Hardness testing

Hardness is not a fundamental property of a material, but rather defined as 'the resistance the material exhibits to permanent deformation by penetration of another harder material'. The principal purpose of the hardness test is to determine the suitability of a material, or the particular treatment to which the material has been subjected. The quantitative value of hardness should always be evaluated in relation to:

the type of indenter and its geometry
the given load on the indenter
a specific loading time profile and a specific load duration.

Methods

Rockwell

Hardness method	Rockwell
Description	Calculated by measuring the depth of an indent, after an indenter has been forced into the specimen material at a given load. HR = N - (h/E), where N = 100, E = 0.002mm and h = permanent depth.
Indenter and indent specifications	Conical diamond, complies to EN ISO standard Lasting impression depth (HR) = N - h/E, where N is 100 units, E is 0.002mm, and h is permanent depth
Load range	15 – 150kgf
Methods and applications	HRA 60kgf	Cemented carbides, thin steel and shallow case-hardened steel
	HRC 150kgf	Steel, hard cast irons, pearlitic malleable iron, titanium, deep cased hardened steel
	HRD 100kgf	Thin and medium case-hardened steel and pearlitic malleable iron
	HR15N 15kgf HR30N 30kgf HR45N 45kgf	Use superficial rockwell scales where regular scales are problematic
Sample requirements	Must not be mounted None or ground
Testing considerations	Geometric correction is available for small diameter cylindrical or ball shaped samples Indentation depth must be less than 10 times the sample thickness
Strengths	Rapid test, ~10 seconds Direct read-out - little/no sample preparation Non-destructive
Weaknesses	Multiple test scales needed to cover entire hardness range Many conversion tables exist - can be material dependent
SOP	Hardness on Struers DuraJet SOP (PDF, 91.5 KB)
Instruction manual	DuraJet Instruction Manual (PDF, 1.9 MB)
Resources	Knowledge on Rockwell Hardness Testing Standards; ASTM E18, ASTM D785, EN ISO 6508, EN ISO 2039-2 Troubleshooting
Booking address	*G SEBE ENG Lab Eqpmt KE2.101 Rockwell Hardness tester

Brinell Depth

Hardness method	Brinell Depth
Description	Modified Brinell hardness HBT. Residual indentation depth h from test indentation under pre-load measured in mm.
Indenter and indent specifications	Tungsten Carbide Ball – 1mm dia. Or 5mm dia.
Load range	5 – 250kgf
Methods and applications	Plastics Testing	49.03N, 132.9N, 357.9N, 961N
	Brinell Depth	HBD 1/5, 1/10, 1/30 HBD 5/125, 5/250
	Carbon Testing	HR 5/7, 5/15, 5/20, 5/40, 5/60, 5/100, 5/150
Sample requirements	None or ground
Testing considerations	Conversion of values to other hardness methods is not the same for each material
Strengths	Rapid test, ~10 seconds Direct read-out - little/no sample preparation Tensile strength for steel can be measured reliably
Weaknesses	Not a genuine Brinell Test Many conversion tables exist - can be material dependent
SOP	Hardness on Struers DuraJet SOP (PDF, 91.5 KB)
Instruction manual	DuraJet Instruction Manual (PDF, 1.9 MB)
Resources	Standards; EN ISO 2039-1
Booking address	*G SEBE ENG Lab Eqpmt KE2.101 Rockwell Hardness Tester

Knoop

Hardness method	Knoop
Description	Calculated by measuring the size of the long diagonal of an indent left by introducing an asymmetrical pyramidal diamond with a given load into the sample material. HK = 14229 (F/L²), where F = test load in gf, and L = long diagonal length in mm.
Indenter and indent specifications	Asymmetrical (rhombic) diamond pyramid Depth of indentation = 1/30 of diagonal length Ratio between long and short diagonal 7:1
Load range	10gf - 1kgf (Micro)
Methods and applications	HV 0.01, 0.025, 0.05, 0.1, 0.2, 0.3, 0.5, 1 - Brittle materials, thin layers, coatings
Sample requirements	Must be reflective Highly polished Less than 50kg
Testing considerations	Geometric correction is available for small diameter cylindrical or ball shaped samples. Material thickness: ASTM must be ≥ 10 times the sample thickness. ISO must be ≥ 1/3 the longer diagonal length Indentation spacing must be ≥ 2.5 diagonal length from the edge and ≥ 3 diagonal length between. Inaccuracy is high when diagonal lengths are < 20µm. Indentation time = 10 – 15 seconds.
Strengths	One scale covers the entire hardness range A wide range of test forces to suit every application Testing on very small or thin samples Lower measurement error compared to Vicker's Results are mainly test force independent over 100g
Weaknesses	Requires a highly finished sample surface for accurate measurement, as measured optically Slow - testing can take 30-60 seconds (excluding sample preparation time)
SOP	Hardness on Struers DuraScan SOP (PDF, 91.0 KB)
Instruction manual	DuraScan instruction manual.pdf (PDF, 3.1 MB)
Resources	Knowledge on Knoop hardness testing Standards; ASTM E384 - https://www.astm.org/ EN ISO 4545 JIS Z 2251 Troubleshooting
Booking address	*G SEBE ENG Lab Eqpmt KE2.101 Vickers Hardness tester

Instrument factors can be categorised by applied load, indentation, indenter and others. The below table shows the different parameters of each factor that can affect hardness testing.

Factors that influence hardness testing

Instrument

Instrument factors can be categorised by applied load, indentation, indenter and others. The below table shows the different parameters of each factor that can affect hardness testing.

Applied Load	Indentation	Indenter	Others
Accuracy Repeatability	Speed Inertia Angle Time Spacing	Lateral movement Shape deviations Damage Material	Anvil, table support Spindle Deflection of the sample Levelling of the machine

Applied Load

Indentation

Indenter

Others

Accuracy

Repeatability

Speed

Inertia

Angle

Time

Spacing

Lateral movement

Shape deviations

Damage

Material

Anvil, table support

Spindle

Deflection of the sample

Levelling of the machine

Material

Measurement

Measurement factors can be categorised by procedure used, verification system and others. The below table shows the different parameters of each factor that can affect hardness testing.

Procedure Used	Verification system	Others
Applied method (HB, HK, HR, HV) Feasibility of method Standard to be followed (ASTM, ISO, JIS)	Calibration of loading system Magnification of objective lenses Resolution of objective lenses Inadequate image quality Uniformity of illumination	Vibrations Dirt, dust, debris

Procedure Used

Verification system

Others

Applied method (HB, HK, HR, HV)

Feasibility of method

Standard to be followed (ASTM, ISO, JIS)

Calibration of loading system

Magnification of objective lenses

Resolution of objective lenses

Inadequate image quality

Uniformity of illumination

Vibrations

Dirt, dust, debris

Examples of typical sources of error include:

Brighter illumination shows an indent to be smaller than it actually is, which leads to higher hardness values.
An unfocused image will cause inaccurate measurement of the diagonals, leading to an inaccurate hardness value.

Operator

Environmental

Conversion tables

There are many conversion tables that exist. Many are material dependent. Care must be taken when using conversion tables to ensure hardness values are being reported accurately.

Hardness testing

Methods

Rockwell

Brinell Depth

Knoop

Factors that influence hardness testing

Instrument

Material

Measurement

Operator

Environmental

Conversion tables

Resources