3.
Hardness is not necessarily an indication of
strength , although for some materials such as
steel, a harder steel is a stronger steel.
Measure of a material’s ability to resist surface
indentation or scratching
A difficult property to describe in terms of first
principles Þ value depends greatly on
method of testing.
Different testing methods Þ different scales and
values
ME101: Materials Science and
Technology
4.
Brinell Hardness Test: 10mm diameter ball
with a load of 500, 1000 or 3000kg
Rockwell Hardness Test: A cone shape
indenter; the depth of penetration is measured.
Vickers Hardness Test: Pyramid shape indenter
ME101: Materials Science and
Technology
8. 1.
2.
3.
Press a 10mm (3/8") diameter ball into
material with a known amount of load.
Measure diameter of the indentation.
BHN =
Load
=
2L
Surface Area πD[D-(D2-d2)1/2]
1. L = Load placed on ball, usually 3000 kg , but 1500 kg, and
500 kg can also be used.
2. D = Diameter of steel ball ( = 10 mm)
3. d = diameter of dent, measured by looking thru a Brinell
microscope.
ME101: Materials Science and
Technology
10. Tensile Strength (MPa)
Tensile Strength = 3.4 BHN
Correlations between the
Brinell hardness number (BHN)
and tensile strength of carbon
steels.
ME101: Materials Science and
Technology
Hardness (HB)
11.
Sample must be ten times thicker than the
indentation depth (sample usually should be at least
3/8" thick).
Test is most accurate if the indentation depth is 2.5 5.0 mm. Adjust load to achieve this.
Test is no good if BHN > 650
ME101: Materials Science and
Technology
12.
Widely used and well
accepted
Large ball gives good
average reading with
a single test.
Accurate
Easy to learn and use
Destructive
Non-portable
High initial cost
($5,000)
Error due to operator
reading Brinell
Microscope (10%
max)
ME101: Materials Science and
Technology
14. Machine measures depth of penetration and computes
hardness
ME101: Materials Science and
Technology
15.
Select Scale - load and
indentor depending on
the scale
Press a point into
material
- Diamond Point (Brale)
- 1/16" ball
- 1/8" ball
- ¼” ball
ME101: Materials Science and
Technology
17.
Sample must be ten times thicker than the
indentation depth (sample usually should
be at least 1/8" thick).
Need 3 tests (minimum) to avoid
inaccuracies due to impurities, hard spots.
Test is most accurate if the Rockwell
Hardness is between 0 and 100. Adjust
scale to achieve this.
For Steel:
If HRa > 60, use HRc scale
If HRa < 60, use HRb scale
ME101: Materials Science and
Technology
18.
Widely used and well
accepted
Little operator
subjectivity
Accurate
Fast
Destructive
Non-Portable
Initial cost ($5,000)
ME101: Materials Science and
Technology
20. DPH/VHN/VPH/VH = 2PSin(θ/2)/L2
P
P: applied load in kg, 5-120 kg
L: average diagonal length, mm
(typically from a few µm to 1 mm)
θ
θ: angle between opposite faces of
indenter; ≡ 136°
L
Range: 5 (extremely soft metals) 1500 (extremely hard materials)
1.854P
HV =
2
L
A
Vickers Hardness = F/A =
3σ
ME101: Materials Science and
Technology
21.
Continuous hardness from soft (5 DPH) to hard
materials (1500 DPH)
DPH independent on load value unlike BHN
Careful surface preparation required
Slow due to careful measurement
Small indentation compared to BHN
Pin cushion and Barrel indentations possible
ME101: Materials Science and
Technology
23.
Knoop hardness Test: Pyramid shape
indenter
Scleroscope: rebound height
Durometer: The resistance to penetration
(elastic deformation)
Relationship between Hardness and
Strength
TS = K h ( HB )
where K h = 500 in lb/in
ME101: Materials Science and
Technology
= 3.45 in MPa
2
24.
Micro Hardness Tests
Major : Minor = 7 : 1
P :
Ap :
L :
C :
14.2P
HK =
2
L
Applied load = 25 gf- 300 gf
Unrecovered Proj. area of indentations, mm 2
Length of long diagonal, mm
A constant supplied by the manufacturer
(C=0.07028 for 172° 30' between
long edges and 130° 0' between short edges)
ME101: Materials Science and
Technology
25.
Accurate
Useful for elongated and
anisotropic constituents.
Requires load to be
normal to surface plane
parallel surfaces.
Can be done on
mounted specimens
Slow
Sensitive to surface
condition
Subject to error in
diagonal measurement
ME101: Materials Science and
Technology
26. Microhardness variation in a 9Cr-1 Mo steel weldment as a function of position.
Note the structural changes and the corresponding changes in hardness as the
fusion line is traversed.
ME101: Materials Science and
Technology
28.
The Shore (Durometer) test provides an empirical hardness
value that doesn't correlate to other properties or
fundamental characteristics.
Shore Hardness, using either the Shore A or Shore D scale,
is the preferred method for rubbers/elastomers and is also
commonly used for 'softer' plastics such as polyolefins,
fluoropolymers, and vinyls. The Shore A scale is used for
'softer' rubbers while the Shore D scale is used for 'harder'
ones.
Because of the resilience of rubbers and plastics, the
hardness reading may change over time - so the
indentation time is sometimes reported along with the
hardness number. The ASTM test number is ASTM D2240
number
while the analogous ISO test method is ISO 868.
ME101: Materials Science and
Technology
32. The Brinell’s Hardness Test of an alloy steel was measured
to be 355. What is the diameter of the indentation if a load
of 2000 kg was used. Also compute the tensile strength of the
material.
D= 10 mm
d = 2.65 mm
Tensile Strength = 1207 MPa
ME101: Materials Science and
Technology
33. The Brinell hardness of an alloy steel is 355. Compute the
diameter of the indentation if a load of 200 kg was used and estimate the
Solution
corresponding tensile strength of the material.
BHN = ( 2 P )
{
π D D −
(
)
D2 − d 2
}
Substituting the values from the problem statement yields :
355 = ( 2 ) ( 2000 )
{
10π 10 −
(
)
10 2 − d 2
}
which after some algebra gives d = 2.65mm
ME101: Materials Science and
Technology
35.
Indentation Hardness used for steel
It is indicative of ultimate tensile strength
as opposed to scratch or rebound hardness
Atoms move out of the way to create indentation
Two main types: Brinell and Rockwell
36.
37.
A spherical indenter (1 cm diameter) is shot
with 29 kN force at the target
Frequently the indenter is steel, but for harder
materials it is replaced with a tungsten carbide
sphere
The diameter of the indentation is recorded
The indentation diameter can be correlated
with the volume of the indentation.
39.
ASTM and ISO use the HB value. It can be
HBS (Hardness, Brinell, Steel) or the HBW
(Hardness, Brinell, Tungsten)
HBW = 0.102 BHN
Sometimes written as HBW 10/3000
(Tungsten, 10 mm diameter, 3,000 kg force)
43. Scale Code
Load
Indenter
Use
120° diamond cone
Tungsten
carbide
A
HRA 60 kgf
B
Al, brass, and
HRB 100 kgf 1/16 in diameter steel sphere soft steels
C
D
HRC 150 kgf 120° diamond cone
HRD 100 kgf 120° diamond cone
E
HRE 100 kgf 1/8 in diameter steel sphere
F
HRF 60 kgf
G
HRG 150 kgf 1/16 in diameter steel sphere
1/16 in diameter steel sphere
Harder steels
