Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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SPECIFICATION
The present invention is in the field of nondestructive
testing using magnetic particles wherein a magnetized
workpiece is treated with a dry dispersion of magnetizable
particles in a light, fluffy inert dry powder vehicle which
prevents agglomeration of the magnetic particles, but is
readily removable from the surface once the magnetic par-
ticles have become deposited along flaws in the surface of
the workpiece.
Techniques employing magnetic particles have been used
for flaw detection for considerable periods of time. Such
non-destructive testing methods basically include the
step of supporting a magnetizable workpiece and passing a
direct or alternating current therethrough in order to
magnetize the same and then contemporaneously applying
magnetic particles either dry or in suspension in water or
oil. The stray magnetic fields produced at any surface
flaws serve to attract the magnetic particles so that they
are readily visible upon inspection. More recently, the
magnetic particles have been combined with fluorescent
particles either in the form of a water or oil suspension
so that the workpiece was inspected under filtered ultra-
violet or blac~ light to observe any concentration pattern
of fluorescent particles caused by a surface discontinuity.
Prior art patents referring to this type of inspection include
Switzer Patent No. 2,267,999 and Kazenas Patent No. 2,936,287.
These patents relate, respectively, to lacquer bonded and
resin bonded fluorescent magnetic particles for use in this
type of inspection process.
One of the more important areas in which magnetic
particle inspection is used is that of inspecting steel
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billets in a continuous manner. Normally, the billet i5
inspected for longitudinal seams and, in the case of
fluorescent particles, the inspection takes place under black
light in a darkened inspection area.
One of the difficulties with the conventional method
of magnetic particle inspection whereinthe particles are
applied dry is the fact that the particles tend to agglom-
erate and are difficult to maintain in discrete form. The
wet method of application whether using water or a hydro-
carbon oil has its own drawbacks. For one, there is the
problem of freezing of the suspending liquid, or at least
an increase in viscosity of the liquid, during periods of
cold temperatures. Consequently, it has been common practice
to add antifreeze solutions to the bath. Secondly, it is not
an easy task to remove the excess liquid from the surface
ofthe piece to avoid unwanted background. In addition, there
is the problem, particularly with a water suspension, of
the particles settling out in pipelines and the like
resulting in clogging and loss of magnetic particles. In
addition to this, it is not possible to process hot billets
with dispersions containing magnetizable particles in
conventional liquid suspending media.
The present invention pro~ides an improved suspending
medium for magnetic particles to be used in magnetic particle
inspection methods, whether such particles are used as such
or they are adhered to or coated with a fluorescent material.
In accordance withthe new method, we apply suspensions or
dispersions of magnetic material in fluffy, extremely light,
relativel~ inert and dry powder vehicles. Typical among such -
vehicles are the very finely divided aluminum oxides produced
by flame hydrolysis of anhydrous aluminum chloride.
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More specifically, the magnetizable particles
constitute from about 0.5 to 25% by weight of the dispersion
and preferably about 2 to 20% by weig~ n a preferred
form of the invention, the dry powder vehicle consists pre-
dominantly of particles less than 1 micron in maximum dimen-
sion.
In the present invention, magnetic particles which may
be of the fluorescent type or not are combined with a fluffy
and light powder by agitating the two mechanically or by
means of air currents to effect uniform distribution of the
magnetic material through the dispersion. The light, fluffy
and non-sticking powders used in accordance with the present
invention exhibit practically no "scraping" action with respect
to the indications being formed by the magnetic particles.
Since the powders do not stick to the surface of the billet,
excess powders merely fall of the surface during processing
or they can be blown off with a gentle stream of air. It has
been further found that processing and handling of the fluffy
suspensions can be simpler than involved with the less bulky
and heavier products.
The method of the present invention is particularly
applicable to the use of the type of improved fluorescent
magnetic powders described in Borrows Canadian Serial No.
351,690 and assigned to the same assignee as the present
application. In the aforementioned copending application
there is described a composition for nondestructive testing
of magnetizable workpieces employing discrete magnetic
particles having a ferromagnetic particle core, fluorescent
pigment particles attached to the core, and a cascading
opacifier associated with the core, the fluorescent pigment
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particles being at least 2 microns in maximum dimension.
In the preferred form of the invention, the cascading opac-
ifier is fluoranthene, and t~e core particles have a maximum
dimension of from 25 to 150 microns. The core particles ~nd
daylight fluorescent pigment particles are held together
by means of encapsulation with a filmforming resin which also
serves as the carrier for the opacifier.
The preferred fluffy vehicle is a finely divided alum-
inum oxide produced by flame hydrolysis of anhydrous aluminum
oxide. One such material is available commercially as
~Aluminum Oxide C" marketed by Degussa, Inc. This material
is said to ha~e a very large specific surface area. Crystallo-
graphic analysis indicates that the material s primarily a
gamma aluminum oxide having a slight delta structure. The
improved aluminum oxide has a strong tendency to acquire a
positive charge which may account for its lack of adherence
to the test surfaces.
The magnetizable particles constitute from about 0.5
to 25% by weight of the composition, and preferably from 2
to 20% by weight. The dispersion can be made up by mech-
anically agitating the two materials together or by using
a suitable current of air. The application of the particles
is substantially the same as the present technique for apply-
ing magnetizable particles in dry form, namely, suspending
the dispersion in air and carrying the same to the surface
of the part. The suspension should be applied in such a way
that the powders reach the magnetized surface in a uniform
cloud with a minimum of motion. Under these conditions, the
particles come under the influence of the leakage fields
while suspended in air, and have three-dimensional mobility.
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This condition can be best achi~ved when the magnetized
surface is vertical or overhead. Since the fluffy, dry
particles do not adhere to the surface, it is usually not
necessary to use outside forces to re ve excess particles
from the surface, although a gentle stream of air can be used
if desired. The workpiece is then inspected either under
ordinary light or under ultraviolet radiation for clusters
of the magnetizable particles. Where magnetizable particles
of the type described in the aforementioned Borrows applica-
tion are used, the inspection can proceed simultaneously
in white light and in ultraviolet light.
The use of the improved composition of the present
invention in the magnetic particle inspection method has
several distinct advantages. For one, the problems associ-
ated with low temperatures are avoided inasmuch as no anti-
freeze additions are necessary. The relatively hydrophobic
fluffy powders are also superior to other materials in that
they do not absorb moisture from the air. Furthermore, it
is easier to fix the formed indications on dry billets than
it is on wet ones. It i9 also possible to process hot billets
with the improved composition of the present invention,
since the carrier powders will not melt or decompose up to a
temperature of at least 300F.
It should be evident that various modifications can be
made tothe described embodiments without departing from the
scope of the present invention.
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