Czichos H., Saito T., Smith L.E. (Eds.) Handbook of Metrology and Testing

Подождите немного. Документ загружается.

368 Part C Materials Properties Measurement

testing standards related to a speciﬁc hardness test in-

clude different parts specifying requirements for the

1. Test method

2. Veriﬁcation and calibration of testing machines

3. Calibration of reference blocks.

An overview of the different well-known conventional

methods of hardness testing is given in Fig. 7.26.

7.3.1 Conventional Hardness Test Methods

(Brinell, Rockwell, Vickers and Knoop)

The most commonly used indentation hardness tests

used today are the Brinell, Rockwell, Vickers, and

Knoop methods. Beginning with the introduction of

the Brinell hardness test in 1900, these methods were

developed over the next four decades, each bringing

an improvement to testing applications. The Rockwell

method greatly increased the speed of testing for indus-

try, the Vickers method provided a continuous hardness

scale from the softest to hardest metals, and the Knoop

method provided greater measurement sensitivity at

the lowest test forces. Since their development, these

methods have been improved and their applications ex-

panded, particularly in the cases of the Brinell and

Rockwell methods.

Fig. 7.27 Principle of the Brinell hardness test

Many manufactured products are made of differ-

ent types of materials, varying in hardness, strength,

size and thickness. To accommodate the testing of

these diverse products, each conventional hardness

method has deﬁned a range of standard force lev-

els to be used in conjunction with several different

types of indenters. Each combination of indenter type

and applied force level has been designated as a dis-

tinct hardness scale of the speciﬁc hardness test

method.

Brinell Hardness Test (HBW)

Principle of the Brinell Hardness Test [7.168, 169].

An indenter (hardmetal ball with diameter D) is forced

into the surface of a test piece and the diameter of the

indentation d left in the surface after removal of the

force F is measured (Fig. 7.27). The Brinell hardness

is proportional to the quotient obtained by dividing the

test force by the curved surface area of the indenta-

tion. The indentation is assumed to be spherical with

a radius corresponding to half of the diameter of the

ball

Brinell hardness HBW

=Constant ×

Test force

Surface area of indentation

=0.102 ×

π D(D −

√

−d

)

, (7.24)

where F is in N and D and d are in mm.

Designation of the Brinell Hardness Test. The Brinell

hardness is denoted by HBW followed by numbers rep-

resenting the ball diameter, applied test force, and the

duration time of the test force. Note that, in former stan-

dards, in cases when a steel ball had been used, the

Brinell hardness was denoted by HB or HBS.

Example. 600 HBW 1/30/20

600 Brinell hardness value

HBW Brinell hardness symbol

1/ Ball diameter in mm

30 Applied test force (294.2N=30 kgf)

/20 Duration time of test force (20 s) if not indicated

in the designation (10–15 s)

Application of the Brinell Hardness Test. For the appli-

cation of the Brinell hardness test, the most commonly

used Brinell scales for different testing conditions are

giveninTable7.6. Other combinations of test forces and

ball sizes are allowed, usually by special agreement.

Part C 7.3

Mechanical Properties 7.3 Hardness 369

Table 7.6 Standard test forces for the different testing conditions

Hardness symbol Ball diameter D (mm) Force : diameter ratio

0.102F/D

(N/mm

)

Nominal value of test force

F (kN)

HBW 10/3000 10 30 29.42

HBW 10/1500 10 15 14.71

HBW 10/1000 10 10 9.807

HBW 10/500 10 5 4.903

HBW 10/250 10 2.5 2.452

HBW 10/100 10 1 0.9807

HBW 5/750 5 30 7.355

HBW 5/250 5 10 2.452

HBW 5/125 5 5 1.226

HBW 5/62.5 5 2.5 0.6129

HBW 5/25 5 1 0.2452

HBW 2.5/187.5 2.5 30 1.839

HBW 2.5/62.5 2.5 10 0.6129

HBW 2.5/31.25 2.5 5 0.3065

HBW 2.5/15.62 2.5 2.5 0.1532

HBW 2.5/6.25 2.5 1 0.06129

HBW 1/30 1 30 0.2942

HBW 1/10 1 10 0.09807

HBW 1/5 1 5 0.04903

HBW 1/2, 5 1 2.5 0.02452

HBW 1/1 1 1 0.009807

Advantages of the Brinell Hardness Test.

•

Suitable for hardness tests even under rough work-

shop conditions if large ball indenters and high test

forces are used.

•

Suitable for hardness tests on inhomogeneous

materials because of the large test indentations, pro-

vided that the extent of the inhomogeneity is small

in comparison to the test indentation.

•

Suitable for hardness tests on large blanks such as

forged pieces, castings, hot-rolled or hot-pressed

and heat-treated components.

•

Relatively little surface preparation is required if

large ball indenters and high test forces are used.

•

Measurement is usually not affected by movement

of the specimen in the direction in which the test

force is acting.

•

Simple, robust and low-cost indenters.

Disadvantages of the Brinell Hardness Test.

•

Restriction of application range to a maximum

Brinell hardness of 650 HBW.

•

Restriction when testing small and thin-walled spec-

imens.

•

Relatively long test time due to the measurement of

the indentation diameter.

•

Relatively serious damage to the specimen due to

the large test indentation.

•

Measurement of many indentations can lead to op-

erator fatigue and increased measurement error.

Rockwell Hardness Test (HR)

Principle of the Rockwell Hardness Test [7.170,171]. The

general Rockwell test procedure is the same regardless

of the Rockwell scale or indenter being used. The in-

denter is brought into contact with the material to be

tested and a preliminary force is applied to the indenter.

The preliminary force is held constant for a speciﬁed

time duration, after which the depth of indentation is

measured. An additional force is then applied at a spec-

iﬁed rate to increase the applied force to the total force

level. The total force is held constant for a speciﬁed

time duration, after which the additional force is re-

moved, returning to the preliminary force level. After

holding the preliminary force constant for a speciﬁed

time duration, the depth of indentation is measured for

a second time, followed by removal of the indenter from

Part C 7.3

370 Part C Materials Properties Measurement

+ F

Fig. 7.28 Rockwell principle diagram (Key: 1 – Indenta-

tion depth by preliminary force F

; 2 – Indentation depth

by additional test force F

; 3 – Elastic recovery just after

removal of additional test force F

; 4 – Permanent indenta-

tion depth h; 5 – Surface of test piece; 6 – Reference plane

for measurement; 7 – Position of indenter)

the test material (Fig. 7.28). The difference in the two

depth measurements is calculated as h in mm. From the

value of h and the two constant numbers N and S (Ta-

ble 7.7), the Rockwell hardness is calculated following

the formula

Rockwell hardness = N −

. (7.25)

Designation of the Rockwell Hardness Test. The Rock-

well hardness is denoted by the symbol HR, followed by

a letter indicating the scale, and either an S or W to in-

dicate the type of ball used (S = steel; W = hardmetal,

tungsten carbide alloy) (Table 7.7).

Example. 70 HR 30N W

70 Rockwell hardness value

HR Rockwell hardness symbol

30N Rockwell scale symbol (Table 7.7)

W Indication of type of ball used, S = steel,

W = hardmetal

Application of the Rockwell Hardness Test. For the

application of the Rockwell hardness test, the indenter

types and speciﬁed test forces, as well as, typical appli-

cations for the different scales are given in Table 7.7.

ASTM International deﬁnes thirty different Rockwell

scales [7.170] while the ISO [7.171] deﬁnes a subset

of these scales (Table 7.7).

Advantages of the Rockwell Hardness Test.

•

Relatively short test time because the hardness value

is automatically displayed immediately following

the indentation process.

•

The test may be automated.

•

Relatively low procurement costs for the testing

machine because no optical measuring device is

necessary.

•

No operator inﬂuence of evaluation because the

hardness value is displayed directly.

•

Relatively short time needed to train operator.

Disadvantages of the Rockwell Hardness Test.

•

Possibility of measurement errors due to movement

of the test piece and poorly seated or worn machine

components during the application of the test forces.

•

Less possibility of testing materials with surface

layer hardening as a consequence of relatively high

test forces.

•

Sensitivity of the diamond indenter to damage, thus

producing a risk of incorrect measurements

•

Relatively low sensitivity on the difference in hard-

ness.

•

Signiﬁcant inﬂuence of the shape of the conical di-

amond indenter on the test result (especially of the

tip).

Vickers Hardness Test (HV)

Principle of the Vickers Hardness Test [7.172–174]. Adi-

amond indenter in the form of a right pyramid with

a square base and with a speciﬁed angle between oppo-

site faces at the vertex is forced into the surface of a test

piece followed by measurement of the diagonal length

of the indentation left in the surface after removal of the

test force F (Fig. 7.29).

Designation of the Vickers Hardness Test. The Vick-

ers hardness is denoted by the symbol HV followed

by numbers representing the applied test force, and the

duration time of the test force.

Example. 640 HV 30/20

640 Vickers hardness value

HV Hardness symbol

30 Applied test force (294.2N=30 kgf)

/20 Duration time of test force (20 s) if not indicated in

the designation (10–15 s)

Application of the Vickers Hardness Test. ISO [7.174]

speciﬁes the method of Vickers hardness test for the

three different ranges of test force for metallic mater-

Part C 7.3

Mechanical Properties 7.3 Hardness 371

Table 7.7 Rockwell hardness scales and typical applications [7.175]

Scale

symbol

Indenter type

(ball dimensions

Preliminary

force

Total

force

Typical applications HR calculation

indicate diameter) (N) (N) N S

HRA Spheroconical

diamond

98.07 588.4 Cemented carbides, thin steel, and shallow case hard-

ened steel

100 0.002

HRB Ball – 1.588 mm 98.07 980.7 Copper alloys, soft steels, aluminum alloys, malleable

iron, etc.

130 0.002

HRC Spheroconical

diamond

98.07 1471 Steel, hard cast irons, pearlitic malleable iron, tita-

nium, deep case hardened steel, and other materials

harder than HRB 100

100 0.002

HRD Spheroconical

diamond

98.07 980.7 Thin steel and medium case hardened steel, and

pearlitic malleable iron

100 0.002

HRE Ball – 3.175 mm 98.07 980.7 Cast iron, aluminum and magnesium alloys, and bear-

ing metals

130 0.002

HRF Ball – 1.588 mm 98.07 588.4 Annealed copper alloys, and thin soft sheet metals 130 0.002

HRG Ball – 1.588 mm 98.07 1471 Malleable irons, copper-nickel-zinc and cupro-nickel

alloys

130 0.002

HRH Ball – 3.175 mm 98.07 588.4 Aluminum, zinc, and lead 130 0.002

HRK Ball – 3.175 mm 98.07 1471 Bearing metals and other very soft or thin materials, 130 0.002

HRL

Ball – 6.350 mm 98.07 588.4 use smallest ball and heaviest load that does not give 130 0.002

HRM

Ball – 6.350 mm 98.07 980.7 anvil effect 130 0.002

HRP

Ball – 6.350 mm 98.07 1471 130 0.002

HRR

Ball – 12.70 mm 98.07 588.4 130 0.002

HRS

Ball – 12.70 mm 98.07 980.7 130 0.002

HRV

Ball – 12.70 mm 98.07 1471 130 0.002

HR15N Spheroconical

diamond

29.42 147.1 Similar to A, C and D scales, but for thinner gage

material or case depth

100 0.001

HR30N Spheroconical

diamond

29.42 294.2 100 0.001

HR45N Spheroconical

diamond

29.42 441.3 100 0.001

HR15T Ball – 1.588 mm 29.42 147.1 Similar to B, F and G scales, but for thinner gage 100 0.001

HR30T Ball – 1.588 mm 29.42 294.2 material 100 0.001

HR45T Ball – 1.588 mm 29.42 441.3 100 0.001

HR15W

Ball – 3.175 mm 29.42 147.1 Very soft material 100 0.001

HR30W

Ball – 3.175 mm 29.42 294.2 100 0.001

HR45W

Ball – 3.175 mm 29.42 441.3 100 0.001

HR15X

Ball – 6.350 mm 29.42 147.1 100 0.001

HR30X

Ball – 6.350 mm 29.42 294.2 100 0.001

HR45X

Ball – 6.350 mm 29.42 441.3 100 0.001

HR15Y

Ball – 12.70 mm 29.42 147.1 100 0.001

HR30Y

Ball – 12.70 mm 29.42 294.2 100 0.001

HR45Y

Ball – 12.70 mm 29.42 441.3 100 0.001

These scales are deﬁned by ASTM International, but are not included in the ISO standards

ials, whereas ASTM International [7.172, 173]divides

the Vickers test into two ranges (Table 7.8).

The Vickers hardness test is speciﬁed in ISO

to apply to lengths of indentation diagonals of

0.020–1.400 mm, whereas ASTM International [7.173]

allows indentation diagonals below 0.020 mm.

Although, for the application of the Vickers hard-

ness test, any force level is allowed between the

Part C 7.3

372 Part C Materials Properties Measurement

a) b)Indenter (diamond pyramid) Vickers indentation

Fig. 7.29a,b Principle of the Vickers hardness test

speciﬁed limits, traditionally used and recommended

test forces for different testing conditions are given

in Table 7.9.

Table 7.8 Ranges of Vickers hardness tests

ISO

Vickers range

(ISO 6507-1)

Nominal ranges of

test force, F (N)

Hardness range

Vickers

hardness

F ≥ 49.03 ≥ HV 5

Low force

Vickers

hardness

1.961 ≤ F < 49.03 HV 0.2toHV 5

Vickers

microhardness

0.09807 ≤ F < 1.961 HV 0.01

to HV 0.2

ASTM International

Vickers range Nominal ranges of

test force F (N)

Hardness range

Vickers

hardness

(ASTM E 92)

9.807 ≤ F < 1177 HV 1toHV 120

Vickers micro-

indentation

hardness

(ASTM E 384)

0.0098 ≤ F < 9.807 HV 0.001 to HV 1

Table 7.9 Recommended test forces

Hardness test

Low-force hardness test Microhardness test

Hardness

symbol

Nominal value of

the test force F (N)

Hardness

symbol

Nominal value of

the test force F (N)

Hardness

symbol

Nominal value of

the test force F (N)

HV 5 49.03 HV 0.2 1.961 HV 0.01 0.09807

HV 10 98.07 HV 0.3 2.942 HV 0.015 0.147

HV 20 196.1 HV 0.5 4.903 HV 0.02 0.1961

HV 30 294.2 HV 1 9.807 HV 0.025 0.2452

HV 50 490.3 HV 2 19.61 HV 0.05 0.4903

HV 100 980.7 HV 3 29.42 HV 0.1 0.9807

Nominal test forces greater than 980.7 N may be applied

Advantages of the Vickers Hardness Test.

•

Practically no limit to the use of the method due to

the hardness of the test piece.

•

Testing thin sheets, small test pieces or test surfaces,

thin-walled tubes, thin, hard and plated coatings is

possible.

•

In most cases, the small indentation has no inﬂuence

on the function or appearance of tested materials or

products.

•

The hardness value is usually independent of the test

force in the range of HV 0.2 and above.

•

No incorrect measurement if the test piece yields to

a limited extent in the direction of the test force.

•

Can be reported in terms of stress values.

Disadvantages of the Vickers Hardness Test.

•

High effort in preparing a suitable test surface.

•

Relatively long test time due to the measurement of

the diagonal lengths.

•

Sensitivity of the diamond indenter to damage.

•

If the test indentations are small, dependence of the

hardness on the shape deviations of the indenter and

the preparation of the test surfaces.

•

Very sensitive to effects of vibration, especially in

the microhardness range.

•

Relatively large variation in measurement depend-

ing on the operator of microscope. Especially for

low force hardness testing.

For the application of the Vickers hardness test for the

different ranges of test force see [7.176–179].

Knoop Hardness Test (HK)

Principle of the Knoop Hardness Test [7.173, 180].

A diamond indenter, in the form of a rhombic-based

pyramid with speciﬁed angles between opposite faces

at the vertex, is forced into the surface of a test piece

followed by measurement of the long diagonal of the in-

Part C 7.3

Mechanical Properties 7.3 Hardness 373

dentation remaining in the surface after removal of the

test force F (Figs. 7.30, 7.31,Table7.10).

The Knoop hardness is proportional to the quotient

obtained by dividing the test force by the projected area

of the indentation, which is assumed to be a rhombic-

based pyramid, and having at the vertex the same angles

as the indenter.

Designation of the Knoop Hardness. The Knoop hard-

ness is denoted by the symbol HK followed by numbers

representing the applied test force, and the duration time

of the test force.

Example. 640 HK 0.1/20

640 Knoop hardness value

HK Hardness symbol

0.1 Applied test force (0.9807 N =0.1 kgf)

/20 Duration time of test force (20 s) if not indicated in

the designation (10–15 s)

Application of the Knoop Hardness Test. For the appli-

cation of the Knoop hardness test, traditionally used and

recommended test forces for different testing conditions

are given in Table 7.11.

Advantages of the Knoop Hardness Test.

•

Particularly suitable for narrow test pieces, such as

wire, due to the large diagonal length ratio of ap-

proximately 7 :1.

•

Better suited to thin test pieces or platings than the

Vickers test method because the indentation depth

is smaller by a factor of four for the same diagonal

length.

Table 7.10 Symbols and designations for the Knoop hardness test

Symbol Designation

F Test force (N)

d Length of the long diagonal (mm)

c Indenter constant, relating projected area of the indentation to the square of the length of the long diagonal

Indenter constant, c =

tan

2tan

, ideally c =0.07028, where α and β are the angles between the opposite edges at the

vertex of the diamond pyramid (Fig. 7.30)

Knoop hardness = Gravitational constant

Test force

Projected area of indentation

=0.102 ×

=1.451

: Gravitational constant =

9.80665

≈0.102, where g

is the acceleration of gravity

Fig. 7.30 Symbols and designations for the Knoop hardness

test

1μm max.

Fig. 7.31 Symbols and designations for the Knoop hard-

ness test

•

Particularly suitable for brittle materials because of

lower tendency to cracking.

•

Particularly suited to investigating the anisotropy of

a material because the Knoop test is dependent on

Part C 7.3

374 Part C Materials Properties Measurement

Table 7.11 Recommended test forces

Hardness Symbol Nominal value of the test

force F (N)

HK 0.01 0.09807

HK 0.02 0.1961

HK 0.025 0.2452

HK 0.05 0.4903

HK 0.1 0.9807

HK 0.2 1.961

HK 0.3 2.942

HK 0.5 4.903

HK 1 9.807

the direction selected for the long diagonal in such

cases.

•

In most cases, the small indentation has no inﬂuence

on the function or appearance of tested materials or

products.

•

Practically no limit on the application of the method

from the hardness of the material tested.

Disadvantages of the Knoop Hardness Test.

•

Time effort to achieve a sufﬁciently ﬁne test surface.

•

Considerable dependency of the hardness on the

test force, with an especially strong inﬂuence of the

preparation of the test surface.

•

Sensitivity to damage of the diamond indenter.

•

Expensive alignment of the test surface to achieve

symmetrical test indentations.

•

Relatively long test time due to the measurement of

the diagonal length.

•

Measurement of the diagonal length is more difﬁcult

than in the Vickers test method as a consequence of

the indenter geometry.

7.3.2 Selecting a Conventional Hardness

Test Method and Hardness Scale

The selection of a speciﬁc hardness testing method

is substantially inﬂuenced by many factors as dis-

cussed below. Unfortunately, there is no one hardness

test that meets the optimum conditions for all appli-

cations. Although the Brinell, Rockwell, Vickers and

Knoop hardness testing methods have been developed

to be capable of testing most materials, in many test-

ing circumstances the size or depth of the indentation is

generally the primary consideration when choosing the

appropriate test forces and type of indenter to be uti-

lized by the hardness test method. The combinations of

test forces and indenter types are known as a hardness

method’s speciﬁc hardness scales. The hardness scales

have been deﬁned to provide suitable measurement sen-

sitivity and indentation depths that are appropriate for

different materials, strengths and dimensions of test

pieces. The largest and deepest indentations are gen-

erally obtained from the heavy-force Brinell scales,

followed by the commonly used heavy-force Rockwell

and Vickers scales, while small to extremely small in-

dentations can be obtained from the commonly used

lower-force Rockwell, Vickers and Knoop scales.

Strength of the Test Piece

A material’s resistance to penetration or its strength is of

primary importance when selecting a hardness method

or hardness scale. An appropriate test force and indenter

must be chosen to produce a large enough indentation

in the test material to enable the hardness testing ma-

chine’s measuring system to resolve and measure the

depth or size of the indentation with sufﬁcient sensitiv-

ity to produce a meaningful hardness result, but at the

same time to not penetrate too deeply beyond the inden-

ter’s useful depth. On the other hand, a suitable indenter

of the appropriate material, size and shape must be cho-

sen to prevent the indenter from being damaged when

testing high-strength materials. For example, hardened

and tempered steel material generally requires hardness

methods that employ higher test forces and a pointed

or conical diamond indenter, whereas testing cemented

carbides with high forces could lead to the diamond

being fractured.

Test-Piece Size, Shape, Weight, Accessibility,

Dimensions, and Thickness

The choice of a hardness test method may be inﬂu-

enced by the size, shape or weight of the test piece. The

designs of some typical standard hardness testing ma-

chines may prevent the testing of very large and heavy

test pieces or unusually shaped test pieces, and may

require special machine designs, special designs of in-

denters or additional test piece support ﬁxtures. These

types of designs are usually limited to the Brinell, Rock-

well and heavy-force Vickers machines. In contrast,

small sample sizes may require mounting in a support

material which is generally only suitable for the Knoop

and low-force Vickers hardness methods.

The dimensions of the test piece may also have to

be considered for another reason. As a hardness mea-

surement is being made, the material surrounding the in-

dentation is plastically deformed, with the deformation

extending well to the sides and below the indentation. If

a hardness measurement is made too near the edge of the

Part C 7.3

Mechanical Properties 7.3 Hardness 375

test material, the deformation surrounding the indenta-

tion may extend to the edge and push out the material,

thus affecting the measured hardness value. Similarly, if

the deformation extends completely through the thick-

ness of thin test material, then the deformed material

may ﬂow at the interface with the supporting anvil, or

the anvil material may contribute to the hardness mea-

surement. These problems can inﬂuence the deformation

process likely causing the test to give erroneous hardness

results. Consequently, in cases where a hardness test is

to be made on narrow-width or thin material or mater-

ial having a small area, an appropriate hardness test and

scale must be chosen that produces indentations small

enough to prevent this edge interaction. Also keep in

mind that the area surrounding a previously made inden-

tation may also affect a new hardness test due to induced

residual stress and areas of workhardening surrounding

the indentation. Therefore, the number of indentations

that will be made must be taken into consideration when

small test areas are involved.

Material Composition and Homogeneity

The size and location of metallurgical features within

the test material should be considered when choosing

the hardness test method and hardness scale. For mater-

ials that are not homogeneous, an appropriate hardness

test and scale should be chosen that will produce a suf-

ﬁciently large indentation representative of the material

as a whole. For example, forgings and cast iron are typ-

ically tested using the Brinell method. If the subject of

interest is a material artifact, such as a decarburization

zone or a thin material layer, then a hardness test and

scale should be chosen that produces a small or shal-

low indentation. In an analogous way to the issues of

test-piece size, the closeness to adjacent regions of a dif-

fering hardness, such as the heat-affected zone of a weld,

should also be considered. If the deformation zone sur-

rounding an indentation extends into these regions, the

test measurement may be inﬂuenced. In such cases, an

appropriate hardness test and scale should be chosen that

uses test forces and indenter that produce a small enough

indentation to avoid the inﬂuence of these areas.

Permissible Damage

One of the greatest attributes of a hardness test is that it

is essentially a nondestructive test, leaving only a small

indentation which usually does not detract from a prod-

uct’s function or appearance. However, there are some

applications for which a hardness indentation of a cer-

tain size could be detrimental to a product’s service or

appearance. For example, it is possible that an inden-

tation could act as an initiation point for a fracture in

a part subjected to cyclic loading, or a large visible in-

dentation could affect the appearance of a product. In

these cases, smaller indentations may be the solution.

Test Surface Preparation

When the ability to prepare the testing surface is not

possible or practical or is limited, a hardness method

and hardness scale must be chosen that is less sensi-

tive to the level of surface preparation. The degree of

surface roughness that can be tolerated by the different

hardness testing methods is generally dependent on the

force levels to be applied, indenter size and resultant

indentation size. In general, the larger the indentation

size and depth, the less sensitive is the hardness test to

the level of surface roughness and imperfections, and

the more likely the measurement will represent the true

hardness value of a material. This is particularly true

for the Brinell, Vickers and Knoop methods because of

the need to accurately resolve and measure the resultant

indentation. The larger the sizes of surface anomalies

with respect to the indentation size, the more likely it

becomes that errors in the indentation measurement and

the hardness result will increase.

An important feature of the Rockwell test method,

which bases the hardness result on depth measurement

rather than indentation size, is the use of the prelimi-

nary force as part of the testing cycle. Application of the

preliminary force acts to push the indenter through mi-

nor surface imperfections and to crush residual foreign

particles present on the test surface. By establishing

a reference beneath the surface prior to making the ﬁrst

depth measurement, it allows testing of materials with

slight surface ﬂaws while maintaining much of the test

accuracy.

As a general guide, the high-force scales of the

Brinell test method requires the least surface prepa-

ration, such as a surface that has been ﬁled, ground,

machined or polished with no. 000 emery paper. Mater-

ial tested by the Rockwell test method usually does not

need to be polished, but should be smooth, clean and

free of scale. The Brinell test method and the heavy-

force Vickers scales producing small indentations are

likely to require polished surfaces to allow accurate

measurement of the indentation sizes.

Permissible Measurement Uncertainty

The measurement uncertainty of a hardness test is in-

ﬂuenced by many factors, including, among others, the

operator, the machine’s repeatability and reproducibil-

ity, and the testing environment (Sect. 7.3.3). With

Part C 7.3

376 Part C Materials Properties Measurement

respect to choosing a hardness test method, the num-

ber of different operators that would potentially use the

hardness machine should be considered. Two of the

larger sources of hardness measurement error are re-

lated to the operator’s measurement of an indentation,

such as is the case for Brinell, Vickers and Knoop meth-

ods, and the operation of manually controlled hardness

testers. The variability between operators contributes

to the measurement uncertainty by increasing the lack

of repeatability and reproducibility in the measurement

system. If a low measurement uncertainty is of high

concern, then it may be beneﬁcial to use automatically

controlled hardness machines or automated indentation

measuring systems, or to use the Rockwell hardness

method, which lessens the inﬂuence of the operator by

basing the hardness result on indentation depth as meas-

ured by the machine.

Speed of Testing Desired

For many industrial processes, time is one of the most

important parameters. In circumstances where hard-

ness testing is an integral part of a process and many

tests must be performed, the time required to complete

each test becomes an extremely important considera-

tion. For a hardness test, the time required for each

step of a measurement must be considered from the

preparation of the test piece, to the indentation process,

to the measurements needed to determine the hardness

result, and ﬁnally to the calculation of the hardness

value. The Brinell, Vickers and Knoop methods are two-

step processes: (1) the indentation process, followed by

(2) measurement of the indentation. The Rockwell hard-

ness method was speciﬁcally designed to reduce the

time needed for a hardness test by eliminating the sec-

ond step of having to measure the size of the indent after

the indentation process. The Rockwell hardness method

quickly and automatically calculates and displays the

hardness result based on the machine’s measurement of

indentation depth. Instrumented indentation testing is

likely to take the longest time to complete due to the

care and time required for preparing the test sample,

running the indentation cycle, and analyzing the data.

Economic Viability and the Availability

of Machines and Equipment

Other nontechnical but nevertheless important consider-

ations in choosing a hardness method are the economics

and practicality of the selection. Economic limitations

often drive the choice of which hardness method is cho-

sen for use. It may be a difﬁcult choice in weighing

the beneﬁts of the best hardness method for an appli-

cation against its cost. This may be particularly true

when existing hardness equipment currently exists in-

house. Finally, the volume of testing and the necessary

training of operators should be taken into account when

selecting a test.

Guidance on selecting hardness test methods is

given in international hardness test method standards,

such as those published by ISO and ASTM Interna-

tional. When several hardness methods are acceptable

for testing a material, generally, the most commonly

used method for the type of material or product should

be chosen. In circumstances where the user wants to

compare measurements with previously obtained hard-

ness data, the same hardness test method and scale

should be chosen as was used for the previous testing as

long as a valid test can be obtained. This is preferred to

testing with one hardness test method and scale and then

converting the data to another hardness scale by way of

published conversion tables. Converted data is never as

accurate as the original measurement. This also holds

true when the hardness method and scale is speciﬁed in

a product speciﬁcation or contract. The speciﬁed hard-

ness method or scale should be used whenever possible

as long as a valid test can be obtained.

7.3.3 Measurement Uncertainty

in Hardness Testing (HR, HBW, HV, HK)

The concept of measurement uncertainty is relatively

new to users of hardness testing, particularly as applied

to the conventional hardness tests, Rockwell, Brinell,

Vickers and Knoop. Historically, industry has relied on

limiting hardness measurement error by adhering to er-

ror tolerances speciﬁed by national and international

hardness test method standards. Test method standards

specify tolerances for components of the hardness ma-

chine, such as the allowed error when applying the test

forces and the error in measuring the indentation size or

penetration depth. Tolerances are also speciﬁed for the

performance of the hardness machine limiting the ma-

chine’s measurement error with respect to the certiﬁed

values of hardness block reference standards.

Hardness measurement uncertainty is very different

from the tolerances speciﬁed by test method stan-

dards. Uncertainty does not limit measurement error.

It provides an estimation of the error in a hardness

measurement value with respect to the true hardness of

the tested material. In recent years, with the advent of

accreditation programs for testing and calibration labo-

ratories, measurement uncertainty has gained increasing

importance for hardness laboratories since most accred-

Part C 7.3

Mechanical Properties 7.3 Hardness 377

itation programs require a laboratory to determine their

own measurement uncertainty.

Estimating Hardness Uncertainty –

Direct Approach

The Guide to the Expression of Uncertainty in Measure-

ment (GUM)[7.181] provides guidance for estimating

the uncertainty in a measurement value. One procedure

is a direct approach to quantify the signiﬁcant inﬂu-

ence quantities of the test (for example: force error, test

speed, indenter shape, etc.), determine the biases and

uncertainties of each inﬂuence quantity, correct for the

biases, determine and evaluate sensitivity coefﬁcients

to convert from the units of the input quantity to the

units of the measurand, and ﬁnally combine the uncer-

tainties to provide an overall uncertainty in the resultant

measurement value.

A hardness test involves the time-dependent appli-

cation of forces to speciﬁcally shaped indenters, and,

for some types of harness tests, simultaneously mea-

suring the resultant indentation depths. This is done

using machines that can range from entirely mechanical

to various combinations of electronic and mechanical

components. As a result, it is often difﬁcult to identify

all of the signiﬁcant inﬂuence quantities that contribute

to the measurement uncertainty. For example, error

sources may exist due to the internal workings of the

mechanical components of the hardness machine or the

indenter, which are not easily measurable. Complicating

this, the determination of some sensitivity coefﬁcients is

a lengthy and difﬁcult undertaking since these conver-

sion factors may be dependent on the hardness scale and

the material being tested which can only be determined

by experimental testing. An additional complication is

that the accepted practice for calibrating a hardness

machine is to not correct for biases in the operational

components of the machine as long as they are within

the speciﬁed tolerance limits.

Although this technique of assessing the separate

parameters of the hardness testing machine for their

individual error contributions is an effective method

for determining hardness measurement uncertainty, it

may be best applied at the highest calibration levels,

such as by National Metrology Institutes, which have

the time and resources to assess each of the sources

of error. However, it may present an overwhelming

challenge to many industrial hardness laboratories us-

ing the conventional hardness tests. Detailed procedures

for calculating uncertainty by this method are too in-

volved to be described here; however, recommended

procedures have been published as a document of the

European Cooperation for Accreditation of Laborato-

ries (EAL)[7.182].

Estimating Hardness Uncertainty –

Indirect Approach

A second procedure for determining hardness un-

certainty is an indirect approach based on familiar

procedures and practices of hardness testing facilities

and laboratories. Most of the contributions of uncer-

tainty can be estimated from the results of hardness

tests on reference materials and the material under test.

Hardness reference materials provide traceability to the

national hardness standards maintained by the world’s

National Metrology Institutes (NMIs) and are necessary

to establish a starting point for the uncertainty analysis

as per the GUM. In some cases, where reference ma-

terials are not available from a NMI, suitable reference

materials may be obtained from a secondary reference

source.

This indirect approach views the hardness ma-

chine and indenter as a single measuring device, and

considers uncertainties associated with the overall mea-

surement performance of the hardness machine. It

may be more applicable for industrial laboratories in

determining the measurement uncertainty for the con-

ventional types of hardness tests. It is important to

understand that this procedure is appropriate only when

a hardness machine is operating within all operational

and performance tolerances to ensure that a large er-

ror in one machine component is not being offset by

another large, but opposite, error in a second com-

ponent. These opposite errors could then combine to

produce satisfactory hardness results when testing ref-

erence blocks even when the machine is not operating

properly.

This approach considers uncertainties due to the

lack of repeatability of the hardness machine, the re-

producibility of the hardness measurement over time,

the resolution of the hardness machine display or in-

dentation measuring instrument, the nonuniformity of

the material under test, the uncertainty of the certiﬁed

value of the reference standards, and the measurement

bias as compared to reference standards.

•

The repeatability of the hardness machine is its abil-

ity to continually produce the same hardness value

each time a measurement is made over a short time

period and under constant testing conditions, in-

cluding the same operator. All testing instruments,

including hardness machines, exhibit some degree

of a lack of repeatability.

Part C 7.3