🔩 Magnetic particle testing (MT) of welds — magnetization in two perpendicular directions, tangential field 2–6 kA/m

Materials and NDT PN-EN ISO 17638

Overview

The standard specifies techniques for the detection of surface imperfections in welds in ferromagnetic materials, including the heat-affected zones, by the magnetic particle method. The techniques are suitable for most welding processes and joint configurations; variations of the basic techniques giving higher or lower test sensitivity are described in Annex A. The standard does not specify acceptance levels for the indications — these are given in ISO 23278 or in product standards or application standards. The second edition, ISO 17638:2016, cancels and replaces the first edition, ISO 17638:2003, which has been technically revised.

The method is a surface method and uses the phenomenon of leakage of the induced magnetic field at the location of a discontinuity. The field can be induced only in ferromagnetic materials, so the test cannot be performed on non-magnetic materials, for example austenitic steel. Surface discontinuities and relatively large subsurface discontinuities are detected: with alternating-current magnetization down to about 2 mm below the surface, with direct-current magnetization even down to 3 mm. The method detects discontinuities from about 3 mm long, from 0.001 mm wide and 0.1 mm deep; detectability increases with the magnetic permeability of the material. Surface cracks, lack of fusion, laps, tears, non-metallic inclusions, gas pores and corrosion pits are revealed.

The standard refers to ISO 3059 (viewing conditions), ISO 9934-1 (general principles), ISO 9934-2 (detection media) and ISO 9934-3 (equipment), and takes its terms from ISO 12707 and ISO 17635. Testing and evaluation of the results for final acceptance should be carried out by qualified personnel; qualification in accordance with ISO 9712 or an equivalent standard at the appropriate level in the relevant industrial sector is recommended.

Method principle

The area under test is magnetized so that a magnetic flux flows in the ferromagnetic material. A discontinuity locally increases the magnetic reluctance and deflects the field lines; when it lies on the surface or just below it, the field runs partly outside the material, and in this area ferromagnetic powder accumulates, giving an indication. Detectability depends on the angle between the main axis of the imperfection and the field direction: it is highest when they are perpendicular, while an imperfection parallel to the field may remain undetected. Therefore, to detect imperfections of all orientations, welds are magnetized in two directions approximately perpendicular to each other, with a deviation of at most 30°. For structural steels in welds a tangential field of 2 kA/m to 6 kA/m (rms value) is recommended. The indications are qualitative — they make it possible to locate a defect but not to determine its depth.

Applications

• Detection of surface cracks and lack of fusion in welds and heat-affected zones of ferromagnetic steels
• Inspection of castings, forgings and components in the automotive, aerospace, railway and shipbuilding industries
• Testing of gear wheels, axles, shafts, gears, pins, chains and hooks
• Testing of components of mountain and mining hoists
• In-process inspection and in-service testing, also in field conditions

Key parameters

Standard

Standard number

PN-EN ISO 17638:2017-01

Title (PL)

Badania nieniszczące spoin — Badania magnetyczno-proszkowe

Title (EN)

Non-destructive testing of welds — Magnetic particle testing

Step-by-step procedure

1. Arrangements before testing

   Specify the test procedure, the requirements for certification of NDT personnel, the extent of test coverage, the manufacturing stage of the component, the test techniques used, the check of system performance, any demagnetization, the acceptance level and the action to be taken with unacceptable indications.

2. Supplementary information

   Where necessary, collect the type and designation of the parent material and the weld metal, the welding process, the location and extent of the welds to be tested, the joint preparation and dimensions, the location and extent of repairs, post-weld treatment and the surface condition.

3. Surface preparation

   The areas to be tested must be dry, unless detection media intended for wet surfaces are used. Where necessary, improve the surface condition with abrasive paper or local grinding so that accurate interpretation of the indications is possible; cleaning must not harm the material, the surface finish or the detection media. Use the detection media within the temperature range specified by the manufacturer.

4. Magnetization

   Use an alternating-current electromagnetic yoke, current flow equipment with prods, or an adjacent or threading conductor or a coil. With prods, take measures to limit overheating, burning and arcing at the contact points; remove arcing marks and check the area by a suitable method.

5. Verification of magnetization

   Confirm sufficient flux density at the surface: by testing a reference component with fine discontinuities in the least favourable locations, by measuring the tangential field strength with a Hall probe as close to the surface as possible, by calculating the approximate current value for the recommended tangential field, or by another method based on recognised principles.

6. Directions and area of testing

   Magnetize in two directions approximately perpendicular (deviation up to 30°). Take into account that near the pole pieces and prod tips there is an untestable area due to excessive field strength, visible as "furring" of the particles; ensure proper overlapping of the tested areas.

7. Application of the medium and viewing

   Apply dry powder or suspension; with the wet method it is advantageous to incline the surface so that the excess runs off. Observe the indications under conditions in accordance with ISO 3059. Recognise spurious indications resulting from poor surface preparation, poor field induction, geometric discontinuities (notches, gaps, changes of cross-section, threads) and structural inhomogeneities.

8. Evaluation, documentation and demagnetization

   Evaluate the indications against the adopted acceptance level (e.g. ISO 23278), document them with a sketch or photograph, prepare the test report and, where required, demagnetize the component and remove the detection medium.

Required equipment and apparatus

Reagents, media and consumables

Health and safety (OHS)

• The standard requires particular attention to toxic, flammable and volatile materials, electrical safety and unfiltered UV radiation
• The test produces strong magnetic fields near the object and the equipment — items sensitive to the field should be removed from the zone
• Prods: overheating, surface burns and arcing with poor contact; do not use oil-based suspensions with current flow magnetization because of the fire risk
• Water-based suspensions may cause surface corrosion and electrical short circuits
• Vapours of oil-based suspensions are harmful to health and the environment — ventilation, gloves, respiratory protection

Frequently asked questions