Why this Study ?
• Carbon content changes magnetization effort.
• Waveform choice affects depth of defect detection.
• Large/heavy components need correct kAT and field adequacy.
• Residual magnetism impacts machining, bearings, assembly safety.
• Demagnetization is not universal; harder steels need stronger or DC reversing demag.
• Standards give ranges, but specific guidance for different steels is sparse.
Materials & Specimens:
Steel | Carbon | Magnetic Behaviour |
AISI 1020 | Low (0.21%) | Easy to magnetize & demag |
AISI 1050 | Medium (0.48%) | Moderate behaviour |
AISI EN31 | High (1.09%) | Hard to magnetize & demag |
- Rod Geometry: 200 mm length × 50 mm diameter
- Defects of Geometry :
- Location from surface (depth of top of FBH): 0.5 mm / 1.5 mm / 2.5 mm / 3.5 mm
- Defect depth (FBH height): 19 mm
- Defect diameter: 1 mm
Magnetization Setup
MPI Machine — Model: ABM-4000M capable of delivering up to 4000 A magnetizing current, used for testing.
Circular magnetization (Head / Tail Stock)
Waveforms: AC vs HWDC
Fluorescent wet particles, UV inspection
Magnetizing current applied in steps from 600 A up to 1800 A, increasing until field adequacy is demonstrated
using shim/indicator as per ASME requirements.
Fluorescent Magnetic Particle Media Used :
Product: FlawGlo FMI 800
Key Properties:
- Ultra-bright particles for very fine discontinuity detection
- Particle size: 2–5 µm (avg. ~3 µm) — high sensitivity
- Concentration: 1.25 g/L (optimized suspension)
- Processing Temperature Range: 0–49°C
- Flash Point: 100°C (safe handling)
- Compliance / Standards Supported: ASME BPVC, ASTM E709, ASTM E1444, ASTM E3024
Comparison: AC vs HWDC Circular Magnetization :
Fluorescent MPI – Material: AISI 1020 Steel
• Surface Sensitivity (D1 – 0.5 mm)
1. Both AC and HWDC provide strong detection consistently at all current levels
2. Shimp indication strong at 600 A for both methods
• Slightly Subsurface Defects (D2 – 1.5 mm)
• HWDC outperforms AC
1. Light indication begins at 600 A with HWDC vs 900 A with AC
2. Moderate-to-Strong detection from 900–1800 A with HWDC
• AC remains mostly Light to Moderate even at higher currents
• Deeper Subsurface Defects (D3 – 2.5 mm &
D4 – 3.5 mm)
1. AC: No indications at any current level
• HWDC:
1. Moderate visibility of D3 begins at 1500 A
2. Light visibility of D4 at 1500–1800 A
Indication Results :
Comparison: AC vs HWDC Circular Magnetization
Fluorescent MPI – Material: AISI 1050 Steel
- Surface Defect Sensitivity (D1 – 0.5 mm)
- AC: Best Shim Visibility at 900 A
- Only Light at 600 A
- Strong indications from 900 A onward
- HWDC: Better Shim Visibility at 600 A
- Light at 600 A
- Strong and consistent from 900 A onward
- Subsurface Defect Sensitivity (D2 – 1.5 mm, D3 – 2.5 mm, D4 – 3.5 mm) AC:
- No visibility at any current level (surface-only sensitivity)
- HWDC:
- D2 (1.5 mm): Light at 900 A → Moderate/Strong at 1200–1800 A
- D3 (2.5 mm): Light to Moderate indications begin at 1200–1800 A
- D4 (3.5 mm): No response observed → deeper depth limit
Indication Results :
Comparison: AC vs HWDC Circular Magnetization
Fluorescent MPI | Material: EN 31
• Shim Indication
1. AC: Strong shim indication achieved at 900 A
2.HWDC: Strong shim indication achieved at 600 A
• Surface Sensitivity (D1 – 0.5 mm)
1. AC: Light at 600–900 A → Strong above 1500 A
2. HWDC: Light at 600 A → Strong from 1200 A onward
• Subsurface Sensitivity
1. AC:
• D2 (1.5 mm): Only Light at 1800 A
• D3 & D4: Not detected
2. HWDC:
• D2 (1.5 mm): Light → Strong (900–1800 A)
• No detection of D3 & D4
Indication Results :
Magnetization & Residual Field Assessment
- Specimens were magnetized using Circular Magnetization for MPI inspection
- Magnetic field was circumferential, not aligned with Gauss meter sensitivity
- Therefore, residual field could not be properly detected
- To accurately measure magnetic retention, a Longitudinal Magnetization shot of 1800 A was applied
- Re-oriented the field along the length of the component
- Allowed the Gauss meter to correctly measure the residual magnetic field
Residual Field After 1800 A Longitudinal Magnetization
Material | Residual Field (Gauss) |
1020 Steel | 11.74 G |
1050 Steel | 23.01 G |
EN31 Steel | 50.83 G |
Demagnetization Performance Evaluation
First Demagnetization Attempt
- Performed using 11 KAT Demagnetizing Coil
- Current applied: 2200 A
Objective: Reduce residual field to an acceptable level (<1–2 Gauss)
Material | Residual Field (Gauss) |
1020 steel | 0.95 G |
1050 Steel | 1.61 G |
EN31 Steel | 4.56 G |
Interpretation:
- 1020 & 1050 nearly at acceptable residual limits
- EN31 still above acceptable value → Further demagnetization required
Demagnetization Performance Evaluation
Improved Demagnetization Trials
Performed using larger AT (Ampere-Turn) demag coils:
Coil Setup | 1020 | 1050 | EN 31 |
350 x 350 mm / 3300 AT | 0.85 G | 1.10 G | 3.50 G |
400 x 400 mm / 6300 AT | 0.84 G | 0.95 G | 2.60 G |
Conclusion
- Higher Ampere-turns + Larger coil aperture = More effective demagnetization
- EN31 shows highest magnetic retention, requiring stronger demag settings
- 6300 AT coil achieved lowest residual readings → Recommended for EN31 or hardened steels
Conclusion – Magnetization & Demagnetization Study:
- Accurate residual field measurement requires proper field orientation
✔ Longitudinal magnetization enabled correct Gauss evaluation - Material grade affects magnetism retention
✔ EN31 (high carbon steel) shows the highest residual field
✔ 1020 and 1050 easier to demagnetize
- Demagnetization performance improves with coil strength and aperture
✔ 6300 AT coil demonstrated the most effective demag for all materials
- Final residual fields were reduced to safe and acceptable levels
✔ Components can be released for service after verification
In Summary
To ensure reliable MPI inspection and component safety:
Choose magnetization and demagnetization techniques based on Material properties, Magnetization direction, expected defect orientation and required Gauss limits.
