A fabrication shop has good welders. Yet a customer has rejected a batch of platform frames: some fillet welds are undersized, two frames are out of square after welding, and the inspector has found porosity in a weld on a load-bearing bracket. The welders are frustrated, because each of them did what they always do. The problem is that what they always do differs from welder to welder, the drawings did not say clearly what was needed, settings were left to judgement and nobody checked fit-up before welding started.
Welding quality depends on skilled people, but it is produced by a system: the right process for the job, joints designed and specified clearly, documented procedures that set the variables, welders qualified to those procedures, controlled fit-up and distortion, and inspection matched to the risk. Shops that build that system produce consistent welds whoever is on the shift. Shops that rely on individual skill alone produce good welds most of the time and expensive problems some of the time.
This article explains the main arc welding processes and how to choose between them, the basics of joint design and specification, welding procedures and qualifications, distortion and cracking, inspection methods, and practical steps for a fabrication business. It is general information. Structural, pressure and other safety-critical welding must comply with the applicable standards, and design of welded connections should be checked by a qualified engineer.
The main arc welding processes
| Process | Common names | How it works | Typical strengths |
|---|---|---|---|
| Gas metal arc welding (GMAW) | MIG, MAG | Continuously fed solid wire with shielding gas | Fast, productive, easy to learn; most common in fabrication shops |
| Flux-cored arc welding (FCAW) | Flux-core | Tubular wire with flux inside, with or without gas | High deposition, good on thicker sections and outdoors with self-shielded wire |
| Shielded metal arc welding (SMAW) | Stick, MMAW | Flux-coated consumable electrode | Portable, versatile, good for site work and repairs |
| Gas tungsten arc welding (GTAW) | TIG | Non-consumable tungsten electrode with separate filler | Precise and clean; thin material, stainless steel, aluminium and critical root runs |
| Submerged arc welding (SAW) | Sub-arc | Wire fed under a blanket of granular flux | Very high deposition on long, straight, thick welds, usually mechanised |
Other processes, such as laser, plasma and resistance spot welding, have specialised roles. The choice depends on material and thickness, joint access, position, required quality, productivity, site or shop conditions and the skills available.
Consumables matter as much as the process. Filler wires and electrodes are classified by their strength and chemistry, and must suit the parent material. Shielding gases and fluxes protect the molten metal from oxygen and nitrogen, which cause porosity and brittleness. Moisture in electrodes and fluxes introduces hydrogen, a major cause of cracking, which is why some electrodes must be stored and dried under controlled conditions.
Butt welds and fillet welds
Almost all welds are either butt (groove) welds or fillet welds.
- A full-penetration butt weld, made with a filler at least as strong as the plate, by a qualified welder to a correct procedure, can be treated as being as strong as the plate it joins.
- A fillet weld joins parts meeting at an angle, such as a tee or lap. It carries load through its throat, the shortest distance from the root to the face. For a standard equal-leg fillet, the throat is about 0.707 times the leg length, so a 6 mm fillet has a throat of about 4.2 mm. Drawings specify the leg length; calculations use the throat.
Fillet welds under bending or twisting loads need more careful analysis than simple direct loads, because stress varies around the weld group. Engineers often treat the weld as a line to calculate this, then size the weld from the result. Welding both sides of a plate, where access allows, avoids the eccentric load and doubled stress of a single-sided fillet in bending. Intermittent welds often provide enough strength with less heat and distortion than a continuous run.
Design to standard weld sizes, and avoid oversizing. A fillet larger than needed adds weld metal, time, heat and distortion without useful strength.
Specify welds clearly on drawings
Welds should be specified using standard welding symbols, which state the weld type, size, length, pitch for intermittent welds, which side of the joint, any preparation, the finish and any inspection required. A clear general note can cover routine welds, with specific symbols for structural or critical ones. State the applicable standard and the weld category or quality level where the standard defines them.
Ambiguous drawings, such as “weld all round” with no size, push decisions onto the welder and create disputes at inspection.
Procedures and qualifications
A welding procedure specification (WPS) documents how a particular weld is to be made: process, parent material and thickness range, filler and gas, joint preparation, position, preheat, current, voltage, travel speed and other variables. For many applications, the procedure must be qualified: a test weld is made, tested and the results recorded, often called a procedure qualification record.
Welder qualification shows that an individual welder can produce sound welds using a particular process, position and material range, usually by welding test pieces that are examined and tested.
In Australia, structural steel welding is commonly carried out to the AS/NZS 1554 series, which sets requirements for procedures, welder qualifications, workmanship and inspection, and defines weld categories with different quality requirements. Welder qualification is also addressed in standards such as AS/NZS ISO 9606.1, quality management for welding fabricators in the ISO 3834 series, and pressure equipment welding in its own standards. The contract or design will state which apply; check the current editions.
Even where formal qualification is not required, written procedures with set ranges for the key variables make results consistent and give new welders a clear standard.
Distortion and residual stress
Welding heats metal locally, and as the weld cools and shrinks it pulls the parts out of shape and leaves residual stress. Practical controls:
- Minimise weld volume: right-sized welds, intermittent welds where suitable, efficient joint preparations.
- Balance welding around the neutral axis, alternating sides and using back-step or skip sequences.
- Use fixtures and clamps to hold parts, and pre-set parts slightly so shrinkage pulls them into position.
- Plan the sequence, from the centre outwards, joining sub-assemblies before final assembly.
- Check fit-up before welding: gaps and misalignment increase weld volume and distortion.
Cracking and preheat
Cracks are the most serious weld defect. Hydrogen-assisted cold cracking can occur hours after welding in hardenable steels, thick sections and highly restrained joints. Controls include low-hydrogen consumables stored correctly, clean dry joints, preheat where the steel’s composition and thickness require it, and controlled cooling. Preheat requirements are commonly determined from the steel’s carbon equivalent, a combined measure of the alloying elements that make it prone to hardening, together with thickness and the hydrogen level of the process. Welding procedures should specify preheat where needed.
Inspection matched to risk
A layered approach to inspection:
- Visual inspection of every weld: size, profile, undercut, cracks, porosity, spatter and completeness, using weld gauges. Visual inspection also starts before welding, with checks of fit-up, preparation and cleanliness.
- Surface methods for surface-breaking defects: magnetic particle testing on ferromagnetic steels and dye penetrant testing on other materials.
- Volumetric methods for internal soundness: ultrasonic testing or radiographic testing, where the standard, design or customer requires it.
- Leak testing for pressure-containing and sealed assemblies.
- Proof testing where a structure must demonstrate its capacity before use.
The applicable standard sets acceptance criteria for each weld category. Inspectors performing non-destructive testing should be appropriately qualified. Record the results, especially for structural and critical welds. The inspection and test plans for supplier work article covers agreeing inspection points and evidence with customers.
Common weld defects and their usual causes
| Defect | Typical causes |
|---|---|
| Porosity | Contamination, moisture, inadequate gas cover, drafts |
| Lack of fusion | Low heat input, poor technique or access, incorrect angle |
| Incomplete penetration | Wrong preparation, gap or current |
| Undercut | Excessive current or travel speed, poor angle |
| Slag inclusions | Poor cleaning between runs |
| Cracks | Hydrogen, restraint, hardenable steel without preheat, poor filler choice |
| Undersize or oversize welds | Unclear specification, no gauge checks |
Build welding knowledge into the business
Much welding knowledge lives in experienced welders’ hands and heads. Capture it: record the settings and techniques that work for common joints in the procedures, photograph good and bad examples for training, keep test pieces as references, and pair new welders with experienced ones on the joints that matter most. When a defect occurs, record its cause and the fix, so the same lesson is not learned twice. Repeated defects on the same joint usually point to the joint design, the fit-up or the procedure rather than the individual welder, and are worth investigating properly; the zero-defect manufacturing in a small factory article covers prevention rather than inspection as the route to fewer defects.
Health and safety in welding
Welding brings serious hazards: fume, ultraviolet radiation, electric shock, burns, fire and confined spaces. Welding fume is classified by the International Agency for Research on Cancer as carcinogenic to humans, so control of fume through extraction, ventilation and respiratory protection is essential, alongside screens, protective clothing, safe electrical practice and hot work permits. Safe Work Australia and state regulators publish guidance on welding hazards, and the general duty to manage risks so far as is reasonably practicable applies.
A worked example
This is an illustrative example. A 30-person fabricator builds steel access platforms and stairs. A major customer has rejected a batch of frames for undersized fillet welds, out-of-square frames and porosity found on a bracket weld. Weld repairs are consuming about 30 hours a month.
Diagnosis. Drawings use “weld all round” notes without sizes. Each welder sets their own machine. Fit-up is not checked, and some gaps are 4 mm where 1 mm was intended, so welders fill them with extra weld. Gas cylinders are run in a drafty corner of the shop.
Changes.
- Drawings are updated with standard weld symbols, sizes and the applicable standard and weld category.
- Written procedures are prepared for the four most common joints, with set ranges for current, voltage, wire feed, gas flow and travel speed, and qualified where the contract requires.
- Welders are qualified to the procedures they use, and a skills matrix shows who is qualified for what.
- Fit-up checks are introduced, with gap and squareness limits and a fixture for the main frame.
- Welding sequence for frames is standardised to balance shrinkage.
- Inspection is organised in layers: welders check their own welds with fillet gauges, a supervisor visually inspects all welds before painting and critical brackets get magnetic particle testing.
- Screens are installed around the welding bays, improving gas cover and fume extraction at the same time.
Result. After three months, weld repair time falls to about 6 hours a month, frames come off the fixture within squareness limits, and the customer’s next audit finds no weld nonconformances. New welders reach the required standard faster because the procedures and fixtures make the expected result clear.
Applying this in an Australian fabrication business
- Choose processes to suit material, thickness, position, access and productivity.
- Specify welds clearly with standard symbols, sizes and categories.
- Write and qualify procedures for common and critical joints.
- Qualify welders and keep a skills matrix.
- Control fit-up and sequence to limit distortion.
- Manage hydrogen with correct consumable storage and preheat where required.
- Inspect in layers, matched to the weld’s risk and the standard.
- Control fume and other hazards under WHS law.
- Work to AS/NZS 1554 and other applicable standards where specified.
Where welding quality goes wrong
- Unclear weld specifications on drawings.
- Settings left to individual judgement.
- No fit-up checks, so gaps are filled with extra weld.
- Oversized welds adding heat and distortion.
- Damp consumables and drafts across the arc.
- Inspection only at the end, after painting.
- Fume treated as a nuisance rather than a serious health hazard.
Questions to ask about your welding
- Do our drawings specify every structural weld clearly?
- Do we have written procedures for our common joints, and are they followed?
- Which welders are qualified for which procedures?
- How do we control fit-up and distortion?
- What inspection does each weld receive, and is it matched to the risk?
- How well do we control welding fume?
Bringing it together
Consistent welding comes from a system around skilled people. Choose the right process for each job, design joints to standard sizes and specify them clearly, and set the welding variables in written procedures that are qualified where required. Qualify welders, control fit-up and sequence to limit distortion, manage hydrogen to prevent cracking and inspect in layers matched to the risk and the applicable standard. Protect people from fume and other welding hazards. Shops that build this system produce welds that pass inspection first time, regardless of who is on the shift.
Source: KEVOS editorial notes, drawing on earlier KEVOS engineering handbooks on welding processes, consumables and parameter control, welded joint design and failure prevention, and material testing and verification of weld quality. The worked example is illustrative. This article is general information; follow the applicable welding standards and have welded connections designed by qualified engineers.
