Hardness is defined as the “Resistance to Deformation”. When doing a hardness test, the hardness tester applies a fixed load and the depth of penetration is measured by different means and converted to different units. More simply put, when using a fixed force (load) and a given indenter, the smaller the indentation, the harder the material. Indentation hardness value is obtained by measuring the depth or the area of the indentation using one of over 12 different test methods.
Why do a hardness test?
- Easy to perform
- Quick – 1 to 30 seconds
- Relatively inexpensive
- Finished parts can be tested – but not ruined
- Virtually any size and shape can be tested
- Practical QC device – incoming, outgoing
The most common uses for hardness tests is to verify the heat treatment of a part and to determine if a material has the properties necessary for its intended use. Establishing a correlation between the hardness result and the desired material property allows this, making hardness tests very useful in industrial and R&D applications.
The Brinell Test
The Brinell test was invented in Sweden in 1900 by Dr. J. A. Brinell. This is the oldest form of hardensss testing used today. It is frequently used to determine the hardness of forgings and castings that have a grain structure too course for Rockwell testing. Brinell is frequently done on large parts. The values are considered test force independent as long as the ball size/test force relationship is the same. By varying the test force and ball size, nearly all metals can be tested using a Brinell test. A typical range of Brinell testing in this country is 500 to 3000kg with 5 or 10mm carbide balls. The test methods are defined in the following standards, ASTM E10 and ISO 6506. The indenter is pressed into the sample by an accurate 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. 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.
- 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.
- 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.
- This is a rather slow test. Testing can take 30 seconds not counting the sample preparation time.
The Rockwell Test
The Rockwell test was invented by Stanley P. Rockwell. He was a metallurgist for a large ball bearing company and he wanted a fast non-destructive way to determine if the heat treatment process being performed on the bearing races was successful. At the time the only tests available were Vickers, Brinell and Scleroscope. The Vickers test was time consuming, Brinell caused large indents for the parts and the Scleroscope was difficult to use on smaller parts. Thus the Rockwell test was invented. The simple sequence of tests force application proved to be a major advance in the world of hardness testing. It allowed technicians to test on a variety of sized parts in a short period of time. Rockwell test methods are defined in the following standards:
- ASTM E18 Metals
- ISO 6508 Metals
- ASTM D785 Plastics
There are two types of Rockwell tests:
Rockwell: the minor load is 10 kgf, the major load is 60, 100, or 150 kgf.
The Superficial Rockwell: the minor load is 3 kgf and major loads are 15, 30 or 45 kgf.
In both tests the indenter can be either a diamond cone or steel ball, depending on the characteristics of material being tested. Rockwell hardness values are expressed as a combination of a hardness number and a scale symbol representing the indenter and the minor and major loads. The hardness number is expressed by the symbol HR and the scale designation. There are 30 different scales. The majority of applications are covered by the Rockwell C and B scales for testing steel, brass, and other metals. However, the increasing use of materials other than steel and brass as well as thin materials necessitates a basic knowledge of the factors that must be considered in choosing the correct scale to ensure an accurate Rockwell test. The choice is not only between the regular hardness test and superficial hardness test, with three different major loads for each, but also between the diamond indenter and the 1/16, 1/8, 1/4 and 1/2 in. diameter steel ball indenters. If no specification exists or there is doubt about the suitability of the specified scale, an analysis should be made that control scale selection.