Measurement of Hardness – Hardness Testing

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 Mohs 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. \

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
Rockwell hardness test principle
Fig. 1.Rockwell Principle

Rockwell Hardness Scales

Scale	Indenter	Minor Load F0 kgf	Major Load F1 kgf	Total Load F kgf	Value of E
A	Diamond cone	10	50	60	100
B	1/16″ steel ball	10	90	100	130
C	Diamond cone	10	140	150	100
D	Diamond cone	10	90	100	100
E	1/8″ steel ball	10	90	100	130
F	1/16″ steel ball	10	50	60	130
G	1/16″ steel ball	10	140	150	130
H	1/8″ steel ball	10	50	60	130
K	1/8″ steel ball	10	140	150	130
L	1/4″ steel ball	10	50	60	130
M	1/4″ steel ball	10	90	100	130
P	1/4″ steel ball	10	140	150	130
R	1/2″ steel ball	10	50	60	130
S	1/2″ steel ball	10	90	100	130
V	1/2″ steel ball	10	140	150	130

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).

LINKS TO:
Hardness Testing
Rockwell Hardness Test
Rockwell Superficial Hardness Test
Brinell Hardness Test
Vickers Hardness Test
Microhardness Test
Mohs Hardness Test
Scleroscope and other hardness testing methods

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)
Estimated Hardness Equivalent Chart related to Rockwell C and Vickers (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)
Table of Minimum Test Piece Thickness for Rockwell Hardness Testing using Ball Indenters
Table of Minimum Test Piece Thickness for Rockwell Hardness Testing using Diamond Indenters
HV, MPa and GPa Conversion Calculator

Hardness Testing Methods:

Rockwell Hardness Test

Rockwell Superficial Hardness Test

Brinell Hardness Test

Vickers Hardness Test

Microhardness Test

Mohs Hardness Test

Scleroscope and other hardness testing 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 indenters, 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)

Estimated Hardness Equivalent Chart related to Rockwell C and Vickers (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)

The Brinell scale characterizes the indentation hardness of materials through the scale of penetration of an indenter, loaded on a material test-piece. It is one of several definitions of hardness in materials science.

Proposed by Swedish engineer Johan August Brinell in 1900, it was the first widely used and standardised hardness test in engineering and metallurgy. The large size of indentation and possible damage to test-piece limits its usefulness.

The typical test uses a 10 millimetres (0.39 in) diameter steel ball as an indenter with a 3,000 kgf (29,000 N; 6,600 lbf) force. For softer materials, a smaller force is used; for harder materials, a tungsten carbide ball is substituted for the steel ball. The indentation is measured and hardness calculated as:

$\mbox{BHN}=\frac{2P}{\pi D ({D-\sqrt{(D^2-d^2)})}}$

where:

P = applied force (kgf)

D = diameter of indenter (mm)

d = diameter of indentation (mm)

The BHN can be converted into the ultimate tensile strength (UTS), although the relationship is dependent on the material, and therefore determined empirically. The relationship is based on Meyer’s index (n) from Meyer’s law. If Meyer’s index is less than 2.2 then the ratio of UTS to BHN is 0.36. If Meyer’s index is greater than 2.2, then the ratio increases.^[1]

BHN is designated by the most commonly used test standards (ASTM E10-08^[2] and ISO 6506-1:2005^[3]) as HBW (H from hardness, B from brinell and W from the material of the indenter, tungsten (wolfram) carbide). In former standards HB or HBS were used to refer to measurements made with steel indenters.

HBW is calculated in both standards using the SI units as

$\mbox{HBW}=0.102 \frac{2F}{\pi D ({D-\sqrt{(D^2-d^2)})}}$

where:

F = applied force (N)

D = diameter of indenter (mm)

d = diameter of indentation (mm)

When quoting a Brinell hardness number (BHN or more commonly HB), the conditions of the test used to obtain the number must be specified. The standard format for specifying tests can be seen in the example “HBW 10/3000″. “HBW” means that a tungsten carbide (from the chemical symbol for tungsten) ball indenter was used, as opposed to “HBS”, which means a hardened steel ball. The “10″ is the ball diameter in millimeters. The “3000″ is the force in kilograms force.

Brinell hardness numbers
Material	Hardness
Softwood (e.g., pine)	1.6 HBS 10/100
Hardwood	2.6–7.0 HBS 1.6 10/100
Aluminium	15 HB
Copper	35 HB
Mild steel	120 HB
18-8 (304) stainless steel annealed	200 HB^[4]
Glass	1550 HB
Hardened tool steel	1500–1900 HB
Rhenium diboride	4600 HB
Note: Standard test conditions unless otherwise stated