Chapter 6 of 10 · 10 min read
Filling and capping the joint
Count the passes a pipe joint needs, compare processes by arc minutes per joint, control interpass cleaning, starts and heat, and lay a cap of the right profile without paying for excess weld metal.
Once the root and hot pass are in, the rest of the joint is filled with weld metal pass by pass until it stands just proud of the pipe surface, and then capped. Filling is less dramatic than the root, but it is where most of the weld metal and much of the time goes, and where slag inclusions, lack of side fusion and porosity are born. The cap is what the inspector sees first and what the client judges the work by. A joint with a beautiful root can still be rejected for a slag line in the fill or undercut at the cap.
This chapter counts the passes a joint needs from its groove area, compares processes by the arc minutes and paid time they take per joint, covers the cleaning, sequencing and heat control between passes, and sets out what a good cap looks like and why a high one costs money.
How many passes
The number of passes depends on the area of the groove and the area each pass fills, which depends on the process, the electrode or wire size, and the position. In the overhead and vertical parts of a fixed pipe, passes are smaller than in the flat position, because the pool must be kept small enough not to fall out.
- Total weld area
- about 62 mm²
- Area per pass
- about 16 mm² on average
- passes ≈ 62 ÷ 16 ≈ 3.9, so about four passes
- in practice: root, hot pass, one fill pass and a cap, the cap often laid as two beads side by side on wider grooves
- on DN 150 Schedule 80, about 124 mm², the same passes give about eight
The count matters because every pass brings its own start, stop, cleaning and inspection, and each is a chance for a defect. A joint with twice as many passes takes more than twice as long, because the cleaning and starts multiply, and has more places for slag to hide. On heavy walls, larger electrodes or higher-deposition processes for the fill reduce the number of passes; narrow-groove preparations, as Chapter 3 noted, reduce the area to be filled.
Arc minutes per joint
The arc time follows from the weld metal and the deposition rate of each process in the positions used.
The gas tungsten arc root makes the joint slower in arc time, because the tungsten arc deposits filler slowly, but it gives the best root, and on stainless or quality lines that is what the procedure asks for. Wire processes for the root and fill halve the arc time compared with covered electrodes. In the shop, with the pipe rolling under a mechanised head, the arc time falls further, because each pass can be laid in the flat position at a high current without stopping.
From arc time to paid time
Arc time is not paid time. Between and around the arc, the welder cleans each pass, changes electrodes, grinds starts and stops, moves round the pipe, checks the work, and waits for the joint to cool if the interpass temperature is limited. The operator factor, the share of paid time with the arc burning, is low in pipe welding, often a quarter or less in the field, because of all these interruptions. A joint with about 14 minutes of arc time at a 25% operator factor takes about 56 minutes of welding time, before the fit-up of Chapter 3 is added. That is how the hours per joint of Chapter 4 are built up, and Arc Welding by the Numbers covers operator factor in general.
Consumables per joint
The consumable bought is more than the metal deposited, because electrodes leave stubs and lose their coatings as slag, wires lose flux, and spatter and grinding waste some of everything. The ratio of weld metal deposited to consumable used, the deposition efficiency, is lowest for covered electrodes, often around 60% once stubs are included, and highest for solid wires. At 60%, the DN 150 joint’s 0.25 kg of weld metal needs about 0.42 kg of electrodes. The dairy project’s 640 carbon steel joints of about that size would need about 265 kg of electrodes if all were welded that way, which is the figure the store orders from, plus a margin for repairs and waste.
Cleaning between passes
Slag left on a pass is trapped by the next one and becomes an inclusion, a defect radiography finds easily. Covered electrode and flux-cored passes are cleaned thoroughly between passes, chipped and wire brushed, and the edges of each pass, where slag lodges against the groove wall, are ground if the bead has left a sharp valley. The worst place for slag is at the toe of the hot pass against the bevel, and a light grind there before the fill pays for itself many times. Gas-shielded wire and tungsten arc passes leave little or no slag but can leave a thin oxide film or silicon islands, which are brushed off.
Starts and stops
Each start and stop of a pass is a potential defect: a start can be cold, lacking fusion for the first few millimetres, and a stop leaves a crater that can crack or hold porosity. On a pipe joint, each pass may have two or more stops, where the welder has repositioned or changed electrodes. Good practice grinds each stop to a taper, restarts slightly ahead of it and runs back over it, and staggers the starts and stops of successive passes so that they do not stack up in one place through the thickness. On fixed pipe, the welder usually starts each half of the joint at the bottom, overlapping the start of the other half, and finishes at the top, tying in the two halves.
Heat and interpass temperature
Each pass adds heat to the joint. On small, light-walled pipe, the joint can become very hot after a few passes, which coarsens the grain, lowers toughness, and on stainless steels can harm corrosion resistance and increase distortion. Procedures set a maximum interpass temperature, the temperature of the joint when the next pass starts, and the welder checks it with a temperature-indicating crayon or a contact thermometer. On heavier and hardenable pipe, there may also be a minimum temperature, the preheat, which the joint must not fall below between passes (Chapter 8). The welder working within both limits is the welder working at the pace the steel allows.
Weaving and stringers
Passes can be laid as narrow stringer beads, with little side-to-side movement, or as weaves, with the electrode or torch moved across the groove. Weaving fills a wide groove in fewer passes but puts more heat into each one and can trap slag at the sides if the weave is too wide. Stringers give better control of heat and fusion, and are usual in the overhead part of fixed pipe and on stainless. The procedure may limit weave width, and on impact-tested pipe it usually does, because heat input affects toughness.
Two welders on one joint
On large pipe, two welders often work a joint at the same time, on opposite sides, each welding a half. That halves the time to complete the joint and balances the heat and shrinkage round the circumference, so that the pipe does not pull out of line. The two must coordinate their starts and stops, and their passes, so that the joint is filled evenly, and both must be qualified for the procedure. On pipelines, the crews that follow each other along the line each lay one pass, which is the same idea extended along a whole string of joints.
The cap
The cap is the final pass or set of passes. It should fill the groove completely, with no part of the joint below the pipe surface, overlap each bevel edge by a little, blend smoothly into the pipe without undercut at its toes, and stand only slightly proud. Codes set a maximum reinforcement height, which depends on the wall thickness, because a high cap concentrates stress at its toes and is no stronger for its extra metal.
- Avoid: Groove filledno part of the joint below the pipe surface anywhere round the circumference
- Avoid: Widthcap overlaps each bevel edge evenly; no unfused bevel visible
- Avoid: Reinforcementwithin the procedure's height limit; smooth, not peaked
- Avoid: Toesblended into the pipe; no undercut beyond the limit; no overlap
- Avoid: Surfaceno cracks, porosity, slag or arc strikes
- Avoid: Starts and stopstied in smoothly; no craters left unfilled
- Do: ? Identificationwelder's mark and joint number applied as the weld log requires
A cap built up high wastes weld metal and time and, where the code limits reinforcement or the client wants a smooth finish, has to be ground back. On the DN 150 joint, the cap and root reinforcement are about a quarter of the joint’s weld metal; doubling the cap’s height adds roughly another eighth to the weld metal and arc time, and more if it must then be ground.
Arc strikes
An arc struck on the pipe surface outside the groove, by accident or to test the electrode, leaves a small spot of melted and rapidly cooled metal. On carbon steel it can be a hard, brittle spot with microcracks; on alloy and high-strength pipe, a crack starter. Codes generally require arc strikes to be ground out and checked. Welders on pipe are taught to strike only in the groove, and the inspector looks for arc strikes as part of every visual inspection.
Looking after the consumables
Low-hydrogen covered electrodes, used for the fill and cap of many carbon steel and alloy pipe procedures, absorb moisture from the air, and damp electrodes put hydrogen into the weld, which can cause cracking in hardenable steels and porosity in any. They are stored in sealed packs or heated ovens and issued to welders in heated quivers, and electrodes left out for longer than the maker allows are redried or discarded as the maker directs. Flux-cored wires and fluxes have their own storage needs, and tungsten arc filler rods must be kept clean and identified, because a stainless rod of the wrong grade looks exactly like the right one. The contractor’s store keeps a log of oven temperatures and issues, so that the consumables in every joint are known to be in the condition the procedure assumed.
The running case: the steam line welds
This is an illustration. The contractor’s procedure for the dairy’s carbon steel steam lines was a gas tungsten arc root and hot pass, followed by covered electrode fill and cap, uphill, with a maximum interpass temperature set from the procedure qualification. In the shop, larger spools were filled with flux-cored wire on positioners under a second procedure. The supervisor tracked the arc time and paid time per joint for the first two weeks and found field joints taking about 2.4 hours against the estimate’s 2.2, mostly because of long cleaning and grinding of the fill passes. A change to smaller-diameter electrodes in the overhead position, which gave cleaner, flatter beads, brought the time back to the estimate and reduced the slag indications on radiographs.
Cap profile and inspection
A smooth, regular cap is not only a matter of appearance. Radiographs and ultrasonic tests are easier to interpret when the cap is even, because irregular caps produce confusing images and signals, and the inspector may then ask for the cap to be ground before testing. Visual inspection of the cap is the first and cheapest examination every weld receives, and a cap with uniform ripples and width tells an experienced inspector that the welder had control of the joint.
A failure: the slag line
This is an illustration. On an earlier job, a welder was filling DN 200 joints with covered electrodes in the fixed position and, under time pressure, was cleaning only the top of each pass, leaving slag in the valleys at the bevel walls. The caps looked excellent. Radiography of the first sample showed long slag lines along both sides of the joints, and the sampling rate was increased; eleven joints had to be repaired by grinding out the fill and rewelding.
The repairs cost far more than the minutes saved on cleaning, and because the code increased the sampling after the failures, the extra radiography cost as much again. The contractor now has supervisors watch interpass cleaning on new welders’ first joints, and grinds the bevel toes after the hot pass as a standard step.
Exercise: passes and arc time for a heavy wall
A DN 150 Schedule 80 joint, with a 10.97 mm wall, has a total weld area of about 124 mm² and needs about 0.48 kg of weld metal. If the average pass fills about 16 mm², how many passes are there? At a position-averaged deposition rate of 1.1 kg/h and an operator factor of 25%, what are the arc time and the welding time? How many kilograms of electrodes are needed at 60% deposition efficiency?
The joint needs about 124 ÷ 16 ≈ 8 passes. The arc time is 0.48 ÷ 1.1 ≈ 0.44 hours, about 26 minutes, and at 25% operator factor the welding time is about 0.44 ÷ 0.25 ≈ 1.75 hours, nearly twice that of the Schedule 40 joint, before fit-up. The electrodes needed are about 0.48 ÷ 0.6 ≈ 0.8 kg per joint.
The heavier wall nearly doubles the weld metal, the passes, the time and the consumables of a joint with the same diameter. The estimator who counts joints without checking the schedule underestimates such a line badly.
