Materials, Fabrication and Product Technology
What you must know
Questions in this domain link how a part is made to which discontinuities are probable, their orientation, and therefore which method and direction can find them. Expect: crystal structure and slip, grain size, alloying and hardenability, strength vs toughness vs hardness, heat treatment (anneal, normalise, quench-and-temper, solution treat and age), and the characteristic flaws of each process:
- Casting – shrinkage (often irregular and intergranular), gas porosity (rounded), hot tears, cold shuts, sand/inclusion entrapment, coarse columnar grain.
- Rolling – laminations, seams, laps; inclusions flattened into planes parallel to the surface.
- Forging – laps, bursts, flakes, die mismatch, fibre direction.
- Welding – porosity, slag, incomplete fusion, incomplete penetration, undercut, hot (solidification) cracking, cold/hydrogen cracking, reheat cracking, lamellar tearing.
Traps
- Cracks formed during or immediately after solidification are hot cracks, metallurgical and shrinkage driven; cracking that appears hours later in a hard, restrained joint containing hydrogen is cold cracking. A stem describing delayed cracking wants hydrogen control (preheat, low-hydrogen electrodes, post-heat).
- Lamellar tearing requires through-thickness tensile stress - T-joints and corner joints in thick plate containing inclusion stringers. It is stepped and roughly parallel to the plate surface, so angle beams routed through the weld often miss it.
- Porosity is rounded; slag is elongated, irregular and often in straight lines.
- Coarse or anisotropic grain (austenitic weld metal, cast structures) scatters ultrasound - high attenuation, grass and poor signal-to-noise. Answer: lower frequency, larger element, and refracted longitudinal rather than shear waves.
Worked example / rule of thumb
A 50 mm nozzle welded into a thick C-Mn shell with severe restraint: suspect lamellar tearing. Scan with a 0 degree (straight beam) probe from the accessible shell surface adjacent to the weld, because the discontinuity plane is parallel to the plate surface.
Rule of thumb: the inspection direction must intersect the plane of the discontinuity as close to 90 degrees as geometry allows.