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Magnetic Particle Testing (MT) is a non-destructive testing technology used to detect surface and near-surface defects (cracks, folds, inclusions, etc.) of ferromagnetic materials (such as crane hooks). Its principle is to use a magnetic field to generate a leakage magnetic field at the defect, which absorbs magnetic particles to form a visible indication.

1. Scope of application

1. Applicable materials : ferromagnetic materials (carbon steel, alloy steel, etc.).

2. Detection defect type :

   • Surface cracks (such as fatigue cracks, stress cracks)

   • Near surface defects (depth ≤ 5 mm)

   • Welding defects (such as lack of fusion, porosity)

3. Applicable standards :

   • GB/T 15822-2020 Non-destructive testing - magnetic particle testing

   • JB/T 6061-2007 Test pieces for non-destructive testing of magnetic particle

   • TSG Q7015-2016 (Rules for Periodic Inspection of Lifting Machinery)

2. Testing equipment and materials

3. Preparation before testing

1. Hook surface treatment

• Cleaning : Remove oil, rust and paint (grind to Ra≤12.5μm).

• Drying : Make sure there is no moisture on the surface to avoid affecting the adsorption of magnetic powder.

2. Magnetization method selection

3. Sensitivity Verification

• Use A1 test piece (30/100μm) to stick on the test area, and the magnetic traces should be clearly displayed after magnetization.

4. Testing steps

1. magnetization

   • Yoke method : pole spacing ≤ 200mm, magnetization time 1~3 seconds.

   • Continuous method : Magnetizing while applying magnetic powder (recommended).

   • Residual magnetism method : only applicable to materials with high residual magnetism (magnetization is required before powder spraying).

2. Applying magnetic powder

   • Wet method : spray magnetic suspension (concentration 1.5~3.0g/L).

   • Dry method : suitable for rough surfaces (such as thread roots).

3. Observation and Recording

   • Under natural light (non-fluorescent magnetic powder): black/red magnetic traces.

   • Under UV light (fluorescent magnetic powder): bright green magnetic traces (more sensitive).

   • Take photos and archive : record the defect location, length and shape.

4. Demagnetization (if necessary)

   • If the hook needs to be machined later or to avoid magnetic field interference, it needs to be demagnetized with a demagnetizer.

V. Defect Assessment and Acceptance Criteria

1. Common defect magnetic trace characteristics

2. Acceptance criteria (GB/T 15822)

• Not allowed :

  • Any visible cracks or transverse defects .

  • Linear defect length ＞2mm (critical stress area).

• Allowed :

  • Single point defect ≤ 1mm , spacing ≥ 10mm .

6. Safety precautions

1. Explosion-proof requirements : Oil-based carrier fluids are prohibited in flammable environments.

2. UV protection : Wear UV-proof glasses during fluorescent testing.

3. Equipment inspection : The yoke is well insulated to avoid the risk of electric shock.

7. Magnetic Particle Testing vs. Ultrasonic Testing (UT)

Recommendation : MT+UT combined testing should be used for the key stress-bearing areas of the hook (hook neck and hook bottom) .

8. Typical Cases

• Case 1 : crane hook fracture accident in a steel plant

  • MT testing found : 3mm fatigue crack in the hook neck transition area.

  • Cause : Long-term overload + corrosion leads to crack expansion.

• Case 2 : Cracking of weld seam of port hook anti-drop device

  • MT detection found : unfused defects in the heat affected zone of welding.

IX. Future Development Trends

1. Automated magnetic particle testing : robot scanning + AI image recognition.

2. High-sensitivity magnetic powder : Nano magnetic powder improves the detection rate of tiny defects.

Summarize

Magnetic particle testing is the preferred method for detecting surface defects on crane hooks . It is easy to operate, low cost and highly sensitive. It must be strictly implemented according to the standards and combined with ultrasonic testing (UT) for all-round testing to ensure the safe use of the hook.