ME 215 – Engineering Materials I
Chapter 5
Hardness and Hardness Testing (Part I)
Mechanical
M
h i lE
Engineering
i
i
University of Gaziantep
Dr. A.
D
A Tolga
T l Bozdana
B d
www.gantep.edu.tr/~bozdana
Introduction
h Hardness is defined as the resistance of a material to
permanent deformation such as indentation, wear, abrasion,
or scratch.
t h It is
i a mechanical
h i l property
t related
l t d to:
t
ƒ wear resistance of material
Hardness Tests:
● Indentation Tests
+ Macro Level
ƒ ability
abilit of material to abrade or indent another material
– Brinell
ƒ resistance of material to permanent (plastic) deformation
– Rockwell
– Vickers
h There are many hardness tests for various materials.
Selection of the appropriate hardness test depends upon
relative hardness of material to be tested and amount of
damage to be tolerated on material surface.
+ Micro Level
– Vickers Diamond
– Knoop Diamond
● Scratch
S
Test
h Hardness testers are based on “arbitrary definitions” as:
● Shore Scleroscope
1 Resistance to permanent indentation (indentation test)
1.
● Ultrasonic Test
2. Resistance to scratching (scratch test)
3 Energy absorption under impact loads (dynamic test)
3.
4. Rebound of a falling weight (rebound test)
● Hot Hardness Test
● Durometer
1
Hardness Testing by Indentation
h This is the most employed method in which a hard indenter (a small sphere, pyramid,
or cone) is pressed onto the surface being tested under a specific load for a definite
ti
time
i t
interval,
l and
d size
i or depth
d th off the
th corresponding
di indentation
i d t ti is
i measured.
d
h Shape and size of the indenter and magnitude of the load are selected in accordance
with purpose of the test, structural properties of the material, state of the surface
g tested, and size of the part.
being
h Indentation type hardness testers are classified based on the level of destruction of
the surface being tested:
ƒ Macrohardness tests (load ≥ 1 kg): Brinell, Rockwell, Vickers
ƒ Microhardness tests (load < 1 kg): Vickers Diamond, Knoop Diamond
h IIn microhardness
i h d
t t the
tests,
th indentation
i d t ti size
i is
i very smallll that
th t a powerful
f l microscope
i
is required for the measurements. This method is more tedious than macrohardness
testing and used for testing very thin materials (down to 0.01
testing,
0 01 mm),
mm) extremely small
parts, thin superficially hardened parts, plated surfaces, and research purposes.
2
Brinell Test
h Brinell test is carried out by
indenting the surface with
steel ball (usually Ø10 mm)
under the load of 3000 kg
g 1).
) For soft
for 10-15 s ((Fig.
materials, load is reduced to
1500 or 500 kg for avoiding
too deep indentations.
F
Ød
Figure 1
BHN =
F
(π D 2 ) ( D −
D2 − d 2
)
F : applied load (kg)
D : diameter
di
t off b
ball
ll indenter
i d t (mm)
(
)
d : diameter of impression (mm)
h Brinell Hardness Number (BHN, HB) is determined
based on the extent of indentation. Any combination
of applied load and ball diameter can be used as
long as the ratio of F/D2 is constant (e.g.
(e g 3000/102
and 187.5/2.52 give the same ratio of 30).
Dr J.
J A.
A Brinell) employs
h Brinell tester (invented by Dr.
hydraulic system to apply the required load with
g
((Fig.
g 2).
) Loads of 187.5,, 250,, 500,, 750,,
dead weights
1000, 1500, 2000, 2500, 3000 kg can be used using
balls with diameters of 2.5, 5, 10 mm.
Figure 2
Brinell Test
h For obtaining reliable test results, the followings must be observed:
1. Loading speed: The rate of loading should not exceed 500 kg/s. Applying the load
too rapidly will add extra loading to the nominal load due to inertia of loading system.
g time: It is 10-15 s for iron and steel whereas at least 30 s for other metals.
2. Loading
An error would result from allowing insufficient time for plastic flow to take place.
3 Measurement of impression: Division of scale of measuring device must permit
3.
direct measurement of the indentation diameter down to 0.1 mm.
4 Thickness: Part thickness (t) must be 10 times greater than depth of indentation (h)
4.
so that no bulge or other marks should appear on the other side: t > 10h
5 S
5.
Spacing
i
off indentations:
i d t ti
Di t
Distance
f
from
i d t ti center
indentation
t to
t edge
d off partt or other
th
indentation (L) should be 2.5 times greater than diameter of indentation (d): L > 2.5d
6. Radius of curvature: For indenting on a curved surface, the minimum radius of
curvature (Rmin) should not be less than 25 mm for using Ø10 mm ball: Rmin > 25 mm
7. Selection of load: The load should be selected such that the ratio of diameter of
indentation to diameter of the ball (d/D) is kept within certain limits: 0.25 < d/D < 0.60
Brinell Test
h The most suitable F/D2 ratio depends
on the average hardness of material to
b tested.
be
t t d The
Th recommended
d d ratios
ti
for various materials are given in table
(recommended after Meyer analysis).
analysis)
Approximate HB F/D2 Representative Material
above 100
30 Steels, cast iron
200 to 300
10 Copper and copper alloys
alloys,
aluminum alloys
15 to 100
5 Aluminum
4 tto 6
1 Lead,
L d titin andd titin alloys
ll
y suffix denotes the hardness number obtained using
g ball of Ø10 mm
h HB without any
and load of 3000 kg with duration of 10-15 seconds (e.g. “350 HB”). On the other
hand, “75 HB/5 500/30” indicates the hardness value of 75 measured with a ball of
Ø5 mm and a load of 500 kg applied for 30 seconds.
h IIn order
d to
t standardize
t d di the
th test
t t results,
lt standard
t d d method
th d off testing
t ti has
h been
b
i
issued
d
by various institutions such as TS139 (TSE) and E10-73 (ASTM):
ƒ Deviation in diameter of Ø10 mm ball is limited to 0.005
0 005 mm (10 ± 0.005
0 005 mm).
mm)
ƒ Use of steel ball is limited to materials with the maximum hardness of 450 HB.
ƒ For
F harder
h d materials,
t i l it is
i essential
ti l to
t use a carbide
bid ball.
b ll
ƒ Under all circumstances, Brinell method is limited up to 630 HB.
5
Rockwell Test
h Invented by S. P. Rockwell,
Rockwell test is consisting of
measuring additional depth to
which a steel ball or a Brale
penetrator is forced
diamond p
by heavy (major) load beyond
depth of a previously applied
light (minor) load (Fig. 3).
h This concept
p aims elimination
of measurement errors due to
surface imperfections around
the periphery of indentation.
Figure 3
h As the result of application
pp
of minor load, an initial indentation of depth
p ((δm) is made
on the surface, which also serves as the datum line before the major load is applied.
The major load is applied without removing the minor load, and the penetrator is
forced beyond the depth of previously applied load by the depth (δM). The major load
is removed after certain time, and the depth of permanent indentation is measured.
6
Rockwell Test
h Fig. 4 illustrates measurement of Rockwell hardness (HR) based on different scales.
The datum line is specified by the initial depth due to minor load (δm). Incremental
depth (δM) is due to major load while the minor load is still in position.
position After the major
load is applied and removed, the reading on dial gauge is the hardness value.
h Removal of the addiditonal load allows a partial recovery,
recovery reducing the depth of
penetration. Permanent increase in depth of penetration (e) due to applying and
g the major
j load is used to deduce the hardness number,, as g
given below.
removing
HR = K − e
HR : Rockwell hardness number
e : increase in depth of penetration
p
1 unit / 0.002 mm for brale penetrator
1 unit / 0.001 mm for ball penetrator
K : constant depending on scale
100 for brale penetrator
130 for ball penetrator
Figure 4
h For example,
example after a Rockwell hardness test,
test an additional depth of 0.08
0 08 mm means:
(0.08 mm) x (1 unit / 0.002 mm) = 40 units (for brale penetrator) → HR = 100 - 40 = 60 (for regular testers)
(0.08 mm) x (1 unit / 0.001 mm) = 80 units (for ball penetrator) → HR = 130 - 80 = 50 (for superficial testers) 7
Rockwell Test
h Types
T
off penetrators
t t
used
d in
i Rockwell
R k ll test:
t t
ƒ Diamond sphero-conical (Brale) penetrator (having
angle of 120
120° with spherical tip of 0.2 mm radius) is
used for hardened steels and cemented carbides.
Regular (Normal) Test
Superficial Test
Scale Penetrator Load Scale Penetrator Load
Letter Type
(kgf) Letter Type
(kgf)
B
1/16" ball
100
15N
N Brale
15
C
Brale
150
30N
N Brale
30
A
Brale
60
45N
N Brale
45
D
Brale
100
15T
1/16" ball
15
E
1/8" ball
100
30T
1/16" ball
30
F
1/16" ball
60
45T
1/16" ball
45
G
1/16" ball
b ll
150
15W
1/8" ball
b ll
15
H
1/8" ball
60
30W
1/8" ball
30
K
1/8" ball
150
45W
1/8" ball
45
L
1/4" ball
60
15X
1/4" ball
15
M
1/4" ball
100
30X
1/4" ball
30
P
1/4" ball
150
45X
1/4" ball
45
h C60 or 60RC means Rockwell hardness of 60 on
scale C under load of 150 kg with brale penetrator.
R
1/2" ball
60
15Y
1/2" ball
15
S
1/2" ball
1/2
100
30Y
1/2" ball
1/2
30
h 30N80 indicates superficial hardness of 80 on scale
30N under load of 30 kg with brale penetrator.
V
1/2" ball
150
45Y
1/2" ball
45
ƒ Steel ball penetrator (having diameter of 1/16, 1/8,
1/4, 1/2 inches) is used for steels, copper alloys,
aluminum, plastics, and likewise.
h Rockwell testing falls into two categories:
ƒ Regular testing: The minor load is always 10 kg.
M j load
Major
l d depends
d
d upon type
t
off penetrator.
t t
ƒ Superficial testing: Used for shallow indentations
(due to smaller loads and more sensitive depth
measuring). The minor load is always 3 kg.
h Hardness is shown by scale letter and number:
8
Rockwell Test
h The most suitable Rockwell scale is chosen according to following factors:
1. Material type: Table shows the listing (ASTM E18), providing a valuable source of
reference for regular scales for typical materials with applicable scales.
Superficial scales are: N scale is for materials similar to those on C & D scales with
thinner gauge or case depth.
depth T scale is for materials similar to those on B,
B F,
F G
scales. W, X, Y scales are for soft materials (the smallest ball is recommended).
Scale Load (kgf) Application
B
100
Copper alloys, soft steels, aluminum alloys, malleable iron, etc.
C
150
Steel, hard cast irons, pearlitic malleable iron, titanium,
deep case hardened steel, other materials harder than B100
A
60
Cemented carbides, thin steel, shallow case-hardened steel
D
100
Thin steel
steel, medium case-hardened steel
steel, pearlitic malleable iron
E
100
Cast iron, aluminum and magnesium alloys, bearing materials
F
60
Annealed copper alloys, thin soft metals
G
150
Malleable irons, copper-nickel-zinc and cupro-nickel alloys
(upper limit G92 to avoid possible flattening of ball)
H
60
Aluminum zinc,
Aluminum,
zinc lead
OTHER
Bearing materials and other very soft or thin materials
(smallest ball and heaviest load must be used whenever possible)
9
Rockwell Test
2. Material thickness: For a given thickness, any hardness greater than that thickness
can be tested. For a given hardness, any thickness greater than that hardness can
be tested on the indicated scale.
scale Note that “X”
X refers to no indicated hardness.
hardness
Thickness
(mm)
0.15
0.20
0.25
0.30
0.36
0.41
0.46
0.51
0.56
0.61
0.66
0.71
0 76
0.76
0.81
0.86
0 91
0.91
0.97
1.02
Regular
A D C
86
84
82
78
76
71
61
60
X
X
X
X
X
77
75
72
68
63
58
51
43
X
X
X
69
67
65
62
57
52
45
37
28
20
Superficial
15N 30N 45N
92
90
88
83 82 77
76 80 74
68 74 72
X 66 68
X 57 63
X 47 58
X
X 51
X
X 37
X
X 20
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
Thickness
Regular
Superficial
(mm)
F
B G 15T 30T 45T
0.13
93
0.25
90 87
0.38
78 77 77
0.51
100
X 58 62
0.64
92 92 90 X
X 26
0.76
67 68 69 X
X
X
0.89
X 44 46 X
X
X
1.02
X 20 22 X
X
X
10
Rockwell Test
3. Spacing of indentations: In all types of indentation tests, material in the vicinity of
indentation area is cold-worked. The test result will be affected if another indentation
i placed
is
l
d within
ithi this
thi cold-worked
ld
k d area.
It is recommended to allow minimum distance of
2 5d from
2.5d
f
th center
the
t off indentation
i d t ti to
t the
th edge
d off partt
as well as minimum distance of 3d from the center
of indentation to the center of adjacent indentation.
indentation
4 Scale limitations: In accordance
4.
with the values of coefficient K,
p y of tester is numbered from
display
0 to 100 units for Brale scales and
0 to 130 units for ball scales by
offsetting the corresponding scale
by 30 units (i.e. B scale in Fig. 4).
Figure 4
11
Rockwell Test
5. Radius of curvature of surface: Compared with a flat surface, a convex surface
has less lateral resistance to penetrating force and penetrator will sink further into
material Thus,
material.
Thus hardness value will be lower on convex surface than on flat surface
of the same material. For concave surfaces, the opposite is true.
The difference is negligible above diameters
of 25 mm. Otherwise, correction is required.
Th correction
The
ti
t bl
tables
f curved
for
d surfaces
f
(e.g. cylindrical specimens) are given in
standards TS140 and ASTM E18.
E18
12
Vickers Test
h A diamond indenter (in the form of a right pyramid with a square base and an angle
of 136° between opposite faces) is forced into material under a certain load (Fig. 5).
After removing the load,
load two diagonals (d1 and d2) of the indentation are measured
and their arithmetic mean (d) is calculated.
h Vickers hardness is denoted by VHN (TS 207) or
HV (ASTM E92 or BS 427):
2 F sin (θ 2 )
2
=
VHN =
1.8544
F
d
(
)
d2
F : applied load (kg)
d : mean of diagonal impression (mm)
θ : face angle of the pyramid (136°)
h Hardness value is followed by a suffix designating
load and another suffix indicating time of loading if
different than 10-15 s (e.g. 455 VHN/30/20 refers to
Vickers hardness of 455 obtained by load of 30 kg
applied for 20 seconds).
Figure 5
Vickers Test
h Vickers hardness number is nearly independent of load for homogeneous materials as
the ratio between diagonals of indentation remains constant. Loads of 1 to 120 kg are
applied Hardness number is constant when diamond pyramid is used with loads of 5 kg
applied.
or higher (although it may be load dependent at lower test loads).
h Vickers test provides better accuracy than Brinell or Rockwell since the diamond pyramid
has a large angle and diagonals of indentation (d1 and d2) are about 7 times larger than
depth of indentation (h) especially for high hardness metals. Thus, higher accuracy can
be obtained even if indentation depth is small, which makes this test especially suitable
for measurement in thin layers and very hard alloys.
h Vickers hardness test involves the following advantages:
ƒ Soft as well as hard metals can be tested.
ƒ Tests
T t may also
l be
b conducted
d t d in
i micro
i
ranges.
ƒ Vickers macrohardness test is independent of the applied load.
ƒ The py
pyramidal impression
p
damages
g the surface only
y slightly.
g y
h Vickers and Brinell tests are similar to each other in principle and hardness values:
ƒ Both calculate the hardness as load/area of impression.
ƒ Vickers uses diamond indenter with angle of 136°, resembling ball indenter in Brinell.
ƒ Values of HB and HV of the same test piece are close to each other up to HB400.
14
Microhardness Testing
h Microhardness refers to
indentation tests made
with loads up to 1 kg
using Vickers or Knoop
g 6).
)
indenters ((Fig.
Figure 6
Vickers indenter
Knoop indenter
h It is crucial that surface
to be tested should be
lapped flat and be free
from scratches.
h After the indentation is made, its dimensions
are measured by means of a high-resolution
high resolution
graticule under the microscope (Fig. 7).
hU
Used
d for
f smallll precison
i
parts,
t surface
f
l
layers,
thin materials, small radius wires, constituents,
near edges,
edges and so on.
on Metallographic finish is
necessary for the loads of 100 grams or less.
Knoop
Vickers
Figure 7
15
Microhardness Testing
h Vickers Hardness Number
(VHN) was calculated as:
(
VHN = 1.8544 F d
2
)
h Knoop Hardness Number (KHN) is determined by:
(
)
(
KHN = F A = F CL2 = 14.23 F L2
)
F : applied load (kg)
A : unrecovered projected area of indentation (mm)
L : length of the longer diagonal (mm)
C : a constant relating A to the square of L
h As compared to Vickers indenter, Knoop indenter produces about three times greater
di
diagonal
l length
l
th and
d about
b t half
h lf off indentation
i d t ti depth.
d th Thus,
Th
Vi k
Vickers
i d t is
indenter
i less
l
sensitive to surface conditions for the same load, but more sensitive to errors in
measuring the indentation.
indentation
h In microhardness testing, the hardness number is dependent on the applied load.
The effect is particularly significant for Knoop indenter at loads less than 500 g and
for Vickers indenter at loads less than 100 g. Such loads during testing must be
applied with
ith great care.
care
16