qualified procedure and NDE.

What causes porosity and lack of fusion in a longitudinal seam?

Porosity usually points to gas or contamination: moisture in electrodes, flux or base metal, oil or paint in the groove, inadequate shielding, or a leak in the gas line. Lack of fusion points to too little heat for the joint: travel speed too high, current too low, wrong torch angle, or the bead placed outside the prepared groove. On a long seam, a defect that repeats at a constant pitch usually indicates a machine or consumable problem rather than random contamination.

How is longitudinal weld joint efficiency different from girth weld efficiency?

Codes such as ASME B31.3 assign a longitudinal weld joint quality factor (Ej) to a pipe seam based on how it was made and how thoroughly it was examined; seamless or fully radiographed longitudinal welds can qualify for higher factors than an electric resistance welded seam examined only by other means. Girth welds are generally not derated in the same way because they are made to the same procedure and examined in the field with the same controls. The factor directly affects the allowable stress or required wall thickness.

What safety rules matter most when welding a long seam inside a pipe or tank?

Work inside a pipe or vessel is confined space work under 29 CFR 1910.146 and needs a permit, monitoring and a rescue plan. Ventilation must keep fume below the limits in 1910.1000, and hexavalent chromium from stainless steel has a permissible exposure limit of 5 micrograms per cubic metre as an 8-hour time-weighted average under 29 CFR 1910.1026. Hot metal must be marked after welding to warn others, and fire watch arrangements must follow NFPA 51B as referenced in OSHA’s welding subpart.