French (Fr)English (United Kingdom)

DPC NEWS: a website dedicated to Penetrant Testing and Magnetic Testing



visits on site since April 2008

Log in


Receive HTML?

Some magnetic particle testing industrial uses

Written by Administrator
Saturday, 01 January 2011 13:18

Dear Readers,
This paper is the fruit of our long industrial experience; we are well aware that it is far from being complete, and that it may contain some inaccuracies.
This is why we would like to get your comments, your remarks and your suggestions to improve and to complete it.
We deeply thank you in advance for your kind contribution.
The Authors: Pierre CHEMIN and Patrick DUBOSC

January 2011

Document updated in March 2011

I - Introduction

Magnetic Particle Testing (MT) is used to detect open-to-surface or subsurface (generally down to some mm deep) discontinuities, only on ferromagnetic materials.

Due to that, it can be said that MT is more "restrictive" than Penetrant Testing (PT) but is preferred when applicable as being much faster, among other points.

While indications are sharper than with PT because there is no “bleeding”, indications may be non-relevant (non-significant), for example in some angles, threads and areas of diameters changes. 

Inspection of mechanically welded assemblies is one of the most important MT uses.

Many industries, such as transport (aviation, automotive, railroad, ski lifts), energy, steel metalworking, mechanics, Defence, medical, etc, use MT during manufacturing as well as for maintenance.

This paper details different areas of MT applications, during manufacturing and MRO (maintenance), such as energy, transport (air, rail, road, sea) on structures as well as on mechanical systems.

Quite often, we suggest which detecting media to use in order to meet the acceptance criteria.

As a further information we give examples of specific uses such as high and low temperatures MT, etc.

II - “Checkup” of parts/systems

A sound structural or mechanical part is the one which comes with 100% of the metallurgical characteristics and the anticipated fatigue limits in accordance with the alloy characteristics after any heat treatment or other surface treatment, considering both its shape/dimensions and the stresses it will have to overcome while in service.

Let us call it the “health diagnosis”: it is a main part of the “good practices” when manufacturing a part, and it is often mandatory previous to entering service. This “diagnosis” may also be performed, and is in addition often mandatory, while the part is in service to detect any early fatigue or corrosion.

This testing shall allow for accepting the part for a safe use or rejecting it without any doubt.

A similar process is performed after any defect removal process that would affect the 100% performance of the part.

Magnetic Particle Testing is one of the Non Destructive Testing (NDT) methods which may be used by trained people either on the production line or in the maintenance shop or on site to ensure the part is in a safe condition.

During World War I, Major William E. HOKE noticed that fine magnetic particles (coloured metal chips) due to machining of steel parts tended to go to some areas while “drawing” very clear lines. That is how he discovered a method to find cracks, now called MT (1).

III - The expanding use of magnetic particle testing

Since World War I, many failures led to catastrophes in different industries, taking human lives and impairing the good name of companies and manufacturers. This gave some emphasis to using a simple, easy to understand, cheap NDT method which, along the decades, improved the quality and the sensitivity of its products to an incredible level while lowering the impact on the workers’ health and on the environment.

Nowadays, MT expanding use may be explained by its reliability. However, along the years a major change gradually occurred due to the increasing use of non-ferromagnetic alloys which cannot be inspected using MT, such as in:
• Aerospace industry: titanium, aluminium and lithium-aluminium alloys, plastics, carbon, composites, ceramics, plasma coatings, parts formed by powder metallurgy.
• Automotive industry: plastics, composites, aluminium alloys.
• Nuclear industry: austenitic stainless steels.
• Railways: Aluminium duplex (two levels) carriages of TGV and AGV (very high speed) trains.

Therefore, it is easy to understand why nowadays Penetrant Testing (PT) use is on a stronger path of development than Magnetic Particle Testing (MT).

Though MT is reliable, shows an incredible sensitivity for tiny discontinuities detection and gives reproducible results with a high confidence, inappropriate processing may lead to deceptive results.

This is one of the main reasons to the design and constant improvement of automatic MT processes.

More and more users ask for multifunction magnetic benches able to process small parts as well as large ones. To accomplish that, the manufacturers’ engineering and design departments shall fulfil contradictory conditions and have many new ideas!

IV – Sensitivity levels of detection media

Unlike for Penetrant Testing, there is no third party rating of the sensitivities of the MT detection media.

Around ten years ago, a European military shipbuilder tried to establish such a classification using the ISO 9934-2(2) standard number 1 and number 2 reference blocks. The result was two classes: good or bad. However, it has been impossible to find the best one among the good ones.

In the absence of any classification in standards and specifications, many primes have published their own lists of approved products.

Faced to the lack of official lists, quite often the user, or even the purchase department, chooses the detection media.
If no staff is competent enough to thoroughly choose, we may bet that the cheapest product will be the winner, with the risk to not detect some defects.
The cheapest materials are rarely the right choice.

Take time to read the paper titled: ‘‘Rising Costs and Price War - To the Debit of Quality, in Spite of European Standardisation?’’(3).

V - When is MT carried out?

Parts soundness may be impaired during manufacturing and during running time.

V.1 - Manufacturing

Step after step parts are processed with a whole set of costly physicochemical operations leading most of the time to high expenses of workforce, operations which may induce defects. That is why, instead of carrying a final inspection only, it is cheaper and more rationale to inspect parts after every step which may induce defects so as to reject defective parts before the following step, just to spare costly and time-consuming machining or treatments of defective parts.

The first step is even to check the materials on arrival before any machining or whatever process.

The choice of high-mechanical-characteristics alloys by the engineering and design departments, the new methods of manufacturing and forming, the constant concern to use fewer materials for the same mechanical performance, the constant increase in parts’ and systems’ dimensions, plus other requirements (better corrosion resistance, easier recycling of parts after decommissioning, etc.) lead to higher statistical figures for failures or to faster damage.

Many years ago, inspections began by sampling parts. This statistical method made a lot of people uneasy due to the uncertainty to prevent 100% of the risks.

On the other hand, many were those thinking that inspection was expensive, useless, and a way to increase prices without increasing the quality of the equipment.
However, changes in regulations, involving more and more civil and criminal liabilities, the more and more important attention given by all the media to accidents, the tough international competition, led to a general awareness: inspections shall become tighter to improve at least safety and reliability.

Manufacturers have finally come to the conclusion that investing in NDT equipment, paying inspectors, inspecting 100% of the safety parts was a good investment.

NDTs are, in fact, a good opportunity for savings and allow for:
• Increasing productivity.
• Lowering manufacturing costs.
• Improving the brand image and the name of Companies.
• Gaining or keeping the customers’ confidence.
• Lowering costs incurred by the free-of-charge replacement of defective parts while in service under warranty.
• And many other reasons.

V.2 - Maintenance

Any damage or failure of running equipment is a disaster that can have considerable consequences.

Without help from NDT, it is somewhat impossible to anticipate a failure.

In the least damaging cases, these failures lead to an outing of order of machines or installations, production lines, to the equipment downtime due to its repair, atop of paying workers while they cannot work.

Failure of a part can also lead to a catastrophic way of events which destroy other parts or equipment otherwise in good condition, requiring far more expensive repairs and longer downtimes

In more serious cases, non-detected cracks or incipient cracks can lead not only to material disasters, but also to human life losses. Dramatic examples come to mind about aircraft, cars, trains, chemical industry (Bhopal being the most dramatic accident ever), etc.

The golden rule in maintenance comes as a motto:”better prevent than cure”. To prevent, here, it is to use NDT.

VI - General MT uses

MT processes are versatile enough to allow for inspection of many parts one at a time, or only one part, or entire large equipment or only some areas of large parts. However, inspection of complex-shaped or very large parts may require several steps.

Black magnetic inks together with white contrast aid paints are the most suitable for inspection of mechanically welded assemblies while fluorescent magnetic inks are often preferred for inspection of mechanical components.

For ‘‘on-the-spot” inspections, spray cans are very convenient.

Fluorescent oil-based magnetic inks are used for inspection of the most critical parts in aerospace industry, for instance, while in the automotive industry water-based inks are also used.

VII - Industrial fields and MT use

It is impossible to name all the industrial fields where MT is used. We are to give only some examples.

VII.1 - Welds

Hand-held electromagnets are by far the preferred way. When no mains is available or for safety reasons, permanent magnets may be used.

Along the weld bead, the heat affected zones (HAZ) shall be checked: this is a 3 to 5 cm (1 to 2 inch) large area on each side of the weld bead.

White contrast paint and a black magnetic ink are the most widely used MT materials then.

The contrast paint somehow allows for “smoothing" the area under inspection and then eases the free movement of the magnetic particles attracted by the flux leaks due to discontinuities. It also increases the contrast when using nonfluorescent magnetic particles (generally red or black).

Sometimes a fluorescent magnetic ink is used especially in vessels/reservoirs to prevent applying then wiping off the white contrast paint in a confined area.

Low voltage (42 or 48 volt, sometimes 24 volt) hand-held electromagnets are mandatory when in vessels/reservoirs to meet safety requirements.

Magnetization conditions are checked with a tangential magnetic field meter or flux indicators such as the Berthold penetrameter, the ASME pie gage, etc.

MT is one of the main NDT methods for steel metalworking, pipelines, gas transport, etc.

Though MT materials in spray cans are more expensive than the same products in bulk, using spray cans for this application is, in fact, cheaper if we take into account materials losses and labour costs.

VII.2 - Aerospace

Aircraft structural elements are mainly manufactured from light alloys (aluminium, aluminium-lithium, titanium) and composite materials.

Less and less ferromagnetic materials are used for mechanical components. Where non-ferromagnetic materials replace ferromagnetic ones, Magnetic Particle Testing (MT) is very often replaced by PT. However, over the last 4 or 5 years many magnetic benches have been supplied to the aerospace industries as single parts are bigger and bigger; further, new regulations on hygiene and safety for people close to strong magnetic fields have been enforced, along mandatory inspections in which a lot of parameters shall be recorded as per primes’ specifications and so, written in NADCAP questionnaires.

Only oil-based fluorescent magnetic inks are used.

Parts are processed on MT benches and magnetization conditions are checked with a tangential magnetic field digital meter.

After inspection, a residual magnetization may be found in parts, often above the acceptable figures as per the relevant specifications and the technical requirements that come with the customer’s order. Parts need to be demagnetized down to the right level, using a demagnetisation unit or any other relevant method. The residual magnetization of parts is checked with a residual magnetic field digital meter. Some parts such as bearing cages may require a residual magnetization less than 0.1 mT. It is a pity that such a low magnetic flux density figure cannot be achieved for sure: one must consider the uncertainty of measurement due to the direction of the part in the Earth magnetic field, due also to the quality of the meter and its calibration. Keep in mind that the Earth magnetic field, depending on the location, may be as high as 60 A/m (0.075 mT). In Paris, France, it is 33 A/m (0.04 mT), the horizontal vector being at 16 A/m (0.02 mT). This means an uncertainty of 20%, when 0.1 mT is asked for ... just because of the Earth magnetic field! Add many percentage points due to the calibration, display given at +/-1 digit by every manufacturer ... You may begin to scratch your head about how to meet such a requirement!!!

- VII.2.1 - Structural elements

As written above, ferromagnetic materials are less and less used for structural elements; that is why MT is less and less used for this application.

Let us take an example:
Some time ago, bolts attaching the wheel halves and the engine pylons were made of carbon steel, while nowadays, the non-ferromagnetic Inconel® is the alloy of choice for these parts.

- VII.2.2 - Gas turbines parts

Some blades are still made of ferromagnetic alloys. They are then MTested, as well as the turbines and compressors shafts.

- VII.2.3 - Other mechanical components

Following components are MTested:
• Undercarriage structures (landing gears).
• Gearings.
• etc.

- VII.2.4 - Maintenance

MT is widely used in overhaul and repair of aero-engines for fatigue cracks detection on components made of ferromagnetic alloys such as: some blades, compressor and turbine blades, gears, etc.

Same as above for undercarriage structures (landing gears), gearings, etc.

VII.3 - Nuclear

In pressurized water reactors (PWR), many parts of the primary circuit are made of stainless steel. Nevertheless, what we call in French “the two viroles” (“ferrules”) as well as the cover and the bottom of the reactor are made of carbon steel; two successive layers of stainless steel are then “fused” on the inside surface of the individual parts to prevent corrosion. The secondary circuit is made of many carbon steel parts which are MT inspected, when manufactured as well as during maintenance on sites. Hand-held electromagnets plus white contrast paint and black magnetic inks are widely used though, sometimes, other magnetising techniques and fluorescent magnetic inks are preferred.

Spray cans are widely used in nuclear industry, as it is the best way to prevent any accidental pollution of the MT chemicals, by chlorine, fluorine or sulphur containing materials.

- VII.3.1 - RCC-M Code

In France, the basic document for the entire nuclear industry is the RCC-M code. Any material entering a nuclear power plant must be PMUC (the French acronym for: materials and equipment approved for use in nuclear plants).

Due to a large use of austenitic stainless steels based on nickel, all chemicals must show a low level of halogens (Fluorine + Chlorine + Bromine): less than 200 ppm, as well as for sulphur, less than 200 ppm. Sampling and analysis shall be performed as per the EDF (French Electricity Board) pertaining procedures.

These sampling/analyses are carried out by laboratories approved by EDF according to the PMUC D5713 D5713/DJX/RB DJX 90 0295 Index 1 procedure exclusively.
The analysis is made after combustion of entirely organic materials and after mineralization of chemicals comprising a mineral ingredient, such as the developers. This analysis is carried out on the entire material and not on the residue after heating, as described in the ASME code for PT.

- VII.3.2 - ASME Code (Boiler and Pressure Vessel Code)
In the best of our knowledge, ASME Code does not require any halogens and sulphur analysis of MT materials.

VII. 4 – Industrial gas turbines maintenance

We are to write only about blades inspection during maintenance.

As time allowed for maintenance is quite short, there is no time to tear down blades, check them,  reassembly them and carry out the shaft rebalancing to prevent any unbalance which would be the cause of extremely dangerous vibrations which could lead to catastrophic damages. Therefore, blades are checked on-site as follows.

The electric current goes in several turns of a copper cable wound up to form a flat coil sometimes referred to as “the racket” due to its shape. The racket is put close to the blades. A water-based fluorescent magnetic ink is used.

Some specifications require a 1,200 A (RMS value) AC in a 10 mm dia. (000 American wire gauge), i.e. 70 mm² cable; such a current may be delivered from a portable current generator. The cable warms and the current may be “ON” for only 4 seconds with a duty cycle of 10%, i.e. 4 seconds every 40 seconds. Temperatures above 100°C (212°F), if the system in too intensively used, may destroy the insulating sheath.

Other specifications require a 5,000 A half wave rectified direct current (HWDC) delivered by a mobile, hence heavier, current generator, in a 90 mm² cable (a bit more than 000) or even a 120 mm² cable (a bit more than 0000). Once again, the “ON” time is only a few seconds per minute.
To prevent unreasonable heating of cables, current generators come with a thermal circuit breaker.

VII.5 - Maintenance in hydroelectric power plants

As an example, let us have a look to the maintenance of the PELTON turbines in which a mobile wheel is fitted with curved blades called "buckets" secured on its rim. Inspection of the blades is carried out using a current generator and a fluorescent magnetic ink.

VII.6 - Automotive

- VII.6.1 - Manufacturing
Many castings and forgings are MTed: connecting rods, gears, crankshafts, steering racks, etc. on manual, semi-automatic or automatic benches with fluorescent water-based or oil-based magnetic inks.

We may add inspection of trucks’ leaf springs with a fluorescent oil-based magnetic ink.

- VII.6.2 - Maintenance
For engines overhaul MT is used to check mechanical components such as crankshafts and camshafts with a fluorescent magnetic ink on MT benches.

Magnetic testing is also frequently used during car racing events or periodically. All the parts of the car are dismantled and when it is possible, they are inspected on a magnetic bench.

VII.7 - Railways

- VII.7.1 - Manufacturing
MT is used to check safety parts: axles, bogies, wheels, crankshafts, camshafts, cogwheels etc. Most of these parts are inspected with oil-based fluorescent magnetic though water-based inks are sometimes preferred.

Carriages and wagons coupling hooks are inspected with water- or oil-based fluorescent inks.

Rail tankers also are checked using MT, as they carry chemicals and hydrocarbons, pressurised gases, toxic or corrosive liquids, etc. Therefore, NDT is mandatory.

- VII.7.2 - Maintenance

The same above parts are checked during maintenance.

VII.8 - Marine

- VII.8.1 - Manufacturing

MT is widely used for inspection of welds beads and safety parts.

Welds on hulls and other structural elements are inspected using hand-held electromagnets or sometimes with a permanent magnet and a white contrast paint plus a black magnetic ink. However, PT is used on the nuclear submarines titanium hulls.

Mechanical components of the propulsion line are inspected, including transmission shafts, crankshafts, connecting rods, etc. using a fluorescent oil-based magnetic ink.

- VII.8.2 - Repair

MT is used for inspection of Diesel engines, gas turbines, nuclear power plants, hull weld beads, tanks components as well as propulsion components such as driving shafts, propeller bearing shafts, etc.

VII-9 - Defence

In manufacturing as well in maintenance, MT is widely used on trucks, aircraft and fleet ships, tanks, etc.

All NDT inspections are of the utmost importance as all this equipment shall be available any time for missions in any weather conditions.

VII.10 - Oil fields and services

MT is one of the major NDT methods used on oil and gas exploration or production rigs the world over. As always, its easiness, low cost, availability (permanent yokes, hand-held electromagnets) are among the reasons why so many spray cans are used daily on North Sea rigs ... and elsewhere.

So many parts are inspected while manufactured it is an almost impossible task to list them all. Among them: drilling heads and pipes but also pumps, valves.

Preventative maintenance is another reason to perform MT on a large scale as any in-service break leads to production shutdown with costly consequences.
Imagine an exploration pipe which breaks at 1,000 meters (ca 3,300 feet) underground ... or at 500 meters (ca 1,600 feet) under the sea bottom, 1,500 meters (ca 5,000 feet) deep!

- VII.10.1 - Underwater inspections

MT is used to inspect immersed structures such as oil rigs and offshore drilling platforms. Specifically trained divers carry out inspections.

The magnetic ink is prepared by mixing a dual-purpose magnetic powder (seen as red in daylight and fluorescent orange under UV-A radiation), or a golden coloured one (gold colour is easily seeable at some dozens yards deep in water), with surfactants in water. The “continuous method” is used, i.e. the magnetic ink is applied with a specific hand sprayer during magnetization.

Equipment comes from two directions: either standard equipment modified for subwater use, or equipment specifically designed for underwater use.

→ VII.10.1.1 - “Standard equipment modified for subwater use”
This is the cheapest way but generally not the most convenient for the diver.

A waterproof hand-held electromagnet connected to a half wave rectified direct current (HWDC) mains is a kind of standard equipment. It shall be disconnected when not in water, as there is no button switch on the electromagnet. When in water it is cooled by water, but out of water it could warm up to unacceptable temperatures.
It comes with a several dozen meter (yard) cable plugged to a current transformer rectifier put on a lighter floating on water.

Ultraviolet (UV-A) sources are not watertight; they shall be put in a waterproof case fitted with a glass window which lets the UV-A radiation go through. Polycarbonate shall not be used as it blocks UV-A radiation. Many UV-A sources produce heat: this must be thought of when designing the equipment. Light-emitting diodes (LEDs) may overcome the problem.

→ VII.10.1.2 - Specific equipment
Some few companies have a range of equipment specifically designed for underwater MT which makes it easier for the diver to perform his task.

A system comprises a 12 volt battery, a hand-held electromagnet and a UV-A xenon source or a UV-A LEDs source, all with button switches.

→ VII.10.1.3 - New technique
A new technique is described in a just published newsletter (in French only). Here is an adaptation of this document(4).

It is known that seawater is relatively transparent to blue light and much less to UV-A radiation. Below 10 meters deep, there is almost only blue photons, the red-orange photons being partially absorbed.

As, nowadays, there are many waterproof light sources for underwater lighting based on powerful white light-emitting diodes (LED) that need only to be filtered, and actinic blue light sources designed for oceanic science such as coral breeding, there is an ample choice of sources.

The blue actinic light (420 - 460 nm) has also a powerful effect on some dyes that fluoresce in the yellow/orange (560 - 660 nm) range; the green light (540-560 nm) may even give a red fluorescence (660 - 700 nm).
It occurs that some dual-purpose magnetic particles, red under white light and orange under UV-A radiation, emit a much stronger fluorescence when under blue actinic light.

Using dual-purpose particles along these sources allows then for the improving of MT inspection and of viewing conditions, even in daylight, as these particles may also be used under white light.

A yellow filter, needed to block the glaring blue beam, is exactly the right thing for underwater examination: below 10 meters underwater, the photons are mainly blue. The yellow filter is then a kind of veil, and the diver is as in darkness, even if it is 2 pm. Better for the divers’ and ships’ safety than working at night.

VII.11 - Metallurgy

Among the parts of interest: bars, pipes, profiles.

On unmachined parts, dry magnetic powders are used while magnetic inks are used on machined parts.

A fluorescent magnetic ink is generally used on forgings and castings.

Many rollers of roller-machines are checked using a magnetic ink for inspection either in white light, or under (UV-A) ultraviolet radiation. As rollers always turn in the same direction in the Earth magnetic field, they become magnetized.
This magnetization may modify the direction and the figure of the magnetic field produced in the part during MT; use a tangential magnetic field meter to check."Purists" only (mainly in aerospace industries) demagnetize these parts before MT. Some services companies have the means to demagnetize rollers.

VII.12 - Ski lifts

MT is largely used in maintenance.

Added to the inspection during manufacturing and commissioning, ski lifts (T-bar, chairs, gondola and cabin lifts, etc.) are inspected on a mandatory yearly basis. Welded assemblies (pylons for example), forgings, castings, idler wheels, etc are inspected by MT, mostly with a hand-held electromagnet or a permanent magnet (when mains is not available), a white contrast paint and a black magnetic ink.

Some parts most important for safety, such as axles or fasteners, are inspected with a fluorescent magnetic ink under UV-A radiation.

VII.13 - Lifting devices

Friction of slings leads to metal smearing on contact areas of the cranes hooks, therefore PT cannot be used.

MT with an oil-based fluorescent magnetic ink gives good results.

In some countries, mandatory inspection of the forklifts’ forks is limited to a visual testing of performance when loaded.

This test is as follows: put one or several cast iron weights on each fork, depending on the maximum load acceptable by the fork under test. The inspector only looks for a crack appearing in the curved area connecting the vertical and the horizontal branches of the fork. This area bears the most of the efforts when in use and cracks will eventually come in this area, perpendicular to the efforts direction. At its very beginning a fatigue crack is so tiny it cannot be seen by the naked eye.

MT is an easy way to detect these cracks even at their beginning. A hand-held electromagnet, a white contrast paint and a black magnetic ink: that’s it! Within seconds, the inspector detects the crack…even if the fork is not under stress

VII.14 - Public works & mines

Public works equipment faces harsh conditions when in service and require thorough inspections to prevent accidents involving equipment or leading to injuries.

Parts of cranes’ structure are inspected using a hand-held electromagnet, a white contrast paint and a black magnetic ink.

VII.15 - Plants maintenance

In many plants, sugar factories and cement plants being only examples, welds and mechanical components, pinions or reduction gearbox cases, shall be inspected during down times. Magnetic particles inspections are generally carried out with a hand-held electromagnet, a white contrast paint and a black magnetic ink, sometimes with a fluorescent magnetic ink.

In quite a different area, demagnetization of rollers of rotary printing press is carried out after MT.

VII.16 - Amusement ride maintenance

On roller coasters and the other high-risk merry-go-rounds, a large number of stressed parts are made of carbon steel. That’s why MT is the NDT method of choice. Some amusement parks have their own NDT department.

VII.17 - Machining

Machining cracks are detected using a fluorescent magnetic ink.

VII.18 - Heat treatments

Heat treatments may give rise to cracks which may be tiny, quench cracks for example. If the material is checked by MT, these cracks can be detected if a fluorescent magnetic ink is used.

VII.19 - Sintered metals

Sintered metal parts show a high but controlled porosity. The void ratio may be such that PT is not usable: the very important background may make it very difficult to see discontinuities indications as contrast would be impaired.

In some cases, a fluorescent magnetic ink is the right choice.

VIII – Other applications

Other techniques are used:

• High temperature MT.
• Low temperature MT.

Some examples are described underneath.

VIII.1 - High temperature (HT) MT

Water-based or oil-based magnetic inks cannot be used at more than 80° C (ca 175°F). Another organic carrier could be used but probably not above 120°C (ca 250°F).

A good choice is the dry powder technique. Colour contrast dry magnetic powders, depending on type and colour, may be used at temperatures up to 200-300°C (ca 390-570°F) and even higher than 400°C (ca 750°F) for some of them. When at high temperature for a time some dry magnetic particles may be altered: colour fades, they stick on each other and they may give rise to smoke. Some types at more than 482°C (900°F) or passed through a flame may smoulder or spark.

High temperature MT is also very useful during the periodic inspections of units which cannot be cooled or for which any stop leads to risks for safety or to expensive production losses.

Many examples are available: nuclear or coal/gas/oil fired power plants, chemical and petrochemical plants, industrial boilers with their piping and heat exchangers, oil refineries, etc.

The surface temperature is measured with a calibrated digital thermometer fitted with either a surface probe or a thermocouple.

VIII.2 - Low temperature (LT) MT

Water- based magnetic inks are not designed to be used at temperatures lower than 0°C (32°F).

Oil-based magnetic inks may be used down to a temperature as low as -40°C (-40°F). Other organic carriers may be used, such as light petroleum distillates, alcohols, etc. to work at lower temperatures. However, keep in mind that some UV-A sources may not work at very low temperatures: better to check this point before going to the North or South Poles!

It is also always possible to use the dry powder technique.

Spray cans propellent is mostly LPG (liquid petroleum gas) or DME (dimethylether), both being liquefied gases. When temperatures are very low, the pressure in spray cans using liquefied gases may dramatically drop, more than in spray cans propelled with compressed gases, such as carbon dioxide CO2. This is why the spray cans shall be kept at a minimal temperature of + 10°C (50°F); quite easy to do: just keep spray cans along the body in underwear, for instance.

IX - Limits to PT use

As any NDT method, MT has its own limits, which we will explain in a soon-to-be-published document.


This paper is not supposed to be all-comprehensive, its purpose is just to show how much MT is useful in many industries.

Note for our readers

The above-mentioned pieces of information are based on our current knowledge and on the results of our long experience. However, our readers’ attention is drawn to the fact that they are given for information only, and they do not come as a specification/recommendation: we cannot be held responsible in any way.


(1) Pierre Chemin and Patrick Dubosc, Magnetic Particle Testing History: On our Website.

(2) ISO 9934-2:2002 Non-destructive testing - Magnetic particle testing - Part 2: Detection media, International Organization for Standardization, Geneva, Switzerland, 2002.

(3) Nathanael RIESS, Andre IVANKOV, Harri HAACKE, Rising Costs and Price War - To the Debit of Quality, in Spite of European Standardisation? (Quality Requires its Price; Critical Views; Negative Examples by Means Of MT- and PT-Inspection Media), 9th European Conference on NDT, Berlin (Germany), September 2006.

(4) Babbco Info N° 206 La fluorescence sous un autre jour (Editor's note: Fluorescence under a different light), february 2011 on this Website.

• RCC-M Design and Conception Rules for Mechanical components of PWR Nuclear Islands, 1st Addendum (December 2008) available in French and English versions, 2nd Addendum (December 2009) available in French version, AFCEN, 1 Place Jean Miller, 92084 La Défense cedex, France.

D.5713/DJX/RB 90 0295 Indice 1, Procédure de détermination des teneurs en halogènes et en soufre des produits et matériaux utilisables en centrales – PMUC (Editor's note: Procedure for measuring halogens and sulphur contents of materials and equipment usable in nuclear plants - PMUC), EDF, Groupe des Laboratoires (GDL), Service Contrôles Physico-Chimiques (SCPC), 21 Allée Privée, Carrefour Pleyel, F-93206 Saint-Denis cedex, France, September 23,1994.

• ASME ASME's Boiler and Pressure Vessel Code (BPVC), West Caldwell, New Jersey, 2010 Edition.

Last Updated ( Saturday, 12 October 2013 14:17 )