metallurgical testing

Metallurgical Testing

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

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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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Tensile Test

- 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 Test

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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.

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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 deformation5/27/2015

Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh.

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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. 5/27/2015

Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh.

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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

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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


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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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