Brinell hardness Tester

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 3000Kg test force and a 10mm diameter carbide ball. Aluminum and other softer alloys are frequently tested using a 500Kg 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 1mm carbide ball and a test force as low as 1kg. These low load tests are commonly referred to as baby Brinell tests.

Standards

Brinell Test methods are defined in the following standards:

• ASTM E10
• ISO 6506

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.

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. A typical Brinell hardness is specified as follows:

356HBW

Where 356 is the calculated hardness and the W indicates that a carbide ball was used. Note- Previous standards allowed a steel ball and had an S designation. Steel balls are no longer allowed.

Applications

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.

Strengths

1. 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.
2. A wide range of test forces and ball sizes to suit every application.
3. Nondestructive, sample can normally be reused.

Weaknesses

1. 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.
2. Slow. Testing can take 30 seconds not counting the sample preparation time.

Rockwell Hardness Test

Rockwell hardness testing system is the workhorse of hardness testing - designed to meet the toughest challenges: clamp large parts, operate in poor environments, and test high volumes - all with less service problems and less operator skill requirements.

Vickers hardness test

The Vickers hardness test was developed in 1924 by Smith and Sandland at Vickers Ltd as an alternative to the Brinell method to measure the hardness of materials. The Vickers test is often easier to use than other hardness tests since the required calculations are independent of the size of the indenter, and the indenter can be used for all materials irrespective of hardness. The basic principle, as with all common measures of hardness, is to observe the questioned material's ability to resist plastic deformation from a standard source. The Vickers test can be used for all metals and has one of the widest scales among hardness tests. The unit of hardness given by the test is known as the Vickers Pyramid Number (HV) or Diamond Pyramid Hardness (DPH). The hardness number can be converted into units of pascals, but should not be confused with a pressure, which also has units of pascals. The hardness number is determined by the load over the surface area of the indentation and not the area normal to the force, and is therefore not a pressure.

The hardness number is not really a true property of the material and is an empirical value that should be seen in conjunction with the experimental methods and hardness scale used. When doing the hardness tests the distance between indentations must be more than 2.5 indentation diameters apart to avoid interaction between the work-hardened regions.

The yield strength of the material can be approximated as:

.

where c is a constant determined by geometrical factors usually ranging between 2 and 4.

Types of Hardness Test

Four types of hardness :

1. Brinell
Indenter : 10 mm sphere of steel or tungsten carbide

2. Vickers
Indenter : Diamond pyramid

3. Knoop microhardness
Indenter : Diamond pyramid

4. Rockwell
A, C, D Indenter : Diamond cone
B, F, G Indenter : 1/16 in diameter steel sphere
E Indenter : 1/8 in diameter steel sphere

Definition of Hardness Tester

The hardness test measures the resistance of a material to an indenter or cutting tool. The indenter is usually a ball, pyramid, or cone made of a material much harder than that being tested. A load is applied by slowly pressing the indenter at right angles to the surface being tested for a given period of time. An empirical hardness number may be calculated from knowledge of the load and the cross-sectional area or depth of the impression.
Tests are never taken near the edge of a sample nor any closer to an existing impression than three times the diameter of that impression. The thickness of the specimen should be at least ten and one-half times the depth of the impression.