CONTENTS
1. INTRODUCTION 01
2. PRINCIPLES OF DYE PENETRANT TESTING 04
3. CLASSIFICATION OF DYE PENETRANT TESTING METHODS 10
4. FLOW CHART OF VARIOUS PROCESS COMBINATIONS 13
5. FLOW CHART FOR PROCESS SEQUENCE OF VISIBLE 14
DYE AND FLOURESCENT TYPE PENETRANTS, 6. DUAL SENSITIVITY PENETRANTS PROCESS SEQUENCE 15
7. TYPES OF PENETRANTS 17
8. THE PROCESS OF EMULSIFICATION 23
9. DEVELOPERS 32
10. ULTRAVIOLET LIGHT 38
11. CLEANING OF TEST SURFACE 40
12. STANDARD SPECIMEN 43
13. SYSTEM PERFORMANCE CHECK / KDM PANELS 44
14. EVALUATION 55
15. INSPECTION 51
16. TEST PROCESS 56
INTRODUCTION LIQUID PENETRANT TESTING
Liquid penetrant test method enhances the visibility of surface breaking flaws such as cracks, fissures, crevices and pores. It can be used very successfully regardless of component size and can tolerate complicated part geometry. Liquid penetrants can seep into various types of minute surface pores or openings by capillary action, because of this, the process is well suited for detection of all types of surface cracks, laps, porosity, shrinkage areas, laminations and similar type of discontinuities. It is extensively used for the inspection of cast and wrought and welded products of both ferrous and non ferrous metals, powder metallurgy parts , ceramics, plastics, and glass objects.
Penetrant testing is used on metals such as aluminium, magnesium, brass, copper, cast iron, steel, stainless steel, titanium and other common alloys. It can also test other materials, including glazed ceramics, plastics, molded rubber, powdered metal products and glass.
Some limitations are, The discontinuity to be detected must be open to the surface and the interior free from foreign materials. The test surface should not be porous. The material under test must not be susceptible to damage from the liquids used for the examination The test process has temperature limitations [ 10 to 520 C ]. [ using special materials, the range is -15 to 2000 C
part temperatures ]. As Special requirements, Penetrant materials must be designed with a low sulfur and halogen content to avoid harmful effects on the test parts. Stainless steels are especially susceptible to corrosion when exposed to chlorine and Carbon steels to sulfur. Titanium is extremely
susceptible to embrittlement when in contact with halogens. High Nickel alloys are also affected by sulpher and halogens. These harmful chemicals can be found in penetrant materials but are limited to 1% by weight of content.
Common penetrant materials attack PVC, making it brittle, which leads to cracking. Liquid oxygen compatible penetrant materials must be used when testing parts that will be in contact with either liquid or gaseous oxygen.
Method : A penetrating liquid is applied to the clean and dry test surface and allowed to enter the discontinuity opening over a period of time. The liquid soaks into material flaws that are open at the test surface.
After a suitable penetration time, the excess surface penetrant is removed and the test surface is dried. A developer is then applied in a thin uniform coating which acts as blotter and draws some of the entrapped penetrant out of the discontinuities. The penetrant stains the developer and the indication becomes visible. The surface flaw becomes increasingly visible to the eye, because the dye spreads in the developer and effectively broadens the trace.
The dye indicator appears either red or blue on a white background for color contrast penetrants or appears Yellow – green or orange - red on a dark violet background, when the surface is illuminated by an ultraviolet lamp for a fluorescent test process. The sensitivity of the test depends on the size of the discontinuity opening and not on the length. Crack width is the more important determinant of a penetrant's ability to detect cracks. Tight narrow cracks, regardless of length, are more difficult for penetrants to find. Fatigue cracks and forging cracks are tighter and require more judicious processing and higher sensitivity penetrants to locate than casting cracks of the same length. The crack limit for this inspection method is approximately 0.5 µm crack width.
Detection of surface flaws depends on the general condition and finish of the test surface. The depth of the anticipated defect should be three times deeper than the general surface roughness "valley" depth. Defect detect- ability can be further diminished by normally acceptable surface conditions such as
tool marks, scratches, scale, edges, grooves, notches, rough weld surfaces etc, where penetrant is trapped.
Although the dye penetrant test is a very easy process, it is also very easy to make mistakes and make the test ineffective. Care must be exercised by the operator in order to perform all the steps of the examination correctly.
Sensitivity – equal or better than MT – better than RT for surface discontinuities 1 μm x 10 μm x 50 μm can be detected. Applications – on all materials – metals(ferrous and nonferrous)
nonmetals (rubber ,plastic etc) all type of defects (open to surface)
Normally not applicable to porous materials (unfired ceramics and powder metallurgical parts) Presently Filtered particle penetrant for porous materials has been developed.
Rough surfaces - > 125 μm-background poses problem in using penetrant testing. Highly coloured (visible or fluorescent) organic dye liquid which is also surface active in nature (called penetrants) is applied on to the clean surface of the component and allowed sufficient time for penetration into discontinuities. The excess surface penetrant on the component is removed. This leaves a clean surface of the component with pentrant residing in the discontinuities. At this point Of time, developr, which is highly absorptive in nature, is applied. The developer brings back or bleeds out the penetrant thereby providing an indication in a contrasting background of white colour of developer.
1.Preparation of part
3. Removal of excess surface penetrant 4. Development
5. Inspection and Evaluation 6. Post Cleaning
Physical principles
Inspection and Interpretation Application of developer
Penetrant application
Fig.1. Principle of Penetrant Testing
Clean surface of Component
liquid penetrant inspection depends mainly on penetrant effectively wetting the surface of a solid work piece or specimen, flowing over that surface to form a continuous and reasonably uniform coating and the migrating into cavities that are open to the surface.
So this ability of given penetrant to flow over a surface and enter surface cavities depends principally on the following:
Cleanliness of the surfaces
Configuration of the cavity
Cleanliness of the cavity size of surface opening of the cavity
Surface tension of the fluid
Ability of the liquid to wet the surface
Contact angle of the liquid
The cohesive forces between molecules of liquid causes the surface tension, a best example for this phenomenon is tendency of liquid over the surface such as droplet Of water, these contracts into sphere, in such droplet, surface tension is counter balanced by internal hydrostatic pressure of the fluid.
When the liquid comes into contact with a solid surface, a cohesive forces responsible for surface tension competes with the adhesive forces between the molecules of the liquid and solid surfaces,, these two forces jointly determines the contact angle between the liquid and wet surface,b There are are three types of liquid wets the solid surface in relation to contact angle,
If contact angle is < 90 degrees liquid having good wetting ability
If contact angle is > 90 degrees liquid having poor wetting ability
The property of a liquid called capillarity is explained as the process of increase in the meniscus or of the fluid in a thin tube over the water level when this tube is immersed in water in container, this capillary rise is also closely related to wetting angle between the liquid that wets the tube and the inner surface of the tube, Wetting angle between capillary tube And liquid Less than 90 degrees then meniscus of the liquid will be concave and liquid level in the tube raises in the tube over the liquid level in a container – having good wetting ability
Physical principles
Capillary pressure:
h = 2T cos
θ
w
ρ
g
Wetting ability
good, poor, worst
θ
<90°
= 90° > 90°
Wetting angle between capillary tube And liquid greater than 90 degrees then meniscus of the liquid will be convex and liquid level in the tube reduces in the tube over the liquid level in a container – having poor wetting ability
This is due to hydrostatic pressure immediately under the meniscus is reduced by the distribution of surface tension in the concave surface, and the liquid is pushed up by the hydraulically transmitted pressure of the atmosphere at the free surface of the liquid outside the capillary tube.
The height to which the liquid rises is directly proportional to the surface tension of the liquid and to the cosine angle of contact, and it is inversely proportional to the density of the liquid and the radius of the capillary tube, This above explained similar phenomena is applicable for the penetrants entering the discontinuity and seeping out when developer is applied.The term viscosity is also related to rate at which the liquid will flow and it is having negligible effect in penetrating ability. highly viscous liquids are not suitable for not suitable for penetrants, since rate of flow is slower one consequently requires longer time to migrate into fine flaws.
The principle of liquid penetrant testing is based on the ability of some liquids to enter a discontinuity opening and then re-emerge from it when the excess penetrant is removed from the surface. Capillary action is the means by
.
A liquid drop on a ideally smooth, clean surface at equilibrium
γL γSL γSG poor wetting θ>90 Good wetting θ< 90 γL γSL γL COSθ γSG
L = LIQUID- GAS, SG= SOLID-GAS, SL= LIQUID-SOLID FOR
GOOD SPREADING γSG > γSL
The above description is for a liquid drop on a solid surface and above
the liquid is gaseous medium
which a liquid enters a discontinuity opening. This action is what causes a piece of sponge to absorb liquid
Capillary action is a phenomenon in which water or other liquids will rise above the normal liquid level in a small bore or capillary tube due to the attraction of the molecules in the liquid for each other and for the wall of the tube [ cohesion and adhesion ].
Cohesion is interaction between two surfaces of the same material in contact, makes them cling together [ with two different materials the similar phenomenon is called adhesion ]. According to kinetic theory, cohesion is caused by attraction between particles at the atomic or molecular level. Surface
tension, which causes liquids to form spherical droplets, is caused by cohesion.
The distance the liquid will rise up the tube of a given diameter and material is a function of three factors; Surface tension, Wetting ability, Tube open or close at the top end. Liquid will rise less in a closed end tube.
The practical circumstances, a penetrant encounters during testing is more complex. Cracks, for example are not capillary tubes, but simulate the basic interaction between a liquid and a solid surface, which is responsible for the migration of penetrant into its open space. This same interaction acts again and penetrant emerges from the discontinuity when the excess penetrant is
removed from the surface
Factors influencing Penetrant infiltration into discontinuities
1. surface tension of the liquid penetrant 2. discontinuity configuration constant 3. surface coatings and contaminants 4. additives and contaminants in the liquid
penetrant
5. mechanical obstructions 6. temperature of the test object
7. roughness of the interior walls of the
discontinuity
Normally, the penetrants need to have a very low contact angle and the commercial penetrants have contact angles between 0 –5.. Contact angle depends on the solid surface to be wetted. Water-glass has a contact angle of 0 deg. compared to water-silver which is 90 deg
Once the liquid wets the surface, the ability of the liquid t Surface tension Is one of the most important properties which determines the penetrating ability of a liquid. Low surface tension liquids provide better penetration and spreads well on part surface.
Wetting ability is also determines the penetrating ability of a liquid. Ability to wet or spread on the surface is related directly to the contact angle between the liquid and the surface at the point of contact. To have a good wetting ability, the contact angle must be small. Penetrants used for testing have contact angle of 50 or less.
Brightness The dye in the penetrant should be highly stable and bright enough to be visible in very thin film. Volatility is the speed with which a liquid evaporates.
Penetrant should be non volatile to allow long penetration and inspection time. The penetrant must not dry during the examination period. Flash point is the temperature at which flammable vapor is given off. For safety purpose, a penetrant should have higher or no flash point.
Chemical Innertness is the ability of a material not to interact when mixed with or brought into contact with other materials. Penetrant should be as inert and non corrosive as possible towards the materials to be tested.
Solubility is the ability of a material to be dissolved into another material. Penetrant must be soluble in order to be easily removed from the surface of the part being examined. Creep is the ability of small amounts of liquid in discontinuities to com back out to form an indication.
Tolerance for Contamination is the ability to tolerate small amounts of foreign substances and not affect unfavorably the action of a penetrant. A penetrant is a compound of several ingredients and a little water, acids, detergents and degreaser solvents may upset the balance and cause the penetrant to lose some or all of its important properties. Toxicity, Skin irritation and odor No penetrant should contain poisonous, corrosive or skin irritating material or have an offensive odor.
Penetrant properties points to remember
1. Low surface tension and contact angle, capable of smoothly and evenly spread on the part surface.
2. Low viscosity, capable of entering small discontinuity openings faster. 3. Non volatile.
4. Easy removal from part surface but not from the discontinuities. The penetrant should get removed with no dye precipitation.
5. Capable of emerging from discontinuities after excess penetrant is removed.
6. High visibility and contrast in small quantities and thin films on part surface.
7. Does not corrode the test surface. 8. Must be Non Toxic
The rise in the capillary or enter into the openings is determined by surface tension (T)
Hence the main properties of penetrant are T and θ . The ability of the liquid to spread or wet the solid surface is related to the contact angle θ, which quantifies the resultant adhesive and cohesive forces, The contact angle is defined as the angle between the solid surface and the tangent drawn to the liquid at the point of contact. It can be seen that spreading ability and contact angle are inversely related.
Colour contrast ratio of visible dye is ~1:10 (The light reflected by the white background to bright red of the dye). Colour contrast ratio of the fluorescent dye is ~1:100 (light emitted by the indication to the light emitted by the dark background) Because of this colour contrast ratio, the indication is better seen in the case of fluorescent indication . The human eye brings an effect called halation effect, the ability to magnify the indication property of penetrant is dye solouble capability to produce easily visible indications,finally this penetrants must be easily removed by suitable solvents or remover or emulsifier without precipitating the dye,
The performance of a penetrant is achieved by a combination of controlled physical and chemical properties ;
Viscosity Is related to the rate at which a liquid will flow under some applied force. It affects the speed of penetration through the discontinuity opening. A low viscosity liquid is used for faster penetration and also less drag out in immersion.
There are mainly three types of penetrants namely
Liquid Penetrant è TYPE
∗ Fluorescent >>> Type (I) ; Visible >>> Type (II)
∗
Excess Penetrant Removal è Method
∗ Water Washable Method (A)
∗ Post-Emulsifiable, Liphophilic Method (B)
∗ Solvent removable Method (C)
* Post-Emulsifiable, Hydrophilic Method (D)
Developer è Form
∗ Dry Developer >> Form (a)
∗ Water-soluble >> Form (b)
∗ Water-suspended >> Form (c)
∗ Non-aqueous >> Form (d)
Solvents for pre-cleaning è CLASS
∗ halognated è Non-Flammable Class (1)
∗ non-halognated è Flammable Class (2) Application of penetrant
Dwell time : The total time the penetrant is contact with the test surface including the time required for application and for drain .
Dwell time = application time + drain time
Normally 5-30 mts. Depends on the size and nature of discontinuity and the material and surface condition of the material etc. The only requirement is that a thin layer of penetrant should be present for the specified time (dwell time) on the surface to be inspected
Removal of excess penetrant
The excess surface penetrant on the surface is carefully removed without affecting the penetrant that is residing in the discontinuity. In any NDT method the Signal to Noise ratio is important and this should be as high as possible S- Volume of penetrant that has entered into the discontinuity N- Unremoved excess surface penetrant in the discontinuity
This is an important step as the unremoved excess surface penetrant will affect subsequently by affecting the contrast of the indication (excessive background) and if any penetrant in the discontinuity is disturbed the volume of the penetrant indication will get reduced (The volume of penetrant is already very small)
It is obvious that in PT to achieve a high S/N ratio, the excess penetrant on the surface should be removed as completely as possible and the penetrant is the discontinuity should not be lost by overwashing
Penetrant removal methods Dissolve and remove
1. Disperse (emulsify) and remove by water washable/post emulsification 2. Simply water wash if penetrant contains built in Emulsifier (WW)
In each case, care need be exercised so as to completely remove the excess penetrant on the surface completely and at the same time the penetrant in the discontinuity is not affected. Wash – don’t over wash .
In the case, welds, when need be inspected without flushing the crown, the surface oughness poses problems giving background colouration thereby affecting the visibility of the ndication. Hence Solvent removable and water washable
variations are normally employed. Here overwashing tendency of these methods, istaken to advantage, meaning, the surface is much free from the background colouration. Of course, the sensitivity suffers. Sensitivity depends on the amount of retained penetrant in the discontinuity
DEVELOPING
After the removal of the excess surface penetrant , in developing step, the penetrant from the discontinuity is brought to the surface so as to form an visible indication.
Developer functions. The developer functions are
1.To assist the natural seepage of the penetrant in the discontinuity and extract or blot out the penetrant so as to form an visual indication (mechanism of visual indication)
2. To provide a contrasting base which enhances the detection an indication. Blue-black to yellowish green: white to red
3. To spread the penetrant so as to increase the apparent size of the indication 4. To mask some confusing indications
Mechanism of developing action :
Natural seepage is assisted and the absoptive developers blot out more penetrant from the discontinuity. The thickness of the penetrant layer is increased to the levels above the threshold visibility .( Some fluorescent penetrant indications can be seen without developing with high intensity black light -3000 microwatt/sq.cm) Developer has two variations: Dry and Wet. In the case of wet, aqueous and non aqueous.As the sensitivity of wet developers are better than, dry developers and since in weld inspection, less sensitive methods namely water washable and solvent removable are employed , it is better to use wet developers (mostly nonaqueous) in weld inspection in spite surface roughness. Normally for high surface roughness, dry powder is better suited .
VISIBLE FLOURESCENT
DUAL
SENSITIVITY
WATER
EMULSIFIE
R SOLVENT WATER SOLVENT
WATER WET DRY NONAQUEOUS WET WATER SUSPENDED PARTICLE WATER SOLOUBLE
FLOW CHART FOR PROCESS SEQUENCE OF VISIBLE DYE AND FLOURESCENT TYPE PENETRANTS,
WATER
WASHABLE POST EMULSIFIED SOLVENT REMOVAL
APPLY PENETRANT APPLY PENETRANT APPLY PENETRANT APPLY EMULSIFIER
WATER WASH SOLVENT WIPE
DRY APPLY WATER BASED WET DEVELOPER APPLY DRY POWDER DEVELOPER APPLY NON AQUEOUS WE DEVELOPE R APPLY DRY POWDER DEVELOPER DRY INSPECT POST CLEAN
DUAL SENSITIVITY PENETRANTS PROCESS SEQUENCE DUAL SENSITIVITY PENETRANTS PROCESS SEQUENCE PRECLEAN APPLY PENETRANT SOLVENT WIPE WATER WASH DRY APPLY DRY POWDER DEVELOPER APPLY NON AQUEOUS WET DEVELOPER INSPECT POST CLEAN
Developing action at the end of developer time reveals an indication. In this step the indications are interpreted as to the origin of the indication namely whether True or false indications and if true indication whether relevant or nonrelevant and if
relevant as to the nature of discontinuities linear or volumetric. Inspection environement depends on the type of dye - Visible or fluorescent dye white light for visible and black light for fluoresecent.
Types of penetrants
Penetrants are generally a chemical solution of highly stable dyes, either visible or fluorescent, in a mixture of surface active agents with a blend of highly refined hydrocarbon distillate. The carrier liquid is practically colorless and transparent. The dye provides a high contrast indication against the background.
Petroleum distillates, which are relatively nonvolatile and used to dilute both water washable and non - water washable penetrants, contribute to penetrant performance in several ways. As a diluent, a light petroleum distillate dissolves light organic soils on surfaces and in flaws, and it assists crack penetration with its low surface tension and affinity for metal surfaces. Also, petroleum distillates naturally fluoresce a light blue color and, in a minor way, contribute to penetrant brightness. Petroleum distillates are source of halogens and sulphur.
Water based fluorescent penetrants [ using water as diluent which represents at least 50% of a penetrant ] have been developed for easy disposal, previously used as non-certified applications, are now approved. Water based fluorescent penetrants are restricted to water washable method and available in sensitivity level 1 and 2. The penetrants are liquid oxygen compatible, suitable for use as a leak detector and for inspection of plastics. The penetrants are compatible with water based precleaners, e.g., hot alkaline, which for environmental reasons have replaced petroleum solvents and vapor degreasers. Being compatible with the aqueous cleaner, these penetrants have tolerance for rinse water carry over. Also, flaw entrapped water is less likely to impair the inspection process, such as would happen when flaw entrapped water repelled a petroleum based penetrant.
Water in water based penetrants will evaporate, and regular, testing with a refractometer is necessary to measure the loss. Periodic water additions are part of the maintenance criteria for these penetrants. Longer penetration time [ 30 mins ] improves sensitivity and dry developers are less suitable for these penetrants.
Selection of penetrant type depends on,
• Sensitivity required. [ tightness of the opening ]
• Number of articles to be tested.
• Surface condition of the part to be tested.
• Configuration of the test specimen.
• Availability of water, electricity, compressed air and other equipments.
• Suitability in environment where the test will be performed.
• Cost of inspection.
In weld inspection, lower sensitivity methods,namely water Washable and solvent removable are employed due to surface roughness of the weld. Hence, use of high sensitivity penetrant namely fluorescent dye penetrant may be preferred.
Visible Dye :
Visible dye or color contrast penetrant inspection makes use of a dye that is visible in ordinary light. These penetrants contain a highly stable bright red or purple dye, so that the indications produce a definite contrast with the white background of the developer powder. The dyes are visible in very thin film. Color contrast penetrant can not achieve the level of sensitivity which is possible with the fluorescent penetrants because of higher viscosity and inability to enter very tight cracks.
As a general rule of thumb, visible penetrant examination is roughly equal to fluorescent penetrant examination of level 1 sensitivity. Discontinuities in the range of 50 µ in NiCr panels are routinely detected and under the right conditions and processing 30 µ discontinuities can be highlighted. The
advantage of using a visible penetrant is that it can be used with ordinary shop lighting and from a small portable solvent removable kit at any location. They are specially suitable for field applications and where darkening the inspection area is impractical.
Sensitivity of penetrants
All fluorescent methods are more sensitive
than visible dye penetrant
Penetrant Type
Properties
Visisble dye
Fluorescent
dye
I
ndication
Bright red
colour
Yellowish
green(on
excitation
)
Background
White colour
of the
developer
Dark (max. 2
ft.candles)
Colour contrast ratio ~1:10
~1:100
Visibility(seeability) Medium
Very high
Halation
effect(ability of the
eye to magnify the
indication)
Normal
High
Sensitivity
Medium
High
In weld inspection, lower sensitivity methods,
namely water Washable and solvent removable are
employed due to surface roughness of the weld.
Hence, use of high sensitivity penetrant namely
fluorescent dye penetrant may be preferred.
Visible penetrants are less vulnerable to contamination from cleaning fluid that can significantly reduce the brightness of a fluorescent indication. Visible penetrants are used in maintenance, repair and in manufacturing processes where a visible indication helps in locating the area for subsequent repair work, such as large rough castings. These penetrants are also used for through leak testing of heat exchangers and tanks, where penetrant is applied to one side and developer to the other.
Effect of Temperature: Penetrants are normally developed for use from about 10 to 520 C. However they can be used as low as 40 C but penetration time
must be increased by 2 to 3 times. At low temperature, penetrant becomes highly viscous and sluggish.
Ideal operating temperatures for normal use of penetrants lie between 100
C and 380 C. Penetrant testing at higher part temperature has positive
advantage. The molecular movement of a liquid is directly related to the liquid's absolute temperature. The higher the temperature, the greater the movement and viscosity decreases. Thus, a penetrant, in contact with a heated part, will be less viscous and fast penetrate into flaws by displacing any gas or liquid from the interior of the discontinuity.
Heat vaporizes discontinuity entrapped solvents and moisture, which might otherwise interfere with penetration. Heat liquefies heavy oils and waxes present from previous processing, facilitating penetrant displacement. Thus, a heated penetrant will more effectively fill a crack. In addition, when penetrants are heated, dwell time may be reduced because of faster penetration. During the dwell time, the part temperature will fall and the viscosity of the penetrant will increase. This will resist over removal. Similarly, the developing step is also faster, penetrant molecules are more readily absorbed by the developer layer, and the developer dries faster. When inspecting heated surfaces with water washable penetrants, it would be prudent to wipe the surface with a solvent or cleaner resistant to high temperatures, since boiling water may not rinse excess penetrant satisfactorily. High temp certified penetrant materials shall be used.
Fluorescent Dye [ Type I ] : Fluorescent penetrant uses minerals or chemical compounds which emit visible light when exposed to ultraviolet light. Fluorescent penetrant’s response is maximum when exposed to 365 nm wavelength ultraviolet light.
The dye absorbs this energy and emits between 520 nm to 620 nm wavelengths which is visible as brilliant yellow-green. Orange - red type penetrants are also available. The efficiency of fluorescent dyes in converting UV light to visible light may be reduced by prolonged exposure to UV light. Elevated temperature can reduce the fluorescence of penetrant in open tanks.
Fluorescent penetrants are designed for different sensitivity levels ;
Level 1/2 Ultra low, for castings and rough surfaces.
Level 1 Low sensitivity 50μm NiCr crack panels.
Level 2 Normal sensitivity 40μm, general purpose use.
Level 3 High sensitivity 20 - 30μm.
Level 4 Ultra high sensitivity 10μm extremely critical use. The actual certifications for sensitivity level ½ to 4, involves the use of a series of titanium and nickel alloy panels containing very small, laboratory generated fatigue cracks. Fluorescent penetrants are more sensitive than visible dye because they have lower viscosity, can penetrate smaller openings, and
have excellent visibility. It is easier to detect something glowing in the dark than to observe a small colored area on a white background. Fluorescent indications are many times brighter than their dark surroundings. The brightness of indications depends on the incident UV - light intensity and minimum 1000 micro watts / cm2 is required at the test surface. Background
white light illumination is to be limited to 20 Lux.
Fluorescent penetrants require a developer with lower particle concentration, because the white background provided by the developer is not necessary. Developer is required only for the blotting action. Fluorescent indications are visible in very thin film. For these penetrants, removal of excess penetrant by solvent wiping requires more care and is a problem when a large surface area is to be cleaned. Water washable type will reduce this problem.
For fluorescent penetrant testing, it is very important to ensure that the part surface is completely free from nitric, sulphuric, chromic acids, alkaline permanganate, acid chromate solutions, acid ferric solutions, peroxides, persulphates etc. These common cleaners all degrade or kill the fluorescence completely.
Dual sensitivity penetrants [ Type III ] : These penetrant contains a combination of red visible dye and orange fluorescent dye, such as ‘By Lux’ from Sherwin. The test object is first viewed under white light for red dye indications followed by ultraviolet light in a darkened area for orange indications. These penetrants provide two levels of inspection sensitivity. One advantage is that flaws located during UV examination are marked by visible
indications for repeat examination or repair work.
Another advantage is that flaws found in one mode can be verified in the second mode. However, the brightness of the visible red color and the fluorescent color are less than the individual visible dye and fluorescent penetrants.
Important : Fluorescent penetrant should never be used on surfaces which have been processed previously with color contrast penetrant. In many cases residues of dyes persist in a defect, the fluorescent process suffers not only from the residue reducing the amount able to get into the defect, residues of color dyes will compete for light and effectively kill the fluorescence of any entrapped fluorescent penetrant in subsequent tests. 1% visible penetrant can stop the fluorescence. If fluorescent inspection is required, then the part shall be cleaned thoroughly to remove any color penetrant residue from the interior of the discontinuities. Applying a reversible penetrant developer, which contains a fluorescent dye, reacts with,
and thus is quenched by, the red dye. When the surface is viewed under UV illumination, the residual spots of red dye can be detected, which stands out as dark spots against a fluorescent background. Once the removal of red penetrant dye has been completed and verified by this procedure, the developer is removed by rinsing with water.
Fluorescent Brightness : is the amount of visible light given off when a fluorescent dye is exposed to UV light. It depends on ;
* the thickness of the penetrant film. * the intensity of the UV light.
* amount of fluorescent dye and its capability to absorb UV light.
* the efficiency of the dye in converting the released electrons to visible light.
Penetrants were examined by placing a drop of used and new penetrant next to each other on a paper towel for a visual comparison. Most penetrant specifications require replacement when the brightness drops by more than 10%. Under test situations, the human eye may detect a change of about 15% or more in brightness. Fluorescent brightness measurement can be performed using a approved Photo fluorometer
Note : The general rule is to use the lowest sensitivity fluorescent penetrant that reveals the discontinuities of interest.
Post Emulsification penetrants :
These penetrants are not directly water washable and the danger of over- washing the penetrant from the discontinuities is reduced. An emulsifying agent is used that makes the surface penetrant soluble in water so that the excess penetrant can be removed by water rinse and then a developer can be applied. These penetrants have better penetrating ability than water washable penetrants and can detect minute flaws. These processes are widely used with fluorescent penetrants but much less with color contrast penetrant. Emulsifiers are liquids used to render the excess penetrant water washable. The manufacturers carefully formulate the emulsifiers depending upon the penetrant to be used. Penetrants and emulsifiers are used as a system, and emulsifier from one manufacturer may not perform adequately on a different manufacturer’s penetrant.
There are two types of emulsifiers used in the removal processes ; They are Lipophilic emulsifiers and Hydrophilic removers [ detergents ] .Lipophilic emulsifiers [ Method B ] are oil based and contain blends of surfactants, esters and high boiling point hydrocarbon distillates. They are colored other than green to allow easy identification. These emulsifiers are used as supplied, and function by mechanical and chemical action.
The commonest method for application of lipophilic emulsifiers is direct immersion followed by drainage. After the emulsifier has coated the surface of the part, mechanical action starts to removes some of the excess penetrant as the mixture drains from the part. During the emulsification time, the emulsifier diffuses into the remaining penetrant film on the surface of the part and
render it water washable. The emulsifier is fast acting, thus making the emulsification [ or contact time ] very critical. The emulsifier continues to act as long as it is in contact with the penetrant, therefore, the rinse operation should take place quickly to avoid over - emulsification. The component may be immersed rapidly in a large volume of water to stop the action of the emulsifying agent. On removal from the water, the wash is completed by spray rinsing.
Spray application followed by drainage has also been used successfully. Brush application of lipophilic emulsifiers is prohibited, because this would mechanically mix the emulsifier into the penetrant resulting in non uniform emulsification. Flowing the emulsifier on the component surface is not recommended because it may not be possible to cover the component surfaces rapidly enough to ensure uniform emulsification. It is essential that complex shaped components are rotated during the drainage stage so that the various surfaces receive similar processing.
The three properties of lipophilic emulsifiers that control the washing characteristics are activity, viscosity, and water tolerance. Specific contact times for lipophilic emulsifiers should be established for each application. The contact time can vary between 60 and 180 seconds depending on the type of the emulsifying agent, the penetrant in use and the surface condition of the components. Lipophilic emulsifiers are used at temperatures between 15 and 250 C. Lipophylic emulsifier are miscible with penetrants in all concentrations.
However, if the concentration of penetrant contamination in the emulsifier becomes too great, the mixture will not function affectively as a remover. A specification requirement is that, lipophilic emulsifiers be capable of tolerating 20% penetrant contamination without a reduction in performance. The emulsifier is to be replaced when its cleaning action is less than that of new material. Since lipophilic emulsifiers are oil - based, they have a limited tolerance for water. When the tolerance level is reached, the emulsifier starts to thicken and will eventually form a gel as more water is added. Specification requires that
lipophilic emulsifiers be formulated to function adequately with at least 5% water contamination and that lipophilic emulsifiers be replaced when the water concentration reaches the limit.
Flow diagram for PE lipophilic
Method D removal process ASTM E 165
Pre clean dry penetrantApply dwell
Dip in Emulsifier Time is critical!! rinse dry Non Aqueous developer inspect OK Post clean reject
Hydrophilic removers are applied by immersion in an aqueous solution. The concentration for immersion varies between the range of 2.5% to 10%, 20% and up to 30 % depending on brands and conditions of agitation. The manufacturer of the emulsifier can provide the proper information concerning this concentration. Concentrations higher than recommended by the emulsifier manufacturer or the qualified percentage is prohibited. The immersion time varies from 20 seconds to 5 minutes depending on the penetrant, emulsifier concentration, surface roughness and agitation. The surface active agent in the remover combines with a small quantity of penetrant from the surface and prevents the penetrant from recombining with the remaining surface penetrant. A slight agitation is necessary to remove the colloidal suspension of penetrant – emulsifier from the surface and to expose fresh penetrant. The preferred method of agitation is mechanical.
contaminants. When application is by immersion, hydrophilic emulsifiers malfunction as a result of penetrant accumulation. Contamination increases when part geometry includes cavities, or areas which entrap penetrant to be carried over to the emulsifier tank. Emulsifier with lower concentration gets contaminated faster. The 30% concentration, if used, tolerates some three times more penetrant contamination as the 10% concentration does. Everything being equal, a 30% concentration lasts three times longer than a 10% concentration, and is economical because of less frequent disposals. Hydrophilic emulsifiers are infested by fungus and algae and the tanks must remain covered when not in active use. 30% concentration has more resistance to this infestation.
Hydrophilic remover solutions are also applied by spray or as foam. The concentration of the solution tend to be much lower up to 5%. However, higher concentration may be used depending on the manufacturers recommendation. Immediately following the remover spraying, a freshwater rinse of the entire part is required to stop the action of any remover remaining on the surface of the part. Hydrophilic removers minimize background fluorescence on part surfaces as well as bleed out of excess penetrant from hollow parts.
The concentration of the detergent solution is critical to the success of the process and must be controlled. Water loss in hydrophilic emulsifiers results in more concentrated and more active solutions, a cause for over-emulsification with resultant failure of the penetrant process. To monitor the concentration, the refractive index of the working solution is generally measured and compared against graphs generated by measuring the refractive index of solutions of known concentrations, using a Refractometer [ an optical device with a reading scale ]. A drop of the working solution is placed on the proper spot on the refractometer and a reading is made on the scale and then converted into percentage by using a table or a graph applicable for the particular remover and supplied by the emulsifier manufacturer. The graph may be generated in the workstation using prepared solutions of the standard
remover from 50% of the working strength to 50 % above. A minimum of 5 solutions of different strengths should be prepared for this purpose.
Hydrophilic emulsifier Method D
Pre clean dry Apply
penetrant dwell rinsePre
Collect The bulk Excess penetrant Inspect With BL Apply developer Inspect With BL Post clean
Solvent removable penetrant process :
Penetrants which cannot be removed directly with water can be removed by use of organic solvents. The solvents used are quite versatile and dries quickly without a residue and a developer can be applied on the dry surface. Normally, the same type of solvent is used for pre cleaning and for removing excess penetrant from the test surface.
Typical applications involve in situ inspection or testing of a specific location on the component when it is not practical to remove the penetrant using water, which would involve subsequent drying of the component.
Three types of solvent removers are used :
Volatile hydrocarbon distillates.
Volatile aliphatic alcohols.
The flammable cleaners are potential fire hazards but are free from halogens, while the non flammable cleaners contain halogens, which render them unsuitable for some applications. Solvent removable penetrant may be applied to the test surface by spraying, dipping, flooding or brushing. For a small test area, brushing controls applied penetrant and removal of excess penetrant is easier.
Excess penetrant removal is performed by wiping the part surface in one direction only, with a clean and lint free cloth or absorbent paper. The proper procedure is to make a single pass, then fold the cloth to expose a clean surface for each succeeding wipe. This will remove most of the penetrant. The remaining traces are then removed by wiping with a new cloth or paper lightly wetted with the solvent remover. After each pass, the surface of the cloth is examined. If there is more than a trace of penetrant on the cloth, the cloth is folded to expose a clean surface, remoistened with the solvent, and the surface is wiped again. The process is repeated until there is little or no trace of penetrant on the cloth. Cleaning should be done quickly, using a minimum of cleaner. If the operation is prolonged or if excessive amount of the cleaner is used, some penetrant may be removed from discontinuities. The surface is then viewed under suitable light to ensure that the excess penetrant is removed. For application of developer, the surface is dried by evaporation or wiping.
Non flammable, volatile solvents can be used in the vapor phase in automatic processing stations [ like vapor degreasing ]. The solvent should be redistilled frequently to avoid heavy contamination with penetrant, since the penetrant will start to co-distil with the solvent and cause heavy background.
Note : The volatile organic solvents used for penetrant removal are the most aggressive and great care must be taken in their use to prevent over-removal. For manual processing, the solvent remover must be used by wiping with a cloth. Use of a solvent remover by spraying or flooding will results in over wash or dilution of the penetrant and the indications will become diffused and weak. The optimum sensitivity is achieved when the brightest indications appear against a thin developer contrasting background.
When normal cleaning cannot sufficiently remove penetrant from as-welded surfaces, machined parts with sharp inside corners, or parts with depressions or pits, such areas can be further cleaned with a commercially available cotton swab or with a cotton swab on a toothpick wetted with the solvent.
some hints
1. Due to surface roughness of unflushed welds, it is better to use, water washable and solvent removable methods to avoid undesirable background. 2. In the case of Solvent removable pentrants, usual precaution of wiping with
solvent moistened lint free cloth may not yield the desired results because of excessive background. Sometimes, the solvent is directly applied to the weld to overcome this difficulty. Of course fine cracks may be missed
3. In the case of TIG and MIG where better surface is obtained, it is better to go solvent wiping method
4. Inspection with multiple pass welds with penetrants requires that each weld bead is inspected prior to the next pass to provide the next the best reliability. Cleaning after each testing
5. 5.Water washable fluorescent penetrants can be used on large pressure vessels or other large structures by washing with a hose and air drying. Developent with solvent suspended developer can provide good sensitivity Water washable penetrants can be washed more readily from most weld
6. 6. It is important to remove all the of the slag from welds before penetrant testing. Grinding off the ripples on rough welds is desirable. Grinding should not smear the metal
7. 7. Brazed joints can discontinuities similar to weld beads that can be detected by PT. A braze that does not wet the surface is indication of a poor joint. Developers :
Normally a developer is used to complete the penetrant process. Developers are fine white powders applied as dry or with a liquid carrier to
form a thin uniform coating on the test surface. Developer acts like a sponge with very fine random capillary paths. When the penetrant contacts the developer, it spreads through these paths by capillary action [ known as blotting or reverse capillary action ]. The penetrant gradually diffuses into the developer and indications become larger than the discontinuity opening. The white color of the developer provides contrast with color contrast penetrants and also provides a darker background for fluorescent indications.
It should be noted that the developer itself does not produce indications but simply absorbs the penetrant already present below it and makes it more visible. Developer should be applied as soon as possible or within 30 minutes after the removal of the excess penetrant.
Dry developers [ Form A ] : [ blends of chemically inert white powders ]
Dry developers are a non toxic fluffy absorbent white powder that is used mostly with florescent penetrant process. These dry powders do not normally provide sufficient white background for contrast with color contrast penetrants. This may be achieved by use of electrostatic spray application but is not always controlled easily. Dry powders must be used on dry surface only. Dry developers are the least sensitive.
The dry powders should be used in a closed container or, if sprayed, inside a booth with good extraction. Where the throughput of components is low, components can be placed on a grid in a tank and powder applied using a scoop or from a pear shaped rubber puffer. Excess powder is then removed by gentle air blast, shaking or tapping. Dry powder may also be applied by brushing on rough test surfaces.
Dry powder is best applied by placing the component in a dust storm cabinet which may be of the tunnel type or top loading. The cabinet is sealed to be dust proof. Most cabinets work on a preset cycle so that once started the cabinet cannot be opened until the cycle is complete. The dry powder developer is then agitated with dry and oil free compressed air, which blows the powder into a cloud around the component to coat the surface. It is
advisable to incorporate an " extract and return " system with the cabinets, since the fine particles take a very long time to settle from the air. Dust chambers should be fitted with heaters to maintain a dry inside atmosphere. Installing a fan at the bottom of the container to blow the powder eliminates most of the problems associated with the use of compressed air.
Dry powders can be applied after charging them in a electrostatic spray system. Some dry powders are of a chemical composition which does not become sufficiently charged and this point should be checked before attempting to use this method of application. Dry developers should cling to dry metallic surfaces in a fine film of dust. The adherence of the powders should not be excessive, as the amount of black light available to energize indications will be reduced.
In automatic processing systems, dry powders are applied by passing the part through a conveyor in a fluidized bed tunnel system. This application relies on producing a controlled air / powder mixture which passes over the components and coat the surfaces.Extremely fine dry developer powders are available for application by dipping the dried part into it. This process has the disadvantage that the developer gradually becomes contaminated by fluorescent flecks and produces false indications. For purpose of storage and applications, the dry powders should not be hygroscopic and they should remain dry. If they pickup moisture when stored in areas of high humidity, they agglomerate or lump up and lose their ability to flow and dust the surface adequately. The developer should be checked daily for caking and contamination. Dry powder developers can dry the skin and irritate the eyes and the respiratory system. Use of rubber gloves and respirators is desirable.
Developer properties
Good blotting action. Must be easily wetted by penetrant and draw the maximum amount of penetrant from the defect.
Fine grained
Show indications from small amount of penetrant.
Mask out interfering penetrant and part surface background.
To form a thin and uniform coating.
Non fluorescent.
Easily cleaned from the surface.
Non flammable.
Non toxic and non corrosive.
Developer selection :
Select wet developer in preference to dry on very smooth test surfaces.
Select dry developer in preference to wet on very rough test surfaces. Cleaning and re inspecting a rough surface is difficult, if a wet developer was used for a prior inspection.
Wet developers can not be used reliably where part configuration accumulates developer at certain locations such as threads. Excess developer can mask out indications. Dry developer can be used.
Solvent suspended developers are very effective for flaws with narrow opening, but are not satisfactory for finding wide shallow defects.
Aqueous [ Wet ] developers :
These developers are used in penetrant inspection stations. Aqueous developer uses water as the carrier of the developer particles. The developer is used as a solution [ Form B ] or as suspension [ Form C ].
Water based developer includes a wetting agent, corrosion inhibition system and dispersing medium. These developers are applied directly on a water washed surface and after drying a solvent washed surface. Parts are dried immediately after excess developer has drained from the component. These developers are less sensitive when compared to non - aqueous developers.
Water Soluble developers [ Form B ] : [ Blends of white crystalline powders to be dissolved in water ]
Supplied as soluble white powders to be dissolved in water at a concentration recommended by the manufacturer [ 40 - 60 g / lit ] to prepare a transparent working solution. The prepared bath is completely soluble and does not require agitation. If the solutions are stored in open tanks, precipitate may form due to loss of water by evaporation. The concentration of the solution is monitored by measuring the specific gravity with a hydrometer. Water soluble developers are not recommended for use with water washable penetrants because of the potential to wash the penetrant from within the flaw unless the developer application is very carefully controlled.
Water solution developers can be applied by immersion followed by drainage in a stainless steel tank. The developer may be applied by flow-on, brush or a wet spray. When spray application is used, it is important, that the spray is wet and the droplets coalesce to form a continuous layer on the test surface. The part surface must be dried actively after the developer application to form an uniform layer. The dried developer film is water soluble and can be removed after inspection by simple water rinsing.
Soluble developers contain inorganic compounds which ionizes in the solution.
Dipping different metals in contact with each other and use of uncoated basket produce galvanic corrosion. The contaminated developer solution becomes gray to dark gray in color.
Water Suspended developers [ Form C ] : [ Blends of white inert powders and surfactants for suspension in water ]
supplied as insoluble white powders to be suspended in water at a concentration recommended by the manufacturer [ around 70 g / lit ] to give a working preparation. Water suspension developers may be applied by immersion in an agitated bath or by spraying. The immersion tank should be of stainless steel with mechanical stirrer to prevent settling. Air agitation should be avoided since the developers contain wetting agents and corrosion inhibitors which may become degraded by chemical reaction with the oxygen or carbon dioxide in the air. If the developer is to be applied by spraying, the design and operation of the spray apparatus must allow continuous agitation of the reservoir of developer and the application must allow the suspension to arrive wet at the test surface. Foaming must be avoided because when foam bubbles break they leave holes in the developer coating.
The amount of powder in suspension must be carefully maintained. Too much or too little developer on the test surface can seriously affect sensitivity. It is essential that the developer bath is monitored by checking the specific gravity and the wetting characteristics.
Water suspension developer is applied before drying, the developing time decreases because the heat from the drying operation helps penetrant back to the surface opening and the developer film being already in place, the developing action begins at once.
Non Aqueous Suspension developers [ Form D ] : [ Chemically inert white pigments suspended in volatile liquid carriers ]
These consist of developer particles suspended in a volatile organic solvent. The use of a quick drying solvent as carrier makes non aqueous wet developer the most sensitive for detection of minute cracks. The solvent combines with the penetrant in the cracks, and the result is a less viscous liquid which bleeds into the developer fairly quickly as the solvent evaporates.
Non aqueous suspension developers must be applied by spraying on a dried part surface. Spraying may be by aerosol can or conventional spray gun. Since volatile solvents are used in these developers, efficient extraction facility must be provided in the inspection area.
The suspended particles separate on standing [ settles to the bottom of the container ] and so must be agitated before use to an homogeneous mixture. When aerosol cans are used, this is done by shaking the can. For a spray gun application, the reservoir must be equipped with an efficient agitator. Alternative methods of application, such as immersion, flow-on or brushing on will cause loss of sensitivity.
A developer coat which is just thick enough to mask the metal surface is about right. If the coat is too thick, the penetrant might not blot through to the surface. If it is too thin, it might not blot well enough.
Spraying should be performed from a distance of 8 to 10 inches from the test surface. Holding the can too close to the surface will result in excessive developer being applied every time.