Surface Preparation and Application Sequence

Outcomes

After this chapter, the learner should be able to:

• define an acceptable pre-cleaning condition;
• distinguish continuous and residual application;
• sequence current and media correctly;
• manage shot duration, duty cycle and background;
• select a technique for rough, threaded and high-retentivity components; and
• plan post-cleaning without losing required evidence.

7.1 Surface preparation

The surface must allow particles to move and indications to be seen. Remove loose scale, dirt, oil, grease, weld spatter, thick paint and residues that trap particles or prevent stable pole contact. The required preparation depends on surface profile, expected discontinuity size, media and acceptance sensitivity.

Preparation must not smear metal across a discontinuity or create new cracks. Aggressive grinding can close fine surface openings, add grinding cracks or change local permeability. If grinding is authorised, control the extent and re-clean before testing.

A thin, adherent coating may be examined if the approved technique demonstrates sensitivity. Record coating type and thickness where required. Do not assume a yoke lift test alone proves discontinuity sensitivity through coating.

7.2 Continuous technique

In the continuous technique, particles are present and mobile while the magnetising force is applied. With wet media, establish flow across the area, stop the application while the field remains on, allow excess carrier to drain enough for the indication to form, and then stop the field as defined by the procedure. With dry powder, create a light cloud during magnetisation and gently remove excess while the field remains.

The continuous technique uses the strongest leakage field available during the cycle and is normally the most sensitive approach. AC, HWR, FWR or DC may be used according to discontinuity location and equipment.

The supplied training source describes typical short shots of about 2 to 3 seconds to control heating and equipment duty cycle. That is not a universal timer setting. Component section, contact method, media, equipment rating and procedure determine the permitted duration and repetition.

7.3 Residual technique

In a residual technique, the component is magnetised first, the source is removed, and particles are then applied to the retained field. It requires a material with enough retentivity and a waveform that leaves useful remanence, normally DC or rectified current.

Residual examination may reduce particle build-up at threads, corners and keyways, and it can permit batches of small high-retentivity parts to be processed. Its field is weaker than the applied field, so sensitivity is normally lower and near-surface reliability is poor. Do not assume that an ISO 9934-1 general-principles examination authorises this method: the published scope represented by the supplied framework excludes residual magnetisation. Use requires separate documented authority, procedure validation and agreement under the actual governing document.

7.4 Decreasing-level continuous technique

Rough threads or complex geometry can produce heavy background at a normal setting. A controlled decreasing-level study can establish the lowest field that still reveals the reference condition while reducing geometry furring:

1. apply the validated starting field;
2. observe and record background and reference response;
3. demagnetise between trials where required;
4. reduce the setting in controlled increments;
5. repeat under identical media and viewing conditions; and
6. select and document the lowest level that maintains required sensitivity.

This is technique development, not an improvised attempt to make an inconvenient indication disappear.

7.5 Particle application errors

Common wet-media errors include flooding after the field is off, high spray pressure washing away an indication, incomplete wetting, dried carrier marks and failure to agitate. Common dry-media errors include dumping powder, applying it before the field stabilises, using a strong air stream and inspecting in wind.

For both methods, examine while the indication is in its best condition. Movement, drainage, vibration and post-field handling can change the pattern.

7.6 Shot control and duty cycle

High-current equipment stores and releases substantial energy. Confirm:

• shot timer operation;
• maximum on-time and duty cycle;
• cable and contact temperature;
• secure contact before energising;
• cooldown requirements;
• interlocks and emergency stop; and
• output repeatability.

Repeated shots used to "make it brighter" can overheat the part, arc at contacts and increase background. If indication quality is poor, diagnose the cause.

7.7 Post-cleaning and marking

Do not clean until required indications are evaluated, recorded and dispositioned. Post-cleaning may include removal of particles and contrast coating, corrosion protection, drying and restoration of the surface. Use products compatible with the component and subsequent processes.

Permanent marking must be authorised. Temporary marks should not contaminate the surface or obscure later examination. Any repaired area and retest boundary must remain traceable.

7.8 Field workflow

1. Review scope, procedure and safety information.
2. Identify the component and test area.
3. Confirm material, temperature, coating and surface condition.
4. Clean and prepare without damaging the area.
5. Verify equipment, media and viewing conditions.
6. Mark or plan placements and field directions.
7. Apply the first magnetisation and media sequence.
8. Inspect, classify and record.
9. Repeat with the second field direction and required overlap.
10. Evaluate against acceptance criteria when authorised.
11. Demagnetise if required and verify residual field.
12. Post-clean, protect, report and release.

7.9 Common traps

• Treating cleaning as cosmetic rather than a sensitivity control.
• Applying particles only after a continuous shot has ended.
• Exceeding equipment duty cycle to improve a weak pattern.
• Using residual examination on low-retentivity material without validation.
• Cleaning a rejectable pattern before recording its location.
• Changing current, media and surface preparation at the same time during a troubleshooting trial.

Chapter summary

The order of operations controls MT sensitivity. Continuous examination uses the applied field and is normally most sensitive. Residual examination uses retained flux for specific materials and geometries. Surface preparation, timing, particle mobility and preservation of evidence are part of the technique, not housekeeping extras.