Article
Non-destructive testing (NDT, also called NDE) checks a product for defects without damaging it. Purchase orders for pipes, fittings and flanges often call for one or more NDT methods. Knowing what each one finds helps buyers order only what they need.
The main methods
| Method | Finds | Typical use |
|---|---|---|
| Radiography (RT) | Internal volumetric defects: porosity, slag, lack of fusion | Butt welds in pipe, welded fittings (WX class) |
| Ultrasonic (UT) | Internal defects, laminations, wall thickness | Seamless pipe, forgings, thick welds |
| Dye penetrant (PT) | Surface-breaking cracks and pores | Stainless and non-magnetic materials, weld surfaces, bevels |
| Magnetic particle (MT) | Surface and near-surface cracks | Carbon and low-alloy steel (magnetic only) |
| Eddy current (ET) | Surface and near-surface flaws in tubes and pipe | Alternative to hydrotest for stainless pipe and tube |
Radiography
X-ray or gamma-ray images show differences in density inside a weld. RT is excellent for volumetric flaws and gives a permanent record, but needs safety controls and access to both sides of the part.
Ultrasonic testing
High-frequency sound is sent into the steel and reflections reveal defects. UT is better than RT at finding planar flaws like cracks and laminations, and it is used to measure wall thickness. Phased array UT (PAUT) is increasingly replacing RT on welds.
Dye penetrant testing
A coloured or fluorescent liquid soaks into surface-breaking defects and is drawn out by a developer. Simple and cheap, PT is the standard surface test for stainless steel, which cannot be magnetised.
Magnetic particle testing
The part is magnetised, and iron particles gather at cracks that disturb the magnetic field. MT is fast and sensitive, but only works on ferromagnetic steels.
Ordering tips
- State the method, extent (for example 100% or 10%), standard and acceptance criteria.
- Ask for NDT reports to be attached to the test certificate.
- Choose PT, not MT, for austenitic stainless items.
Frequently asked questions
Can magnetic particle testing be used on stainless steel?
Not on austenitic stainless steels such as 304 and 316, which are non-magnetic. Dye penetrant testing is used instead.
What is the difference between RT and UT?
RT uses radiation to image internal defects, while UT uses sound waves and is better at finding planar flaws and measuring thickness.
Can eddy current replace hydrotesting?
Many pipe specifications, such as ASTM A312, allow a non-destructive electric test like eddy current in place of the hydrostatic test.
Key takeaways
- RT and UT find internal defects; PT and MT find surface defects.
- Use PT for stainless; MT only for magnetic steels.
- Specify method, extent, standard and acceptance criteria.
Need these products?
Sawariya Metal Industries supplies stainless steel pipes, flanges and buttweld pipe fittings from Mumbai to buyers in India and overseas. Send your size, grade, standard and quantity through our enquiry form and we will reply with a clear quotation and the test documents you need. You can also browse our stainless steel pipes, flanges and buttweld fittings.