ME 215 – Engineering Materials I
Chapter 5
Hardness and Hardness Testing (Part I)Hardness and Hardness Testing (Part I)
D A T l B dM h i l E i i Dr. A. Tolga Bozdanawww.gantep.edu.tr/~bozdana
Mechanical EngineeringUniversity of Gaziantep
IntroductionHardness is defined as the resistance of a material topermanent deformation such as indentation, wear, abrasion,
t h It i h i l t l t d t
Hardness Tests:● Indentation Tests
or scratch. It is a mechanical property related to:wear resistance of materialabilit of material to abrade or indent another material
+ Macro Level– Brinellability of material to abrade or indent another material
resistance of material to permanent (plastic) deformation
Brinell– Rockwell
VickersThere are many hardness tests for various materials.Selection of the appropriate hardness test depends upon
– Vickers+ Micro Level
Vickers Diamondrelative hardness of material to be tested and amount ofdamage to be tolerated on material surface.
– Vickers Diamond– Knoop Diamond
S
1Hardness testers are based on “arbitrary definitions” as:Resistance to permanent indentation (indentation test)
● Scratch Test● Shore Scleroscope
1.2.3
Resistance to permanent indentation (indentation test)Resistance to scratching (scratch test)Energy absorption under impact loads (dynamic test)
● Ultrasonic Test● Hot Hardness Test
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3.4.
Energy absorption under impact loads (dynamic test)Rebound of a falling weight (rebound test) ● Durometer
Hardness Testing by IndentationThis 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 definiteti i t l d i d th f th di i d t ti i dtime interval, and size or depth of the corresponding indentation is measured.
Shape and size of the indenter and magnitude of the load are selected in accordanceShape and size of the indenter and magnitude of the load are selected in accordancewith purpose of the test, structural properties of the material, state of the surfacebeing tested, and size of the part.
Indentation type hardness testers are classified based on the level of destruction of
g
the surface being tested:Macrohardness tests (load ≥ 1 kg): Brinell, Rockwell, Vickers
Microhardness tests (load < 1 kg): Vickers Diamond, Knoop Diamond
I i h d t t th i d t ti i i ll th t f l iIn microhardness tests, the indentation size is very small that a powerful microscopeis required for the measurements. This method is more tedious than macrohardnesstesting and used for testing very thin materials (down to 0 01 mm) extremely small
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testing, and used for testing very thin materials (down to 0.01 mm), extremely smallparts, thin superficially hardened parts, plated surfaces, and research purposes.
Brinell TestBrinell test is carried out byindenting the surface withsteel ball (usually Ø10 mm) ( )( )
FBHND D D dπ
=− −2 22
Figure 1F
steel ball (usually Ø10 mm)under the load of 3000 kgfor 10-15 s (Fig. 1). For soft F : applied load (kg)
D di t f b ll i d t ( )
( )( )D D D dπ 2
( g )materials, load is reduced to1500 or 500 kg for avoiding
D : diameter of ball indenter (mm)d : diameter of impression (mm)Ød
too deep indentations.
Brinell Hardness Number (BHN, HB) is determined
Figure 2
based on the extent of indentation. Any combinationof applied load and ball diameter can be used aslong as the ratio of F/D2 is constant (e g 3000/102long as the ratio of F/D2 is constant (e.g. 3000/102
and 187.5/2.52 give the same ratio of 30).
Brinell tester (invented by Dr J A Brinell) employsBrinell tester (invented by Dr. J. A. Brinell) employshydraulic system to apply the required load withdead weights (Fig. 2). Loads of 187.5, 250, 500, 750,g ( g ) , , , ,1000, 1500, 2000, 2500, 3000 kg can be used usingballs with diameters of 2.5, 5, 10 mm.
Brinell TestFor obtaining reliable test results, the followings must be observed:
1. Loading speed: The rate of loading should not exceed 500 kg/s. Applying the loadtoo rapidly will add extra loading to the nominal load due to inertia of loading system.
2. Loading time: It is 10-15 s for iron and steel whereas at least 30 s for other metals.
3 Measurement of impression: Division of scale of measuring device must permit
gAn error would result from allowing insufficient time for plastic flow to take place.
3. Measurement of impression: Division of scale of measuring device must permitdirect 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. Thickness: Part thickness (t) must be 10 times greater than depth of indentation (h)so that no bulge or other marks should appear on the other side: t > 10h
5 S i f i d t ti Di t f i d t ti t t d f t th5. Spacing of indentations: Distance from indentation center to edge of part or otherindentation (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 ofcurvature (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 ofindentation to diameter of the ball (d/D) is kept within certain limits: 0.25 < d/D < 0.60
Brinell TestThe most suitable F/D2 ratio dependson the average hardness of material tob t t d Th d d ti
Approximate HB F/D2 Representative Materialabove 100 30 Steels, cast iron200 to 300 10 Copper and copper alloysbe tested. The recommended ratios
for various materials are given in table(recommended after Meyer analysis)
200 to 300 10 Copper and copper alloys,aluminum alloys
15 to 100 5 Aluminum4 t 6 1 L d ti d ti ll(recommended after Meyer analysis). 4 to 6 1 Lead, tin and tin alloys
HB without any suffix denotes the hardness number obtained using ball of Ø10 mmy gand load of 3000 kg with duration of 10-15 seconds (e.g. “350 HB”). On the otherhand, “75 HB/5 500/30” indicates the hardness value of 75 measured with a ball ofØ5 mm and a load of 500 kg applied for 30 seconds.
I d t t d di th t t lt t d d th d f t ti h b i dIn order to standardize the test results, standard method of testing has been issuedby various institutions such as TS139 (TSE) and E10-73 (ASTM):
Deviation in diameter of Ø10 mm ball is limited to 0 005 mm (10 ± 0 005 mm)Deviation in diameter of Ø10 mm ball is limited to 0.005 mm (10 ± 0.005 mm).
Use of steel ball is limited to materials with the maximum hardness of 450 HB.
F h d t i l it i ti l t bid b ll
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For harder materials, it is essential to use a carbide ball.
Under all circumstances, Brinell method is limited up to 630 HB.
Rockwell TestInvented by S. P. Rockwell,Rockwell test is consisting ofmeasuring additional depth tomeasuring additional depth towhich a steel ball or a Bralediamond penetrator is forcedpby heavy (major) load beyonddepth of a previously appliedlight (minor) load (Fig. 3).
This concept aims eliminationpof measurement errors due tosurface imperfections aroundthe periphery of indentation. Figure 3
As the result of application of minor load, an initial indentation of depth (δm) is madepp p ( m)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
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forced beyond the depth of previously applied load by the depth (δM). The major loadis removed after certain time, and the depth of permanent indentation is measured.
Rockwell TestFig. 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). Incrementaldepth (δ ) is due to major load while the minor load is still in position After the majordepth (δM) is due to major load while the minor load is still in position. After the majorload is applied and removed, the reading on dial gauge is the hardness value.
Removal of the addiditonal load allows a partial recovery reducing the depth ofRemoval of the addiditonal load allows a partial recovery, reducing the depth ofpenetration. Permanent increase in depth of penetration (e) due to applying andremoving the major load is used to deduce the hardness number, as given below.g j , g
eKHR −=HR : Rockwell hardness number
e : increase in depth of penetration1 unit / 0.002 mm for brale penetratorp1 unit / 0.001 mm for ball penetrator
K : constant depending on scale100 for brale penetrator100 for brale penetrator130 for ball penetrator Figure 4
For example after a Rockwell hardness test an additional depth of 0 08 mm means:
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For example, after a Rockwell hardness test, an additional depth of 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)
Rockwell TestT f t t d i R k ll t tTypes of penetrators used in Rockwell test:Diamond sphero-conical (Brale) penetrator (havingangle of 120° with spherical tip of 0.2 mm radius) is
Superficial Test
Scale Penetrator Load
Regular (Normal) Test
Scale Penetrator Loadangle of 120 with spherical tip of 0.2 mm radius) isused for hardened steels and cemented carbides.
Steel ball penetrator (having diameter of 1/16, 1/8,
Letter Type (kgf)15N N Brale 1530N N Brale 30
Letter Type (kgf)B 1/16" ball 100C Brale 1501/4, 1/2 inches) is used for steels, copper alloys,
aluminum, plastics, and likewise.
30N N Brale 3045N N Brale 4515T 1/16" ball 15
C Brale 150A Brale 60D Brale 100
Rockwell testing falls into two categories:Regular testing: The minor load is always 10 kg.M j l d d d t f t t
30T 1/16" ball 3045T 1/16" ball 4515W 1/8" b ll 15
E 1/8" ball 100F 1/16" ball 60G 1/16" b ll 150Major load depends upon type of penetrator.
Superficial testing: Used for shallow indentations(due to smaller loads and more sensitive depth
15W 1/8" ball 1530W 1/8" ball 3045W 1/8" ball 45
G 1/16" ball 150H 1/8" ball 60K 1/8" ball 150(due to smaller loads and more sensitive depth
measuring). The minor load is always 3 kg. 15X 1/4" ball 1530X 1/4" ball 30
L 1/4" ball 60M 1/4" ball 100
Hardness is shown by scale letter and number: 45X 1/4" ball 4515Y 1/2" ball 1530Y 1/2" ball 30
P 1/4" ball 150R 1/2" ball 60S 1/2" ball 100
Hardness is shown by scale letter and number:C60 or 60RC means Rockwell hardness of 60 onscale C under load of 150 kg with brale penetrator.
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30Y 1/2 ball 3045Y 1/2" ball 45
S 1/2 ball 100V 1/2" ball 15030N80 indicates superficial hardness of 80 on scale
30N under load of 30 kg with brale penetrator.
Rockwell TestThe most suitable Rockwell scale is chosen according to following factors:
1. Material type: Table shows the listing (ASTM E18), providing a valuable source ofreference for regular scales for typical materials with applicable scales.Superficial scales are: N scale is for materials similar to those on C & D scales withthinner gauge or case depth T scale is for materials similar to those on B F G
Scale Load (kgf) Application
thinner gauge or case depth. T scale is for materials similar to those on B, F, Gscales. W, X, Y scales are for soft materials (the smallest ball is recommended).
Scale Load (kgf) ApplicationB 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 B100A 60 Cemented carbides, thin steel, shallow case-hardened steelD 100 Thin steel medium case-hardened steel pearlitic malleable ironD 100 Thin steel, medium case-hardened steel, pearlitic malleable ironE 100 Cast iron, aluminum and magnesium alloys, bearing materialsF 60 Annealed copper alloys, thin soft metalsG 150 Malleable irons, copper-nickel-zinc and cupro-nickel alloys
(upper limit G92 to avoid possible flattening of ball)H 60 Aluminum zinc lead
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H 60 Aluminum, zinc, leadOTHER Bearing materials and other very soft or thin materials
(smallest ball and heaviest load must be used whenever possible)
Rockwell Test2. 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 canbe tested on the indicated scale Note that “X” refers to no indicated hardness
Thickness(mm)
Regular SuperficialA D C 15N 30N 45N
be tested on the indicated scale. Note that X refers to no indicated hardness.
Thickness(mm)
Regular SuperficialF B G 15T 30T 45T(mm) A D C 15N 30N 45N
0.15 920.20 900.25 88
(mm) F B G 15T 30T 45T0.13 930.25 90 870.38 78 77 77
0.30 83 82 770.36 76 80 740.41 86 68 74 72
0.51 100 X 58 620.64 92 92 90 X X 260.76 67 68 69 X X X
0.46 84 X 66 680.51 82 77 X 57 630.56 78 75 69 X 47 58
0.89 X 44 46 X X X1.02 X 20 22 X X X
0.61 76 72 67 X X 510.66 71 68 65 X X 370.71 61 63 62 X X 200 76 60 58 57 X X X0.76 60 58 57 X X X0.81 X 51 52 X X X0.86 X 43 45 X X X0 91 X X 37 X X X
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0.91 X X 37 X X X0.97 X X 28 X X X1.02 X X 20 X X X
Rockwell Test3. 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 indentationi l d ithi thi ld k dis placed within this cold-worked area.
It is recommended to allow minimum distance of2 5d f th t f i d t ti t th d f t2.5d from the center of indentation to the edge of partas well as minimum distance of 3d from the centerof indentation to the center of adjacent indentationof indentation to the center of adjacent indentation.
4 Scale limitations: In accordance4. Scale limitations: In accordancewith the values of coefficient K,display of tester is numbered fromp y0 to 100 units for Brale scales and0 to 130 units for ball scales by
Figure 4
offsetting the corresponding scaleby 30 units (i.e. B scale in Fig. 4).
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Figure 4
Rockwell Test5. 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 intomaterial Thus hardness value will be lower on convex surface than on flat surfacematerial. Thus, hardness value will be lower on convex surface than on flat surfaceof the same material. For concave surfaces, the opposite is true.
The difference is negligible above diametersof 25 mm. Otherwise, correction is required.Th ti t bl f d fThe correction tables for curved surfaces(e.g. cylindrical specimens) are given instandards TS140 and ASTM E18standards TS140 and ASTM E18.
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Vickers TestA diamond indenter (in the form of a right pyramid with a square base and an angleof 136° between opposite faces) is forced into material under a certain load (Fig. 5).After removing the load two diagonals (d1 and d2) of the indentation are measuredAfter removing the load, two diagonals (d1 and d2) of the indentation are measuredand their arithmetic mean (d) is calculated.
Vickers hardness is denoted by VHN (TS 207) orVickers hardness is denoted by VHN (TS 207) orHV (ASTM E92 or BS 427):
( ) ( )22
2 sin 21.8544
FVHN F d
dθ
= =
F : applied load (kg)d : mean of diagonal impression (mm)θ : face angle of the pyramid (136°)
Hardness value is followed by a suffix designatingHardness value is followed by a suffix designatingload and another suffix indicating time of loading ifdifferent than 10-15 s (e.g. 455 VHN/30/20 refers toVickers hardness of 455 obtained by load of 30 kgapplied for 20 seconds). Figure 5
Vickers TestVickers hardness number is nearly independent of load for homogeneous materials asthe ratio between diagonals of indentation remains constant. Loads of 1 to 120 kg areapplied Hardness number is constant when diamond pyramid is used with loads of 5 kg
Vickers test provides better accuracy than Brinell or Rockwell since the diamond pyramid
applied. Hardness number is constant when diamond pyramid is used with loads of 5 kgor higher (although it may be load dependent at lower test loads).
Vickers test provides better accuracy than Brinell or Rockwell since the diamond pyramidhas a large angle and diagonals of indentation (d1 and d2) are about 7 times larger thandepth of indentation (h) especially for high hardness metals. Thus, higher accuracy canbe obtained even if indentation depth is small, which makes this test especially suitablefor measurement in thin layers and very hard alloys.
Vickers hardness test involves the following advantages:Soft as well as hard metals can be tested.T t l b d t d i iTests may also be conducted in micro ranges.Vickers macrohardness test is independent of the applied load.The pyramidal impression damages the surface only slightly.py p g y g y
Vickers and Brinell tests are similar to each other in principle and hardness values:Both calculate the hardness as load/area of impression.
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Both calculate the hardness as load/area of impression.Vickers uses diamond indenter with angle of 136°, resembling ball indenter in Brinell.Values of HB and HV of the same test piece are close to each other up to HB400.
Microhardness TestingMicrohardness refers toindentation tests madewith loads up to 1 kg
Vickers indenter Knoop indenterFigure 6
with loads up to 1 kgusing Vickers or Knoopindenters (Fig. 6).( g )
It is crucial that surfaceto be tested should beto be tested should belapped flat and be freefrom scratches.
Knoop VickersAfter the indentation is made, its dimensionsare measured by means of a high resolutionare measured by means of a high-resolutiongraticule under the microscope (Fig. 7).
U d f ll i t f lUsed for small precison parts, surface layers,thin materials, small radius wires, constituents,near edges and so on Metallographic finish is
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Figure 7near edges, and so on. Metallographic finish isnecessary for the loads of 100 grams or less.
Microhardness TestingVickers Hardness Number(VHN) was calculated as:
( ) ( )KHN F A F CL F L= = =2 214 23
Knoop Hardness Number (KHN) is determined by:
( ).VHN F d= 21 8544( ) ( ).KHN F A F CL F L= = = 14 23
F : applied load (kg)A : unrecovered projected area of indentation (mm)A : unrecovered projected area of indentation (mm)L : length of the longer diagonal (mm)C : a constant relating A to the square of L
As compared to Vickers indenter, Knoop indenter produces about three times greaterdi l l th d b t h lf f i d t ti d th Th Vi k i d t i ldiagonal length and about half of indentation depth. Thus, Vickers indenter is lesssensitive to surface conditions for the same load, but more sensitive to errors inmeasuring the indentation
In microhardness testing, the hardness number is dependent on the applied load.
measuring the indentation.
The effect is particularly significant for Knoop indenter at loads less than 500 g andfor Vickers indenter at loads less than 100 g. Such loads during testing must beapplied ith great care
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applied with great care.
