Materials, Welding and Non-Destructive Testing: Toughness, Procedures and Defect Detection
This domain tests the metallurgical and procedural knowledge behind a sound weld: which steel grade a location needs, how a welding procedure is qualified, which defect a given test can find, and how results are accepted or rejected.
Materials
Normal-strength hull structural steel is graded A, B, D and E, with increasing notch toughness verified by impact testing at lower temperatures, which is why higher grades are specified for thick plating, billet strakes and regions of stress concentration. Higher-strength steels such as AH32, DH36 and EH36 carry a minimum yield of 315, 355 and 355 N/mm^2 respectively with grade-dependent toughness. Toughness, not just strength, controls brittle fracture, and the rules specify grade by thickness and position. Aluminium alloys are used for superstructures with dissimilar-metal connections detailed to prevent galvanic corrosion.
Welding procedure control
Three separate documents matter: the welding procedure specification, the procedure qualification record that proves it by test, and the welder qualification test that proves the individual. Consumables are controlled by storage and baking. Preheat and interpass temperature control hydrogen-assisted cracking, and carbon equivalent is the usual screening measure.
Non-destructive testing
- Radiography finds volumetric defects such as porosity and slag, and is suited to butt welds.
- Ultrasonics finds volumetric and planar defects including lack of fusion and lamellar tears, and can size them.
- Magnetic particle testing finds surface and near-surface defects in ferromagnetic material.
- Dye penetrant finds surface-breaking defects in non-porous material, including austenitic stainless steel. Personnel are certified to a recognised scheme such as ISO 9712, and acceptance criteria come from the class rules or ISO 5817.
Worked example
A steel has C 0.16, Mn 1.30, Cr 0.05, Mo 0.01, V 0.01, Ni 0.02, Cu 0.02. Carbon equivalent = 0.16 + 1.30/6 + (0.05+0.01+0.01)/5 + (0.02+0.02)/15 = 0.16 + 0.217 + 0.014 + 0.003 = 0.394. Below the common 0.41 threshold, preheat is not mandatory for moderate thickness, but hydrogen control and interpass limits still apply.
Traps
Using penetrant testing to find subsurface defects; expecting radiography to size a planar lack of fusion reliably; confusing procedure qualification with welder qualification; assuming a stronger steel grade permits thicker unrestrained welds without toughness control.