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Reasons for the impregnation of porous metals
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| What is
the cause of leaks? |
| Cracks, fissures and porosity, as we know,
are the cause of leaks. These defects originate
when metal is fused, especially with alloys
and/or if there are parts with large section
changes. |
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All of
this is due, among other things, to the fact that the material
is not suitable for a particular type of parts, to cooling
defects, to the gas formation or inclusions of extraneous
matter in the casting, which produces cracks, cavities and
porosity.
It may
even be that there are no structural defects in the material,
but because of the thinness of the pieces, they may present
leaks through intermolecular spaces. |
| All of
these defects, which are not frequently discovered at first
sight, produce discards in cast pieces. |
Solutions and advantages of anti-porosity
treatment
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In the event the problem is microporosity resulting in leaks during normal use of the
pieces, this can be treated with our impregnation process, and the pieces are 100% recoverable. Not
only this, but pieces treated with the impregnation
process as an additional phase of production,
will have a guarantee of future durability.
So for certain pieces, the impregnation treatment
must be considered as part of the production
process.
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Types of microporosity in porous metals
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| There are three categories of microporosity
in metal casting: |
| a) Porosity without permeability towards the
outside. In this case, leaks will result only if
mechanical operations are performed. This can affect the
structural strength of the piece, according to its dimensions
and morphology. |
b) Blind porosity.
This is a porosity that starts from the surface and penetrates
the body of the piece. The morphology of this porosity can be
extremely varied; it may have a small mouth and deep
cavities.
The presence of blind porosity negatively affects
the results of superficial treatment.
In fact, the pores
become receptacles for pockets of air, treatment fluids,
cutting oil, etc, which may flow out of the metal, corroding
treated surfaces from underneath, producing the following
problems:
· in the event of oven-treated painting,
there may be a "blistering"effect (bubbles on the surface of
the paint);
· in galvanised pieces, discolouring
may result;
· in anodised pieces, it may result
in "white spotting". |
| c) Permeable
Porosity. This category of porosity is the worst
because it may cause discarding of components that have
already been mechanically processed and which, therefore are
very costly. |
| In taking
the costs of impregnation into account, the largest firms,
especially those in the mechanical sector, have calculated
that with leaks in more than 5% of the pieces, it becomes
interesting to apply the impregnation treatment to 100% of the
production, thus limiting the final control to a single lot
sampling. This percentage varies, of course, depending on the
mechanical processing the pieces are previously subjected
to. |
| It is also possible to effect a control on impregnated
pieces. In this case it must be evaluated whether it is more
economical to impregnate and recover pieces or perform
controls on pieces and impregnate only those that present
leaks. In this case both costs would have to be added
together: control and
impregnation. |
An
important point to take into account with this type of problem
is to define what must be considered a "leak" and how it can
be discovered. A "leak" can be defined as the process of
intake or loss of a fluid through the wall of a piece, which
produces effects that jeopardise the piece itself. If the loss
does not produce this jeopardising effect, it must not be
considered a "leak".
It must be decided whether the leak is
acceptable or not. |
| To arrive
at this conclusion, it is necessary to use a unit of measure.
Since the flow of gas is used in most processes for the
detection of leaks, in order to determine a quantity of fluid,
we must specify a volume and pressure. |
The most commonly used unit is cm3, at
atmospheric pressure per second (cm3atm/sec).
Since leaks are very small in the
majority of cases, a system of negative exponents of 10 is
used. |
| The measurement of a possible leak, in its
industrial application, together with its visual
equivalents, is shown hereunder:
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| 10-1
1cm3/10 sec |
Continuous flow |
| 10-2
1cm3/100 sec |
10
bubbles per second |
| 10-3
3cm3/10 hrs. |
1
bubble per second |
| 10-4
1cm3/3 hrs. |
1
bubble every 10 seconds |
| 10-5
1cm3/24 hrs. |
... |
| 10-6
1cm3/2 weeks |
... |
| 10-7
3cm3/yr. |
... |
| 10-8
1cm3/3yrs. |
... |
| 10-9
1cm3/30 yrs. |
... |
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| A bubble is considered as having a volume of 1 mm3. |
It must be
kept in mind that the maximum leak acceptable for a product
depends upon its nature and upon the fact that static systems
require more restrictive specifications than dynamic
ones.
For instance, in a dynamic chemical process, a leak
of 10-1 at 1 cm3 in N.C. may be
tolerable. |
Every producer must determine the
admissible level of leak and perform tests under conditions
similar to those of actual use, since different conditions of
temperature or fluid would make the test results
unreliable.
Leaks greater than 10-1 must be discovered with
visual or acoustic methods. |
It must be kept in mind, additionally, that searching
for leaks 100 or 1000 times smaller than acceptable limits
only adds to expenses, without improving the dependability of
the product.
Nevertheless, it is recommended in tests to
work with values twice those of normal use, in order to obtain
a reasonable degree of leak
discovery. | |
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