Macro sections and visual inspection

Two of the most informative weld examinations need no machine more complicated than a saw, a polisher and a good light. A macro section, a slice through the weld that is polished and etched, shows the inside of the weld at a glance: how deep it penetrated, whether it fused into both sides, how many runs it took, how wide the heat-affected zone is, and whether there are pores, slag or cracks. Visual inspection, done properly, examines every production weld for the imperfections that show on the surface, which are many of the ones that matter. One looks inside a sample; the other looks at the outside of everything. Between them they catch a large share of the problems that matter, at a small fraction of the cost of the more elaborate tests.

This chapter reads the macro section from the hopper procedure qualification, then sets out how the fabricator inspects every production weld visually, what it finds, and what visual inspection cannot find.

The macro section

A macro section is cut across the weld, usually from the qualification plate, ground flat, polished and etched with a mild acid that attacks the different zones of the weld at different rates. Under good light and low magnification, typically up to about ten times, the etched surface shows:

• the weld metal, often with the boundaries between individual runs visible;
• the fusion line, where weld metal meets parent metal;
• the heat-affected zone, a band of altered parent metal beside the fusion line;
• the parent metal, with its rolling structure;
• any imperfections: lack of fusion, incomplete penetration, porosity, slag inclusions, cracks and undercut.

ISO 17639 sets out macroscopic and microscopic examination of welds. Macro examination is quick and cheap, and it is one of the most persuasive pieces of evidence in a qualification report, because anyone can see what the weld looks like inside.

The hopper procedure’s macro

This is an illustration. The macro section from the hopper procedure qualification plate showed:

Figure 1Macro examination of the hopper procedure qualification plate

• Avoid: Penetrationfull penetration through the 12 mm plate; root fused on both sides
• Avoid: Fusionfusion along both bevel faces and between all runs
• Avoid: Runsseven runs: root, hot pass, four fill runs and a two-run cap
• Avoid: Heat-affected zoneabout 2.5 to 3 mm wide each side, widest near the cap
• Avoid: Imperfectionsone pore about 0.4 mm diameter in the cap; nothing else visible
• Do: ? Root profileslight root concavity, about 0.3 mm, to be assessed against the criteria

Each observation adds to the evidence from the other tests. Full penetration and fusion along both bevel faces confirm what the parent-metal tensile breaks and clean root bends suggested. The run count and sequence confirm that the procedure was welded as written. The heat-affected zone’s width helps place hardness indentations and impact notches (Chapters 6 and 7). And the single small pore in the cap matches the small opening found in face bend B3 in Chapter 5, a reminder that the cap’s shielding is worth watching.

Measuring on a macro

Because the macro is a true cross-section, dimensions can be measured on it with a scale or a calibrated image. For a fillet weld, the leg lengths, the actual throat and the depth of penetration can all be measured, which is how the profile factor of 1.1 used in Arc Welding by the Numbers was confirmed. For a butt weld, the cap reinforcement, root protrusion or concavity, misalignment between the plates and the size of any imperfection can be measured.

This is an illustration. A macro of a production fillet weld attaching a hopper stiffener showed legs of 6.4 and 6.1 mm and a measured throat of 4.5 mm, against a specified 6 mm fillet with a design throat of 4.24 mm. The weld was adequate, with a throat comfortably above the design value, and very slightly oversize, consistent with the weekly weld-size measurements.

Macros of production joints

In some industries, production joints themselves are occasionally cut up and sectioned, a deliberate sacrifice of a part to check that production welds match the qualification plate. Car body makers section spot and arc welds from sample bodies; pipeline contractors cut out sample girth welds early in a project; and fabricators of repetitive welded parts may section one part from a batch. The hopper fabricator does not section production hoppers, but its monthly production test plates, welded alongside the hoppers with the same procedure, welders and consumables, serve the same purpose: their macros check what the production welds probably look like inside, as the failure below shows.

Quality levels for imperfections

Whether an imperfection is acceptable depends on its type, its size relative to the plate thickness or weld size, and the quality level required. ISO 5817, adopted in Australia as AS/NZS ISO 5817, defines quality levels for imperfections in fusion-welded joints in steel and some other metals, designated B, C and D, with B the most demanding. The application standard or the customer’s specification chooses the level, often by the consequence of failure and the type of loading. For structural steel welding in Australia, the AS/NZS 1554 series sets its own weld categories and acceptance criteria. Those criteria must be read and applied as written; this book does not reproduce them.

Visual inspection

Visual inspection is the most widely used weld examination, the cheapest, and the most underrated. ISO 17637 describes the visual testing of fusion-welded joints, including the conditions under which it is done. Properly done, it is not a glance at a finished weld but a structured examination at three stages:

1. Before welding: joint preparation, bevel angles, root gap and face, alignment, cleanliness, tack welds and, where needed, preheat.
2. During welding: run sequence, interpass cleaning, interpass temperature, and the root and each run before it is covered.
3. After welding: the finished weld’s size, profile and surface imperfections, measured with gauges where needed.

Inspection before and during welding prevents defects; inspection after welding finds those that got through. Many of the defects that destructive and non-destructive tests find later could have been prevented by a check before or during welding, such as the misaligned root that failed the bend test in Chapter 5.

Conditions for good visual inspection

Visual inspection is only as good as the conditions it is done in. The weld must be clean, with slag, spatter and heavy scale removed, because slag hides undercut and cracks. The light must be good enough and at an angle that shows the surface’s relief; a torch held low to the surface throws undercut and overlap into shadow where flat overhead light hides them. The inspector needs access close enough to see, which on a hopper means working inside it as well as outside, with mirrors for places the eye cannot reach directly and a magnifier for suspected cracks. ISO 17637 addresses the conditions under which visual testing is carried out.

The inspector matters as much as the conditions. Visual inspection is a skill: knowing what each imperfection looks like, where it is likely to be, and how to measure it with the right gauge. Inspectors are trained and, for many applications, certified for the work, and they check their own eyesight periodically. Independence matters too: an inspector who is also responsible for the welding programme’s schedule faces a conflict when a weld needs repair. The hopper fabricator’s inspector reports to its quality manager, not to the production supervisor.

Photographs help. A photograph of each recorded indication, with a scale, before and after repair, turns a written record into evidence that a customer’s auditor can check, and it helps train welders by showing them what their own imperfections look like.

Microscopic examination

Where the macro shows something that needs explaining, a micro section takes the examination further. The same polished surface, etched more carefully and examined under a microscope at magnifications from tens to hundreds of times, reveals the microstructure: the grain size in the heat-affected zone, whether hard martensite has formed, how a crack has run through the grains, and whether an inclusion is slag or oxide. Micro examination is mainly an investigative tool, used when a weld has failed a test or in service, or when a new steel or procedure is being developed, rather than a routine qualification test.

What visual inspection finds

Visual inspection finds surface-breaking and profile imperfections:

• undercut, a groove melted into the parent metal at the weld toe;
• overlap, weld metal lying on the parent surface without fusing;
• surface porosity and craters, especially at stops and starts;
• surface cracks, including crater cracks;
• incomplete fill and excessive reinforcement or convexity;
• misalignment between the joined parts;
• spatter and arc strikes, the small burns where an arc was struck outside the joint;
• incorrect weld size, measured with a fillet or weld gauge.

These matter more than their small size suggests. Undercut, overlap and arc strikes are stress concentrations where fatigue cracks start, and surface cracks can grow. A structure under repeated loading is far more sensitive to them than one under steady load, and a hopper that is filled and emptied many times a day sees exactly that kind of loading.

The hopper fabricator’s visual inspection

This is an illustration. Each hopper has about 24 m of butt weld in its shell and about 40 m of stiffener fillet welds. The fabricator’s inspector examines every weld after welding, with a torch and gauges, and records each indication by type, location and size. Across the first ten hoppers, 240 m of butt weld, the inspector recorded:

Figure 2Visual indications on 240 m of hopper butt weld, first ten hoppers

Undercut at cap toe12 number of indications

Surface porosity or crater5 number of indications

Arc strike outside the joint4 number of indications

Excess reinforcement3 number of indications

Crater crack1 number of indications

That is 25 indications in 240 m, about 0.10 per metre, or about one every 10 m of weld. Undercut at the cap toes was nearly half of them. The fabricator traced it to one welder’s cap technique, with too high a current and too steep an electrode angle on the final run; after retraining, the undercut rate on the next ten hoppers fell to two indications in 240 m. The crater crack was ground out and repaired, and the welder’s restart technique was reviewed.

The point of recording indications by type is that the record becomes process information. A list of repaired welds is a cost; a count of indications by type and welder is a guide to where training, settings or procedures need attention.

What visual inspection cannot find

Visual inspection sees only the surface. Lack of fusion inside the joint, porosity and slag below the surface, and cracks that do not break the surface are invisible to it. A weld can look perfect and contain a serious internal defect. For that reason, visual inspection is combined with non-destructive testing for welds whose consequence of failure justifies it (Chapter 9), and with destructive tests on qualification and production test plates.

A failure: the perfect-looking weld

This is an illustration. A production test plate, welded alongside a batch of hoppers to the production procedure and examined visually, was flawless: an even cap, no undercut, no porosity, correct reinforcement. Its tensile specimen broke in the parent above the requirement. Its macro section, though, showed a band of lack of fusion between the third fill run and the bevel face on one side, about 2 mm deep, entirely hidden inside the weld.

The macro had found what neither the visual inspection nor the tensile test could, because the defect lay away from the tensile specimen’s cross-section and below the weld’s surface. The welder’s record showed that the third fill run had been made after a break, on a joint that had cooled below the procedure’s minimum interpass temperature, and with a fast travel speed to make up time. The fabricator reviewed the affected batch: the hopper seams welded by the same welder that shift were ultrasonically tested in full, and two short lengths with lack of fusion were found and repaired. The production procedure’s interpass temperature control was reinforced with temperature crayons at each joint.

Exercise: a defect rate

On three later hoppers, with 72 m of butt weld, the inspector records six visual indications. Calculate the indication rate per metre and the average length of weld between indications. Compare it with the first ten hoppers’ rate of about 0.10 per metre and with the two indications in 240 m after retraining.

The rate is 6 ÷ 72 ≈ 0.083 per metre, one indication about every 12 m. That is a little below the first ten hoppers’ 0.10 per metre but well above the post-retraining rate of 2 ÷ 240 ≈ 0.008 per metre, one every 120 m.

The rise from the post-retraining level is a signal worth investigating: a new welder, a different batch of consumables, a change in position or access, or a lapse in technique. Six indications is a small number, and the rate could be partly chance, but the record tells the fabricator to look at which welds and which welders produced them before the trend continues.