Hardness Tests

1- Introduction

The fundamentals of the various hardening tests are based on a number of
different arbitrary definitions of hardness.

Some of these definitions are:-

1- Resistance to permanent indentation under static or dynamic loads (Indentation Hardness).
2- Energy absorption under impact loads (portable rebound hardness tester).
3- Resistance to scratching (Scratch Hardness).
4- Resistance to abrasion (Wear Hardness).
5- Resistance to cutting or drilling ( Machinability ).


1.2. Applicability of hardness test

The results of hardness test may be utilized as:-

1- Similar material may be graded according to hardness and a particular grade as indicated by a hardness test may be specified for some services.
It should be observed that a hardness number can not be utilized directly in design or analysis as can tensile strength for example.

2- The quality level of materials or products may be checked or controlled by hardness test.
They may be applied to determine the uniformity of samples of metals, or the uniformity of results of some heat treatment or case hardening.


3- By establishing a correlation between hardness number and some other desired properties .e.g. tensile strength. Simple hardness test

may serve to control the uniformity of the tensile strength and to indicate rapidly weather more complete tests are warranted.

Relationship between hardness number and tensile strength.

Extrapolation form empirical relations should rarely be made and then only with great cautions.

Ultimate tensile strength (in MPa) = 3.4 BHN

or Ultimate tensile strength (in kgf/mm ) = 0.346 BHN

1.3. Characteristics of hardness tests:-

1- Simplicity.
2- They are relative inexpensive.
3- Require little experience for their conduction.
4- Some times nondestructive.
5- Some hardness test may be applied for inspection of finished parts.

1.4. Type of Hardness Tests:

- The most commonly used hardness tests for metals are Brinell, Rockwell, and Vickers tests.

- However the increasing use of very hard steel and hardened steel surface has brought into use a number of other tests, such as these made with shore scleroscope, Vickers and Rockwell machine.

- Also the need for determining the hardness of very thin materials, very small size particles the hardness gradient over small distances has lead to the development of the so called micro hardness tests.

2 – Scratch Hardness Testing

Scratch hardness is the test in which a scale is set up in terms of several common materials, each of which will just scratch the material of next lower hardness number.
The scale in this test is called Moh's scale.

3. Static Indentation Hardness Tests

3.1. The Brinell hardness test
The Brinell test consists in pressing a hardened steel ball into test specimen.
It is a customary to use a ball with 10 mm in dia. loads used




3.1.1 The principal feature for hydraulic – Type Brinell machine

1. The specimen is placed on the anvil and raised to contact with bell.
2. Load is applied by pumping oil into the main cylinder, and then the main piston or plunger down word and presses the bell into the specimen .

3. The load gauge is used to give an indication of the load, when the desired load is applied the balance weight on the top the machine is lifted by action of small piston, this insures that an over load is not applied to the ball.

3.1.2 The precaution in Brinell test:

1- It is not adapted to testing extremely hard material because the ball is self deforms to match. (Max. permanent change not more than 0.01 mm by a 10 mm ball).


2- It is not used for testing thin piece, such as razor plates, because the usually indentation may be greater than the thickness of pieces.

3- It is not adapted to testing case hardened surface, because the depth of indentation may be greater than the thickness of the case, and because the yielding of the soft case invalidates the results, also for such surface the indentation is almost surrounded by a crack that may cause
fatigue failure of the part is used in service.

4- The Brinell test should not be used for parts marring of the surface of
which impairs their value (not to destroy the surface quality).

3.1.3 Brinell procedure:-

To make a test:-

1- The surface of the specimen should be flat and well polished.

2- For some specimens, the indentation may be made more distinct by using balls highly etched with HNO3.

3- In the standard test, the full load applied for 15 sec for ferrous metals and 30 sec for softer metals, after which interval the load is released and the diameter of the indentation is measured to the nearest 0.02 mm with the microscope.

4- Often, however, a 30 sec interval is used for ferrous metals and a 60 sec interval for softer metals.

5- The thickness of the specimen requires being at least 10 times the depth of the indentation.

t = depth of indentation

Minimum specimen thickness in Brinell hardness test

6- If an indentation is made too near the edge of the specimen, it may be too large and unsymmetrical, if made to close to previous one, it may be too large owing to lack of sufficient supporting material or too small owing to work hardening of the first indentation.

7- The error may be neglected if the distance of the indentation to the edge of the specimen or to the center of adjacent indentation is equals or greater than 2.5 times the diameter of the indentation.

d = diameter of indentation
Indentation-to-indentation or indentation-to-edge distance.

8- If the compressive properties of a flat specimen are not uniform owing perhaps to direction of rolling or to cooling stresses, a non circular indentation will result. In this case, the average B.H. of the material may be obtained if the diameter is taken as the average in four directions roughly 45° apart.

9- To measure the B.H.N. for a curved surface the indentation having uniform properties but not have a circular boundary, for radius of the specimen not less than 25 mm, the diameter of indentation may be taken as the smaller radii ( smaller than 25 mm ) a flat spot may be prepared on the surface of the specimen.

3.1.3 The parameter found by a B.H. test.

Brinell hardness number

It is the pressure per unit area [Kg/mm2 ] of the indentation remaining after the load is removed.

It is obtained by dividing the applied load by the area of the surface of the indentation which assumed to be spherical.

B.H.N = load on ball / area of indentation



-For small thin specimen, it is sometimes necessary to make B.H test with a ball less than 10 mm diameter.

Examples:

1- Ordinary steel with 3000 Kg load the range BHN from 100 to 500.
2- For medium carbon structural steels (0.23 % C) BHN (130-160)
3- For very hard special steels BHN is 800 – 900. It is not
recommended to use Brinell hardness test for hard materials with BHN more than 650.
4- Cu-alloys BHN 50 – 160 and for pure Cu BHN is less than 50.
5- Al- alloys BHN is 30 – 160 and for Sn-alloys BHN is 10 – 30 and
for Pb-alloys BHN is 3 – 10.

3.1.4 Source of errors and precautions in B.H. test

1) A rapid rate of applying the load affects the diameter of the indentation in two ways:
a) The effect of inertia of the piston and weights and the friction of
the plunger cause rise of the load above 3000 Kg and enlarge the
indentation
b) A rapid rate of penetration allow, less time for the plastic flow of
the material, resulting in decreasing the size of indentation.

- The flow for most metals is quite rapid during the first 30 sec under required load. It is much less rapid in the interval from30–120 sec.

- The BHN for most materials vary, less than 1 % for loading intervals between 30 and 120 sec.

- Error due to inertia of the plunger can be very much greater than
that due to an insufficient period of sustaining the load to permit
plastic flow to occur.

2- The error in the BHN is less than 1 % as long as the error in diameter does not exceed 0.01 mm

The error in reading the diameter of indentation may be due to:

a) an error in reading the microscope.

b) indefiniteness of the boundary of the indentation. The error in reading a modern Brinell microscope should not exceed 0.02 mm.

3- The variations from standard size and shape of modern well made balls are usually too low. However, the flattening of the ball particularly when the hardness of the specimen approaches that of the ball may be serious errors.

For testing a materials with BHN greater than about 400, the ball should frequently checked for distortion, and for testing materials BHN over 450, balls of harder material than steel should be used (tungsten carbide WC balls) for BHN values up to 630, which is the upper limit of
application of Brinell hardness testers .

3.2 ROCKWELL TEST

3.2.1 The main difference between Brinell and Rockwell test

1) Rockwell test is used for materials which have a high hardness.
2) The resulting indentation in Rockwell test is smaller.
3) The indenter and the loads are smaller than that of the Brinell test.
4) Rockwell test is faster than Brinell test, as it gives direct reading.
5) In Rockwell the depth is taken as a measure of the hardness number while in Brinell the indentation diameter id the base of the BHN.

3.2.2 Rockwell is similar to Brinell in that:

1) It is an indentation hardness test.
2) The load used is a static load.
3) The hardness number is inversely proportional to the indentation
depth. In the field of metal testing there are two main methods to measuring the Rockwell hardness of a metal namely Rockwell-C method Rockwell-B method.

3.2.3 Rockwell – C method (RC)

- It's applied for hard materials.
- The indenter is a diamond cone with a rounded apex having an apex
angle of 120° and the radius of rounding is 0.2mm.
- The total load used is usually 150 kg (10 kg minor load + 140 kg major load)

- The hardness found in this case has the notation HRC (C = cone).
- The useful range is between HRC 20 and HRC 70.

3.2.4 Rockwell – B method (RB):-

- It is used for relatively softer materials.
- The indenter is a steel ball having 0.16 mm or 3.6 mm diameter.
- Major loads usually 60, 100 and 150 kg are used.
- The useful range is between HRB 0 and HRB 100.
Rockwell carbide ball indenters with different ball diameter

3.2.5 Procedure of Rockwell test

1- A major load of 10 kg is first applied which causes an initial indentation depth (h1) that sets the indenter in the material and holds it in position.

2- The dial is set at zero mark, after that the major load is applied to make an indentation depth of (h2- elastic + plastic).

3- The major load is removed while the minor load is still in position, the residual is plastic deformation.

4- h3 is the plastic depth due to the major load. The hardness number is calculated from the knowledge of h3.

HRB = 130 – depth of indentation / 0.002
HRC = 100 – depth of indentation / 0.002

The A scale is usually provided with a diamond ball of 1.6 mm diameter and a major load of 60 Kg. This scale is usable for soft materials (annealed brass) to hard materials (cemented carbide).

3.2.6 Rockwell precautions

1- The test surface should be flat and free from scales, oxide films, pits, and foreign materials that may affect the result.

2- Oiled surfaces generally give slightly lower reading than dry ones because of reduced friction under the indenter.

3- The bottom surface should free from scale, dirt or other foreign materials that might crush or flow under the test pressure and so affect the result.

4- The thickness of piece tested should have no bulge pieces or other marking opposite the indentation. For hard materials the thickness may be little as about 0.025 mm.

5- All hardness tests should be made on a single thickness of the materials, regardless of the thickness of the piece. The use of more than one piece of thin materials to give a adequate thickness, does not yield the same result.

6- By testing curved surface, small flat spot should be prepared before making the indentation and correction are to be added to the observed Rockwell values of cylindrical specimens having diameters 0.64 to 3.8 mm from special tables.

7- If the table on which the Rockwell hardness tested is mounted is subjected to vibrations, the hardness number will be too low, since the indenter will sink more in the material than when such vibration are absent.

8- If curved plates are tested the concave side should face the indenter to avoid the flattening of the piece on the anvil.

9- It is advisable to check the indenter regularly to see that they don’t become blunted or shipped.


3.3 Rockwell superficial hardness tester

This tester is special purpose machine. The common uses of Rockwell
superficial are:
1- When a very shallow indentation is required.
2- To know the hardness of the specimen close to the surface.

Special uses
1- For testing the nitride steel, softy razor blades, and lightly carburized pieces.

2- For testing brass, bronze and steel sheet.

ROCKWELL- T

- 0.16 mm steel bell diameter, Used for testing thin sheets.
- 15, 30, or 45 kg load.

ROCKWELL- N

- diamond ball 0.16 mm,
- 15, 30, or 45 kg load.
- Unhardened materials, such as metals softer than hardened steel, or where shallow indentations are desired.

The relative sizes of the indentations made by Brinell (B), ordinary Rockwell C (C) and Rockwell superficial hardness tester (A) are illustrated in the following figure.


A = superficial indentation
- Each hardness number = 0.001 mm depth.
- Minor load = 3kg and major load 15, 30 or 45 kg.
- Diamond cone.

B = Indentation of Brinell Hardness tester
- 3000 kg load
- Depth of indentation ~ 0.025 mm for one point of the hardness number.
- Steel ball 10 mm diameter.

C = indentation of the common Rockwell C
- Total load = 150 kg, - Diamond cone.
- Depth of indentation ~ 0.013 mm for one point of the hardness number.