Methods at a glance
| Method | Detects | Suitable for | Limits |
|---|---|---|---|
| Visual testing (VT) | Surface condition and visible discontinuities | All castings | Surface only |
| Liquid penetrant testing (PT) | Surface-breaking cracks, cold shuts, porosity | All non-porous materials, including austenitic steels and non-ferrous alloys | Open-to-surface defects only; surface must be clean |
| Magnetic particle testing (MT) | Surface and near-surface cracks | Ferromagnetic materials: carbon, low-alloy, martensitic and many duplex steels | Not applicable to austenitic steels and non-magnetic alloys |
| Radiographic testing (RT) | Internal shrinkage, gas porosity, inclusions | Volumetric examination of most castings | Planar defects parallel to the film are poorly shown; radiation safety |
| Ultrasonic testing (UT) | Internal discontinuities, wall thickness | Fine-grained steels with suitable geometry | Coarse-grained austenitic and nickel alloy castings attenuate and scatter sound |
| Computed tomography (CT) | Three-dimensional internal defects and geometry | Complex, small to medium castings | Cost, part size and wall thickness limits |
| Pressure and leak testing | Through-wall leakage | Pressure-retaining and fluid-carrying parts | Proves tightness only |
Liquid penetrant testing
Penetrant testing is the most widely used surface method for investment castings because many are austenitic or non-ferrous. Colour-contrast or fluorescent penetrants are used; fluorescent systems are more sensitive. The surface condition matters: blasting can close fine discontinuities, so the sequence of blasting, pickling and testing is specified. General principles are given in ISO 3452-1 and ASTM E165; EN 1371-2 defines severity levels specifically for investment castings.
Magnetic particle testing
A magnetic field is induced in the part; discontinuities at or near the surface distort the field and collect fine magnetic particles. The method is fast and sensitive for ferromagnetic steels. EN 1369 gives severity levels for castings; ASTM E709 is the corresponding guide.
Radiographic testing and computed tomography
X-rays or gamma rays pass through the casting and are recorded on film or a digital detector. Differences in thickness and density appear as differences in image density. Radiographs are evaluated against reference radiographs such as ASTM E192 for thin-walled investment steel castings or ASTM E446 for heavier steel sections, or against severity levels of the applicable EN standards. Computed tomography reconstructs a three-dimensional image and is increasingly used for first-sample qualification and failure analysis.
Ultrasonic testing
Ultrasonic testing is used on suitable steel castings for volumetric examination and wall-thickness measurement. Its applicability to austenitic stainless and nickel alloy castings is limited by their coarse cast grain, and it should be specified for such materials only after its feasibility has been confirmed.
Specifying NDT
An NDT requirement is complete when it states:
- the method and the procedure standard;
- the extent: which castings (all, sampling plan, first samples) and which areas of each casting;
- the stage of manufacture, for example after final heat treatment;
- the acceptance standard and severity level, possibly different for critical and non-critical zones;
- qualification of personnel, commonly to ISO 9712 or an equivalent scheme;
- the report required.
Critical zones are best marked on the drawing. Demanding the highest severity level on every surface increases cost without improving function.
Who performs the testing
Depending on the method and order, NDT is carried out by the manufacturer or by qualified external providers, and may be witnessed by a third party. This is defined in the inspection and test plan; see quality assurance.