Welding Metallurgy, Processes and Discontinuities

Learning outcomes

The learner should be able to distinguish fusion and solid-state joining, connect heat input and joint geometry to microstructure, classify weld discontinuities by formation mechanism, recognise delayed cracking and plan NDT around orientation and timing.

11.1 Process and joint foundations

Arc processes such as shielded metal arc, gas metal arc, flux-cored arc, gas tungsten arc and submerged arc welding create a molten pool using different electrodes, shielding and deposition rates. Resistance, laser and electron-beam welding deliver heat differently. Brazing and soldering use a lower-melting filler without melting the parent metal. Friction, diffusion and ultrasonic joining are solid-state processes.

Joint preparation controls access, penetration, root support and contraction. Butt, fillet, T, lap and nozzle joints create different stress and inspection geometry. Welding procedure qualification establishes a controlled range of variables; the NDT technician should verify the relevant weld map, process, thickness, material and post-weld condition.

Use joint terminology consistently. The weld face or cap is the exposed deposited surface; toes are the transitions between face and parent material; the root is opposite the face at the joint origin; fusion faces bound the prepared joint; weld metal is the fused deposit and melted parent contribution; and the heat-affected zone is unmelted parent material changed by the thermal cycle. Fillet weld size may be described by leg length and design or actual throat. These dimensions must come from the governing drawing or fabrication standard rather than a sketch measured by eye.

11.2 Heat input and thermal cycle

For a simple arc-welding estimate:

H = eta V I / v

where H is effective heat input per unit length, eta is process efficiency, V voltage, I current and v travel speed. Units must be consistent. Many welding documents use a stated formula with unit-conversion factors, and that controlled formula governs.

Higher heat input generally produces a wider thermal field and slower cooling, but process, preheat, thickness and interpass conditions also matter. The fusion zone solidifies directionally. The heat-affected zone can contain coarse-grained, transformed, intercritical, tempered or softened regions. Residual stress and distortion result from restrained non-uniform contraction.

11.3 Formation-based discontinuity groups

• Fusion and penetration: incomplete fusion at sidewall, inter-run or root; incomplete penetration at the joint root.
• Cracking: solidification cracking, liquation cracking, hydrogen-assisted cold cracking, reheat cracking, lamellar tearing and crater cracking.
• Cavities: gas porosity, wormholes and shrinkage cavities.
• Solid inclusions: slag, flux, tungsten or oxide.
• Profile and dimension: undercut, overlap, excessive reinforcement, root concavity, burn-through, misalignment and incorrect weld size.

Hydrogen-assisted cracking requires susceptible microstructure, diffusible hydrogen and tensile stress. It may appear after welding, so the governing document may impose delayed inspection. Lamellar tearing occurs in parent plate through-thickness under shrinkage strain and is associated with elongated inclusions. Solidification cracking is usually located in the weld metal and forms while the last liquid films are weak.

11.4 NDT strategy and interpretation

VT establishes profile, access and surface conditions. PT or MT addresses surface-breaking cracking, with MT limited to ferromagnetic material. UT and PAUT are strong for favourably oriented planar fusion conditions but require scan planning around cap, root and geometry. RT is useful for many volumetric and profile conditions, but tight planar lack of fusion may be missed if the beam is poorly aligned.

An indication at the root is not automatically incomplete penetration; root geometry, backing, mismatch and mode conversion can produce responses. Interpretation uses location, orientation, movement, complementary scans and construction evidence.

Practical checkpoint

On a weld cross-section, mark fusion zone, heat-affected subregions, root, toes and parent plate. For eight discontinuity types, state mechanism, likely orientation, when it forms, preferred NDT and a confusing geometric response. Include any required inspection delay.

Common errors and misconceptions

Several common errors recur in this topic: calling every root response a crack; ignoring delayed inspection for hydrogen cracking; assuming high heat input is always beneficial or always harmful; using radiography as proof that every planar fusion face is sound; and evaluating an indication before confirming which acceptance document applies.

Chapter summary

Welding combines process variables, joint geometry, rapid solidification and a local thermal cycle. Discontinuities should be understood by mechanism and orientation. NDT timing, access and method must address both weld metal and heat-affected parent material.