Hardness Test: Everything You Need to Know

1. Overview

Hardness: the ability to resist local indentation deformation or scratch fracture.

Two kinds of Mohs hardness sequence tables

Order

Material

Order

Material

1

talc

1

talc

2

gupse

2

gupse

3

calcite

3

calcite

4

fluorite

4

fluorite

5

apatite

5

apatite

6

orthoclase

6

orthoclase

7

quartz

7

SiO2 glass

8

topaz

8

quartz

9

corindon

9

topaz

10

adamas

10

garnet

11

Fused zirconia

12

corindon

13

silicon carbide

14

Carbonization shed

15

diamond

2. Brinell hardness

(1) Principle

Press the spherical indenter with diameter D into the metal surface with a load F of a certain size, and keep it for a period of time to form a spherical indentation.

The load value that can be borne on the unit indentation area is the Brinell hardness of the metal material.

Measuring indentation diameter

Indenter material:

  • Hard alloy ball (HBW) HB=450~650
  • Hardened steel ball (HBS) HB<450

(2) Representation method

For example: 280HBS10/3000/30
1kgf=9.81N

  • 280 – Hardness value
  • HBS – Hardness symbol
  • 10 – Diameter of steel ball mm
  • 3000 – Load size kgf
  • 30 – Load holding time s

General conditions: 10mm steel ball diameter; 3000kg load; 10s pressure holding time, namely HB280

(3) Test steps

(4) Selection of F and D (principle of geometric similarity of indentation)

When measuring Brinell hardness with indenters of different diameters and loads of different sizes, the principle of geometric similarity must be met to obtain the same HB value, that is, the opening angleφ of the indentation is equal.

Method: The same HB shall be measured for samples with the same material but different thickness, or materials with different hardness and softness.

When selecting D and F, F/D2 shall be the same.

Principle of geometrical similarity of indentation:

It can be seen that as long as F/D remains constant, HB only depends on the pressing angle φ.

F/D2 ratio: 30,15,10,5,2.5,1.25,1

According to the engineering regulations, the ratio of F/D2 is 30, 10 and 2.5, which are selected according to the material hardness and sample thickness.

See various standards and test specifications for details.

Fig. 1-21 Application of similarity principle

Selection Table of Brinell Hardness Test P/D2

Material type

Brinell hardness number/HB

Sample thickness/mm

Relationship between load P and indenter diameter D

Diameter of indenter D/nm

Load P/kgf

Load holding time/s

Ferrous metal

140~450

6~3

4-2

 <2

P=30D2

10

5

2.5

3000

750

187.5

10

<140

>6

 6~3

<3

P=10D2

10

5

2.5

1000

250

62.5

10

Nonferrous metals

>130

6~3

4-2

<2

P=30D2

10

5

2.5

3000

750

187.5

30

36~130

9~3

6~2

<3

P=10D2

10

5

2.5

1000

250

62.5

30

8-35

>6

6~3

<3

P=2.5D2

10

5

2.5

250

62.5

15.6

60

The experiment shows that HB is stable and comparable when 0.25D<d<0.5D.

(5) Load holding time:

If it has influence on the test, it shall be carried out in strict accordance with the regulations, generally 10s and 30s.

(6) Characteristics and Application of Brinell Hardness

With large indentation area and high measurement accuracy, it is suitable for coarse or heterogeneous materials;

The indentation is large, and the finished product inspection is difficult.

It is mainly used for raw material inspection:

The indenter material is limited to the softer material (HB450~650);

The efficiency of indentation measurement is low.

3. Rockwell hardness

Test principle: Use indentation depth to reflect material hardness.

In order to use the same hardness tester to adapt to different soft and hard materials, different indenters and loads are used in many grades.

Common Grade C, HRC: the total load is 150kgf, and the 120 ° diamond cone indenter is loaded twice.

First, the initial load P1=10kgf is added to make the indenter contact the material surface well, and then the main load P2=140kgf is added.

After removing P2, the indentation depth is h.

Fig. 3-17 Schematic Diagram of the Principle and Test Process of Rockwell Hardness Test

(a) Add preload (b) Add main load (c) Unload main load

Hardness symbolHead usedTotal test force NScope of applicationApplied range
HRADiamond cone588.420-88Carbide, hard alloy, quenched tool steel, shallow case hardening steel
HRBφ 1.588mm steel ball980.720-100Mild steel, copper alloy, aluminum alloy, malleable cast iron
HRCDiamond cone147120-70Quenched steel, quenched and tempered steel, deep case hardened steel

Indenter: 120 diamond cone or hardened steel ball

Rockwell hardness definition:

0.002mm residual indentation depth is a Rockwell hardness unit.

K – constant, 130 for steel ball indenter and 100 for diamond indenter

Table 3-6 Test Specification and Application of Rockwell Hardness

Ruler

Type of indenter

Initial test force/N

Main test force/N

Total test force/N

Constant K

Hardness range

application examples

A

Diamond circular dimension

100

500

600

100

60~85

High hardness thin parts and cemented carbides

B

φ1.588mm steel ball

900

1000

130

25~100

Non ferrous metals, malleable cast iron and other materials

C

Diamond circular dimension

1400

1500

100

20~67

Heat treated structural steel and tool steel

D

Diamond cone

900

1000

100

40-77

Surface hardened steel

E

φ3.175mm steel ball

900

1000

130

70~100

Plastic

F

φ1.588mmm steel ball

500

600

130

40~100

Non ferrous metals

G

φ1.588mm steel ball

1400

1500

130

31~94

Pearlitic steel, copper, nickel, zinc alloy

H

φ3.175mm steel ball

500

600

130

Annealed copper alloy

K

φ3.175mm steel ball

1400

1500

130

40~100

Non ferrous metals and plastics

Soft metal and non-metallic soft materials

High hardness thin parts and cemented carbides

Non ferrous metals, malleable cast iron and other materials

L

φ6.350mm steel ball

500

600

130

M

φ6.350mm steel ball

900

1000

130

P

φ6.350mm steel ball

1400

1500

130

R

φ12.70mm steel ball

500

600

130

Heat treated structural steel and tool steel

S

φ12.70mm steel ball

900

1000

130

V

φ12.70mm steel ball

1400

1500

130

Characteristics and Application of Rockwell Hardness

() Direct reading of hardness value, high efficiency, suitable for batch inspection;

(2) The indentation is small and basically “nondestructive”, which is suitable for finished product inspection;

(3) The indentation is small, and the representativeness is poor, which is not suitable for coarse or non-uniform materials;

(4) It is divided into various scales and has a wide range of applications;

(5) The Rockwell hardness values of various scales are not comparable.

4. Vickers hardness

1. Principle

Press a diamond pyramid into the metal surface with a certain load F to form a pyramid indentation.

The load value on the unit indentation area is the Vickers hardness of the metal material.

When the unit of test force F is kgf:

When the unit of test force F is N:

Indenter material: diamond pyramid with an included angle of 136 °

2. Representation method

For example: 270HV30/20, if the holding time is 10-15s, it can be recorded as 270HV

  • 270 – Hardness value
  • 30 – Load size kgf
  • 20 – Load holding time s

3. Microhardness

Vickers hardness with very small load, the load is 5-200gf.

Indicated by Hm, it can be used to test the hardness of single grain or phase.

Vickers hardness test

Low load Vickers test

Micro Vickers hardness test

Hardness symbol

Test force/N

Hardness symbol

Test force/N

Hardness symbol

Test force/N

HV5

49.03

HVO.2

1.961

HVO.01

0.09807

HV10

98.07

HVO.3

2.942

HVO.015

0.1471

HV20

196.1

HVO.5

4.903

HVO.02

0.1961

HV30

294.2

HV1

9.807

HVO.025

0.2452

HV50

490.3

HV2

19.61

HVO.05

0.4903

HV100

980.7

HV3

29.42

HVO.1

0.9807

Note: 1. The Vickers hardness test can use a test force greater than 980.7N;

2. The micro Vickers test force is recommended.

Characteristics and Application of Vickers Hardness

(1) The geometrical shape of the indentation is always similar, and the load is optional;

(2) The corner cone indentation contour is clear and the measurement accuracy is high;

(3) The diamond indenter has a wide range of applications and can provide continuous and consistent hardness scales for various materials:

(4) The indentation measurement efficiency is low, and it is not suitable for on-site batch inspection;

(5) The indentation is small and not suitable for coarse or heterogeneous materials;

However, if the metallographic specimen is made, the hardness or hardness distribution of various phases can be measured.

5. Improvement of hardness strength relationship and test method

(1) Hardness test characteristics

① The stress state is very soft (α>2), which is widely applicable;

Hardness of some materials

Material

Condition

Hardness/(kgf/mm ²

Metallic Materials

99.5% aluminum

annealing

20

 

 

cold rolling

40

 

Aluminum alloy (A-Zn Mg Cu)

Mild steel (tc=0.2%)

annealing

60

 

 

Precipitation hardening

170

 

Bearing steel

Aluminum alloy (A-Zn Mg Cu)

normalizing

120

 

 

cold rolling

200

 

Mild steel (tc=0.2%)

normalizing

200

 

 

Quenching (830 ℃)

900

 

 

Tempering (150 ℃)

750

ceramic materials

WC

agglutination

1500~2400

 

Cermet (Co=6%, allowance WC)

20℃

1500

 

 

750℃

1000

 

Al2O3

 

~1500

 

B4C

 

2500~3700

Material

Condition

Hardness/(kgf/mm ²

BN (cubic meter)

 

7500

diamond

 

6000-10000

Glass

 

 

Silica

 

700-750

Soda lime glass

 

540~580

optical glass

 

550-600

Polymer

 

 

High pressure polyethylene

 

40-70

Phenolic plastic (filler)

 

30

polystyrene

 

17

organic glass

 

16

polyvinyl chloride

 

14~17

ABS

 

8-10

polycarbonate

 

9-10

Polyoxymethylene

 

10~11

Polytetraethylene oxide

 

10~13

polysulfone

 

10~13

Covalent bond ≥ ionic bond>metal bond>hydrogen bond>Van’s bond

② The method is simple, nondestructive and suitable for field inspection;

③ The physical meaning is not clear, and it is difficult to design quantitatively.

(2) Relationship between hardness and strength

σb≈KH

Steel: K=0.33~0.36

Copper alloy, stainless steel, etc.: K=0.4~0.55

Relationship between hardness and strength of annealed metals

Name of metal and alloy

HB

σb/MPa

k(σb/HB)

σ-1/MPa

σ(σ-1/HB)

Non ferrous metals

Ferrous metal

Non ferrous metals

copper

47

220.30

4.68

68.40

1.45

aluminium alloy

138

455.70

3.30

162.68

1.18

Duralumin

116

454.23

3.91

144.45

1.24

Ferrous metal

Industrial pure iron

87

300.76

3.45

159.54

1.83

20 steel

141

478.53

3.39

212.66

1.50

45 steel

182

637.98

3.50

278.02

1.52

18 Steel

211

753.42

3.57

264.30

1.25

T12 steel

224

792.91

3.53

338.78

1.51

1Cr18Ni9

175

902.28

5.15

364.56

2.08

2Cr13

194

660.81

3.40

318.99

1.64

Note: Unit of hardness!

(3) Nano indentation test

During the loading process, the elastic deformation first occurs on the surface of the specimen, and with the further increase of the load, the plastic deformation starts to appear and gradually increases;

The unloading process is mainly the process of elastic deformation recovery, and the plastic deformation finally makes the sample surface form an indentation.

Load displacement curve of nano indentation

Principle of nano indentation test

  • H – Nano hardness;
  • S – Contact stiffness;
  • A – Contact area;
  • β – Constants related to the geometry of the indenter;
  • Er – equivalent modulus

There are important differences between nano hardness and traditional hardness:

First of all, the two definitions are different.

Nanohardness: the instantaneous force borne by a unit area on the projection of the surface area of the base indentation during the indentation process of the sample, which is a measure of the sample’s ability to withstand the contact load;

Vickers hardness is defined as the average force per unit area on the surface area of the indentation retained after the unloading of the indenter, which reflects the ability of the specimen to resist linear residual deformation.

In the process of hardness measurement, for the process dominated by plastic deformation, the results of the two definitions are similar: for the contact process dominated by elastic deformation, the results will be different.

In the process of pure elastic contact, the residual contact area is very small, so the traditional hardness definition will lead to infinite hardness, and the true hardness value of the sample cannot be obtained.

Secondly, the measurement ranges of the two are different.

Traditional hardness measurement is only applicable to large size samples: not only because of the limitations of the measuring instrument itself, but more importantly, when the indentation is small to the micro nano scale, the residual indentation can no longer correctly reflect the true hardness of the sample.

New measurement techniques and calculation methods are used for nano hardness measurement, which can more accurately reflect the hardness characteristics of the sample at the micro nano scale.

The important difference between the two is the calculation of indentation area.

The nano hardness is to measure the indentation depth and then calculate the contact area according to the empirical formula;

The traditional hardness measurement method is to obtain the surface area of the indentation according to the photos of the indentation after unloading.

(4) Nanoindentation test method

The basic components of nano hardness tester can be divided into several parts, such as control system, moving coil system, loading system and indenter.

The indenters are generally all diamond indenters, which are divided into triangular cones or four-edge dimensions.

During the test, the initial parameters are input first, and the subsequent detection process is completely controlled by the microcomputer automatically.

By changing the current in the moving coil system, the loading system and the action of the indenter can be manipulated.

The measurement and control of the indenter pressing load are completed by the strain gauge.

At the same time, the strain gauge also feeds back the signal to the moving coil system to achieve closed-loop control, so that the test can be completed according to the input parameter settings.

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