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Hardness Test

ME 215 Laboratory

Introduction

Hardness is determined by the resistance to plastic deformations and fracture of interatomic and crystalline bonds in the material, which also determines the mechanical strength of a material.

There are three standard methods for measuring the hardness which depends upon the manner in which the test is conducted.

These are:

1- Scratch hardness measurement.

2- Rebound hardness measurement.

3- Indentation hardness measurement

Indentation hardness measurement

A load is applied by pressing the indenter at right angles to the surface being tested.

The three commonly used indentation hardness tests is:

BRINELL HARDNESS TEST sphere indenter of 10 mm in dia,, varying load, measure the size of indentation Dr. J. A. Brinell invented the Brinell test in Sweden in 1900

ROCKWELL HARDNESS TEST using different scale (various sized indenter, different loads) Stanley P. Rockwell invented the Rockwell hardness test

VICKERS PYRAMID HARDNESS TEST diamond used as indenter The Vickers (HV) test was developed in England is 1925

BRINELL HARDNESS TEST

In the Brinell hardness test a hardened steel or tungsten carbide ball of diameter D is forced in to the surface of the material. The hardness is then calculated from the diameter of the indentation left in the surface, d, following removal of the indenter.

For Brinell numbers less than 450 the steel ball can be used.

For Brinell number values over 450 the tungsten carbide ball should be used.

The Brinell number is defined as:

Brinell Test Method

1. The indenter is pressed into the sample by an accurately controlled test force.
2. The force is maintained for a specific dwell time, normally 10 - 15 seconds.
3. After the dwell time is complete, the indenter is removed leaving a round indent in the sample.
4. The size of the indent is determined.
5. The Brinell hardness number is a function of the test force divided by the curved surface area of the indent.

Brinell Hardness Measurements

Material Aluminum Brass Mild Steel

P kg 15625 kg 15625 kg 15625 kg

D mm 2.5 mm 2.5 mm 2.5 mm

d mm 451.83 um 432.44 um 329.00 um

BHN kg/mm^2

Principal of the Rockwell Test

1. Select image to enlarge The indenter moves down into position on the part surface
2. A minor load is applied and a zero reference position is established
3. The major load is applied for a specified time period (dwell time) beyond zero
4. The major load is released leaving the minor load applied

The resulting Rockwell number represents the difference in depth from the zero reference position as a result of the application of the major load.

Rockwell Hardness Test

Rockwell hardness test: measures the depth of indentation

HR = E – e Material Aluminum Brass Mild Steel

Specimen Scale * Scale * Scale *

1 56.8 70.2 82.

2 54.3 66.6 88.

3 56.4 69.7 86.

HR

Vickers Test Method

All Vickers ranges use a 136° pyramidal diamond indenter that

forms a square indent

1. The indenter is pressed into the sample by an accurately controlled test force.
2. The force is maintained for a specific dwell time, normally 10 – 15 seconds.
3. After the dwell time is complete, the indenter is removed leaving an indent in the sample that appears square shaped on the surface.
4. The size of the indent is determined optically by measuring the two diagonals of the square indent.
5. The Vickers hardness number is a function of the test force divided by the surface area of the indent
6. The average of the two diagonals

is used in the following formula to calculate the Vickers hardness Docsity.com

Vickers Harness Measurements

..

Load kg

Length of diagonal (1) mm

Length of diagonal (2) mm

Average Length of diagonal

VHN

calculated by machine 1 d 1 =

d 2 =

d =

0.5 d 1 =

d 2 =

d =

0.3 d 1 =

d 2 =

d =

Average value of VHN Calculated