Vickers Test

Vickers Test
The Vickers (HV) test was developed in England is 1925 and was formally known as the Diamond Pyramid Hardness (DPH) test. The Vickers test has two distinct force ranges, micro (10g to 1000g) and macro (1kg to 100kg), to cover all testing requirements. The indenter is the same for both ranges therefore Vickers hardness values are continuous over the total range of hardness for metals (typically HV100 to HV1000). With the exception of test forces below 200g, Vickers values are generally considered test force independent. In other words, if the material tested is uniform, the Vickers values will be the same if tested using a 500g force or a 50kg force. Below 200g, caution must be used when trying to compare results.

STANDARDS

Vickers test methods are defined in the following standards:

• ASTM E384 - micro force ranges - 10kg to 1kg
• ASTM E92 - macro force ranges - 1kg to 100kg
• ISO 6507- 1,2,3 - micro and macro ranges

VICKERS TEST METHOD

All Vickers ranges use a 136° pyramidal diamond indenter that forms a square indent.

• 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 an indent in the sample that appears square shaped on the surface.
• The size of the indent is determined optically by measuring the two diagonals of the square indent.
• The Vickers hardness number is a function of the test force divided by the surface area of the indent. The average of the two diagonals is used in the following formula to calculate the Vickers hardness.

HV = Constant x test force / indent diagonal squared




The constant is a function of the indenter geometry and the units of force and diagonal. The Vickers number, which normally ranges from HV 100 to HV1000 for metals, will increase as the sample gets harder. Tables are available to make the calculation simple, while all digital test instruments do it automatically. A typical Vickers hardness is specified as follows:

356HV0.5

Where 356 is the calculated hardness and 0.5 is the test force in kg.

APPLICATIONS

Because of the wide test force range, the Vickers test can be used on almost any metallic material. The part size is only limited by the testing instrument's capactiy

Strengths

• One scale covers the entire hardness range
• A wide range of test forces to suit every application
• Nondestructive, sample can normally be reused

Weaknesses

• The main drawback of the Vickers test is the need to optically measure the indent size. This requires that the test point be highly finished to be able to see the indent well enough to make an accurate measurement
• Slow. Testing can take 30 seconds not counting the sample preparation time

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Rockwell Hardness Test

Rockwell Hardness Test
Stanley P. Rockwell invented the Rockwell hardness test. 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 they were doing on the bearing races was successful. The only hardness tests he had available at time were Vickers, Brinell and Scleroscope. The Vickers test was too time consuming, Brinell indents were too big for his parts and the Scleroscope was difficult to use, especially on his small parts.

To satisfy his needs he invented the Rockwell test method. This simple sequence of test force application proved to be a major advance in the world of hardness testing. It enabled the user to perform an accurate hardness test on a variety of sized parts in just a few seconds.


Rockwell test methods are defined in the following standards:

• ASTM E18 Metals
• ISO 6508 Metals
• ASTM D785 Plastics

TYPES OF THE ROCKWELL TEST

There are two types of Rockwell Tests:

1. Rockwell: the minor load is 10 kgf, the major load is 60, 100 or 150 kgf.
2. Superficial Rockwell: the minor load is 3 kgf and major loads are 15, 30, or 45 kgf.

In both tests, the indenter may be either a diamond cone or steel ball, depending on the characteristics of the material being tested.

ROCKWELL SCALES

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 of the following factors that control scale selection:

• Type of material
• Specimen thickness
• Test location
• Scale limitations

PRINCIPAL OF THE ROCKWELL TEST

• The indenter moves down into position on the part surface
• A minor load is applied and a zero reference position is established
• The major load is applied for a specified time period (dwell time) beyond zero
• The major load is released leaving the minor load applied

The resulting Rockwell number represents the difference in depth from the zero reference position as a result of the application of the major load.

APPLICATIONS

With the two test ranges available, the Rockwell test can be used on almost any metal sample as well as some hard plastics. The test can normally be performed in less than 10 seconds and the indent is usually small enough to allow the part to be used. Some parts with a critical hardness specification are tested 100%.

Weaknesses

• Multiple test scales (30) needed to cover the full range of metal hardness
• Conversions between scales can be material dependent
• Samples must be clean and have a smooth test point to get good results

Strengths

• Rapid test, usually less than 10 seconds
• Direct readout, no questionable optical measurements required
• Non-destructive, part normally can be reused

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Knoop Test

Knoop Test

Knoop (HK) hardness was developed by at the National Bureau of Standards (now NIST) in 1939. The indenter used is a rhombic-based pyramidal diamond that produces an elongated diamond shaped indent. Knoop tests are mainly done at test forces from 10g to 1000g, so a high powered microscope is necessary to measure the indent size. Because of this, Knoop tests have mainly been known as microhardness tests. The newer standards more accurately use the term microindentation tests. The magnifications required to measure Knoop indents dictate a highly polished test surface. To achieve this surface, the samples are normally mounted and metallurgically polished, therefore Knoop is almost always a destructive test.

STANDARDS

Knoop test methods are defined in ASTM E384

KNOOP TEST METHOD

Knoop testing is done with a rhombic-based pyramidal diamond indenter that forms an elongated diamond shaped indent.


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 an elongated diamond shaped indent in the sample.
The size of the indent is determined optically by measuring the longest diagonal of the diamond shaped indent.
The Knoop hardness number is a function of the test force divided by the projected area of the indent. The diagonal is used in the following formula to calculate the Knoop hardness.


HK = Constant X test force / indent diagonal squared







The constant is a function of the indenter geometry and the units of force and diagonal. The Knoop number, which normally ranges from HK 60 to HK1000 for metals, will increase as the sample gets harder. Tables are available to make the calculation simple, while all digital test instruments do it automatically. A typical Knoop hardness is specified as follows:

450HK0.5
Where the 450 is the calculated hardness and 0.5 is the test force in kg.

APPLICATIONS

Because of the wide test force range, the Knoop test can be used on almost any metallic material. The part size is only limited by the testing instrument's capacity.

Strengths

• The elongated diamond indenter and low test forces allows testing very small parts or material features not capable if being tested any other way.
• One scale covers the entire hardness range.
• Test results a mainly test force independent over 100g.
• A wide range of test forces to suit every application.

Weaknesses

• The main drawback of the Knoop test is the need to optically measure the indent size. This requires that the test point be highly polished to be able to see the indent well enough to make an accurate measurement.
• Slow. Testing can take 30 seconds not counting the sample preparation time.

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