Experiment no. 7Object: To perform the following hardness tests
on the given specimens:
a) Rockwell Hardness Test
b) Vickers Hardness Test
c) Brinell Hardness Test
Equipment: Rockwell Hardness Tester, Vickers Hardness Tester,
Brinell Hardness Tester and testing specimens
Theory
Hardness while not lending itself readily to a comprehensive,
exact definition, may be generally termed as that property of a
material which denotes resistance to permanent deformation. It is
also defined as resistance of material to penetration under
localized pressure.A number of different definitions of hardness
form the basis for the various hardness tests now in use. Some of
the definitions are:
1. Penetration Hardness
2. Rebound Hardness Energy absorption under impact loads.
3. Resistance to scratching Scratch Hardness
4. Resistance to abrasion Wear Hardness5. Resistance to cutting
or drilling Machinability.
These definitions generally develop with the necessity for some
way of expressing quantitively, the performance requirements under
different conditions of service. Although all hardness measures
are, no doubt, functions of inter-atomic forces, the various
hardness tests dont bring these fundamental forces into play in the
same way and hence different values of hardness obtained for the
same material under different types of tests. It must however be
emphasized at any time hardness is only on index to some particular
property of the material.
Indentation hardness tests which are widely used and accepted,
usually consist of pressing an indentor of standard shape and size
into the material to be tested, under a given load. The hardness
number is then based on depth of indentation produced by a given
load. The hardness testing machines based on indentation principle
consist mainly of the following parts.
1. An indentor of definite shape and size.
2. A means of applying the known load.3. A means of measuring
the dimension of the impression made by the indenter.
It may be noted that the property of hardness is only a relative
value of the material and has no absolute value. Similar materials
may be graded according to hardness and a particular grade may be
specified for same type of surface. The hardness number cannot be
utilized directly in design or analysis like tensile, compressive
or shear strengths:
SCOPE AND APPLICATION OF HARDNESS TESTS
These tests have a wide field of use, although as commercial
tests, they are perhaps more commonly applied to metals than to any
other class of material. The results of a hardness test may be
utilized as follows:
1.Similar materials may be graded according to hardness, and a
particular grade, as indicated by a hardness test may be specified
for some one type of service.
However, the degree of hardness chosen depends upon previous
experience with materials under the given service and not upon any
instruisic significance of the hardness number.
2.Hardness tests are useful in process control and research. The
progress of annealing and the results of cold working can be
followed quickly and easily by hardness measurement. Thus,
annealing time temperature relationships can be established for
cold worked materials and the limits of strain hardening for
annealed pieces which are being cold worked can be determined. The
success or failure of heat treatment cycles (including surface
hardening for steel can be ascertained by measuring the hardness
after quenching and again after tempering. Hardness measurements
can be used to determine the best combinations of time and
temperature for temperature and precipitation hardening. Simple
hardness measurements can occasionally aid in separating mixed
batches of components, enabling detection of foreign compositions
and of parts that have been either improperly heat treated or not
heat treated at all.3.By establishing a correlation between
hardness and some other desired property e.g. tensile strength,
simple hardness tests may serve to control the uniformly of the
tensile strength and to indicate rapidly whether more complete
tests are warranted.
Penetration and rebound type tests: Because of simplicity, these
have become one of the important quality control tests for metals.
These are relatively inexpensive, require little experience for
their conduct and are non-destructive.
Abrasion or wear tests have found their principal use in
connection with paving materials and a number of such tests have
been standardized. Scratch test is one of the earliest measures of
hardness and is based on the hardness of materials. The scale known
as Mohrs scale begins with talc as 1 and ends with diamonds as 10.
Iron and steel range in hardness from 4 to 5, depending on the
alloy content and heat treatment.For determining the mach-inability
of metals, various special tests have been proposed. The hardness
reported as the depth of hole made by special drill in a given time
while running at a constant speed and pressure is sometimes called
Baur drill test.
Rockwell Hardness Test: The Rockswell hardness is a penetration
type of hardness test and the hardness, thus determined is a
function of the depth of impression made by the penetration on the
specimen.
Requirement of specimens: Two types specimen holders are
generally provided one to hold flat specimens and other to hold
round specimens. As far as possible, the specimens should be finely
machined and there should not be local surface irregularities.
Description of machine: The Rockwell hardness tester may be
either hand operated or motor operated, but because the loads to be
applied in the test are quite small, hand operated machines in
which the loads are manually handled are quite commonly in use. For
ease of conducting the test, portable type of Rockwell Hardness
Testers, which can be easily moved from place to place, are also
available. The principle of working of all these machines, however,
is the same.
The load is applied through a system of weights and levers and
the Penetrator may either be a steel ball or it may be a diamond
cone with a somewhat rounded point. The hardness value as read from
a specially graduated dial indicater, is an arbitrary number that
is related inversely to the depth of indentation. It should be
noted that there is no Rockwell hardness value designated by a
number alone, because it is necessary to indicate which indenter
and load have been employed in working the test. The dial of all
Rockwell Hardness testers have two sets of figures, one red and the
other black, which differ by 30 hardness number. The red figures
are the B scale and are used for readings obtained with ball
indenters and the black figures are the C scale which is used by
only with diamond cone. The B scale is for the testing materials of
medium hardness such as low and medium carbon steels in the
annealed conditions. The working range of this scale is from 0 to
100. If the ball indenter is used to test material harder than
about B 100, there is danger that it will be flattened.
Furthermore, because of its shape (i.e. round), the ball is not so
sensitive as the rounded conical indenter to differences in
hardness of hard specimens. If the 1/16 dia ball is used on
materials softer than Bo, there is danger that the cap of the
indenter that holds the ball in place will make contact with the
specimen.
The C scale is the one most commonly used for materials harder
that B100. The hardest steels run about C 70. Any inaccuracies that
occur in grinding the diamond cone to its proper shape have
proportionally greater effect on small indentations, and it should
therefore not be used below C 20. A scale is the same as scale C.
Indenter used is also the same. However, load used in this case is
small. This scale is used for case hardened steels.
In general, a scale should be selected to employ the smallest
ball that can properly be used because of the loss of sensitivity
as the size of indenter increases. An exception to this is when
soft non-homogeneous material is to be tested in which case, it may
be preferable to use a larger ball that makes an indentation of
greater area, thus obtaining more of an average hardness.Rockwell
Hardness Testing Machine used in the laboratory in shown in Fig.4
and its various components are also labelled. This machine
essentially consists of (i) a measuring dial with three scales A, B
& C. One of these scales shall be used with a particular type
of indenter as specified in Table 1. (ii) A diamond holder to hold
the indenter (iii) A main screw, which moves up and down and the
specimen is placed on the platform of this screw. (iv) A lever for
applying the load (v) A zero setting screw for initial adjustment
of the pointer to zero of A & C scales or 30 of B scale. (vi)
Hand wheel for moving the screw up and down.
Table 1
Loads and Indenters for Rockwell Hardness TesterA Scale
B ScaleC Scale
Initial Load10 kg10 kg10 kg
Major Load60kg60+100 = 160 kg60+100+50 = 310 kg
IndenterDiamond 1200Steel Ball
1/16 dia
Diamond 1200
Initial pointer position on dial at
o outer scaleB 30
inner scaleo outer scale
Suitable forTests of case hardened steelsAnnealed or hardened
and tempered steel, Non-ferrous metalsAnnealed or hardened and
tempered or case hardness steel
PROCEDURE
1. Put the weights on the plunger, according to the Rockwell
scale as specified in Table 1.
2. Keep the lever pointing towards the operators side
3. Place specimen securely on the testing platform of the main
screw.
4. Use the zero adjusting screw to roughly bring the small
pointer to zero and the long pointer to zero of outer scale (B-30
inner scale).
5. Turn the hand wheel clockwise so that the specimen pushes the
indenter and will show a reading 0w dial gauge. The small pointer
will be at 3 and the long pointer close to zero of the outer scale.
Fine adjustment is made by zero setting screws.
6. The major load should now be applied. To do this, turn the
lever clockwise to other position.
7. As soon as the pointer reaches a steady position, take back
the lever to its original position. By this way, major load is
removed and only minor load remains active.
8. Note down the reading of the long pointer. This gives the
direct reading of the hardness of the specimen.
9. Remove the specimen by turning the hand wheel in the
anticlockwise direction.
PRECAUTIONS:
1. The load must be placed according to the specifications given
in Table.1.2. While pressing the specimen, the small pointer must
not move beyond 3.
3. The long pointer must be set at zero, with the help of zero
setting screw.
4. Handle the indenter carefully
VICKERS HARDNESS TEST
A regular pyramid having a square basis and smoothened off point
made of diamond is pressed in the material to be tested under a
load P. The produced impression is projected on a focussing screen,
while the diagonals of the impression are measured by means of
measuring equipment. The indenter in this case is a square based
diamond pyramid in which the angle between the opposite faces is
136o. The load may be varied from 5 to 120 kg, generally in
increments of 5 kg. The hardness number is determined from the
ratio P/A of the load P in kilograms to the surface area A of the
indentation in square milli-meters.
The test is specially suitable for small, hard and thin
specimens (as thin as 0.006). It is said to be accurate for
hardness as high as 1300 (about 850 Brinell). The Vickers hardness
is a good criterion of the wearing qualities of nitrated steel. One
advantage that is sometimes claimed of the Vickers hardness
measurement is that more accurate measurements can be made of the
diagonals of a square than can be made of the diameter of a
circle.The Vickers Hardness Number (VHN) is a function of the size
of the impression made and is measured by load per unit area and
the expression for it comes out to be
VHN = (Px1.854) d2Where P is the load and d is the diagonal
length of the square impression made. Often tables are provided
from which once the diagonal size of the impression made is known,
Vickers hardness can be directly read.Description of the
machine.
Various types of Vickers hardness testers are available. These
may differ in (i) method of applying load i.e. oil pressure, gear
driven screw etc. (ii) method of operation i.e. hand or power (iii)
method of measuring load i.e piston with weights, bourdon gage,
dynamometer, weights with lever and (iv) size i.e. large
(laboratory size) or portable. Vickers test can also be made in a
universal testing machine if a suitable adapter is available. The
principle of operation of all these machines, however is
similar.
Sometimes the same machine can be used to perform the Brinell
& Vickers test. One such machine manufactured by Blue &
Star is installed in Material Testing Laboratory of C.A.E, PAU,
Ludhiana is shown in Fig. (5). The various components of the
machine are also shown in the diagram.
The machine has a bow shaped frame of heavy construction. The
upper part is covered with a cover. The back of frame is closed
with real wall.
On the substructure of the frame, there is a ball bearing hand
wheel, in the centre bore of which a spindle is screwed in. The
spindle is capable of being adjusted.
Facing the upper part of the frame the supporting table is
inserted in the bore of the spindle. Vertically above it, in the
upper part of the frame there is the swiveling part to which the
thrust piece and the vertical illuminant of the projector are
fastened.
The thrust piece has been designed for fixing a penetrator,
while the objective corresponding to the requirements of the
technique of measuring may be screwed in the vertical illuminant.
The objective is interchangeable.
All parts are enclosed by the open clamping sleeve which is
meant for clamping the specimen against the supporting table during
the test processes and to protect the penetrator and the objective
against damages.
The swiveling part is pivoted in the pressure bush which, for
its part, is carried in six ball bearings applied to fair load
bush. This fair load bush is rigidly connected with the frame. The
eyepiece along with prism of the projector is screwed in the top of
the pressure bush. On the support, there is the solenoid. The tie
rod connects it with swiveling part.
The lever is carried in the fair load bush. It is meant for
transmitting the load produced by the weights to the penetrator
which is inserted in the swiveling part. One part of the double
armed levers rests upon the oil brake which is attached to the
frame while the other part is provided with a balance weight.The
oil brake is intended for uniformly applying the test load. It
consists of cylinderical housing and a piston rod along with piston
which moves in the housing of the oil brake. The piston rod is
provided with two knurled nuts for adjusting and locking the
adjustment. The hexagonal screw and an outlet pipe are meant for
draining the oil out of the oil brake when the oil is being
changed. The hangers are fastened to the level by a fork. They
consist of a rod along with the plate and the weights.
The hardness impressions are measured by means of the measuring
equipment. It consists of the housing, which contains the graduated
focussing screen and the graduated clear screen. Furthermore, a
Vernier scale is applied to the focussing screen. In the normal
position, the two screws may be horizontally shifted by means of a
turning knob. At right there is a graduated micrometer, by means of
which the clear screen that is arranged above the focussing screen
may be separately adjusted. Two marks staggered by 900 are applied
to the periphery of measuring equipment. An illuminating device is
provided for lighting the impression through the vertical
illuminant. A lamp having a power of 30 watts is inserted in a
socket. The switch that may be operated from outside is meant for
switching in the hardness testing machine.Operation of the
machine.
The switch being turned on, the lamp for the projecting and
measuring equipment if flashing shows that the electrical equipment
is alive.
While the hand wheel is turned to the right, the spindle along
with the supporting table and the specimen put on are lifted and
moved against the clamping bush, until the superficial structure of
the specimen is sharply imaged on the focussing screen.
By pushing one of the push buttons corresponding to the load
stages to be adjusted, the appropriate ring of the supporting frame
changes its position.
The controlling current key is pressed. The solenoid is made
alive through the contacts of the pressed key. The solenoid turns
the swiveling part by means of the tie rod up to an adjusted step
by which the penetrater is swiveled in. The belt is released so
that the hangers fastened to the lever are able to lower by its own
weight. The rate of lowering depends upon the adjustment of oil
brake. By lowering, the load hangers displace the lever in the
pressure bush. Thus the inserted penetrator sits on the specimen.
The amount of load adjusted with the push button consequently
becomes effective and makes the impression.
While the lever is displacing, the limit step switch is closed
which after the controlling current key has been released, starts
supplying the electromagnet with current.On the expiry of the
loading time (normally 10 to 30 seconds) depending upon the
material to be tested, the hand lever is pushed down to the lower
position and relieved slowly till it takes its proper position.
Being lighted through the vertical illuminant and the objective
the impression made on the test specimen is imaged on the focussing
screen of the measuring equipment by way of the eye piece and an
eye piece prism. The impression is magnified according to the
objective used.
The hand wheel being turned to the left, the supporting table
with the specimen put on it lowers, so that the specimen may
conveniently be removed.
PROCEDURE1. Corresponding to the method of test and the load
stage required the penetrator- ball holder along with a ball and
thrust piece should be inserted and fastened in the thrust piece of
the machine. The suitable objective is inserted. The hardness test
may be carried out even without the clamping sleeve, if the shape
of specimen requires it. In such a case however, suitable measures
should be taken for holding the specimen on its support.
2. The load stage fixed should be adjusted with the
corresponding push button. The push button which was engaged before
must be released.
3. Adjust the oil break.4. Turn the switch on. The lamp for the
projecting device flashes indicating that the electrical equipment
is alive.
5. Support the standard hardness test plate or the specimen.
Hand wheel should be turned to the right until the surface of the
specimen is sharply imaged on the focussing screen of the measuring
equipment.
6. Adjust the clamping sleeve. The cap nut should be turned to
the left so that the clamping sleeve clamps the specimen.
7. Push the controlling current key and do not release it before
the hand lever starts rising. Then release the controlling current
key. Wait until the hand lever stands still. The time of loading
should be 30 seconds.8. On expiry of loading time, push the hand
lever as far as the step device is engaged. The objective is
switched in, while the impression is being sharply focused on the
measuring equipment.
9. Measure the dimensions of the impression.
EFFECT OF IMPORTANT VARIABLES.The rate of applying the land on
the specimen has a definite effect on the hardness test. If the
land is applied very rapidly, there is momentary increase in load
(that it intended to be applied) due to the inertia of the piston
and weights. This causes an enlargement of the impression made. On
the other hand, the rapid penetration allows less time for the
plastic flow of the material resulting in decreased size of the
indentation. It has been confirmed experimentally that the first
effect of enlargement is much greater than the second effect. By
exercising care in operating the load mechanism, this inertial
effect may be reduced.PRECAUTIONS1. The surface of the specimen
should be flat and reasonably well polished.2. When operating the
hardness tester, never force any part of the operating
mechanism.
3. Tester should rest on solid supports free of vibrations.
4. If it is doubtful that a material can be tested with one type
of indenter, use the next harder indenter. This avoids damage to
the penetrater.
5. Diagonal length of impression (diameter of impression in
Brinell test) should be measured very accurately to reduce the
error in computed hardness.
BRINELL HARNESS TEST
A ball having a diameter D is pressed in the material to be
tested under a load P while diameterd of the produced impression is
measured.
The Brinell Method applies so frequently not only because of the
easy working but also because for some materials tensile strength
can be derived from Brinell hardness. The inter-relation between
the Brinell harness and the tensile strength however, gives only
approximate values and it does not mean that it can replace tensile
test.
The Brinell method applies especially to cast iron, unhardened
steel and light metals. It is restricted to materials having
Brinell hardness of 400 kgm2 in maximum, lest to penetrater (steel
ball) should be deformed.
The Brinell test relies on mechanical or hydraulic loads as
large as 3000 kg acting through a 10-mm hard steel or carbide ball.
In order to compensate for the variations in the response of
materials to the application of the load, the time for which the
load is applied should be specified. The time of loading should be
10 seconds. However for creeping materials such as zinc, lead etc
the loading time should be longer than 10 seconds. Any loading time
deviating from 10 seconds should be particularly noted in the test
record. After the load is removed, diameter of the impression made
by the ball is measured in millimeters. The Brinell Hardness Number
(BHN) is the quotient of the load P divided by the area A of the
impression.
B. H. N. = P (D-D2-d2) D/2In practice, BHN is read directly from
a table list in various values of d for various values of P and
D.The Brinell test makes a large impression on the surface of the
piece tested. Unless such a large impression can be tolerated, the
test may be considered destructive. However, the large impression
is advantageous because it gives a more representative result than
a smaller impression which is more sensitive to local soft or hard
homogeneities. The larger size of the impression also renders the
test less sensitive to the presence of rough surface finish and
mill scale.
Tensile strength of a steel specimen can be approximately
estimated by knowing the Brinell hardness number. This is known as
Dohmars law.
Tensile strength
(psi)=515 times B.H.N.
(for B.H.N. less than 175)
=490 times B.H.N.
(for B.H.N. greater than 175)
It should be remembered that Dohmars law gives only approximate
values and that only for steel. Description of machine: The same
machine as described under Vickers test is used for Brinell test.
The indenter in the machine is of course changed from diamond
pyramid to steel ball. The mode of operation is also the same
except that in this case dia of impression is measured instead of
diagonals of the square impression.Procedure: In conducting a test,
the specimen is placed in the anvil and raised by a screw until it
is close to the point of indenter. The load is now slowly applied
(in about 10 seconds) on the indenter and then released. The
dimensions of the impression made are measured by a suitable
arrangement provided in the machine. All steps involved in
operation are the same as discussed under Vickers hardness
testing.Precautions: Same as discussed in Vickers hardness
testing
Brinell Hardness Range: The Brinell hardness range of various
materials is given in Table. 2
Table 2
Following load stages should be used in the Brinell hardness
test:
Material
Hardness Range
Soft iron steel, Steel Casting Malleable Iron, Cast Iron
67 - 500 HB
Light alloys, casting and forging alloys, Die-casting alloys,
Copper, Brass, Brozise, Nickel
22 - 315 HB
Pure Aluminum, Magnesium, Zinc, Cast Brass
11 - 158 HB
Bearing metals
6 - 78 HB
Lead
3 - 39HB
Soft metal at elevated temperature1 - 15HB
Moreover care must be taken that the diameterd of the ball
impression ranges within 0.2 - 0.7 of the ball dia D.Limitations of
Brinell Hardness Test:
1. It cannot be used for very thin specimens.
2. It cannot be used for extremely hard materials, due to
deformation of the indenter.
3. It cannot be used for surface hardened materials since the
depth of penetration may exceed the depth of case.
4. Hardness values of more than 450 to 500 are only broadly
comparative.
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