What Article 7 Requires Before a Magnetic Particle Result Means Anything

Article 7 sets the conditions; the construction code sets accept or reject

Article 7 covers magnetic particle examination of ferromagnetic materials, and like every method article in Section V it stops short of telling you whether an indication is acceptable. It tells you how to establish a magnetic field of the right strength and the right direction, how to prove that field exists, how to apply and remove particles, how much light to work under, how to cover the part and what to write down. The accept or reject line comes from the construction code that invoked Section V.

That split is why an MT report with no field adequacy evidence is worthless even when it shows no indications at all. The absence of indications is only meaningful if the technique was capable of producing them in the first place. A pie gage impression, a shim indication or a probe reading is the evidence that the method was live at the joint. Without it, no relevant indications records nothing more than that somebody sprayed particles on a weld and looked at it.

The architecture repeats across methods, which is why crews who already work to Article 2 for radiography, Article 4 for ultrasonics, Article 6 for liquid penetrant or Article 9 for visual can navigate Article 7 in an afternoon. Procedure, essential variables, calibration, technique, examination, evaluation, documentation, in that order, every time. The Section V compliance overview shows how the method articles sit against one another and against the general requirements article that binds them.

The written procedure and its classified variables

Magnetic particle examination is performed to a written procedure, and Table T-721 classifies each requirement of that procedure as essential or nonessential. Change an essential variable and the procedure must be requalified by demonstration before further use. Change a nonessential variable and the procedure needs a revision or an addendum, but no requalification. Any change to either kind requires the written procedure to be updated, which is the clause most field programs breach without ever noticing they have breached it.

The essential side is where capability lives: the magnetizing technique itself, the current type and amperage, surface preparation, the particles used including whether they are fluorescent or visible and wet or dry, how the particles are applied and how excess is removed, the minimum light intensity, any nonmagnetic surface contrast enhancement, and coating thickness where examination is performed through a coating. Move any of these and the sensitivity that was demonstrated is no longer demonstrated.

The nonessential side covers items such as the demagnetizing technique, post-examination cleaning, personnel qualification requirements and the shapes and sizes to be examined. They still have to be stated in the procedure. A procedure that answers a Table T-721 requirement with as required or per manufacturer has given no value and no range, cannot be demonstrated against anything, and will not survive a serious technical procedure development review.

Five magnetization techniques and what each one sees

Article 7 recognizes five techniques and the procedure names the one in use: the prod technique, longitudinal magnetization using a coil or cable wrap, circular magnetization by direct contact or through a central conductor, the yoke technique, and multidirectional magnetization. Each produces a field in a characteristic direction, and each has its own current rules and its own effective coverage footprint. Substituting one for another mid-job without a procedure revision is a nonconformance even when the substitute is technically the better choice.

Direction is the whole point. Circular fields, whether from direct contact or a central conductor, run circumferentially and reveal discontinuities running along the axis. Longitudinal fields from a coil run along the axis and reveal circumferential discontinuities. A yoke produces a field between its legs and reveals discontinuities running across that line. Choosing a technique is choosing which orientations you can see, which is why the second perpendicular pass exists at all.

Multidirectional magnetization drives two or more fields in rapid alternation so that both orientations are covered in a single operation. It buys speed on production parts, and it costs proof: the balance of the fields has to be demonstrated in each direction, using field indicators or artificial flaw shims placed at the locations that actually matter, before the technique is used and again whenever the setup changes. Speed without that demonstration is not coverage.

Prod and circular magnetization: the current numbers

The prod technique has hard numbers. Prod spacing must not exceed 8 in., and spacing below about 3 in. is impractical because particles band around the prod tips and mask the surface between them. Current is set by the spacing actually used: 100 to 125 amperes per inch of prod spacing for sections 3/4 in. thick and greater, and 90 to 110 amperes per inch for sections thinner than 3/4 in. Set the spacing, then read off the current.

Direct contact circular magnetization is set against the part diameter, in the range of 300 to 800 amperes per inch of diameter, where the diameter is the largest cross section perpendicular to the current path. Prods and direct contact both carry an arc-strike and local overheating risk on finished surfaces, which is why many owner specifications prohibit them outright on stainless and on in-service pressure boundaries and push crews toward yokes or a central conductor.

The central conductor technique removes the contact risk by passing current through a bar inside the bore. When the conductor is centered, the field is uniform around the circumference and one setup covers the bore. When it is offset against the bore wall, the examination is limited to a circumferential increment of roughly four times the diameter of the conductor, with a 10 percent overlap between adjacent increments, and the part is rotated to cover the whole circumference.

The yoke technique and the lift test

The yoke is the field workhorse, and it is also the piece of equipment with the simplest daily proof. Alternating current yokes must lift at least 10 lb and direct current or permanent magnet yokes at least 40 lb, in both cases at the maximum pole spacing that will actually be used on the job. That qualifier is the one crews miss. A yoke proved at 4 in. pole spacing has not been proved at 8 in., and the field between the poles falls away fast as the legs open.

The lifting power is verified before the yoke is used each day, and again whenever the yoke has been damaged or repaired. Test weights are weighed on a scale and stenciled with the nominal weight before first use, so that nobody is lifting an unverified lump of steel and calling it a calibration. A weight only needs reverifying if it is damaged in a way that could have removed material from it.

The lift test proves the yoke can produce a field. It does not prove the field is adequate at the examination surface through a coating, across a fillet, or at the spacing and orientation the technician actually used on the joint. That is a separate demonstration with a separate tool, and treating the lift test as though it covered field adequacy is one of the more common technical errors in otherwise well-run MT programs.

Proving field adequacy and direction

Article 7 accepts three means of showing that the field is adequate and correctly oriented. A pie-shaped magnetic particle field indicator, made of low-carbon steel segments furnace-brazed together and copper plated, is placed copper side up on the surface. Artificial flaw shims are attached with the flawed side against the surface. A Hall effect tangential field probe measures the field at the surface directly and returns a number.

The three are not equivalent and they answer different questions. The pie gage and the shims show that a field of usable strength exists and which way it runs, which is what a technician standing at the joint needs to know. Only the probe measures the tangential field as a value, and Article 7 sets the range that reading must fall within. Choose the tool to match the question actually being asked.

Whichever is used, the demonstration belongs on the record. Field adequacy is one of the items Article 7 expects the examination record to carry, alongside the magnetizing technique, the current type and amperage, the examination medium, the light source and the equipment used. A record that lists amperage but no field demonstration cannot show that the amperage produced anything at the surface. That distinction sits at the heart of what makes an NDT report defensible.

Two directions, overlap and what coverage means

A magnetic particle indication forms when the flux runs roughly across the discontinuity. A discontinuity lying parallel to the flux produces little or nothing at all. Article 7 therefore requires at least two examinations of each area, with the lines of flux in the second examination approximately perpendicular to those used in the first. A different technique may be used for the second pass, which is why yoke work is performed in two orientations at every position along a weld.

Coverage is the other half of the requirement. Examinations are conducted with sufficient overlap to ensure 100 percent coverage of the area at the required sensitivity. With a yoke that means the effective area between the poles rather than the span of the legs, and it means stepping along the weld in overlapping bites. With an offset central conductor it means the four-conductor-diameters increment with 10 percent overlap, repeated around the bore.

Surface condition governs whether any of this works. Surfaces must be free of oil, grease, sand, loose rust, loose scale, welding flux, spatter and anything else that would hold particles or mask an indication. As-welded, as-rolled, as-cast and as-forged surfaces are often acceptable as they are. Where surface irregularity would mask indications, conditioning by grinding or machining comes first, and the procedure states which preparation method applies.

Lighting, interpretation and evaluation of indications

Nonfluorescent particle examinations require a minimum of 100 foot-candles, about 1,076 lux, of white light on the surface being examined. Fluorescent examinations require a minimum ultraviolet-A intensity of 1,000 microwatts per square centimeter on the surface, with the ambient white light in the darkened area held to 2 foot-candles, about 21.5 lux, or less. Light meters used to verify these values are calibrated at least annually and whenever they are damaged.

Fluorescent work adds human requirements that get skipped under schedule pressure. Black lights are allowed to warm up for at least five minutes before use or before their intensity is measured. Examiners spend at least five minutes in the darkened area before interpreting, so their eyes adapt. Photochromic and permanently tinted lenses are not worn. Ultraviolet intensity is measured at intervals through the shift and again whenever the workstation changes.

Evaluation itself is short in Article 7 because it hands off. Indications are evaluated against the acceptance standards of the referencing construction code. Localized surface irregularities such as machining marks can produce false indications that a Level II has to recognize and discount. Broad areas of particle accumulation that could mask indications are not acceptable, and those areas are cleaned and reexamined rather than interpreted through the haze.

Demagnetization, records and the calibration trail

Demagnetization is required where residual magnetism could interfere with subsequent processing or with the service function of the part. Residual fields deflect welding arcs, hold swarf on machined surfaces, disturb instrumentation and cause trouble in bearings and seals. Article 7 requires the demagnetizing technique to be described in the procedure and the demagnetization to be recorded; the acceptable residual level comes from the referencing code, the fabrication specification or the end user.

The examination record is expected to carry the magnetizing technique, the current type and amperage, the examination medium, any surface contrast enhancement, the light source, the equipment used, the demonstration of field adequacy, the material and thickness, and a map or description of recorded indications. Rejectable indications are recorded with their type, location and extent. Nonrejectable indications are recorded where the referencing construction code calls for them.

Calibration underpins the whole record. Magnetizing equipment fitted with an ammeter is calibrated at least annually, and after major electrical repair, overhaul or damage, with the meter reading required to sit within 10 percent of full scale against a certified test meter. Light meters run on the same annual cycle. Where an MT program needs an owner for these procedures, reviews and demonstrations, outsourced ASNT Level III consulting is the usual answer: affordable, accessible, fully customizable, quote on request.

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