Materials, Welding, Nondestructive Testing, and Failure Analysis
This domain links three things an inspector uses together: whether the material and weld are fit for the service, whether the NDT method can actually detect the expected defect, and whether a failure interpretation is supported by evidence.
Materials and weldability
Carbon and low-alloy steels dominate; austenitic stainless steels and non-ferrous alloys appear in corrosive service. Weldability is judged by carbon equivalent, CE = C + Mn/6 + (Cr+Mo+V)/5 + (Ni+Cu)/15; higher CE means preheat and hydrogen control. Material substitution requires the original design unit's consent, since strength, toughness and corrosion resistance all change together.
Welding
A qualified welding procedure specification, backed by procedure qualification records, must exist before production welding; welders must be qualified for method, position, thickness and material group. Typical defects: cracks, lack of fusion, incomplete penetration, porosity, slag, undercut. Post-weld heat treatment relieves residual stress, commonly in the 600–650 °C band for carbon and low-alloy steels.
NDT applicability
- RT: volumetric defects - porosity, slag.
- UT: internal planar defects - cracks, lack of fusion; also thickness.
- MT: ferromagnetic materials only, surface and near-surface.
- PT: non-porous materials, surface-breaking defects only. Acceptance levels follow NB/T 47013; when a defect exceeds the level, the standard's extension/retest rules apply rather than a local judgement.
Failure analysis
Modes: ductile and brittle fracture, fatigue (beach marks, initiation at stress concentration), creep (high temperature, dimensional growth, intergranular cavitation), corrosion thinning, hydrogen damage, and stress corrosion cracking.
Worked example
An anhydrous ammonia vessel shows branched cracks on the inner surface near welds, with locally high hardness. Reasoning: branched, weld-adjacent, hard-zone cracking points to stress corrosion cracking. Confirm with hardness survey, metallography and, if needed, crack-depth UT; the fix is not just grinding out - it requires hardness control, post-weld heat treatment, and medium condition control (residual water), otherwise the cracks return.
Traps that cost marks
Specifying MT on austenitic stainless steel; using PT on porous or rough surfaces; diagnosing fatigue where the evidence shows intergranular creep damage.