HARDNESS TEST

HARDNESS TEST

What is Hardness?

Hardness is the property of a material that enables it to resist plastic deformation, usually

by penetration. However, the term hardness may also refer to resistance to bending,

scratching, abrasion or cutting.

Measurement of Hardness:

Hardness is not an intrinsic material property dictated by precise definitions in terms of

fundamental units of mass, length and time. A hardness property value is the result of a

defined measurement procedure.

Hardness of materials has probably long been assessed by resistance to scratching or

cutting. An example would be material B scratches material C, but not material A.

Alternatively, material A scratches material B slightly and scratches material C heavily.

Relative hardness of minerals can be assessed by reference to the Moh's Scale that ranks

the ability of materials to resist scratching by another material. Similar methods of

relative hardness assessment are still commonly used today. An example is the file test

where a file tempered to a desired hardness is rubbed on the test material surface. If the

file slides without biting or marking the surface, the test material would be considered

harder than the file. If the file bites or marks the surface, the test material would be

considered softer than the file.

The above relative hardness tests are limited in practical use and do not provide accurate

numeric data or scales particularly for modern day metals and materials. The usual

method to achieve a hardness value is to measure the depth or area of an indentation left

by an indenter of a specific shape, with a specific force applied for a specific time. There

are three principal standard test methods for expressing the relationship between hardness

and the size of the impression, these being Brinell, Vickers, and Rockwell. For practical

and calibration reasons, each of these methods is divided into a range of scales, defined

by a combination of applied load and indenter geometry.

Hardness Test Methods:

Rockwell Hardness Test

Rockwell Superficial Hardness Test

Brinell Hardness Test

Vickers Hardness Test

Microhardness Test

Moh's Hardness Test

Scleroscope and other hardness test methods

Hardness Conversion or Equivalents:

Hardness conversion between different methods and scales cannot be made

mathematically exact for a wide range of materials. Different loads, different shape of

indeters, homogeneity of specimen, cold working properties and elastic properties all

complicate the problem. All tables and charts should be considered as giving approximate

equivalents, particularly when converting to a method or scale which is not physically

possible for the particular test material and thus cannot be verified. An example would be

converting HV/10 or HR-15N value on a thin coating to the HRC equivalent.

Hardness Conversion Tables and Charts:

Hardness Conversion Table (colour version - may take time to load)

Hardness Conversion Table (non-colour version)

Hardness Conversion Chart (1)

Hardness Conversion Chart (2)

Chart of Brinell, Vickers and Ultimate Tensile Strength Equivalents (1)

Chart of Brinell, Vickers and Ultimate Tensile Strength Equivalents (2)

Hardness Conversion Table related to Rockwell C Hardness Scale (hard materials)

(colour)

Hardness Conversion Table related to Rockwell C Hardness Scale (hard materials)

(non-colour)

Hardness Conversion Chart related to Rockwell C Hardness Scales (hard materials)

Hardness Conversion Table related to Rockwell B Hardness Scale (soft metals)

(colour)

Hardness Conversion Table related to Rockwell B Hardness Scale (soft metals)

(non-colour)

Hardness Conversion Chart related to Rockwell B Hardness Scale (soft metals)

HV, MPa and GPa Conversion Calculator

Rockwell Hardness Test

The Rockwell hardness test method consists of indenting the test material with a diamond

cone or hardened steel ball indenter. The indenter is forced into the test material under a

preliminary minor load F0 (Fig. 1A) usually 10 kgf. When equilibrium has been reached,

an indicating device, which follows the movements of the indenter and so responds to

changes in depth of penetration of the indenter is set to a datum position. While the

preliminary minor load is still applied an additional major load is applied with resulting

increase in penetration (Fig. 1B). When equilibrium has again been reach, the additional

major load is removed but the preliminary minor load is still maintained. Removal of the

additional major load allows a partial recovery, so reducing the depth of penetration (Fig.

1C). The permanent increase in depth of penetration, resulting from the application and

removal of the additional major load is used to calculate the Rockwell hardness number.

HR = E - e

F0 = preliminary minor load in kgf

F1 = additional major load in kgf

F = total load in kgf

e = permanent increase in depth of penetration due to major load F1 measured in units of

0.002 mm

E = a constant depending on form of indenter: 100 units for diamond indenter, 130 units

for steel ball indenter

HR = Rockwell hardness number

D = diameter of steel ball

Fig. ll Principle

Rockwell Hardness Scales

Minor LoadMajor LoadTotal LoadValue of

Scale Indenter

F0 F1 F

E

kgf kgf kgf

A Diamond cone

B 1/16" steel ball

C Diamond cone

D Diamond cone

E

F

H

K

L

M

P

R

S

V

1/8" steel ball

1/16" steel ball

1/8" steel ball

1/8" steel ball

1/4" steel ball

1/4" steel ball

1/4" steel ball

1/2" steel ball

1/2" steel ball

1/2" steel ball

10

10

10

10

10

10

10

10

10

10

10

10

10

10

10

50

90

140

90

90

50

140

50

140

50

90

140

50

90

140

60

100

150

100

100

60

150

60

150

60

100

150

60

100

150

100

130

100

100

130

130

130

130

130

130

130

130

130

130

130

G 1/16" steel ball

Typical Application of Rockwell Hardness Scales

HRA . . . . Cemented carbides, thin steel and shallow case hardened steel

HRB . . . . Copper alloys, soft steels, aluminium alloys, malleable irons, etc

HRC . . . . Steel, hard cast irons, case hardened steel and other materials harder than 100

HRB

HRD . . . . Thin steel and medium case hardened steel and pearlitic malleable iron

HRE . . . . Cast iron, aluminium and magnesium alloys, bearing metals

HRF . . . . Annealed copper alloys, thin soft sheet metals

HRG . . . . Phosphor bronze, beryllium copper, malleable irons HRH . . . . Aluminium,

zinc, lead

HRK . . . . }

HRL . . . . }

HRM . . . .} . . . . Soft bearing metals, plastics and other very soft materials

HRP . . . . }

HRR . . . . }

HRS . . . . }

HRV . . . . }

Advantages of the Rockwell hardness method include the direct Rockwell hardness

number readout and rapid testing time. Disadvantages include many arbitrary non-related

scales and possible effects from the specimen support anvil (try putting a cigarette paper

under a test block and take note of the effect on the hardness reading! Vickers and Brinell

methods don't suffer from this effect).

The Brinell Hardness Test

The Brinell hardness test method consists of indenting the test material with a 10 mm

diameter hardened steel or carbide ball subjected to a load of 3000 kg. For softer

materials the load can be reduced to 1500 kg or 500 kg to avoid excessive indentation.

The full load is normally applied for 10 to 15 seconds in the case of iron and steel and for

at least 30 seconds in the case of other metals. The diameter of the indentation left in the

test material is measured with a low powered microscope. The Brinell harness number is

calculated by dividing the load applied by the surface area of the indentation.

The diameter of the impression is the average of two readings at right angles and the use

of a Brinell hardness number table can simplify the determination of the Brinell hardness.

A well structured Brinell hardness number reveals the test conditions, and looks like this,

"75 HB 10/500/30" which means that a Brinell Hardness of 75 was obtained using a

10mm diameter hardened steel with a 500 kilogram load applied for a period of 30

seconds. On tests of extremely hard metals a tungsten carbide ball is substituted for the

steel ball. Compared to the other hardness test methods, the Brinell ball makes the

deepest and widest indentation, so the test averages the hardness over a wider amount of

material, which will more accurately account for multiple grain structures and any

irregularities in the uniformity of the material. This method is the best for achieving the

bulk or macro-hardness of a material, particularly those materials with heterogeneous

structures.

Vickers Hardness Test

The Vickers hardness test method consists of indenting the test material with a diamond

indenter, in the form of a right pyramid with a square base and an angle of 136 degrees

between opposite faces subjected to a load of 1 to 100 kgf. The full load is normally

applied for 10 to 15 seconds. The two diagonals of the indentation left in the surface of

the material after removal of the load are measured using a microscope and their average

calculated. The area of the sloping surface of the indentation is calculated. The Vickers

hardness is the quotient obtained by dividing the kgf load by the square mm area of

indentation.

F= Load in kgf

d = Arithmetic mean of the two diagonals, d1 and d2 in mm

HV = Vickers hardness

When the mean diagonal of the indentation has been determined the Vickers hardness

may be calculated from the formula, but is more convenient to use conversion tables. The

Vickers hardness should be reported like 800 HV/10, which means a Vickers hardness of

800, was obtained using a 10 kgf force. Several different loading settings give practically

identical hardness numbers on uniform material, which is much better than the arbitrary

changing of scale with the other hardness testing methods. The advantages of the Vickers

hardness test are that extremely accurate readings can be taken, and just one type of

indenter is used for all types of metals and surface treatments. Although thoroughly

adaptable and very precise for testing the softest and hardest of materials, under varying

loads, the Vickers machine is a floor standing unit that is more expensive than the Brinell

or Rockwell machines.

There is now a trend towards reporting Vickers hardness in SI units (MPa or GPa)

particularly in academic papers. Unfortunately, this can cause confusion. Vickers

hardness (e.g. HV/30) value should normally be expressed as a number only (without the

units kgf/mm2). Rigorous application of SI is a problem. Most Vickers hardness testing

machines use forces of 1, 2, 5, 10, 30, 50 and 100 kgf and tables for calculating HV. SI

would involve reporting force in newtons (compare 700 HV/30 to HV/294 N = 6.87 GPa)

which is practically meaningless and messy to engineers and technicians. To convert a

Vickers hardness number the force applied needs converting from kgf to newtons and the

area needs converting form mm2 to m2 to give results in pascals using the formula above.

To convert HV to MPa multiply by 9.807

To convert HV to GPa multiply by 0.009807

The Scleroscope Hardness Test

The Scleroscope test consists of dropping a diamond tipped hammer, which falls inside a

glass tube under the force of its own weight from a fixed height, onto the test specimen.

The height of the rebound travel of the hammer is measured on a graduated scale. The

scale of the rebound is arbitrarily chosen and consists on Shore units, divided into 100

parts, which represent the average rebound from pure hardened high-carbon steel. The

scale is continued higher than 100 to include metals having greater hardness. The Shore

Scleroscope measures hardness in terms of the elasticity of the material and the hardness

number depends on the height to which the hammer rebounds, the harder the material, the

higher the rebound.

The Durometer

The Durometer is a popular instrument for measuring the indentation hardness of rubber

and rubber-like materials. The most popular testers are the Model A used for measuring

softer materials and the Model D for harder materials. The operation of the tester is quite

simple. The material is subjected to a definite pressure applied by a calibrated spring to

an indenter that is either a cone or sphere and an indicating device measures the depth of

indentation.

Moh's Hardness Scale

The Moh's hardness scale for minerals has been used since 1822. It simply consists of 10

minerals arranged in order from 1 to 10. Diamond is rated as the hardest and is indexed as

10; talc as the softest with index number 1. Each mineral in the scale will scratch all those

below it as follows:

Diamond

Corundum

Topaz

Quartz

Aptite

Fluorite

Calcite

Gypsum

Talc

109

8

7

5

4

3

2

1

Orthoclase (Feldspar)6

The steps are not of equal value and the difference in hardness between 9 and 10 is much

greater than between 1 and 2. The hardness is determined by finding which of the

standard minerals the test material will scratch or not scratch; the hardness will lie

between two points on the scale - the first point being the mineral which is scratched and

the next point being the mineral which is not scratched. Some examples of the hardness

of common metals in the Moh's scale are copper between 2 and 3 and tool steel between

7 and 8. This is a simple test, but is not exactly quantitative and the standards are purely

arbitrary numbers.

The materials engineer and metallurgist find little use for the Moh's scale, but it is

possible to sub-divide the scale and some derived methods are still commonly used today.

The file test is useful as a rapid and portable qualitative test for hardened steels, where

convention hardness testers are not available or practical. Files can be tempered back to

give a range of known hardness and then used in a similar fashion to the Moh's method to

evaluate hardness.

Microhardness Test

The term microhardness test usually refers to static indentations made with loads not

exceeding 1 kgf. The indenter is either the Vickers diamond pyramid or the Knoop

elongated diamond pyramid. The procedure for testing is very similar to that of the

standard Vickers hardness test, except that it is done on a microscopic scale with higher

precision instruments. The surface being tested generally requires a metallographic

finish; the smaller the load used, the higher the surface finish required. Precision

microscopes are used to measure the indentations; these usually have a magnification of

around X500 and measure to an accuracy of +0.5 micrometres. Also with the same

observer differences of +0.2 micrometres can usually be resolved. It should, however, be

added that considerable care and experience are necessary to obtain this accuracy.

Knoop Hardness Indenter Indentation

The Knoop hardness number KHN is the ratio of the load applied to the indenter, P (kgf)

to the unrecovered projected area A (mm2)

KHN = F/A = P/CL2

Where:

F = applied load in kgf

A = the unrecovered projected area of the indentation in mm2

L = measured length of long diagonal of indentation in mm

C = 0.07028 = Constant of indenter relating projected area of the indentation to the

square of the length of the long diagonal.

The Knoop indenter is a diamond ground to pyramidal form that produces a diamond

shaped indentation having approximate ratio between long and short diagonals of 7:1.

The depth of indentation is about 1/30 of its length. When measuring the Knoop hardness,

only the longest diagonal of the indentation is measured and this is used in the above

formula with the load used to calculate KHN. Tables of these values are usually a more

convenient way to look-up KHN values from the measurements.

Vickers Pyramid Diamond Indenter Indentation

The Vickers Diamond Pyramid harness number is the applied load (kgf) divided by the

surface area of the indentation (mm2)

Where:

F= Load in kgf

d = Arithmetic mean of the two diagonals, d1 and d2 in mm

HV = Vickers hardness

The Vickers Diamond Pyramid indenter is ground in the form of a squared pyramid with

an angle of 136obetween faces. The depth of indentation is about 1/7 of the diagonal

length. When calculating the Vickers Diamond Pyramid hardness number, both diagonals

of the indentation are measured and the mean of these values is used in the above formula

with the load used to determine the value of HV. Tables of these values are usually a

more convenient way to look-up HV values from the measurements.

Information provided by website:

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Thermal Spray Coatings

Gordon England

Independent Metallurgist and Consultant to the Thermal Spray Coating

Industry

Kingfishers, Folly Lane North

Upper Hale, Farnham

Surrey, GU9 0HU

England

Telephone: 44 (0) 1252 405186

Fax: 44 (0) 1252 405187

Mobile: 07951 120293