Hardness Test(For Materials)

8/12/2019 Hardness Test(For Materials)

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HARDNESS TEST


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Defination Of Hardness The Metals Handbook defines hardness as "Resistance of metal

to plastic deformation, usually by indentation. However, the term

may also refer to stiffness or temper, or to resistance toscratching, abrasion, or cutting. It is the property of a metal,which gives it the ability to resist being permanently, deformed(bent, broken, or have its shape changed), when a load isapplied. The greater the hardness of the metal, the greaterresistance it has to deformation.

In mineralogy the property of matter commonly described as theresistance of a substance to being scratched by anothersubstance. In metallurgy hardness is defined as the ability of amaterial to resist plastic deformation.

The dictionary of Metallurgy defines the indentation hardness asthe resistance of a material to indentation. This is the usual typeof hardness test, in which a pointed or rounded indenter ispressed into a surface under a substantially static load.


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Hardness measurement can be defined as

Macro-,Micro- or Nano- scale according to the forces applied anddisplacements obtained .

Measurement of the macro-hardness of materials is aquick and simple method of obtaining mechanical property datafor the bulk material from a small sample.

It is also widely used for the quality control of surface treatmentsprocesses.

However, when concerned with coatings and surface propertiesof importance to friction and wear processes for instance, themacro-indentation depth would be too large relative to thesurface-scale features.

Where materials have a fine microstructure, are multi-phase,non-homogeneous or prone to cracking, macro-hardness

measurements will be highly variable and will not identifyindividual surface features. It is here that micro-hardnessmeasurements are appropriate.


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Microhardnessis the hardness of a material asdetermined by forcing an indenter such as a Vickers or

Knoop indenter into the surface of the material under15 to 1000 gf load; usually, the indentations are sosmall that they must be measured with a microscope.

Capable of determining hardness of differentmicroconstituents within a structure, or measuring

steep hardness gradients such as those encounteredin casehardening. Conversions from microhardnessvalues to tensile strength and other hardness scales(e.g. Rockwell) are available for many metals andalloys[2].

Micro-indenters works by pressing a tip into a sampleand continuously measuring: applied load, penetrationdepth and cycle time.
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Nano-indentation testsmeasure hardness

by indenting using very small, on the order of

1 nano-Newton, indentation forces and

measuring the depth of the indention that wasmade.

These tests are based on new technology

that allows precise measurement and controlof the indenting forces and precise

measurement of the indentation depths.


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Hardness Measurements

There are three types of Hardness Measurements:

1.Stratch Hardness-ability to strach less hardermaterial.[Mobs Hardness]

2.Indentattion Hardness:

Different types: Rockwell,Brinell,Vicker, Knoopand Meyer Hardness

3. Rebound Hardness:

The indentor is dropped on metal surface and

hardness is measured as energy of impact.


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Rockwell Hardness Test The Rockwell hardness test method consists of indenting

the test material with a diamond cone or hardened steelball indenter. The indenter is forced into the test materialunder a preliminary minor load F0(Fig. 1A) usually 10kgf .

While the preliminary minor load is still applied an

additional major load is applied with resulting increase inpenetration (Fig. 1B).

When equilibrium has again been reach, the additionalmajor load is removed but the preliminary minor load isstill maintained. Removal of the additional major loadallows a partial recovery, so reducing the depth ofpenetration (Fig. 1C).

The permanent increase in depth of penetration,resulting from the application and removal of theadditional major load is used to calculate the Rockwellhardness number


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HR = E - e

F0= preliminary minor load in kgfF1= additional major load in kgfF

= total load in kgfe= permanent increase in depth of penetration due to major loadF1 measured in units of 0.002 mmE = a constant depending on form of indenter: 100 units fordiamond indenter, 130 units for steel ball indenterHR = Rockwell hardness numberD= diameter of steel ball

Fig. 1.Rockwell Principle


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r


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Purpose of Minor load

The first load, known as the "minor", loadof 10 kilograms is applied to the specimen tohelp seat the indenter and remove theeffects, in the test, of any surface

irregularities. In essence, the minor load creates a

uniformly shaped surface for the major loadto be applied to.

The difference in the depth of the indentationbetween the minor and major loads providesthe Rockwell hardness number.


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Rockwell Hardness Scales

Scale Indenter

Minor Load

F0kgf

Major Load

F1kgf

Total Load

Fkgf

Value ofE

A Diamond cone 10 50 60 100

B 1/16" steel ball 10 90 100 130

C Diamond cone 10 140 150 100

D Diamond cone 10 90 100 100

E 1/8" steel ball 10 90 100 130

F 1/16" steel ball 10 50 60 130

G 1/16" steel ball 10 140 150 130

H 1/8" steel ball 10 50 60 130

K 1/8" steel ball 10 140 150 130

L 1/4" steel ball 10 50 60 130

M 1/4" steel ball 10 90 100 130

P 1/4" steel ball 10 140 150 130

R 1/2" steel ball 10 50 60 130

S 1/2" steel ball 10 90 100 130

V 1/2" steel ball 10 140 150 130


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Rockwell Superficial Hardness Tester

This machine works the same as the standard Rockwelltester, but is used to test thin strip, or lightly carburizedsurfaces, small parts or parts that might collapse underthe conditions of the regular test.

The Superficial tester uses a reduced minor load, just 3kilograms, and has the major load reduced to either15 or 45 kilogramsdepending on the indenter, whichare the same ones used for the common scales.

Using the 1/16" diameter, steel ball indenter, a "T" is

added (meaning thin sheet testing) to the superficialhardness designation.


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SCALE TYPICAL APPLICATIONS

For Rockwell Hardness Test

A Cemented carbides, thin steel and shallow casehardened steel

B Copper alloys, soft steels, aluminum alloys,malleable iron, etc.

C Steel, hard cast irons, pearlitic malleable iron,titanium, deep case hardened steel and other materialsharder than B 100

D Thin steel and medium case hardened steel andpearlitic malleable iron

E Cast iron, aluminum and magnesium alloys, bearingmetals

F Annealed copper alloys, thin soft sheet metals

G Phosphor bronze, beryllium copper, malleable irons

H Aluminum, zinc, lead K L M P R S VBearin metals and other ver soft


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ADVANTAGES OF ROCKWELL Test

1.Since two types of indentors are used(diamond coneand steel ball)testing of all the materials is possible.

2.Thick as well as thin(not very thin)materials can betested.(60 kg major load for thin and 150 major load

for thick) 3.Surface condition of component to be tested need

not to be very critical as hardness number is corelatedto difference in depth from minor load to major load.

4.The hardness number are obtained on dial hencefree from personal error.

Test is very fast and does not requires skilledpersonal.


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BRINELL HARDNESS TEST

Brinell hardness is determined by forcing ahard steel or carbide sphere of a specifieddiameter under a specified load into the

surface of a material and measuring thediameter of the indentation left after the test.

The Brinell hardness number, or simply theBrinell number, is obtained by dividing the

load used, in kilograms, by the actual surfacearea of the indentation, in square millimeters


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The hardened steel ball into the surface of the test specimen. Themachine applies a load of 500 kilograms for soft metals such as

copper, brass and thin stock. A 1500 kilogram load is used foraluminum castings, and a 3000 kilogram load is used for materialssuch as iron and steel.

The load is usually applied for 10 to 15 seconds. After theimpression is made, a measurement of the diameter of the resultinground impression is taken. It is measured to plus or minus .05mm

using a low-magnification portable microscope. The hardness is calculated by dividing the load by the area of the

curved surface of the indention.

To make it easier, a calibrated chart is provided, so with thediameter of the indentation the corresponding hardness number canbe referenced.

"75 HB 10/500/30"

On tests of extremely hard metals a tungsten carbide ball issubstituted for the steel ball.


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Requirements of surface for testing by

Brinell Hardness test The surface of specimen should be smooth and free

from surface defects, oils,extraneous materials etc.

The surface on which impression is to be taken shouldbe parallel to bottom surface so as to obtain circular

impression The thickness of specimen to be tested must be

sufficient so that anvil support will not affect thepenetration of the ball (thickness should be 10 timesdepth of impression)

The distance from the edge of test piece to the centre ofthe impression should not be less than 2.5 times thediameter of impression.


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Drawbacks

1.Because of large size of ball(10mm) and high loads

(3000kg)impressions are larger. These impressions

decrease the useful life of the components.

2.Ball indentor is likely to deform while testing hard

materials. The increase in diameter of indentation gives

lower hardness value.


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ADVANTAGES

1.Large sized indentor covers more area and hence

gives average hardness of material.

2.Useful to measure hardness of hetrogenous materialslike cast iron, porous powder metallurgical parts.

It gives continous scale for hardness measurement.


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Vickers Hardness Test

It is the standard method for measuring the

hardness of metals, particularly those with extremely

hard surfaces: the surface is subjected to a standard

pressure for a standard length of time by means of a

pyramid-shaped diamond.

The diagonal of the resulting indention is measured

under a microscope and the Vickers Hardness value

read from a conversion table [9]
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The indenter employed in the Vickers test is a square-based pyramid whose opposite sides meet at the apex atan angle of 136.

The diamond is pressed into the surface of the materialat loads ranging up to approximately 120 kilograms-force, and the size of the impression (usually no morethan 0.5 mm) is measured with the aid of a calibratedmicroscope.


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The Vickers number (HV) is calculated using thefollowing formula:

HV = 1.854(F/D2),

with F being the applied load (measured in kilograms-force) and D2 the area of the indentation (measured insquare millimetres). The applied load is usually specifiedwhen HV is cited.


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Advantage of Vicker Over Brinell Like the Brinell test, the Vickers number is determined

by dividing the load by the surface area of theindentation (H = P/A). The load varies from 1 to 120kilograms.

Rather than using the Brinell's steel ball type indenter,and have to calculate the hemispherical area ofimpression, the Vickers machine uses a penetrator that

is square in shape, but tipped on one corner so it has theappearance of a playing card "diamond".

The Vickers indenter is a 136 degrees square-baseddiamond cone, 136 degree angle is chosen because itcorresponds to most desirable ratio of indentation

diameter to ball diameter of 0.375 in Brinell hardnesstest.the diamond material of the indenter has anadvantage over other indenters because it does notdeform over time and use.

The impression left by the Vickers penetrator is a dark

square on a light background. The Vickers impression ismore easily "read" for area size than the circular


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Requirement of Test

1.The surface of specimen should be smooth and freefrom oxide,dirt and defects because impressions are

very small.

2.The test surface should be parallel to the bottom

surface. 3.Thickness of specimen should be atleast 1.5 times the

diagonal of impression.minimum three readings should

be taken and is average out.


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Advantages

The advantages of the Vickers hardness testare that extremely accurate readings can betaken, and just one type of indenter is used

for all types of metals and surface treatments Impressions are small so it does not damage

the life of components.

Indentor is made of very very hard material-

diamond.


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DISADVANTAGES

1.Because of small size of indentation not

useful to measure hardness of hetrogenous

materials.

Test demands good polish at the surface

Test is slow and subject to personla errot.


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Meyer Hardness Test

Meyer suggested that a more rational

definition of hardness than that proposed by

Brinell.

In this the mean pressure between thesurface of the indenter and the indentation is

equal to the load divided by the projected

area of the indentation.


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Meyer proposed an empirical relation between the loadand the size of the indentation. This relationship is

usually called Meyers law. P = kdn

The parameter nis the slope of the straight lineobtained when log Pis plotted against log d, and kisthe value of Pat d = 1.

Fully annealed metals have a value of nof about 2.5,while nis approximately 2 for fully strain-hardenedmetals.

This parameter is roughly related to the strain-hardening

coefficient in the exponential equation for the true-stress-true-strain curve.


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MICROHARDNESS TESTER/KNOOP

TESTER Many metallurgical problems require the determination of

hardness over very small areas. The measurement ofthe hardness gradient at a carburized surface, thedetermination of the hardness of individual constituentsof a microstructure, or the checking of the hardness of a

delicate watch gear might be typical problems. This test method was devised in 1939 by F. Knoop and

colleagues at the National Bureau of Standards in theUnited States

The Knoop indenter is a diamond ground to a pyramidalform that produces a diamond-shaped indentation withthe long and short diagonals in the approximate ratio of7:1 resulting in a state of plane strain in the deformedregion. The Knoop hardness number (KHN) is the

applied load divided by the unrecovered projected areaof the indentation.


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The final Knoop hardness (HK) is derived from thefollowing formula:

HK = 14.229(F/D2),

with F being the applied load (measured in kilograms-force) and D2 the area of the indentation (measured insquare millimetres).

Knoop hardness numbers are often cited in conjunctionwith specific load values.


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The final Knoop hardness (HK) is derived from the following formula:

HK = 14.229(F/D2),

with F being the applied load (measured in kilograms-force) and D2 the area of theindentation (measured in square millimetres).


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MOHS Hardness

Mohs hardness is defined by how well a substance

will resist scratching by another substance [18].

It is rough measure of the resistance of a smooth

surface to scratching or abrasion, expressed interms of a scale devised (1812) by the German

mineralogist Friedrich Mohs.

The Mohs hardness [19]of a mineral is determined

by observing whether its surface is scratched by asubstance of known or defined hardness.
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Advantage:

1.Simple and quick.

2. Used for identification of minerals in the field

Diadvantage:

It is not suitable for accurately gauging the hardnessof industrial materials such as steel or ceramics as it

gives relative hardness.

Mohs scale is that it is not linear; that is, each

increment of one in the scale does not indicate aproportional increase in hardness


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Hardness Test(For Materials)

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