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Penetrant testing industrial uses

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Written by Administrator
Monday, 01 November 2010 15:42

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

November 2010

I - Introduction

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

In most cases, we suggest the sensitivity level of the penetrant system to use in order to meet the acceptance criteria.

As a further information we give examples of specific uses such as the high temperature penetrant testing, the low temperature penetrant testing, the water-based penetrants and inspection of ceramics and plastics.

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.

Penetrant 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.

PT is known and used since the end of the 19th century, when warm oil and chalk or whiting were used.

III - The ever-expanding use of penetrant testing

Since the 1940s, 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, PT ever-expanding use may be understood by its reliability, but is also due to the more frequent use of alloys that do not show the ferromagnetic properties needed to be inspected by Magnetic Testing (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.

If PT is reliable, of an incredible sensitivity for tiny discontinuities detection and if it gives reproducible results with a high confidence, inappropriate processing may lead to deceptive results.

This is one of the main reasons why PT automatic processing has been designed and improved.

IV - PT processes sensitivity levels

Here we use the ISO 3452:2006 standard PT materials' classification, which was taken, with some adjustments, from the American SAE-AMS 2644 specification; further, we will use the ISO 12706:2009 standard terminology.

V - When is pt 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 PT uses

PT processes are versatile and parts’ shapes or dimensions are not a reason against PT. PT may be used to check many small parts at a time or a single small or large part, or some areas only of large equipments. As examples: the daily inspection of thousands of five mm (less than ¼”) long, 2.5 mm (1/10”) diameter screws; inspection of more than 4 meter (ca 13’) long by 2.6 meter (ca 8.5’) high rocket panels; an impressive tank of 240 000 litres (52,800 Imp gal or ca 63,500 US gal) of penetrant to check BOEING B 747 parts, etc.

Colour contrast penetrant testing using a water-washable penetrant is especially suitable for inspection of smooth surfaces and even rough surfaces such as the partly dressed welds and rough castings. It is also used for inspection of “semi-finished products” such as stainless steel slabs.

The two main fluorescent PT processes use water-washable (WW) penetrants or post-emulsifiable (PE) penetrants. The solvent-removal penetrant process is seldom used, more often for “on-the-spot” inspection with spray cans.

A water-washable fluorescent PT process is used for castings, welds, rough-surface forgings, mainly those in light alloys (aluminium, magnesium but also brass) to detect porosities, cracks, cold shuts, laps, bursts and other open to the surface defects.

PE fluorescent PT processes are used for inspection of the most critical parts, the aerospace industry being by far the largest user.

VII - Sensitivity levels

The most widely used sensitivity levels are:

Level 2: Level 2 PE fluorescent penetrants are quite unusual, while Level 2 WW fluorescent penetrants are the most widely used fluorescent penetrants. The latter show good water washability and a good overwashing resistance. These are the most suitable ones for use on rough or quite absorbent surfaces such as castings, rough specimens with their weld beads, and the preforms for machining.

Level 3: Level 3 PE fluorescent penetrants are preferred against WW ones. These penetrants are used for inspection of investment castings, forgings and rotating components as found in turbo engines.

Level 4: Though they exist, Level 4 WW fluorescent penetrants are nearly never used. The penetrants of choice for this level are the PE ones which show not only an ultra-high sensitivity, but that also give consistent results if accurately used. Everywhere the top sensitivity is needed to find the tiniest discontinuities on critical parts along an exceptional reliability, they are widely accepted. Just a few examples from aerospace industry: turbine and compressor blades, turbine disks, bearings, etc.

VIII - Iindustrial fields and PT use

It is an impossible task to present all the industrial fields where PT is used. We are to give only some examples.

VIII.1 - 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 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.

Aerospace industries allow colour contrast PT for very specific situations: in situ inspection of non-critical parts. Quite often, particular procedures are applied which do not meet the generally accepted specifications' requirements.

Fluorescent penetrant testing accounts for more than 99.999% of all the PT inspections in the aerospace industries. Structural elements are inspected, very often with Level 2 and sometimes Level 3 WW systems. Mechanical components are inspected with Level 3 and Level 4 PE systems.

For instance, the Space Shuttle reliability relies upon PT inspection of thousands parts.

The large structural elements, i.e. the “body” of the rockets/engines as well as almost all the components in contact with liquid hydrogen and liquid oxygen are also safe, thanks to PT.

- VIII.1.1 - Structural elements

Level 2 WW fluorescent penetrants are widely used for inspection of:
• Air frames.
• Wing stringers, wing spars, stiffeners.
• Nozzle guide vanes (NGV).
• Flame tubes.
• Wheel rims.
• Bolts attaching the wheel halves and the engine pylons.
• Metal works such as casings, rings.
• Non-rotating components.
• etc.

For more critical or strained parts Level 3 WW fluorescent penetrants are used, but volumes are significantly lower than Level 2’s.

- VIII.1.2 – Gas turbines parts

PE fluorescent penetrants of various sensitivity levels are used for inspection of:

- Rotating parts of gas turbines such as disks (Level 4) and blades (Level 3 and sometimes 4), rotors.

- VIII.1.3 – Other mechanical components

PE fluorescent penetrants of various sensitivity levels are used for inspection of:
• Undercarriage structures (landing gears).
• Bearings.
• etc.

- VIII.1.4 - Maintenance

PE fluorescent penetrants are widely used in overhaul and repair of aero-engines for crack detection as their sensitivity to detect these discontinuities at an early stage is much higher than WW PT systems’.

Honeycombs and adhesively bonded aircraft structures are inspected in maintenance to help finding where water comes from in areas where water and corrosion may damage the material.

Combustion chamber barrels are inspected before repair.

VIII.2 - Nuclear

Most of the inspections are carried out using colour contrast PT processes as well on site as in the workshop.

Stellite coatings on valves are also inspected this way, though a WW fluorescent Level 2 penetrant would be, by far, the best choice.

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

- VIII.2.1 - 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 equipments 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.

- VIII.2.2 - ASME Code (Boiler and Pressure Vessel Code)

Exactly for the same reason as above, halogens and sulphur content shall be measured. Sampling, analysis method and the maximum allowable content are different. (refer to Section V, Article 6, paragraph T-641, and Article 24 as appropriate).
For instance, bromine is not included in the halogens' content. The acceptable maximum is 1% on the residue, sometimes 0.1%. Keep in mind two points:

• The American method cannot measure very low content. The scope of the document states that it is impossible to measure less than 1,000 ppm. That is why we are somewhat surprised that some customers require, for instance, a maximum of 200 ppm as per the American method ... and that suppliers “agree” to supply certificates giving figures in the 70/80 ppm range!!! In fact, the suppliers write on the certificates the figures got from other accurate sampling/analysis methods with a lower detection threshold to meet the customers’ requirements. Otherwise, the customers would probably choose to work with another supplier!!

• Analyses are carried out on the residues of penetrant materials after heating for 60 minutes at a temperature of 194°F to 212°F (90 – 100°C), or at the boiling temperature if lower (as for solvents for solvents and non-aqueous wet developer). That means that volatile sulphur or halogens may evaporate before analysis. This is not theoretical. Penetrants, for instance, it is their duty, are prone to enter even small and tight discontinuities. If they contain sulphur or halogens, these chemical elements will enter ... and may stay in the discontinuity even after the developer has pulled out some part of the penetrant. Corrosion may occur ... while the analysis performed on the residue after heating may show a suitable content of these elements.

VIII.3 - Automotive

- VIII.3.1 - Manufacturing

Many parts are aluminium - or non-ferromagnetic alloys-based. Non-critical parts may be tested in automatic fluorescent penetrant inspection (FPI) process lines after sampling, while 100% of the critical parts will be tested. Here is a not thorough list of parts:
• Cylinders blocks.
• Engine casings.
• Engine mounts.
• Gearbox casings.
• Steering systems (spindles).
• Suspension components (suspension arms).
• Brake systems (swiveling stirrups).
• Pistons.
• Aluminium or magnesium wheel rims.
• Air cooling systems: leaks.
• Valves.
• Etc.

More and more, WW water-based fluorescent penetrants are used, chosen for their good washability, while keeping a good sensitivity, even if used without any developer. Nevertheless, preliminary tests are necessary, as some water-based penetrants are prone to overwashability, hence have a very low sensitivity and reliability.

These penetrants are generally supplied as concentrates to be diluted with an equal volume of water. This solution is right to detect cracks, shrinkages, puckering and porosities on aluminium castings; but they are used undiluted on safety parts.

On the other hand, stellite plated valves are tested with a Level 2 oil-based WW or PE penetrant.

Leak detection in air-conditioning systems is performed using a dye additive, fluorescent under UV-A radiation. This additive is standard in the entire European production. After a 3 to 5 minute running, it is easy to detect any leak using a UV-A source in dimmed visible light.

- VIII.3.2 - Maintenance

Colour contrast penetrant testing is used by repair workshops for several applications.

Engines MRO shops also use FPI to check many parts such as pistons, engines casing, cylinder heads, etc.

→ VIII.3.2.1 - Oil leak detection on engine casings
First clean the engine’s outside surfaces and, using a spray can, spray a non-aqueous wet developer (NAWD) on the clean areas.
Then, run the engine for 15 to 30 minutes. Then, the oil warms up, its viscosity decreases and the micro-leaks tend to open; the developer pumps the oil by capillary effect. After 15-30 minutes, the leaks will be seen as brownish spots against a white background.
Oil went as a penetrant: it is called through-leak penetrant testing. The same procedure is used to detect leaks on gearbox casings. Just drive the car or the truck for 15 to 20 km (9 to 12 miles) while often changing gears.

→ VIII.3.2.2 - On car racing circuits
Safety parts may be inspected by the teams’ mechanics using a colour-contrast PT.

VIII.4 - Railways

- VIII.4.1 - Manufacturing

Using light alloys for the duplex AGV and TGV (High Speed Train) carriages and stainless steels for passenger wagons also gave a revival to PT since MT is ineffective on these alloys. Here again, PT is used for cracks, porosities and puckering detection.

→ VIII.4.1.1 - Weld inspection
Further to the above examples about passenger cars, rail tankers are also tested, as they carry chemicals and hydrocarbons, pressurised gases, toxic or corrosive liquids, etc. Therefore NDT is mandatory. Leak testing, or leak-through penetrant testing, as described underneath, is often performed in addition to MT and UT.

→ VIII.4.1.2 – Leak Testing (LT)
On newly-built railway carriages, roof watertightness is checked using a water-based water-washable fluorescent penetrant diluted at 50% in water. The penetrant is applied on the roof outside surface. After penetration time, the roof inside surface is checked for leaks on the weld beads using a UV-A light. Should leaks be detected, welds are repaired as appropriate. Before painting, carriage’s outside surfaces are alumina blasted. The penetrant and the PT process shall be designed to have no detrimental effect on the paint and its adhesion.

- VIII.4.2 - Maintenance

In France PT has been used first to check wire wheels that fitted wagons then. Furthermore, Diesel-electric locomotives comprise far fewer ferromagnetic parts than steam engines.

→ VIII.4.2.1 - Valves
On valves seatings very superficial and thin fatigue cracks are likely. Careful and complete valve descaling and cleaning with suitable chemicals are mandatory to allow detection of these cracks. Level 2 WW fluorescent penetrants, or occasionally, a Level 4 PE fluorescent are used.

→ VIII.4.2.2 - Wheels
The web-rim fillet is the location of choice of extremely dangerous fatigue cracks, which may be due to different causes: blowholes, cold shuts, delamination, decarbonization machining cracks, etc.

→ VIII.4.2.3 - Fuel circuits of Diesel-electric locomotives
Leak-through penetrant testing is easy, if a fluorescent additive is added in “homeopathic” dose to the fuel. Inspecting external surfaces of the fuel circuits with a UV-A source, in the right viewing conditions, allows for detecting micro-leaks. This same process may be used for hydraulic systems, using an additive of a different colour, just to prevent any confusion.

→ VIII.4.2.4 - Inspection of Chrome-plated hinge joints connecting rods on the crankshaft of locomotive thermal engines
Failure of this joint would be catastrophic for the engine. This inspection is carried out with a Level 2 or 3 WW fluorescent penetrant.

→ VIII.4.2.5 - Weld inspection in situ under an aluminium alloy chassis
Using a colour contrast solvent-removable thixotropic penetrant.

→ VIII.4.2.6 - Crack detection on gear teeth
In situ gears and reduction gears inspection without disassembling using a Level 2 fluorescent thixotropic penetrant.

→ VIII.4.2.7 - Other parts inspected by PT
Cylinder casings, cylinder heads, pistons, pinions, bearings, etc.

VIII.5 - Marine

- VIII.5.1 - Manufacturing

Colour contrast PT is widely used for inspection of welds beads and safety parts.

- VIII.5.2 - Repair

Colour contrast PT 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.

VIII.6 - Weld bead inspection

This inspection is most usually carried out with a colour contrast PT system is used. Indeed, WW colour contrast penetrants are the best choice for rough surfaces, such as half-dressed welds.

A reliable inspection needs the penetrant to be applied not only on the weld itself, but also on the adjacent surfaces on a ca 25 mm (1”) width.

These 25 mm wide zones are called the heat-affected zone (HAZ).

This is because heat-generated cracks may appear in this area of the welded parts.

The inspector must be aware that PT inspection reliability may be questioned if the two sides of the bead are not enough wet by penetrant.

Savings from a too small width covered with penetrant is then against safety.

Though PT 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 manpower costs.

When using spray cans, usual data are:
• ca 20 metres (22 yards) per hour of weld bead on carbon steel
• ca 30 metres (33 yards) per hour of weld bead on stainless steel
may be checked.

VIII.7 - Leak Testing (LT)

Leak-proof equipments are a permanent requirement in industry and in our daily life.

The multiple damages due to lack of watertightness of vessels and circuits may not lead only to large industrial plants shutdowns, but also may harm or kill human beings.

Leak Testing may be carried out while manufacturing equipment, when commissioning it and, later, during maintenance. There are several NDT methods to detect leaks, including ultrasonic probes able to detect the ultrasonic frequencies from a small leak. Here we detail the lonely ones using penetrants.

- VIII.7.1 - Methods based on liquids' diffusion

→ VIII.7.1.1 – Through-leak Penetrant Testing
By far the most widely used PT technique is the “conventional” technique, i.e. the one when penetrant and developer are applied on the same surface.

The way through-leak penetrant testing is used depends on the application.

Of course, as we are looking for “through defects”, both surfaces shall be cleaned and moisture-free. Idem for the “through defect” itself.

In general industry, say, vessel manufacturing, a water-washable colour contrast penetrant (Level 2 according to ISO 3452-2:2006 standard), either from a spray can or in bulk, is applied on the inside surface or on one side (according to the part geometry) and a non-aqueous wet developer (NAWD) is sprayed from a spray can on the outside surface or on the other side, or on the surface the easiest to be viewed.

In France, to check electric transformers tanks (then designed to use PCBs as coolants) for leaks, inspection of welds is often performed using a water-based, WW fluorescent penetrant. No developer is used, viewing being under UV-A radiation in suitable viewing conditions. However, some transformers’ manufacturers still rely upon the colour contrast technique: an indication which was not previously seen may yet be detected when the transformer is put on the truck for delivery! Some leaks are so tiny, added to a length of more than 30 cm (12”) between the “entrance point” and the “exit point” of the penetrant, that more than one day is needed to detect them! This is an extreme occurrence.

How does the process work? The penetrant enters any micro-channels which, together, make the leak. As a rough guide, the time needed for the penetrant to exit is about5 minutes for a 1 to two mm (40 to 80 mils) thickness, 10 minutes for 2 to five mm (80 to 200 mils) and 20 minutes for 5 to 10 mm (200 to 400 mils) ... with some specific exceptions, as seen just above.

For applications on aerospace parts/circuits, use ONLY penetrants and/or tracers approved by the primes.

Many are the variations:
• The liquid used to fill the vessel/pipes may play the whole part of the penetrant, the developer being applied on the external side: such is the case of oils filling the engine casings.
• The developer can be deleted and replaced by a transparent glass or plastic suction cup, connected to a vacuum pump.

After this test, if a colour contrast penetrant has been used, it is necessary to completely and carefully remove the colour contrast penetrant which could remain on the surface and in the leak before it dries, if the equipment must be painted later. In fact, when painting, solvents and thinners contained in paints could dissolve the remaining traces of the dried penetrant, and unacceptable red spots could appear on the paint layer.

To prevent this, the use of a fluorescent penetrant is a good idea, as remaining traces of penetrant could be detected only when using a UV-A source ... or sometimes in plain raking sunlight since the diffraction due to the dyes are more visible than under illumination at 90°.

→→VIII.7.1.1.1 – Leaks of fluorescent liquids
In any circuit of water, oil, coolant, it may be useful to dissolve a very small amount of a fluorescent tracer compatible with the liquid. Then, we make the liquid circulate into the circuit. It is then easy not only to detect a leak, but also to know where it is, without any shut off or dismantling of the circuit.

Two types of carriers are used: an aqueous carrier or an organic carrier.

VIII.7.1.1.1.1 - Aqueous carrier
Fluorescent dyes are added to water. As water has a high surface tension, its penetrating power is very poor. To overcome this, some surface active agents (surfactants) are also added. Leak detection is very easy when using a UV-A source in suitable viewing conditions; wearing specific goggles could greatly enhance the contrast between the indication and the background. Cooling and air-conditioning systems, tanks and pressurized vessels are checked this way.

The process is as follows:
• Using a metering pump, add the fluorescent dye to get a homogeneous aqueous solution.
• Run for 10 to 20 minutes and check. However, longer running times may be needed for very small leaks.

VIII.7.1.1.1.2 – Organic carrier
In aerospace industry, two additives, one for jet fuels, the other one for lubricants and hydraulic circuits, have been designed. The jet fuel additive must not have any detrimental effect on the inside surface of fuel tanks covered with a slosh coating compliant with the American specification SAE AMS-S-4383 C ‘‘Sealing Compound, Topcoat, Fuel Tank, Buna-N Type’’.

These additives shall be compatible with any of the items comprised in the circuits; for instance, no clogging of oil or fuel filter is acceptable.

Very similar to the aqueous additive’s one, the process is to add gently the organic carrier-based additive in the circuit, to let the equipment run for a specified time and to check as above for any fluorescent indication.

Coolant and air-conditioning circuits may require specifically designed organic-carrier-based additives, as the oils used in these circuits are very different from lubricants or jet fuel. Otherwise, the process is exactly the same.

In short, always use the tracers approved for use in a specific circuit. Even for air-conditioning circuits, depending on the pressurised gas which is used, lubricating oils of the compressors may be different and require the exact additive. Do not think that “an additive is an additive, good for all sorts of applications”.

VIII.8 - Metallurgy

Castings and forgings, as well as rolled, drawn and extruded products, are most often inspected using a Level 2 or 3 WW or PE fluorescent penetrant.

On bronze (copper-tin alloy) or copper-tin-lead alloys castings in which copper is the main ingredient, the structure of these alloys sometimes makes it difficult to interpret indications of defects such as: porosities, cracks and shrinkage.

This is the main reason why European standards rule out Penetrant Testing of these alloys from their field of application.

To improve the surface condition while reducing background, a smooth sand blasting may be convenient, if approved by primes/procedures, but it may close the defects or clog them by metal smearing. A slight chemical etching then often comes after sand blasting to “reopen” the would-be defects.

Another specific use of PT on ferromagnetic cast-iron: after an MT with the current flow technique by prods. In fact, at the contact point of prods on the part surface, a spark (due to poor contact with the surface or because the current was switched “on” BEFORE the good contact prods/surface) may give rise to craters.
Many procedures ask for a PT of these craters using a colour contrast system, after a thorough cleaning of the area. Craters, if not removed, may rapidly induce a network of flaws.

VIII.9 - Ski lifts

Though MT is far more used than PT, this one is used for:
• Steel castings.
• Light alloy castings.
• Steel forgings (bars, disks, rings, hollow rough shaped parts, complex shaped parts).
• Welded assemblies.

VIII.10 – Medical

Metallic prosthetic joints are generally checked with Level 4 PE penetrants.

Level 2 WW fluorescent penetrants, used without any developer, are quite often used on polished ceramics (such as zirconia).

IX – Other fields of use

Other PT developments comprise:
• High temperature PT.
• Low temperature PT.
• Water-based penetrants.

We are going to describe below some of their uses.

IX.1 - High temperature (HT) PT

Most of the PT standards and specifications require for maximum temperatures ca 50°C (ca 122°F) when performing inspection.

This limit is due to would-be irreversible modifications at higher temperatures:

• Evaporation of its most volatile ingredients increasing its viscosity.

• Thermal degradation of some of its components, such as the dyes. This can also lead to the evolution of harmful or toxic vapours.

• Penetrant drying on the part surface making it difficult to remove the excess of penetrant.

• Clogging of defects, which can prevent any efficient further PT if a new test is required or needed, for example for a counter-valuation.

• Partial or complete discoloration of indications, leading to a dramatic drop in sensitivity and a lack of reliability.

You may find underneath some good reasons for High Temp PT system use.

- IX.1.1 – Multiple pass welding

After welding, the area is hot. It is also likely that the part itself was preheated, for instance. If “normal temperature range” penetrant materials are used for PT inspection of every pass, it is necessary to wait for a while for the part to cool down. Imagine the time needed for cooling, for inspection, for reheating at 140°C (285°F), or even 177°C (350°F) for some steels; the waste of energy. Imagine all the tensile stresses due to several heating/cooling cycles.

Multiple pass welding is used when welding thick stainless steel sheets, for instance, in nuclear industry, but also for chemical industry.

- IX.1.2 - Maintenance

High temperature PT 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 power plants or thermal power plants burning coal/gas/oil fired, chemical and petrochemical plants, industrial boilers with their piping and heat exchangers, oil refineries, etc.

HT colour contrast penetrant processes allow for inspection with sensitivity and reliability similar to “normal range temperature” systems.

Such a system comprises:
• A colour contrast penetrant which can be applied by brush, swab and spray or flowing on.
• A penetrant remover for excess penetrant removal since water cannot be used above 80°C (175°F).
• A non-aqueous wet developer (NAWD).

Another advantage of this process is the very short penetration time. The higher the surface temperature, the shorter the penetration time: it may be as short as 30 seconds at 180°C (356°F), 3 minutes at ca 140°C (284°F) approximately.

The process is as follows:

1 - Cleaning (if necessary).
2 - Surface temperature control.
3 - Penetrant application.
4 - Excess penetrant removal.
5 - Developer application.
6 - Inspection.

The surface temperature is measured with a calibrated digital thermometer fitted with either a surface probe or a thermocouple. Some HT penetrant materials withstand temperatures up to 200°C (390°F).

- IX.2 - Low temperature (LT) PT

To carry out PT in winter or in countries where temperature is often below 10 to 15°C (50 to 59°F), a specific colour contrast system may be used down to -30°C (-22°F).

Once again, water is not welcome as a remover; that is why a clean dry lint free swab can be used to remove the bulk of the surface excess penetrant. Then a rag dampened with a very volatile solvent such as acetone, for example, is used to wipe out the area under test.

A very volatile NAWD is used, the carrier being acetone or even hexane (a mixture of isomers containing less than 5% of n-hexane).

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.3 – Water-based fluorescent penetrants

“Conventional” penetrants are based on high boiling hydrocarbons.

Water-based penetrants were designed to allow inspection of:
• Leak testing of Liquid oxygen (LOX) circuits.
• Concrete and similar materials.
• Composites.
• Polished ceramics.
• Synthetic macromolecules (plastics).

- IX.3.1 – Leak testing of Liquid oxygen (LOX) circuits

Liquid oxygen circuits are very important equipments of the rockets’ cryogenic engines.

Residual traces of oil-based penetrant are substances which, if in contact with liquid oxygen, a highly combustive material, can lead to accidents. Similar result if a rag is forgotten in a LOX circuit.

Further, if oil based penetrants are used to inspect parts designed to work later in the vacuum, a slow degassing of hydrocarbons in the vacuum will occur. On the contrary, water from water-based penetrants will evaporate quickly.

- IX.3.2 - Composites

Some composite materials can be PT inspected provided that the penetrant materials used are compatible with composite materials.

PT can be used to detect delaminations and other open to surface defects caused when drilling and milling or by deformation.

- IX.3.3 - Polished ceramics

Water-based fluorescent penetrants are sometimes used without any developer for cracks and porosities detection.

As an alternative, a Level 2 WW oil based fluorescent penetrant may also be used, without any developer.

- IX.3.4 - Synthetic macromolecules (plastics)

Oil based penetrants may degrade, soften, or cause crazing of rubber, thermoplastic or thermosetting synthetic macromolecules (plastics). A main reason why water-based fluorescent penetrants are currently used without any developer for cracks and porosities detection.

However, due to the incredible diversity of synthetic macromolecules, to the complexity of their polymerisation, to their many cross linking degrees and to the many additives that the manufacturers put into, it is always recommended to make preliminary compatibility tests. Indeed, water-based penetrants may have an adverse effect on the substrate. It can also irreversibly dye the part which makes inspection almost impossible because of too much background. This irreversible colouring can make the parts unsightly, hence unmarketable.

Water-based penetrants are used, for example, to detect porosities on polytetrafluoroethylene (PTFE) coatings.

IX.4 – Electroplating and electroless plating

Let us give some examples among the most significant uses.

- IX.4.1 - Hard chromium plating

Hard chromium plating is applied on some steel parts such as hydraulic cylinders, landing gears, etc. This plating is checked both when manufactured and during maintenance.

The American SAE- AMS 2440B ‘‘Inspection of Ground, Chromium Plated Steel Parts’’ specification was designed to assist inspecting facilities with planning and required proper evaluation of defects on hard chromium coating plating.

Cracking on hard chromium plating is known as ‘‘chicken wire’’.

This type of defect is seen, for example, on aircraft landing gear pistons (the exterior surface of pistons is chrome plated).

Cracking may have several origins:
- Flaws in the steel substrate.
- Electroplating operating parameters.
- Machining parameters of the hard chromium plating (a too high strength applied, excessive heating leading to high thermal stress).
- Chromium bath contamination (by oil by example).

Therefore, the metallic substrate shall be inspected before electroplating. If it is a ferromagnetic steel, this inspection is carried out by magnetic testing (MT). After electroplating, if the plating thickness is equal to or more than 75 µm (3 mils), it is impossible to detect reliably cracks in the substrate by MT.

Chicken wire detection on hard chromium plating is carried out using a Level 2 WW fluorescent penetrant. Nevertheless, a Level 4 PE penetrant is needed to detect the smallest flaws. The hydrophilic emulsifier shall be diluted at 1%, or even at 0.5% in water, to prevent any withdrawing of the tiniest quantity of penetrant out of the discontinuity.

- IX.4.2 - Electroless Nickel (EN)

Porosities on EN coatings can be detected with a water-based fluorescent penetrant. Sometimes it may be used without any developer.

IX.5 - Cutting tools

Carbide tipped tools are inspected by fluorescent PT process. WW Level 2, or better, Level 3 penetrants may be used, with or without a developer. PE Level 4 fluorescent penetrants are even used. Preliminary tests help to write the right procedure.

IX.6 - Machining

Machining cracks are fine enough not to be easily detectable by colour contrast PT.

Machining cracks detection requires at least a Level 2 WW fluorescent penetrant used in conjunction with a dry developer or a non-aqueous wet developer (NAWD).

IX.7 - Heat treatments

Heat treatments may create tensile stress concentrations which can induce incipient micro cracks: the quench cracks.

Crack detection requires at least a Level 2 WW or even a Level 4 PE fluorescent penetrant used in conjunction with a dry developer.

IX.8 - Food industry

It may seem impossible to use PT in food industry: chemicals in contact with foodstuffs!!! In fact, PT materials do not come in close contact with foodstuffs, but they are widely used.
For instance, in plants manufacturing cream and yoghourts from milk, very large heat exchangers, made of scores or hundreds very thin stainless steel sheets, are used to warm milk. On one side, there is milk, on the other side, steam brings the heat to the system. In no way steam and milk must come in contact. This could happen if a leak exists in one, or several, sheets.
Leak-through testing is mandatory on a periodic basis, using either colour-contrast products or a fluorescent system, quite often without a developer then.
This is only an example of PT use in food industry ... but probably the one which uses the most penetrant materials...and man-hours. We know a main PT materials supplier for which, some years ago, about 10% of the sales came from these heat exchangers!

IX.9 - Oil fields

PT 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 (no rechargeable batteries which run out of energy, no sensor lost or broken, etc.) are among the reasons why so many spray cans are used daily on North Sea rigs ... and elsewhere.

IX.10 - General metal works

PT is used in the technical trucks which are on Formula 1 circuits, on aircraft carriers, in so many repair shops of farm machineries, for the inspection of cylinders for rolling-machines, truck engines, etc., that it is impossible to name all the applications.

X - Some unusual applications

There are sometimes unexpected uses of PT materials. We are happy to give you some examples.

X.1 - Fingerprints' detection

The Royal Canadian Mounted Police (RCMP) sprays a Level 2 WW fluorescent penetrant on the areas on which fingerprints could have been left by people they are looking for. The penetrant is “glued” by the lipo-acidic skin coating marks left on glass and other surfaces. This process comes in addition to the cyanoacrylate ester process. (Process often shown in series such as ‘‘CIS: Crime Scene Investigation’’. The part is put into a closed cabinet, cyanoacrylate glue is heated and settles down on the ‘‘wrinkles’’ left by the fingers on the surfaces).

X.2 - Laser Measurement

Non-aqueous wet developers (NAWD) in spray cans are suitable for the application described in the following paragraph, provided that the developer pigments do not scratch the surface when the developer coating is removed with a brush after use.

It is applied on parts prior to geometry and/or dimensions measurement by laser scanning. It prevents the troublesome reflection of the laser beam on reflective surfaces (made of aluminium alloys, for instance). This coating ensures an even diffuse reflection independent of the nature and colour of the surface, but reliably reproduces all the contours of the area.

Calibration of devices designed for use in outer space is eased by applying a thin, even layer of some non-aqueous wet developers to average the thermal response of a surface.

X.3 - Crash test

Crash test is a destructive testing used by car manufacturers in which a dummy is covered with a non-aqueous wet developer (NAWD). When head-on or side impacts occur, it is easier to determine the impact areas.

XI - Limits to PT use

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


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.


References

• ISO 3452-2:2006 Non-destructive testing - Penetrant testing - Part 2: Testing of penetrant materials, International Organization for Standardization, Geneva, Switzerland, 2006.

• SAE-AMS 2644E: Inspection Material, Penetrant, October 2006, available from Society of Automotive Engineers (SAE), 400 Commonwealth Drive, Warrendale, Pennsylvania 15096, 2006.

• ISO 12706:2009 Non-destructive testing - Penetrant testing - Vocabulary. International Organization for Standardization, Geneva, Switzerland, 2009.

• 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 Defense 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 equipments 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.

• SAE-AMS 2440B Inspection of Ground, Chromium Plated Steel Parts, available from Society of Automotive Engineers (SAE), 400 Commonwealth Drive, Warrendale, Pennsylvania 15096, July 2007.

• SAE AMS-S-4383 C Sealing Compound, Topcoat, Fuel Tank, Buna-N Type, available from Society of Automotive Engineers (SAE), 400 Commonwealth Drive, Warrendale, Pennsylvania 15096, July 1999.

Last Updated ( Tuesday, 17 April 2012 18:19 )