Welding Engineering
Part analysis, sample trials, WPS/WPQR and fixture design by our IWE team.
We Don't Sell Robots. We Deliver Welding Engineering.
Robentex is not an integrator assembling standard robot arms. With our International Welding Engineers (IWE), every decision from part analysis to automation design is driven by welding engineering.
Six Disciplines, One Responsibility
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Part Analysis
CAD data, material and joint accessibility are reviewed; automation feasibility and cycle targets are reported in engineering terms.
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Sample Welding Trials
Before quotation, trial welds are run on the customer's actual parts; penetration and bead geometry are physically verified.
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Process Selection
The right process (MIG/MAG, TIG, SAW or tandem) is selected for material and section; wire, gas and parameter windows are defined.
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WPS & WPQR
Welding procedure specifications are prepared and qualified; production runs on auditable, documented procedures.
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Fixture Design
Part-specific pneumatic/hydraulic fixtures are designed; the clamping strategy addresses distortion and repeatability together.
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Automation Design
Robots, axes, sensors and safety architecture are integrated as one system; the cell is built around the welding-engineering decisions.
Welding Engineering Is Where Four Fields Meet
Whether a weld is acceptable can only be answered when all four fields are considered together.
Arc Physics & Heat Transfer
How much energy the arc transfers to the workpiece, how it spreads, and how fast the weld cools.
Physical Metallurgy
What the thermal cycle does to the microstructure: grain size, hardness, toughness and cracking tendency.
Mechanical & Structural Design
Joint geometry, weld size, residual stress and the effect of distortion on the structure.
Quality Management & Inspection
Procedure qualification, personnel competence, the inspection plan and defined acceptance criteria.
Weldability Is Not a Fixed Property of a Material
Weldability is defined together with the process, the joint geometry and the service condition. Its three components must be asked separately.
Metallurgical
- Microstructure after the thermal cycle
- Behaviour against cracking and embrittlement
- Retention of corrosion resistance
Operational
- Acceptable bead geometry and penetration
- Wetting and slag detachment
- Positional capability and torch access
Service
- Behaviour at operating temperature
- Performance under fatigue and impact loading
- Life under corrosive conditions
The practical conclusion: no material is simply "weldable". It is weldable with a given WPS, at a given thickness, for a given service condition.
Heat Input: One Number Decides the Weld
Heat input is the energy transferred per unit weld length: Q = (U × I × k) / (v × 1000) kJ/mm. Alongside voltage, current and travel speed, the thermal efficiency factor (k) of the process enters the calculation, because not all electrical power reaches the workpiece. As heat input rises, cooling slows, the heat-affected zone widens and grains coarsen; as it falls, cooling accelerates and hard, crack-prone structures form. In steels the industrial indicator is the t8/5 time: how long the weld zone takes to cool from 800 °C to 500 °C. That range is where austenite transforms into ferrite, bainite or martensite, so t8/5 directly determines the final microstructure. The acceptable window narrows with the material: for high-strength fine-grained steels and duplex stainless steels, heat input is specified with both a lower and an upper limit.
Thermal Efficiency Factor by Process (EN 1011-1)
| Submerged arc welding (12) | k = 1.0 |
|---|---|
| Covered electrode (111), MIG/MAG (13x), flux-cored (13x/14x) | k = 0.8 |
| TIG / GTAW (14x) | k = 0.6 |
| Plasma arc welding (15) | k = 0.6 |
Sub-Zones of the Heat-Affected Zone (HAZ)
| Coarse-grained zone (CGHAZ), ≈1100 °C to melting | Austenite grains coarsen, toughness drops. The most critical zone for crack initiation. |
|---|---|
| Fine-grained zone (FGHAZ), ≈Ac3–1100 °C | Recrystallisation; usually the toughest region of the joint. |
| Intercritical zone (ICHAZ), Ac1–Ac3 | Mixed structure; local hard islands (M-A phase) may form. |
| Subcritical zone (SCHAZ), ≈500 °C–Ac1 | Softening; strength loss in quenched-tempered and thermomechanical steels. |
Cracking Mechanisms and Their Prevention
Each weld crack rests on different physics, so each has a different remedy.
Hydrogen (cold) cracking
- Four conditions must coincide: hydrogen, hard microstructure, tensile stress, temperature below ~200 °C
- Removing any one of them prevents the crack
- Drying and correct storage of consumables
- Removal of oil, paint, rust and moisture from surfaces
- Extending t8/5 through preheating
- It can appear hours after welding; inspection is repeated after the specified waiting time
Hot (solidification) cracking
- At the end of solidification the liquid film at grain boundaries cannot carry the stress
- Keeping sulphur and phosphorus content low
- Controlling the depth-to-width ratio of the bead
- Targeting some delta ferrite in austenitic stainless welds (FN)
- Avoiding excessive travel speed; centreline segregation
Lamellar tearing & reheat cracking
- Through-thickness (Z direction) restraint stress in thick plate
- Use of Z-quality material (EN 10164)
- Redesigning the joint to move load off the Z direction
- Buttering layer application
- Heat input that limits the coarse-grained zone in Cr-Mo-V steels
Welding is a "special process" whose result cannot be fully verified by inspection alone; quality therefore comes from controlling the process, not the product.
Arc Welding Processes and Automation Fit (EN ISO 4063)
| MIG/MAG (GMAW), 131 / 135 | High deposition, very high automation fit. The core process of general fabrication and robotic welding. |
|---|---|
| Flux-cored (FCAW), 136 / 138 | Very high deposition. Thick sections and site welding. |
| TIG (GTAW), 141 | Low deposition, high quality. Root passes, thin sections, stainless steel. |
| Submerged arc (SAW), 121 | Highest deposition. Long straight seams, heavy plate, pipe; high automation on fixed lines. |
| Covered electrode (SMAW), 111 | Not suited to automation. Site work, repair and hard-to-reach areas. |
| Laser / laser-hybrid; 52 / 522 | Narrow, high power density; low distortion, high speed. |
Metal Transfer Mode Drives Both Quality and Cycle Time
How filler metal moves from wire to pool is the variable that governs bead geometry, spatter and positional capability.
Short-Circuit (Dip)
At low current and voltage the wire touches the pool, short-circuits and the droplet detaches. Low heat input: thin sheet and positional welding. Waveform-controlled machines cut spatter sharply.
Globular
Droplets larger than the wire diameter fall under gravity. Unstable and spattery; generally an avoided operating range.
Spray
Above the transition current, droplets are projected axially. High deposition and a smooth bead; for flat positions and thick sections. Requires argon-rich gas.
Pulsed Spray
Current oscillates between background and peak; one droplet per pulse. Average current stays low: positional welding, thin sections, aluminium and stainless.
Distortion Is Prevented at the Design Stage, Not on the Shop Floor
The most effective tool for distortion control is reducing the volume of deposited metal. Post-weld correction does not replace prevention; it completes it.
Design decisions
- Reducing groove angle; narrow-gap preparation
- Double-sided symmetrical grooves
- Avoiding oversized fillet welds; doubling throat thickness roughly quadruples volume
- Moving welds away from stress concentration zones
Process and sequence
- Symmetrical, balanced pass sequence
- Back-step technique
- Rigid fixtures and pre-setting
- Moving to high energy density processes; narrow HAZ, less distortion
Post-weld
- Stress-relief heat treatment
- Mechanical straightening
- Anticipating distortion released during machining
Residual stresses in the weld metal and adjacent HAZ can approach the yield strength of the material; they emerge as distortion when the part is released.
How a WPS Is Qualified
Welding is a process whose result cannot be fully verified by non-destructive testing. Assurance therefore comes from documented qualification of the procedure.
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Preparation
pWPS
Preliminary welding procedure specification: the parameter set foreseen by the engineer is prepared.
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Verification
Qualification Test
A test coupon is welded to EN ISO 15614-1 and subjected to tensile, bend, impact and macro/hardness testing.
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Documentation
WPQR
Welding procedure qualification record: results are documented and ranges of validity (thickness, diameter, material group, position, heat input) are defined.
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Production
WPS
The final specification is issued in EN ISO 15609-1 format and production runs on it.
Weld Quality Levels (EN ISO 5817)
| B; Stringent | Critical structures under fatigue loading, pressure equipment, dynamic loads. |
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| C; Intermediate | General load-bearing steelwork and machine building. |
| D; Moderate | Lightly stressed, statically loaded secondary members. |
| Selection rule | A quality level is a design requirement, not a "grade of quality". Specifying B where it is not needed raises cost and rejection rates; using D where it is not appropriate is a safety risk. |
Non-Destructive Testing Methods (EN ISO 17635)
| Visual testing (VT) | Surface geometry, cracks, undercut, weld size. The first step of every inspection; sees the surface only. |
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| Penetrant testing (PT) | Surface-breaking discontinuities, including on non-magnetic materials. Unreliable on porous or rough surfaces. |
| Magnetic particle (MT) | Surface and near-surface defects. Ferromagnetic materials only. |
| Radiography (RT) | Volumetric defects: porosity, slag. May miss planar cracks perpendicular to the beam; requires radiation safety. |
| Ultrasonic (UT) | Planar internal defects, lack of fusion, cracks. Operator-dependent; difficult in thin sections. |
| Phased array (PAUT) / TOFD | Defect position, size and height with recorded data. High equipment and qualification requirements. |
Most of the Cost Is Decided Before a Single Arc Is Struck
The cost and quality of welded fabrication are settled at the design stage. The principles below reduce both defect risk and cost.
Weld volume
- Minimise the required weld volume
- Reduce the groove angle or use narrow-gap preparation
- Do not oversize fillet welds
- Base joint preparation on the standard (EN ISO 9692)
Robot access
- Model the required torch approach angle during design
- Leave free volume around the torch
- An unreachable weld means going back to manual welding
- Keep tolerances within the acceptance range of the WPS
Structural behaviour
- Move welds away from stress concentrations and section changes
- Under fatigue loading, prioritise smooth weld toes
- Do not place welds too close together; avoid overlapping HAZs
- Design for inspection access: a critical weld that cannot be tested invalidates the plan
Robotic Welding Does Not Change the Rules; It Makes Them Repeatable
The variables a welder compensates for on the fly (gap variation, part tolerance, distortion) must be anticipated and engineered out in a robotic cell.
Arc-On Time
The real productivity measure of a cell is not travel speed but the percentage of the cycle during which the arc is actually burning. A two-station positioner (welding on one side, loading on the other) raises it markedly and is usually the largest single item in the payback calculation.
Positioner and Access
The flat position (PA) allows the highest deposition rate and the best pool control. A positioner is not a convenience but a productivity and quality device; coordinated motion keeps the part in the ideal position throughout the seam.
Sensing and Adaptive Control
Through-arc seam tracking needs no extra hardware. Laser triangulation measures not only positional deviation but also gap volume, enabling adaptive fill. Touch sensing is slow but cheap and reliable.
Calibration and Accuracy
Repeatability (returning to the same point) is usually very high; absolute accuracy (reaching the point defined in CAD) is lower. Teach programming needs only repeatability; offline programming depends on absolute accuracy, so calibration is essential.
Engineering Questions
That is answered by reviewing CAD data, material, thickness, joint geometry and torch access together. Before quotation we run trial welds on your actual parts, physically verifying penetration and bead geometry. A weld the torch cannot reach is the limit of automation; and saying so upfront is the honest answer.
Heat input determines cooling rate, cooling rate determines microstructure, and microstructure determines the toughness and cracking behaviour of the weld. Of two welds that look identical, one may carry the service load and the other may not; the difference is usually invisible and hides in the heat input window.
You could, but it costs. In EN ISO 5817, B is the most stringent level and is intended for critical structures under fatigue loading. Specifying B where it is not required increases inspection scope, repair rates and cost. The correct approach is to derive the quality level from the design requirement.
Preheating is not about warming the surface; it extends the t8/5 time to limit hard martensite formation and gives hydrogen time to diffuse out of the weld zone. A common mistake is heating only the groove locally; standards require heating over a defined width around the joint and through the full section.
In small-batch, high-variety production OLP is the main lever for cutting robot downtime. Its accuracy, however, depends on calibrating the cell to its real geometry: without robot and positioner kinematic calibration and TCP calibration, a path that looks correct in simulation drifts on the shop floor.
For manual and partly mechanised welding, welder qualification (EN ISO 9606-1) applies. For fully mechanised and automatic welding, the relevant qualification is welding operator and setter under EN ISO 14732; that examination covers correct setup, parameter setting and process control rather than manual welding skill.
Send Your Part, Let's Start with a Test
Submit your request for trial welding and a feasibility assessment; our engineering team will get in touch to review your part.