Residual Magnetism After Inspection: What to Ask the Supplier

A statement that a part was “demagnetized after MPI” is not a residual-field record. It does not show where the part was measured, which axis the sensor followed, what instrument mode was used, when the reading was taken or which requirement applied. For a buyer, these fields determine whether the result represents the agreed part geometry and inspected lot.

The review begins after magnetic particle inspection. It does not choose magnetizing current or prove inspection sensitivity. Its purpose is to distinguish inspection-field evidence from post-process residual magnetism, define repeatable measurements and connect them to an order-specific acceptance source.

Separate field direction from residual field strength

Magnetic particle inspection deliberately establishes a magnetic field in a ferromagnetic part. Field direction matters because a discontinuity's orientation relative to that field affects the particle response. The ASTM E3024/E3024M-22a public scope places general-industry MT under a detailed written procedure and keeps product acceptance dependent on the invoked documents.

Tools used during magnetization can help assess whether the active field direction and relative strength are suitable for the inspection technique. An ASNT discussion of quantitative quality indicators describes QQIs in that inspection-field role. A QQI or pie gauge response does not provide a post-demagnetization residual-field magnitude at a defined part location.

Residual field is what remains after the magnetizing current is removed. A post-demagnetization check occurs after the specified demagnetization cycle, if demagnetization is required. Its instrument and sensing principle may report a local field component or magnitude under a particular mode. Record the quantity and mode instead of reducing every display to the word “magnetism.”

Preserve the sign of a signed reading where the plan requires polarity information, and keep absolute magnitude as a separately defined statistic. Record units exactly as the instrument and purchase requirement use them. Do not introduce a universal gauss limit or convert units without documenting the relationship and rounding.

Part orientation and timing belong with the value. Rotating a part or probe relative to the ambient field can change the reported component. A reading taken immediately after a cycle and one taken after handling or another process may not be comparable. State the applicable procedure or specification, the part orientation, measurement time and elapsed process step.

The ASNT MT overview notes demagnetization as a post-inspection consideration for ferromagnetic parts. It does not say every part requires demagnetization or supply a numeric limit. The drawing, contract or product specification must provide that decision.

Agree the measurement locations and conditions

Identify the part number and revision and the serial, heat or lot trace used by the order. Link the magnetic-particle inspection and demagnetization procedure revisions. Then create a location map that names the actual features to be checked. Depending on the controlled plan, those could include ends, bores, shoulders, teeth, journals or other geometry-specific zones.

A single low reading on an easy flat face cannot represent an unmeasured bore, opposite pole or remote end when the plan requires those areas. Magnetic poles and local field components depend on geometry and magnetization history. The location plan should therefore specify coverage based on the actual product requirement, not a generic fixed grid applied to every component.

For each point, document probe contact or standoff and the sensor axis or orientation. A tangential reading, an axial reading and a normal-to-surface reading may describe different components of the local field. Use a repeatable physical reference on the part or drawing so another operator can place the probe in the same way.

Where the instrument reports signed values, define the positive probe direction on that map. Reversing the sensor can reverse the sign without changing the local magnitude. If the specification uses magnitude, show how signed observations were handled rather than deleting polarity. This small convention prevents two operators from calling consistent readings contradictory.

Record part orientation relative to the surrounding environment and how ambient or background field is handled. The measurement procedure may call for a background check, a controlled orientation or another correction practice. Do not subtract a convenient ambient value unless the approved meter procedure defines how.

Identify DC or AC response and peak, RMS or another instrument mode where relevant. These labels are not interchangeable. Name the instrument manufacturer, model and serial number, its selected range and resolution, and the zero check performed for the session. Include calibration status and due date. A vendor's available gauge range describes an instrument capability; it is not an order acceptance limit.

A Magnaflux field-indicator data sheet distinguishes calibrated and non-calibrated models and their stated functions. That difference reinforces the need to identify the actual device and record, rather than accepting “field meter used” as sufficient evidence.

Include operator, date and time and retain repeat readings when the plan calls for them. State whether the decision uses individual points, a maximum or another statistic only as the invoked requirement defines it. A calculated maximum is meaningful only when the required locations and axes were actually measured.

Release-record state	Part, location and measurement	Requirement and trace	Review action
Normal: record aligns	All required locations, probe axes, part orientation, instrument modes, timing and repeats are documented with calibrated equipment.	Part and lot, MPI and demagnetization records, result statistic and order-specific limit source match.	Send the completed record to the authorized disposition owner.
Missing: coverage or trace incomplete	A location, axis, standoff, mode, calibration status, timing or repeat reading is absent.	The lot link, quantity covered, procedure revision or acceptance source is missing.	Request the missing measurement or record; do not let one point represent the unmeasured part.
Conflict: records are not comparable	Before and after readings use different locations, axes or modes, or a repeated demagnetization cycle has no new measurement.	The result is near the invoked limit without required uncertainty treatment, or the stated statistic differs from the order.	Preserve all readings and route the method conflict, retest or deviation to the named owner.

The normal state makes the evidence ready for disposition; it does not itself release material. Missing and conflict states are useful because they identify the exact measurement or trace link that still prevents a decision.

Check the result after demagnetization

First confirm whether the invoked document requires demagnetization and a residual-field check. Do not infer the requirement from the fact that MT occurred. If required, identify the demagnetization method or cycle by its controlled procedure rather than replacing it with an informal description of the equipment.

Before-and-after readings can show the effect of a cycle when the plan requests them, but they must use the same locations, probe orientations, instrument mode and comparable conditions. Otherwise, the difference combines process change with measurement change. Record the post-cycle timing and preserve unsuccessful cycles rather than reporting only the final attempt.

A typical vendor process guide, such as the Magnaflux guide to magnetic particle inspection, places demagnetization and a residual check after indication inspection. It does not establish one universal cycle, location plan or limit. Those remain controlled by the part and order documents.

Compare the reported result using the exact statistic required: each individual location, the maximum of the planned set or another defined rule. Use the agreed units and the acceptance source and revision. A value below an order-specific limit is decision-ready only when its instrument, location, axis, timing and statistic match that requirement.

Consider instrument uncertainty or stated accuracy treatment when a result is near the limit, following the governing procedure. Record overrange or out-of-range displays as such; do not replace them with the end of the instrument's scale. A meter's resolution also should not be mistaken for measurement uncertainty or acceptance margin.

If another demagnetization cycle or retest is proposed, identify who has authority to approve it. Link each cycle to a fresh measurement set. An unauthorized repeat that discards an earlier result weakens traceability. Where the method, limit or uncertainty rule remains unresolved, preserve the conflict and use the formal deviation or concession route.

No practical record should promise zero magnetism. The defensible statement is that measured residual-field results at the agreed locations met, did not meet or could not yet be compared with the invoked order-specific requirement under documented conditions.

Link the result to the inspected lot

Complete the record by linking part, heat, lot or serial identifiers to the quantity covered. Reference the original MPI report, the demagnetization record and the residual-field measurement sheet and version. Include the instrument certificate or calibration reference and identify the operator and qualified reviewer required by the procedure.

A signed sheet without a lot link or quantity cannot show which pieces the readings represent. State the sampling or coverage basis, list exceptions and identify unmeasured pieces. If only samples were checked, do not word the result as though every part was measured.

When multiple pieces are sampled, keep each reading set linked to its individual part or sample identifier. A lot-level summary should be reproducible from those records and should not merge locations from different pieces into a fictional complete map. Any skipped or replaced sample needs its reason and authorization.

Keep the acceptance source and revision with the final field record. Corrected reports should identify what changed, retain superseded versions according to the quality system and preserve the raw readings. Nonconformance or deviation records must link to the affected material and the authorized disposition owner.

Shipping and release status stay separate from measurement status. A complete residual-field record is one input to the order's disposition process. It does not prove protection of bearings, electronics, seals or other downstream systems unless a product-specific requirement defines that relationship. The existing guide to track-roller floating seals and lubrication provides product context without creating a magnetic limit or harm claim.

The useful supplier response is therefore not “zero magnetism” or “demagnetized.” It is a traceable map of readings with location, axis, instrument, mode, timing, repeated-cycle history and the exact acceptance source. That package lets the authorized owner decide the lot while keeping unmeasured areas and unresolved conflicts visible.