Liquid Penetrant Method
NDT Training Program
The American Society for Nondestructive Testing
Liquid Penetrant Testing is a nondestructive means of locating
surface discontinuities based on capillary action.
In the liquid penetrant method, the liquid is applied to the
surface of the specimen, and sufficient time is allowed for
penetration of surface discontinuities. If the discontinuity is
small or narrow, as in a crack or pinhole, capillarity assists the
After sufficient time has passed for the penetrant to enter
the discontinuity, the surface of the part is cleaned.
Capillary action is again employed to act as a blotter to
draw penetrant from the discontinuity.
To insure visibility, the liquid penetrant contains either a colored
dye easily seen in white light, or a fluorescent dye visible under
black (ultraviolet) light.
B. Time allowed for
A. Penetrant applied to
surface. penetrant to seep into
C. Surface penetrant removed.
D. Developer applied to draw penetrant out of opening.
E. Specimen visually examined.
Discontinuities that are subsurface in one stage of production
could be open to the surface at another stage, such as after
grinding or machining.
Nonmetallic inclusions and porosity in the ingot may cause
stringers, seams, forging laps, cold shuts, and the like as the
billet or slab is processed in the manner shown on the next
Anything that could block the penetrant from entering the
discontinuity must be removed.
A list of contaminants that must be removed would
include dirt, grease, rust, scale, acids, and even water.
The cleaning solvent used must be volatile (readily
vaporized) so that it easily evaporates out of the
discontinuity and does not dilute the penetrant.
Surface preparation by shot or sandblasting is not recommended.
Discontinuities that were open to the surface may be closed
by the shot or sandblasting.
Historically, penetrant inspection was called the “Oil and
Whiting Method,” as it used kerosene and a white
powder for the inspection of railroad parts. However, in
the past 40 years the process has been improved
tremendously to the point where it is a reliable and
accurate inspection technique.
The liquid penetrants used in nondestructive testing
can be categorized by the type of dye they contain.
1. Visible dye penetrants contain a colored
(usually red) dye.
2. Fluorescent penetrants contain a fluorescent
3. Dual sensitivity penetrants contain a
combination of visible and fluorescent dyes.
Penetrants can be further categorized by the processes
used to remove the excess penetrant from the specimen.
1. Water-washable penetrants are either self-
emulsifying or removable with plain water.
2. Post-emulsified penetrants require a separate
emulsifier to make the penetrant water washable.
3. Solvent-removable penetrants must be removed
with a solvent which is typical when using visible
dye in pressurized spray cans.
The flow chart below illustrates the processing sequence with
visible dye and fluorescent penetrants.
Dual sensitivity penetrants would follow a processing
sequence similar t that shown below.
The selection of the best process, as listed on pages 5 and 6,
1. Sensitivity required.
2. Number of articles to be tested.
3. Surface condition of part being inspected.
4. Configuration of test specimen.
5. Availability of water, electricity, compressed air,
suitable testing area, etc.
Penetrant testing is successfully used on metals such as
aluminum, magnesium, brass, copper, cast iron, stainless steel,
titanium, and most other common alloys.
It can also be used t test other materials, including
ceramics, plastics, molded rubber, powdered metal
products, or glass.
Penetrant testing is limited by its inability to test materials with
discontinuities that are not open to the surface or having an
extremely porous surface.
1. FALSE 2. FALSE 3. FALSE
4. FALSE 5. FALSE 6. FALSE
7. FALSE 8. TRUE 9. TRUE
10. FALSE 11. TRUE 12. FALSE
13. TRUE 14. TRUE 15. FALSE
16. TRUE 17. TRUE 18. TRUE
This lesson discusses the equipment and material required to
perform the various penetrant tests and the required pre- and
Proper cleaning is essential to liquid penetrant testing for two
1. If the specimen is not clean and dry, penetrant testing
is ineffective. 2.
If all traces of penetrant materials are not removed
after the test, they may have a harmful effect on
the specimen. (Chlorine and sulfur may affect
some alloys.) Immersion tanks
and detergent solutions are common means of assuring that a
specimen surface is both physically and chemically clean.
Vapor degreasing is
particularly effective in the removal of oil, grease, and similar
organic contamination. However, certain alloys have an affinity for
specific elements used in vapor degreasing and if exposed to
them may become structurally damaged.
Steam cleaning is particularly adaptable to the cleaning of large,
Solvent cleaning may be used in immersion tanks or may be
used in a wipe-on and wipe-off technique. Solvent cleaning is
usually less effective than the previous methods of cleaning.
Rust and surface scale can be removed by any good
commercially available acid or alkaline rust remover following
Paint removal is often done in dissolving-type hot-tank paint
strippers, bond release, or solvent paint strippers.
Etching is effective for use on articles that have been ground or
machined. This process uses an acid or an alkaline solution to
open up grinding burrs and remove metal from surface
Surface cleaning processes to be avoided include blast (shot,
sand, grit, or pressure), liquid honing, emery cloth, wire brushes
and metal scrapers. These processes tend to close
discontinuities by peening or cold working the surface of the
Penetrant Test Equipment (Stationary)
Stationary equipment used in liquid penetrant testing varies in
size and is largely dependent upon the size of the test
specimen. Depending on the type and process used, a
stationary system could include the following:
1. Pre-cleaning station (usually in remote area)
2. Penetrant station (tank)
3. Drain station
4. Emulsification station (tank)
5. Rinse station (tank)
6. Developing station (tank)
7. Drying station (usually oven)
8. Inspection station (enclosed booth or table with proper
9. Post-cleaning station (usually in remote area)
SEE PICTURE ON NEXT SLIDE
Penetrant Test Equipment (Portable)
Both visible and fluorescent dye penetrants are available in
kits which can be used at a remote location or when testing a
small portion of a large article.
A visible dye penetrant kit usually contains:
1. Pressurized spray cans of cleaning or removal fluid.
2. Pressurized spray cans of visible dye penetrant.
3. Pressurized spray cans of non-aqueous developer.
4. Wiping cloths and brushes.
A fluorescent dye penetrant kit usually contains:
1. A portable black light and transformer.
2. Pressurized spray cans of cleaning or removal fluid.
3. Pressurized spray cans of fluorescent dye penetrant.
4. Pressurized spray cans of non-aqueous developer.
5. Wiping cloths and brushes.
Black light equipment is required in fluorescent penetrant
testing since it supplies light of correct wavelength to cause the
penetrant to fluoresce. A deep red-purple filter is used to pass
only those wavelengths of light that will activate the fluorescent
material. At least a five-minute heat-up time is required to
reach the correct arc temperature when using mercury arc
The black light emits a special light with wavelengths
that fall between visible and ultraviolet. Provided that
the filter is not broken or cracked, there is no danger of
injury to the human eye. It is suggested that the filter be
checked prior to each use.
Penetrant testing materials can be used in a variety of
combinations. Most materials are available in either
pressurized spray cans or in bulk quantities.
The flow chart below illustrates the different material combinations.
However, care should always be taken to assure that manufacturers’
specifications or company procedures are closely followed.
Penetrant Testing Materials
Penetrant materials are often restricted to specific groups. The
establishment groups of penetrant materials will use the following in
a variety of combinations to obtain the best results:
1. Water-Washable penetrants – Contain an emulsifying
agent which makes them easily removable by a water rinse
or wash. This penetrant material can be obtained with
either a visible or fluorescent dye.
2. Post-Emulsifiable penetrants – Are highly penetrating, oily
visible or fluorescent penetrants which are not soluble in
water. These penetrants must be treated with an emulsifier
before they can be removed by a water rise or wash.
3. Solvent-Removable penetrants - Are oily penetrants that do
not contain an emulsifying agent and are removable only by
solvents specially designed for that purpose.
4. Emulsifiers – When applied to a penetrant-coated specimen
make the resultant mixture removable by water rinse or
wash. Emulsifiers have low penetrant characteristics and
do not remove indications from the specimen surface.
5. Removers (Solvent) – Are designed to be used in conjunction
with specific penetrants. Typical removers are available in
bulk or pressurized spray containers.
6. Dry developers – Are a fluffy, absorbent white powder that is
used in both fluorescent and visible dye penetrant tests. It
functions to draw the penetrant indications to the surface
thus making them visible. 7.
Wet developers – Function similarly to dry developers except
that they are a mixture of a developing powder and water. 8.
Non-aqueous Wet developers – Differ from wet developers in
that the developer powder is mixed with a rapid-drying liquid
Liquid Oxygen (LOX) Compatible Materials – Must be used
when articles inspected are subjected to contact with either
liquid or gaseous oxygen. These materials are specifically
designed to be inert when in the presence of LOX.
10. Low Sulfur and Low Chlorine – Penetrant materials must be
specifically designed to avoid the harmful effects caused on
some nickel and titanium alloys by the sulfur and chlorine
In general, the materials used in penetrant inspection can be
flammable and can cause skin irritations.
In addition, the ultraviolet spectrum of light rays generated from
the mercury arc lamp can cause sunburn and may be injurious
to the eyes. However, if the proper filter for fluorescent dye
inspection is used, the harmful rays will be filtered out.
FIRE – Many penetrant materials are flammable. Safe precautions
requires that penetrant materials used in open tanks
have a flashpoint of greater than 120º F.
SKIN IRRITATION – Skin irritation can be avoided by preventing
unnecessary contact and by the use of gloves,
aprons, and protective hand creams.
AIR POLLUTION – The developing powders are considered
nontoxic but excessive inhalation must be avoided.
Exhaust fans should be installed in any confined area
where dry developers or vapors from the penetrants
1. FALSE 2. TRUE 3. FALSE
4. FALSE 5. FALSE 6. TRUE
7. FALSE 8. FALSE 9. FALSE
10. FALSE 11. FALSE 12. TRUE
13. TRUE 14. FALSE 15. TRUE
16. TRUE 17. TRUE
This lesson discusses surface preparation and penetrant
The effectiveness of liquid penetrant testing is based upon the
ability of the penetrant to enter surface discontinuities. All paint,
carbon, oil, varnish, oxide, plating, water, dirt, and similar coating
must be removed before application of the penetrant.
Liquid penetrant placed on the surface of a specimen does not
merely seep into discontinuities, it is pulled into them by capillary
action. This is the reason one can cover the under surface of an
item with a penetrant and still have a valid test.
The following are typical cleaning methods discussed earlier:
1. Detergent Cleaning 5. Rust and Surface Scale
2. Vapor Degreasing Removal
3. Steam Cleaning 6. Paint Removal
4. Ultrasonic Cleaning 7. Etching
Application of Penetrants
Almost any liquid could be considered a penetrant, but modern
penetrants must have:
1. The ability to hold a dye material in suspension.
2. The ability to spread the dye evenly over the surface.
3. The ability to carry the dye into any discontinuity open to
4. The ability to bring up the dye as it is “coaxed” back to
5. The ability, when desired, to be easily removed.
There are two types of dye used in modern penetrants:
1. Visible – A brightly colored dye that is highly visible under
normal lighting conditions. This type of dye is normally
called visible dye.
2. Fluorescent – An almost colorless dye which emits visible
light rays when reviewed under black light.
A dye with dual sensitivity contains both a visible dye for
examination in white light and a fluorescent dye for a more
sensitive evaluation of small discontinuities.
Penetrants, either fluorescent or visible, can be applied by any
one of the following means:
1. Spraying – Usually using a low pressure circulation pump or
from pressurized spray cans.
2. Brushing – Usually applied with rags, cotton waste, or
3. Immersion – The entire part is dipped into a tank of
4. Pouring – The penetrant is simply poured over the surface.
Penetration (Dwell) Time
The period of time during which the penetrant is permitted to remain
on the specimen is a vital part of the test.
Tight crackline discontinuities may require in excess of 30 minutes
for penetration that will give an adequate indication. However, gross
discontinuities may be suitably penetrated in 3 to 5 minutes.
The temperature of the specimen and temperature of the
penetrant can affect the required dwell time.
Warming the specimen to 70º F or higher accelerates
penetration and shortens dwell time. However, care should
be taken not to overheat the specimen since too much heat
may cause evaporation of the penetrant from the
Dwell times are based on the assumption that the penetrant
will remain wet on the part surface. Additional penetrant may
be applied during dwell time.
The penetrant manufacturer will provide suggested dwell
times for the various penetrants that it produces.
Penetrant Testing Processes
We previously mentioned the two types of penetrants, visible
and fluorescent. For both categories there is a further
1. Water-Washable (visible or fluorescent)
2. Post-Emulsification (visible or fluorescent)
3. Solvent-Removable (visible or fluorescent)
Water-Washable penetrants have a built-in emulsifier
and the penetrant is easily removed by a water rinse.
Care must be taken to insure that the spray volume and
force does not wash penetrant out of the discontinuity.
Water temperatures above 110º F are not recommended
because this may speed up the evaporization of the
The sketch below represents the steps in a Water-Washable
Water-Washable penetrants are usually preferred for use on
articles with a rough surface or if they contain threads or
The built-in emulsifier provides the best penetrant
removal from blind holes and other hard-to-reach
locations but has the disadvantage of poor reliability in
detecting wide or shallow discontinuities.
(See page 6 for advantages and disadvantages)
Solvent-Removable penetrants – They have the advantage
of portability and can be used outdoors without using heavy,
They are excellent for many maintenance inspections
and for checking portions of a larger structure.
Penetrants is often applied from a pressurized spray
can which makes the system very portable.
After the specified dwell time,
the excess penetrant is first
removed by wiping with
absorbent towels and then
cleaned with towels dampened
Solvent is never applied directly to the specimen as it might
wash out or dilute the penetrant in the discontinuity.
(See page 6 for advantages and disadvantages of solvent-
Post-Emulsification penetrants require a two-step removal
process. The emulsifier is usually applied by dipping or
immersion. The amount of dwell time in the emulsifier is in the
range of one to four minutes in accordance with manufacturers’
recommendations and the type of defects expected.
The resultant emulsifier-penetrant mixture is removed by water
rinse. (Emulsification will be covered in more detail in Lesson
Easily washed with water
Good for quantities of small specimens
Good on rough surfaces
Good on keyways and threads
Good on wide range of discontinuities
Fast, single step process
Available in oxygen compatible form
Not reliable for detecting scratches and similar shallow surface
Not reliable on reruns of specimens
Not reliable on anodized surfaces
Acids and chromates affect sensitivity
Penetrant subject to water contamination
No water required
Good on anodized specimens
Good for spot checking
Specimens can be rerun
Removal of excess surface penetrant is time consuming
Materials cannot be used in open tanks Difficult
to use on rough surfaces such as cast magnesium
High sensitivity for very fine discontinuities
Good on wide shallow discontinuities
Easily washed with water after emulsification
Short penetration time
Cannot be easily over washed
Two step process
Equipment required for emulsifier application
Difficult to remove penetrant from threads, keyways, blind holes
and rough surfaces
Leak-through technique – The use of liquid penetrant in this
method is suited for finding leaks in such articles as tanks,
piping, tubing, and hollow castings. The sketch below
illustrates the leak-through technique.
Fixing and Recording Indications
1. Photographs – Typical black and white and color
film are used with the self-developing films
providing the most convenience.
2. Special Wax and Plastic Film Developers have
been developed to absorb and fix the penetrant
indication to form a permanent record.
A. Strippable lacquers are sprayed in several
coats over the indication and when dry can
be “lifted” to provide a permanent record.
B. Special “fixers” are sprayed over the indication
and when dry are lifted with transparent
1. FALSE 2. FALSE 3. FALSE
4. TRUE 5. FALSE 6. TRUE
7. FALSE 8. FALSE 9. TRUE
10. TRUE 11. FALSE 12. SPRAYING
13. DETERGENT, STEAM, VAPOR BREASING, RUST &
This lesson discusses emulsification, penetrant removal, and
Post-Emulsification Penetrant (P.E. Penetrant)
This type of penetrant emulsification is accomplished by
dipping the part in a chemical emulsifier prior to washing. The
emulsifier will break down the penetrant and make it water
The emulsifier can be applied by dipping or spraying, but not
by brushing. The bristles of the brush may enter the
The sketches below and on the next slide represents the steps
in a Post-Emulsification Penetrant Test.
If too short an emulsification time is used, not all penetrant will be
removed, which will cloud over discontinuities.
If too long a time is used, penetrant within the discontinuities will
also become water soluble and be washed away with excess
The P.E. penetrant makes the water wash less critical. The
sketches below compare the effects of water-washable and post-
(A) Water-Washable(B) Post-Emulsification
Some indications may be visible prior to the application of a
developer but this step will ensure that all discontinuities are
visible to the naked eye.
Developing is accomplished when a highly absorbent powder is
applied to the item being tested after excess penetrant is
The penetrant is actually drawn out of the discontinuity by the
strong capillary action of the developer.
As shown on the previous slide, the image of the discontinuity
in the developer will actually be larger than the actual size of
There are two common types of developers in use today – wet
and dry. Both use a white powder and the primary difference
is in the method of application.
1. Wet Developers
A. The non-aqueous developer is held in
suspension in a solvent base and is usually
supplied in pressurized cans.
B. Another type of wet developer holds the white
powder in suspension in a water base.
This type of wet developer is generally used with water-
washable or P.E. penetrants and is applied by dipping or
spraying. After application, a short time is allowed for the
water to evaporate leaving a thin layer of white powder. If an
oven is used, the temperature should not exceed about 225º
F, as this could evaporate penetrant in the discontinuity.
2. Dry Developers
A dry developer is a fluffy white powder that is not
carried in a liquid.
Dry developers are applied directly to the article as a
powder. This is usually done with slight air pressure
or articles may be dipped into a container of dry
It is very necessary to have a dry surface prior to
application of a dry powder. A wet surface will result
in uneven layers of powder.
Dry developer is usually used on fluorescent
Advantages of “Wet” Developers
Better on smooth surfaces where the dry developer will not
When a wide, shallow discontinuity is sought, a wet developer will
leave a more even coat of developer.
Advantages of “Dry” Developers
On rough surfaces, and on sharp fillets, holes, and threaded
articles, the wet developer tends to leave too much developer.
1. TRUE 2. FALSE 3. TRUE
4. TRUE 5. TRUE 6. TRUE
7. FALSE 8. FALSE 9. TRUE
10. TRUE 11. FALSE 12. FALSE
13. FALSE 14. FALSE 15. TRUE
This lesson discusses the final steps in the penetrant process:
inspection, evaluation, post-cleaning, and material control.
Proper lighting should be the first consideration in the
inspection of an article.
1. If a fluorescent dye penetrant is used, a room or
booth with dim light and a black light with proper
intensity are required.
2. If a visible dye penetrant is used, adequate normal
lighting is necessary.
Typical Indications in Penetrant Inspection
All indications found with the liquid penetrant
method will be surface discontinuities, but
the indications may or may not affect the
usefulness of the article.
The most common source of false
indications is poor washing.
The operator can easily tell when a
good rinse is obtained by using a
black light during and after the
fluorescent penetrant removal process.
To avoid false indications, care should be taken so that no outside
contamination such as the following occurs.
1. Penetrant on operator’s hands.
2. Contamination of developer.
3. Penetrant transferred to clean specimen from other indications.
4. Penetrant on inspection table.
Nonrelevant indications are actual surface
discontinuities that in most cases are there by
design. They are caused by some feature of
assembly such as articles that are press-fitted,
keyed, splined, or riveted.
Nonrelevant indications could also include loose
scale or a rough surface on a forging or
(Some typical indications are shown in Chapter 6, pages 8-19 in the training handbook)
True indications are those caused by surface discontinuities
that have been interpreted as not being false or nonrelevant.
True Indications could be divided into five basic categories:
1. Continuous line – This type of indication is often caused by
cracks, cold shuts, forging laps, scratches, or die marks.
2. Intermittent line – These indications could be caused by
any of the discontinuities mentioned above provided
they were very tight or where the part had been peened,
machined, or ground.
3. Round – Usually caused by porosity open to the surface.
4. Small dots – Tiny round indications caused by the porous
nature of the specimen, coarse grain structure, or
5. Diffused or Weak – These indications are difficult to interpret and
often the part must be cleaned and retested. In many cases
the diffused or weak indications turn out to be false
indications caused by an improper penetrant procedure.
Depth Determination of Penetrant Discontinuities
The greater the depth of a discontinuity, the more penetrant it will
hold and the larger and brighter the indication,
After the specimen has been inspected it is very important that it be
thoroughly cleaned. Post-cleaning usually will involve the same types
of cleaning operations were used in pre-cleaning.
Quality Control of Penetrant Test Materials
Aluminum Test Blocks – They provide a good means of evaluating
general purpose penetrants. They should be used for comparisons
only and not for absolute evaluations.
These blocks are non-uniform, and the depth and width of cracks are
uncontrolled. The size of the blocks may vary but are about 3/8 by 2
inches of 2024-T-3 aluminum. About 3 inches long.
The blocks are heated and quenched to produce an overall crack
pattern. They can be reused by reheating, quenching, and careful
Sensitivity Tests – While there have been no simple
quantitative tests developed for measuring penetrant
sensitivity. A simple comparative test is usually adequate.
A small sample of the penetrant from the testing area is placed
on one side of an aluminum test block, and a small sample of
new penetrant on the other side. By visual observation it is
determined if the old penetrant is contaminated to the point
where it must be discarded.
Meniscus Test – Provides a practical test for evaluating the
dye concentration in thin-liquid films.
The test utilizes a flat glass platen and a convex lens. When a
drop of solution is placed between the lens and platen, a
colorless or nonfluorescent spot is formed around the point of
contact. The resultant contact angle simply indicates the ability
of a liquid to wet a surface.
The diameter of the remaining “spot” of colorless penetrant
provides a measure of film thickness which can be used to
compare the dye concentration of penetrants. Prolonged
exposure of a film to ultraviolet light or accidental contamination
by acids or alkalis will affect the dye concentration and
therefore change the spot diameter.
Ceramic Block Test – Consists of an unglazed ceramic disc
which has thousands of micropores and cracks on its surface.
A pencil mark is made on the block and a small amount of a
test penetrant is placed on one side and a reference penetrant
is applied to the other side. After the correct dwell time, the
two penetrants are compared in a side-by-side visual
A reduction in the number or apparent brightness of pore
indications should be observed when comparing a fresh and
old batch of penetrant.
Water Content Test – The ASTM standard D-95 describes a
test where 100 ML of penetrant is placed in a boiling flask with
a similar quantity of moisture-free xylene. The condensate is
collected in a graduated tube to show percent of water by
If percent of water exceeds manufacturers’ recommendations,
the penetrant is discarded.
Viscosity Test – A viscometer is used to measure the viscosity
in centistokes to determine if the penetrant is within the range
recommended by the manufacturer. A typical standard is
Fluorescent Penetrant Fade Test – This test involves the use
of the aluminum test blocks in a side-by-side comparison
The fluorescent penetrant is placed on both sides of the block
and processed normally. One half of the block is then exposed
to a standard black light for one hour, while the other side is
covered with paper.
The fluorescent brilliance of the two sides is observed and,
if one side in noticeably less brilliant, the penetrant is
Water Washability Test – This test evaluates the efficiency of
the emulsifier by comparing two different percentage blends of
penetrant and emulsifier.
A special steel block is placed at a 75º angle and the two blends
are allowed to flow separately down the block. After a five-minute
waiting period, the block is washed and examined for traces of
Developers – Dry Developers are simply visually inspected to
see that they are not lumpy or caked instead of fluffy and light.
Wet developers are often checked using a hydrometer to
assure that the density of the powder in the vehicle is within the
range recommended by the manufacturer.
Filtered Particle Method of Penetrant Inspection – When
testing a porous surface with filtered particles. The increased
area created by a crack will “filter” the penetrant, leaving the
particles on the surface.
1. FALSE 2. TRUE 3. FALSE
4. TRUE 5. FALSE 6. TRUE
7. TRUE 8. TRUE 9. TRUE
10. FALSE 11. TRUE 12. FALSE
13. TRUE 14. FALSE 15. TRUE
16. TRUE 17. TRUE 18. FALSE
This lesson discusses types of discontinuities that can be
evaluated with the liquid penetrant method.
Discontinuities can be divided into three general categories:
Inherent, Processing, and Service.
1. Inherent – They are usually related to discontinuities
found in the molten metal.
Inherent wrought discontinuities relate to the
melting and solidification of the original ingot before it
is formed into slabs, blooms, and billets.
Inherent cast discontinuities relate to the melting
and solidification of a cast article. Usually caused by
inherent variables such as inadequate feeding, gating,
excessive pouring temperature, and entrapped gases.
2. Processing Discontinuities – They are usually related to
the various manufacturing processes such as
machining, forming, extruding, rolling, welding, heat
treating, and plating.
3. Service Discontinuities – They are related to the various
service conditions such as stress corrosion, fatigue,
Remember, when you use the liquid penetrant method, you
can find only those discontinuities which are open to the
However, during the manufacturing process, many
discontinuities that were subsurface can be made open to the
surface by machining, grinding, and the like.
Remember that discontinuities are not necessarily defects.
Any indication that is found by the inspector is called a
discontinuity until it can be identified and evaluated as to the
effect it will have on the service of the part.
Listed on the next slide are some typical discontinuities that
should be recognized when doing any type of nondestructive
test. (A more detailed study of typical discontinuities can be
obtained by reading programmed instruction handbook PI-4-1.)
Discontinuities trapped in the ingot during the steel-making
process may cause additional types of discontinuities as the
steel is used in the manufacture of an article.
There are three main types of discontinuities found in ingots.
1. Porosity – Caused by
entrapped gas in the
2. Nonmetallic Inclusions -
Caused by impurities
accidentally included in the
3. Pipe – Caused by shrinkage
at the center of the ingot as the
molten metal solidifies.
The “HOT TOP” is usually cropped off to remove most of the
When an ingot is further processed into slabs, blooms, and
billets, it is possible for the discontinuities to change size and
As a billet is flattened and spread out, nonmetallic inclusions
may cause a lamination. Pipe and porosity could also cause
laminations in the same manner as shown below:
As a billet is rolled into bar stock, nonmetallic inclusions are
squeezed out into longer and thinner discontinuities called
As a billet is rolled into round bar stock, surface
irregularities may cause seams. Seams are caused by folding
of metal due to improper rolling or by a crack in the billet as
A seam could also occur when the billet is formed into a
rectangular bar as shown below:
Forging Discontinuities occur when metal is hammered or
pressed into shape, usually while the metal is very hot.
A forged part gains strength due to the grain flow taking
the shape of the die, and the process is shown below:
A forging lap is caused by folding of metal on the
surface of the forging, usually when some of the forging
metal is squeezed out between the two dies.
A forging burst is a rupture caused by forging at improper
temperatures. Bursts may be either internal or open to the
surface as shown below.
Casting discontinuities occur when molten metal is poured
into a mold and allowed to solidify.
A cold shut is caused when molten metal is poured
over solidified metal as shown below:
Hot Tears (shrinkage cracks) occur when there is unequal
shrinkage between light and heavy sections as shown below:
Shrinkage cavities are usually caused by lack of enough
molten metal to fill the space created by shrinkage, similar to
pipe in the ingot.
Microshrinkage is usually many small subsurface holes that
appear at the gate of the casting.
Microshrinkage can also occur when the molten metal
must flow from a thin section into a thicker section of a
Blow holes are small holes at the surface of the casting
caused by gas which comes from the mold itself. Many molds
are made of sand, and when molten metal comes into contact
with the mold, the water in the sand is released as steam.
Porosity is caused by entrapped gas. Porosity is usually
subsurface but can occur on the surface depending on the
design of the mold.
Grinding Crack: Are a processing-type discontinuity caused
by stresses which are built up from excess heat created
between grinding wheel and metal.
Grinding cracks will usually occur at right angles to the
rotation of the grinding wheel.
Heat treat cracks are often caused by the stresses built up
during heating and cooling. Unequal cooling between light
and heavy sections may cause heat treat cracks.
Heat treat cracks have no specific direction and usually start
at sharp corners which act as stress concentration points.
Fatigue cracks are service-type discontinuities that are usually
open to the surface where they start from concentration
Fatigue cracks are possible only after the part is placed into
service, but may be the result of porosity, inclusions, or other
discontinuities in a highly stressed metal part.
Welding Discontinuities – The following are types of
1. TRUE 2. TRUE 3. TRUE
4. TRUE 5. FALSE 6. FALSE
7. FALSE 8. FALSE 9. FALSE
10. FALSE 11. FALSE 12. FALSE
13. TRUE 14. TRUE 15. TRUE
16. TRUE 17. TRUE 18. FALSE
Identification and comparison of discontinuities
that can be found with the liquid penetrant
The student is asked to study the photographs
and descriptions of typical discontinuities as
printed in the PT Classroom Training Handbook
Each of the specific discontinuities illustrated are
divided into three general categories: Inherent,
Processing, and Service.
Cold Shut Page 7-10
Fillet Crack Page 7-12
Grinding Cracks Page 7-15
Heat-Affected Zone Cracking Page 7-19
Heat Treat Cracks Page 7-21
Shrinkage Cracks Page 7-24
Thread Cracks Page 7-26
Hydrogen Flake Page 7-31
Lack of Penetration Page 7-40
Laminations Page 7-42
Laps and Seams Page 7-44
Laps and Seams Page 7-46
Microshrinkage Page 7-49
Stress Corrosion Page 7-55
Hot Tears Page 7-63
Intergranular Corrosion Page 7-65
After reviewing the photographs and descriptions of typical
discontinuities, it should be obvious that liquid penetrant
inspection is not the best method in all cases.
The selection of one testing method over another is based
upon variables such as:
1. Type and origin of discontinuity.
2. Material manufacturing process.
3. Accessibility of article.
4. Level of acceptability desired.
5. Equipment available.
Training and Certification
It is important that the technician and supervisor be qualified in
the liquid penetrant method before the technique is used and test
The American Society for Nondestructive Testing recommends the
use of its document Recommended Practice No. SNT-TC-1A.
This document provides the employer with the
necessary guidelines to properly qualify and certify
the NDT technician in all methods.
To comply with this document the employer must
establish a “written practice” which describes in detail
how the technician will be trained, examined, and certified.
The student is advised to study page 6 of the June 1975
edition of SNT-TC-1A to determine the recommended
number of hours of classroom instruction and months of
experience necessary to be certified as a liquid penetrant
Certification of NDT personnel is the responsibility of the employer
and is usually at three levels.
LEVEL I – is qualified to perform specific calibrations, specific tests,
and specific evaluations according to written
LEVEL II - Is qualified to set up and calibrate equipment and to
interpret and evaluate results with respects to codes,
standards, and specifications. Must be able to
prepare written instructions and report test results.
LEVEL III - Must be capable and responsible for establishing
techniques, interpreting codes, and designating the
test method and technique to be used. Must have a
practical background in the technology and be familiar
with other commonly used methods of NDT. The
SNT-TC-1A Document recommends that the NDT Technician be
examined in the following areas:
A. General Examination
B. Specific Examination
C. Practical Examination
ASNT provides a service to the
industry by providing Level III
Examinations in the general and
practical areas. Because of the
individual requirements of the many
industries using NDT, the specific
examination is still the
responsibility of the employer.
The following flow chart indicates
the paths that can be taken to be
certified according to the SNT-TC-
1. FALSE 2. FALSE
3. TRUE 4. FALSE
5. TRUE 6. TRUE
7. FALSE 8. FALSE
9. TRUE 10. TRUE
• Summarize your points.
• State your conclusion. Make it relevant to
And, if applicable:
• Describe options for future consideration.
• Recommend a future strategy, plan