Disadvantages of rockwell hardness test

MIME 1650 Laboratory 4

Hardness Test

Objective

The student will learn to measure the hardness of materials using the Brinell Test, the

Rockwell Test and Micro-hardness Vickers Test techniques.

Agenda

• Introduction to Hardness testers by the TA • Perform Brinell hardness tests on the provided materials • Perform Rockwell hardness tests on the provided materials • Perform Vickers micro-hardness tests on the metallographic weld specimen

Report Requirement

1. Why should a different testing device be used if the Brinell hardness is above 630?

2. Explain the limitations of Brinell and Rockwell testers?

3. Why was it necessary to grind the sample being used in the Rockwell tester but not in the Brinell tester?

4. If you want to measure the hardness of the side of a cylinder, would you use a Brinell or Rockwell tester?

5. Calculate the Brinell Hardness Number (HB) using the formula mentioned in the third page of this handout. Compare the calculated value and tabled value.

6. What are results in Nugget, Heat Affect Zone and Base Metal in hardness?

Instruction

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.

Resistance to scratching or cutting has probably long assessed hardness of materials. 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 many standard test methods for expressing the relationship between hardness and the

size of the impression, such as Brinell, Vickers, Rockwell, Shore, Knoop, etc. 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.

In this lab students will perform Brinell Test, Rockwell Test and Vickers Micro hardness

Test.

Brinell Hardness Test

This is a static test that measures a material’s resistance to surface penetration and gives a

measure of the metal’s surface plasticity or the resistance to permanent deformation. The

Brinell Hardness Test, commonly used for metallic materials, determines hardness by

applying a known load (3000, 1500, or 500 kgf) to the surface of the test specimen via a

hardened steel or diamond ball of prescribed size (10 mm in diameter).

The size of the permanent impression on the specimen surface is converted into a Brinell

Hardness Number (HB) according to the formula:

)( 2

)(mmnIndentatioofAreaSurface

Load(kgf)

22 2

dDD D

P HB

−− π

==

Where D: Diameter of the ball indenter

d: Diameter at the rim of the permanent impression

P: Load

Force

d

D: Indenter Diameter

d: Indentation Diameter Indenter

Specimen

D

Fig. 1 Schematic of the principle of the Brinell indentation process

1 2 3 0

Fig. 2 Brinell indentation with measuring scale in millimeters

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 10

mm diameter hardened steel with a 500-kilogram load applied for a period of 30 seconds.

A measurement is considered valid if the diameter of the permanent impression is in the

range of 2.5 to 4.75 mm (slightly exceeding this limit is tolerated). As a result, the 3000

kgf load yields HB measures between 160 and 600; the 1500 kgf load, HB 80 to 300; and

the 500 kgf load, HB 26 to 100. Smaller loads (e.g., 250, 125, 100 kgf) can be used for

softer metals.

According to the ASTM (American Society for Testing and Materials) Standard E10-66,

a steel ball may be used up to HB 450, and carbide may be used up to HB 630. It is NOT

recommended to use the Brinell test on materials harder than HB 630. These limits were

set to avoid errors introduced by the deformation of the ball indenter itself. Another

precaution when using the Brinell test is that the minimum thickness of the test sample

should be at least 10 times the depth of penetration of the ball. This implies that the

metals less than 8 to 10 mm (3/8 inch) should not be used under the 3000 kgf load. If

thinner samples are tested, the hardness of the anvil affects the results.

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 F1 is applied with

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

additional major load F1 is removed but the preliminary minor load F0 is still maintained.

Removal of the additional major load F1 allows a partial recovery, 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.

eEHR −=

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

Fig. 3. Rockwell Principle

There are two different types of Rockwell Hardness Testers. One is Normal Rockwell

Hardness Tester. Another is the Superficial Rockwell Hardness Tester. The Superficial

tester was developed for thinner specimens. The Rockwell Normal test should not be

used on thin materials (less than 1/16 in), on rough surfaces, or on materials that are not

homogeneous, such as gray cast iron. This is due to the fact that the indenters are small

and any variation in surface roughness or composition could give drastically different

readings. If a hardness measurement is needed for a thin material, or on a material where

a small indentation is desired then the superficial tester should be used.

When reporting Rockwell hardness number it is important to note which scale you

measured on. Rockwell B hardness of 60 (HRB 60), which corresponds approximately to

a tensile strength 50 ksi, is considerably different than a Rockwell C hardness of 60

(HRC 60), which corresponds to a tensile strength of 311 ksi. The proper way to state a

Rockwell C hardness of 60 is HRC 60. The range of accuracy for Rockwell hardness

numbers is between 0 and 100. Rockwell numbers close to 0 or 100 should be checked on

a different scale to verify that they do not exceed this range.

Typical Application of Rockwell Hardness Scales

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

HRB . . .. Copper alloys, soft steels, aluminum 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, aluminum and magnesium alloys, bearing metals

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

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

HRH . . .. Aluminum, 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).

Rockwell Hardness Scales

Table 1

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

Micro-hardness Test

The term micro-hardness 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 accuracy of +0.5 micrometers. Also with the same observer

differences of +0.2 micrometers can usually be resolved. It should, however, be added

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

1. Vickers Test

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

surface area of the indentation (mm 2 )

22

0

854.12

136 sin2

d

F

d

F

HV ≅=

where:

F= Load in kgf

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

HV = Vickers hardness

Fig 4 Vickers Pyramid Diamond Indenter Indentation

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

with an angle of 136 o

between 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.

2. Knoop Test

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

to the unrecovered projected area A (mm 2 )

2 CL

F

A

F KHN ==

where:

F = applied load in kgf

A = the unrecovered projected area of the indentation in mm 2

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.

Fig 6 Knoop Hardness Indenter Indentation

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.

Procedures:

To Rockwell test:

1. Clear the sample surface. If surface is rough, grind it. 2. Select the appropriate indenter and load for HRB or HRC (if in doubt choose brale,

then change if necessary). Record indenter type and load.

3. Apply the minor load (preload) by slowly raising the anvil until the contact between the indenter and sample surface is made.

4. Adjust the zero (set point) by turning the collar wheel so both needles reach zero. If you go past zero, you must lower the part and start again.

5. Apply the major load (test load) by releasing the load lever arm. Allow the needle to come to a complete stop. This may take 10 to 15 seconds

6. Read appropriate scale and record the hardness value in the appropriate scale. 7. Unload by lowering the anvil. 8. Repeat the measurement on the flat and round surfaces for three trails.

To Brinell Test:

1. Adjust load to desired amount 2. Place sample on anvil 3. Raise until sample is touching 10 mm ball 4. Pull load handle and HOLD for 15-20 seconds, record load 5. Unload by raising lever and lowering sample 6. Measure diameter of indentation in measuring device 7. Repeat the measurement for three trials 8. Use two ways to get Brinell Hardness Number (HRB): 9. Check from the attached table. 10. Use formula to calculate.

To Vickers Micro-hardness Test:

1. Test Parameters

• Load: 200 g • Dwell Time 15 sec

2. Tester Setup

• Switch the ‘STAND BY’ on. (Right switch on tester back) • Setup: Press DWELL/ENTER to display test parameters

D1: Model 1 D2: 15 seconds

Press DWELL/ENTER to change settings then press ENTER.

Don’t change the settings if they are already set correctly.

Please don’t change other model settings.

• Turn LOAD SELECTOR KNOB to 200 gf. • Align the two measuring lines, press the RESET button and hold for 1

second to zero out the display.

• Press HV/HK button so that the HV light is on. • If you want to change lighting, hold LIGHT and press INC or DEC to

desired lighting. Then press ENTER to fix it.

CAUTION: Before you operate the tester, double check parameters.

3. Install Sample

• Install the sample to INCLINING DEVICE making sure if it fits securely.

CAUTION: Be careful; don’t hit the microscope lens or indenter.

4. Indenting

• Use10× lens to determine test point and focus your sample. • Always Keep the MAIN LINE in the center of the circle. • Test Points: There are three different locations in your specimen. They are

Weld Nugget, Heat Affect Zone and Base Metal. In each zone make three

indentations. The distance between two indentations must be at least 4

times more than the diagonal length.

• After you choose the test point, press START.

5. Measurement

• Press the TURN button to view the indentation with the 50X lens. (Make sure lens will not hit sample when it turns.)

• Measuring an indentation: o Adjust the inner edge of main line to touch the left edge of the

indentation.

o Turn the right knob to adjust the outer edge of the secondary line to touch the right edge of the indentation to get D1. (See following

figures)

o Press the READ button. D1

o Rotate the eyepiece 90 degree and use the same procedure to measure D2.

Main Line

Circle in Eyepiece

Indentation

D2

o Press READ button again. You will get a HV value in the front panel.

o Measure three times and record the values.

• Repeat step 4 and 5 until you get all of 9 indentations and hardness data. • Take the sample out. • Switch off the STAND-BY and cover tester.

Data Sheet

Brinell Test Date

Rockwell Test (Normal)

Date

Diameter of Indentation (d)

(mm) HB Load

Kg Material

1 2 3 Average Table Formula

Reading

Scale Load

Kg

Indenter Material

1 2 3 Average

Hardness

Data Sheet

Micro-hardness Vickers Test

Load ………………. gf

Dwell time …………. sec

Date …/…/…

HV Reading

Location I II III Average

HV

Nugget

Heat Affect Zone

Base Metal