13-07-2012, 02:08 PM
Hardness Test
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Simply stated, hardness is the resistance of a material to permanent indentation. It is important to recognize that hardness is an empirical test and therefore hardness is not a material property. This is because there are several different hardness tests that will each determine a different hardness value for the same piece of material. Therefore, hardness is test method dependent and every test result has to have a label identifying the test method used.
Hardness is, however, used extensively to characterize materials and to determine if they are suitable for their intended use. All of the hardness tests described in this section involve the use of a specifically shaped indenter, significantly harder than the test sample, that is pressed into the surface of the sample using a specific force. Either the depth or size of the indent is measured to determine a hardness value.
Why Use a Hardness Test?
• Easy to perform
• Quick - 1 to 30 seconds
• Relatively inexpensive
• Non-destructive
• 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.
Hardness Scales
There are five major hardness scales:
• Brinell - HB
• Knoop - HK
• Rockwell - HR
• Shore - HS
• Vickers - HV
Each of these scales involve the use of a specifically shaped diamond, carbide or hardened steel indenter pressed into the material with a known force using a defined test procedure. The hardness values are determined by measuring either the depth of indenter penetration or the size of the resultant indent. All of the scales are arranged so that the hardness values determined increase as the material gets harder. The hardness values are reported using the proper symbol, HR, HV, HK, etc. indicating the test scale performed.
Five Determining Factors
The following five factors can be used to determine the correct hardness test for your application.
• Material - grain size, metal, rubber, etc.
• Approximate Hardness - hardened steel, rubber, etc.
• Shape - thickness, size, etc.
• Heat Treatment – through or casehardened, annealed, etc.
• Production Requirements - sample or 100%
Brinell Hardness 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 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.