Metallurgical Testing

Metallurgical Testing
5/27/2015
Mohd. Hanif Dewan, Chief Engineer and
Maritime Lecturer & Trainer, Bangladesh.
1

Non-Destructive Testing
- This is carried out on components rather than on test
pieces, they are designed to indicate flaws occurring
due or after manufacture. They give no indication of
the mechanical properties of the material.
- Surface flaws may be detected by visual means
aided by dye penetrant or magnetic crack detection.
- Internal flaws may be detected by X-ray or ultrasonic
testing.
- In addition to this there are special equipment able to
exam machine finish.
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Liquid Penetrant Methods
- The surface is first cleaned using an volatile cleaner and
degreaser.
- A fluorescent dye is then applied and a certain time
allowed for it to enter any flaws under capillary action.
Using the cleaning spray, the surface is then wiped clean. -
- An ultra violet light is shone on the surface, any flaws
showing up as the dye fluoresce.
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Dye penetrant method
- The surface is cleaned and the low viscosity
penetrant sprayed on.
- After a set time the surface is again cleaned.
- A developer is then used which coats the surface
in a fine white chalky dust, then the dye seeps out
and stains the developer typically a red colour.
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Magnetic crack detection
- A component is place between two poles of a magnet.
- The lines of magnetism concentrate around flaws.
- Magnetic particles are then applied, in a light oil or dry sprayed, onto
the surface where they indicate the lines of magnetism and any
anomalies/ abnormalities like the below figures.
Limitation of Magnetic test:
This method of testing has a few limitations.
- Firstly it cannot be used on materials which cannot be magnetised
such as austenitic steel and non-ferrous metals.
- Secondly it would not detect a crack which ran parallel to the lines of
magnetism.
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- When a material is tested under a tensile load, it
changes shape by elongating.
- Initially the extension is in proportion to the increasing
tensile load.
- If a graph is plotted showing extension for various
loads, then a straight line is obtained at first.
- If the loading is continued the graph, deviates as
shown.
- Within the limit of the straight line, if the load is
removed the material will return to its original length
which is elastic limit of the specimen.
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Tensile Testing Method

- When the test piece reaches the Yield point (Yu), there is a
failure of the crystalline structure of the metal, not along the
grain boundaries as it has been the case, but through the grains
themselves. This is known as “slip”.
- A partial recovery is made at the lower yield point (YL), then
the extension starts to increase.
- If the load is removed at any stage along the “Load-Extension”
curve after Yield Point (YL), the material will have a
corresponding permanent deformation. This termed
“permanent set”.
- Maximum loading occurs at the “ultimate Load” (S).
and after Yield Point (YL) to Ultimate Load (S) is the plastic limit.
- Ultimate Load (S) to Breaking point (B) this stage local wasting
or extension will start which termed “necking”. Normally this starts
at about the centre of the specimen and will rapidly be followed by
failure up to breaking point (B). 9
Tensile Testing Method

Proof Stress:
For a material which does not have a marked yield point such
as Aluminium, there is a substitute stress specified. This is
termed “the proof stress”.
- Proof stress is determined from a load/extension or
stress/strain graph.
11

Proof Testing Method
• Hard steels and non-ferrous metals (Aluminium)
do not have defined yield limit, therefore a stress,
corresponding to a definite deformation, (0.1% or
0.2%) is commonly used instead of yield limit.
This stress is called proof stress or offset yield
limit (offset yield strength):
• σ0.2%= F0.2% / S0
• The method of obtaining the proof stress is
shown in the picture.
• As the load increase, the specimen continues to
undergo plastic deformation and at a certain
stress value its cross-section decreases due to
“necking”.
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• At point S in the Stress-Strain Diagram the stress
reaches the maximum value, which is called ultimate
tensile strength (tensile strength):
σt= FS / S0
• Continuation of the deformation results in breaking the
specimen - the point B in the diagram (from Ultimate
load S to breaking point B)
• The actual Stress-Strain curve is obtained by taking into
account the true specimen cross-section instead of the
original value.
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Creep Testing
- Creep tests are carried out
under controlled temperature
over an extended period of
time in the order of
10,000hrs.
- The test piece is similar to
the type used for tensile
tests and creep is usually
thought of as being
responsible for extensions
of metal only. In fact creep
can cause compression or
other forms of deformation
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- Temperature of the test is at recrystallization point around
400oC. For other metals the recrystallization temperature is
different (200oC for copper and room temperature for tin and
lead).
- At the start of the test the initial load must be applied
without shock.
- This load, normally well below the strength limit of the
material, will extend the test piece slowly.
- The load is kept steady through the test and the
temperature is maintained accurately.
- Extension is plotted and is seen to proceed in three distinct
stages.
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HARDNESS TESTING
The basis of the Brinell hardness
testing is the resistance to
deformation of a surface by a
loaded steel ball.
Oil is pumped into the chamber
between the pistons until there is
sufficient pressure to raise the
Weight so that it is floating. The ball
is now forced into the specimen
material at the same force. The
loading for steel and metals of
similar hardness is 3,000Kg. The
load is allowed to act for 15 sec to
ensure that plastic flow occurs.
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- The surface diameter of the indentation is measured
with the aid of a microscope which is traversed over
the test piece on a graduated slide with a vernier.
- Cross wires in the microscope, enable the operator
to accurately align the instrument.
- Both the loading and ball diameter (10mm) are
known, by measuring the indentation diameter the
hardness can be calculated.
For softer materials the loading is reduced, Copper
being 1000Kg and Aluminium 500Kg. The diameter of
the indentation must be less than half the ball
diameter.
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- The thickness of the specimen must be not less than
10x the depth of the impression. The edge of the
impression will tend to sink with the ball if the surface
has been work hardened; otherwise the local
deformation will tend to cause piling up of the metal
around the indent
If the hardness test is used on very hard materials, the
steel ball will flatten. This method is not reliable for
reading over 600. It is used in preference to other
methods where the material has large crystals, e.g.
Cast iron.
Mild Steel 130, Cast Iron 200, white cast iron 400,
nitrided surface 750.
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Under low temperature conditions , impact or shock loading on a
material can cause cracking in a material which is normally
ductile at room temperature.
To find out the Critical stressing in a material, Griffith
equation,
sc = Kic / ж Pc
where, sc = the critical stress in a material
Kic = the fracture toughness of a material
Pc = the micro-crack length within the materials
21
BRITTLE FRACTURE TESTING

BRITTLE FRACTURE TESTING
- The presence of these micro cracks (porous
materials or defects) can act to cause
transcrystalline type failures with a bright crystalline
appearance.
- Testing is carried out via the Charpy notched piece
test at various temperatures between -200o to
+200oC
- To reduce the effects of brittle fracture the carbon
content in carbon steels is kept as low as practical.
- Grains within the materials are kept as small as
possible by heat treatment and normalizing.
- Alloying elements may also be added.
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Transition Temperatures
• As temperature decreases a ductile material can become
brittle - ductile-to-brittle transition
– The transition temperature is the temp at which a
material changes from ductile-to-brittle behavior
• Alloying usually increases the ductile-to-brittle
transition temperature. FCC metals remain ductile down
to very low temperatures. For ceramics, this type of
transition occurs at much higher temperatures than for
metals.

Factors which affect the transition temperature are
1. Elements:
- Carbon, silicon, phosphorus and sulphur raise the
temperature.
- Nickel and manganese lower the temperature.
2. Grain size:
- the smaller the grain size the lower the transition
temperature, hence grain refinement is beneficial.
3. Work hardening:
- this appears to increase transition temperature.
4. Notches:
- possibly occurring during assembly e.g. weld defects or
machine marks.
- Notches can increase tendency to brittle fracture.
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18/80 stainless steel
5/27/2015
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It is this property of stainless steel that makes it so suitable
for use in LPG carriers. Hardness Testing

Metallurgical Testing

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