Hardness Test of Metals

ABSTRACT:

Indentation hardness tests such as Brinell, Rockwell, Vickers, Knoop, and instrumented indentation are frequently used methods for determining hardness. The basic concept utilized in all of these tests is that a set force is applied to an indenter in order to determine the resistance of the material to penetration. If the material is hard, a relatively small or shallow indentation will result, whereas if the material is soft, a fairly large or deep indentation will result.

INTRODUCTION:

The purpose of the following experiments is to study the hardness of different types of materials, and to understand the significance of this property in materials. The materials and the aforesaid property have significant use in engineering Technology.

Hardness is defined as a measure of a material’s resistance to deformation (i.e. small dents or scratches).

As said above, a variety of metal alloys were used in the experiments to understand how each of these metals is characterised as a hard metal and to compare the hardness of different metal alloys.

THEORY:

Hardness is usually defined as resistance of a material to penetration. Hard Marterials resist scratches or being worn out by friction with another body. Hardness is primarily a function of the elastic limit (i.e. yield strength) of the material and to lesser extent a function of the work hardening coefficient. The modulus of elasticity also exerts a slight effect on harnes

“The converse of Hardness is Softness”

THERE ARE MANY TESTS AVAILABLE TO THE HARNESS OF MATERIALS.

Some of them are following:

· ROCKWELL

· BRINELL

· VICKERS HARDNESS

Etc.

ROCKWELL HARDNESS TEST:

In addition to a Rockwell Hardness Test, there is a Superficial Rockwell. For each test, a minor load is applied to either a diamond cone or a steel ball indenter positioned on the test material’s surface to establish a zero reference position. Next, a major load is applied for a specified amount of time, leaving the minor load applied upon release. The Rockwell hardness number will be the difference in depth between the zero reference position and the indent due to the major load.

The choice of indenter is dependent upon the characteristics of the test material. The Rockwell Hardness Test applies larger minor and major load values than the Superficial Rockwell, yet both tests offer three different major load options. More than thirty different scales are used between Rockwell and Superficial Rockwell hardness testing due to the various choices and combinations of tests, indenters and major loads.

VICKERS HARDNESS TEST:

The Vickers Hardness Test can be performed on both the micro and macro hardness scales with a maximum test load of 50 kilograms. This type of hardness test is also performed by applying controlled force for a specific amount of time to an indenter, which in this case is a square-based diamond pyramid. The impression measurement and test load are used in the appropriate formula to calculate the Vickers hardness value. Like Brinell and Knoop, this method has one scale that covers its entire hardness range.

OUR JOB WAS TO OBSERVE BRINAL .SO!

OBJECTIVE:

To determine the Hardness by BRINALL HARDNESS TEST of the following specimens:

Copper
Aluminum
D2 Steel
Aluminum Bronze
Nickel Bronze

APPARATUS:

· Vernier Caliper

· Magnifying Glass

· Test Specimens

· Brinall Testing Apparatus.

BRINALL HARNESS TEST

Dr. J. A. Brinell invented the Brinell test in Sweden in 1900. The oldest of the hardness test methods in common use today, the Brinell test is frequently used to determine the hardness of forgings and castings that have a grain structure too course for Rockwell or Vickers testing. Therefore, Brinell tests are frequently done on large parts. By varying the test force and ball size, nearly all metals can be tested using a Brinell test. Brinell values are considered test force independent as long as the ball size/test force relationship is the same.

In the USA, Brinell testing is typically done on iron and steel castings using a 3000 Kg test force and a 10 mm diameter carbide ball. Aluminum and other softer alloys are frequently tested using a 500 Kg test force and a 10 or 5mm carbide ball. Therefore the typical range of Brinell testing in this country is 500 to 3000kg with 5 or 10mm carbide balls. In Europe Brinell testing is done using a much wider range of forces and ball sizes. It's common in Europe to perform Brinell tests on small parts using a 1 mm carbide ball and a test force as low as 1kg. These low load tests are commonly referred to as baby Brinell tests.

Brinell Test Method:

All Brinell tests use a carbide ball indenter. The test procedure is as follows:

The indenter is pressed into the sample by an accurately controlled test force.
The force is maintained for a specific dwell time, normally 10 - 15 seconds.
After the dwell time is complete, the indenter is removed leaving a round indent in the sample.
The size of the indent is determined optically by measuring two diagonals of the round indent using either a portable microscope or one that is integrated with the load application device.
The Brinell hardness number is a function of the test force divided by the curved surface area of the indent. The indentation is considered to be spherical with a radius equal to half the diameter of the ball. The average of the two diagonals is used in the following formula to calculate the Brinell hardness

where:

HB = the Brinell hardness number

F = the imposed load in kg

D = the diameter of the spherical indenter in mm = 10mm

d = diameter of the resulting indenter impression in mm

APPLICATIONS:

The Brinell number, which normally ranges from HB 50 to HB 750 for metals, will increase as the sample gets harder. Tables are available to make the calculation simple. Because of the wide test force range the Brinell test can be used on almost any metallic material. The part size is only limited by the testing instrument's capacity.

STRENGTH:

One scale covers the entire hardness range, although comparable results can only be obtained if the ball size and test force relationship is the same.
A wide range of test forces and ball sizes to suit every application.
“Nondestructive”, sample can normally be reused.

WEAKNESSES:

The main drawback of the Brinell test is the need to optically measure the indent size. This requires that the test point be finished well enough to make an accurate measurement.
Slow. Testing can take 30 seconds not counting the sample preparation time. Laboratory exercise: Brinell hardness test We will measure the hardness of an aluminum plate. The diameter of the spherical indenter in will be 2,5 mm, the imposed load will be 30 kgf, the time of loading will be 30 s. We will measure the indent size with the aid of optical microscope, its division is 1/10 mm.

The Hardness test machines can only be operated by the leader of practice

Select a location for the test Remove oil or dirt, but the surface does not need to be polished.
Make sure the part can be held securely.
Raise the spherical indenter according to the red mark on the scale.
Apply the 30 kgf load and wait 30 seconds Remove the load.
Measure the indent size with the aid of the optical microscope.
Calculate the Brinell hardness with the appropriate equation, use HB
Write a short report on the Brinell results. The standard way to write Brinell hardness of 45 (it is only an example) would be 45 HB.

TABLE OF OBERVATIONS & CALCULATIONS:

MATERIALS	P (Pa)	F (N) *(P0.005)**	d◦ (mm)			d (mm)	HB
MATERIALS	P (Pa)	F (N) *(P0.005)**	d1	d2	d3	d (mm)	HB
COPPER	1500	7.5	4.65	4.4	4.5	4.51	0.44
ALUMINUM	1500	7.5	3.35	3.5	3.3	3.38	0.811
NI-BRONZE	1500	7.5	3.1	3.15	3.15	3.13	0.95
D2 STEEL	1500	7.5	2.5	2.3	2.3	2.36	1.69
AL-BRONZE	1500	7.5	2	2.05	2	2.01	2.34

COMMENTS:

After examining all the different Materials we had, I observed such specific thing that Ductile or less hardend metal such as (Copper Aluminum) has the clear and more deeper penetration than the stiff or hard metal alloy such as (D2 & Ni-Bronze).