Vicker Hardness Test for metals and alloys

Introduction

One of the most widely used methods for hardness measurement is the Vickers hardness test which is a micro-indentation test. In this test an indenter is forced into the surface of the material under the applied load, resulting in an indent. The lengths of the diagonals of the impression produced are measured in terms of HVN by using the formula.

Hardness tests like Rockwell and Brinell are more commonly used in industry but the Vicker hardness test is more specified for coating hardness and for laboratory research purposes.

Basic characteristics which makes Vickers hardness test very efficient for research and development purposes are:

• Can be used for soft and hard materials, small and thin specimens
• Indenter do not deform during normal operation
• Only one scale is used which is linear and consistent
• Can be used for hardness measurement of different phases

Standards for Vickers Hardness test

Following are the standards for Vickers Hardness test, which describes the testing procedure and its specifications.

• ASTM E 384 (For Micro-indentation)
• ASTM E 92 (For Macro-indentation)
• ISO 6507

Vicker Hardness Test Theory

Principle

Hardness measurement using Vicker hardness test is carried out with the help of a filar microscope (standard 4X to 500X). Indentation is formed with the help of indenter using standard load and time for the test. Using the load applied and diameters of impression formed, we get a hardness number. All the length measurements are taken with the help of microscope and final readings as hardness number are achieved at the end.

Working

In the Vickers hardness test, the specimen is indented with a diamond indenter. The indenter is a pyramid-shaped containing angle of 136 in-between opposite faces and 1 to 120 kgf load is applied. Shape and impression of Diamond indentor for Vicker hardness test are given below:

Time for load application is 10-15 seconds usually. Impression formed is usually square based. After removal of load opposite diagonals of square base is measured and average diameter is considered as final as a parameter to find area of square. Hardness number is achieved by dividing the load applied by the area of square indentation. Hardness number can be represented by VHN (Vickers hardness number), VPN (Vickers pyramid number) and DPH (diamond pyramid hardness number).

Vickers hardness number is calculated by the following formula:

Where:

P = Load in Kg.

d = Average length of opposite diagonals of impression in millimeters.

Types of Vickers Hardness Testers

Vicker hardness test can be divided into two types based on applied load;

1.         Micro-indentation hardness testing

2.         Macro-indentation hardness testing

In both tests the pyramidal diamond indenter is forced into the material to produce the impression the only difference is of the amounts of applied loads. In micro-indentation test the hardness of materials is measured on micro scale and the applied load ranges between 1gf and 1000gf. Where in case of macro-indentation hardness test the hardness of the materials is measured on macro scale and the applied load is 1 – 120 kgf.

Only a single hardness scale is the major benefit of Vickers hardness testing. Scale gives a range of hardness from very low to very high hardness. Hardness is simply measured as a function of applied load and lengths of diagonals of impression formed. Tester contains a microscope which automatically measures the length of diagonals. Tester is programed with the formula needed to measure hardness. Thus, tester gives final reading.

How to read Vicker hardness test number

• Vickers hardness number can be presented in different ways like HVN, DPH and VPN.
• One way of representation is 325 HV which implies that material has 325Vickers hardness. In this case no information is provided about load.
• Another method of representation of Vickers hardness number is 440 HV 30. This shows that material has 440Vickers hardness when 30kgf load is applied.
• It can also be represented as 440HV30/20 which means 440Vickers hardness is obtained, when 30kgf load is applied for 20 seconds.

Indentation Defects

There are different types of indentation defects which may produce during Vicker hardness test.

Pincushion indentation

Due to the sinking of material pincushion indentation produces during Vickers hardness test. In annealed metals the Vickers indenter produces an indentation having concave boundaries. In this case overestimation of diagonal lengths occurs.

Barreled Indentation

This defect arises due to the piling up of material around the faces of indenter while performing Vicker hardness test. In work hardened materials the materials flow upward around the indentation and the faces of Vickers indenter. Since no restriction is present for the flow of material then at the corners convex shape indentation produces. In this type of defect underestimation of diagonal lengths occurs.

Why Use Vickers hardness test

• Vicker hardness test involves some of the proper advantages over different hardness testing methods as follows:
• Major advantage is its only scale. No complexity at all.
• Readings are consistent/ linear.
• Indent formed is of tiny size which might be ignorable as it cannot be seen by naked eye.
• Test is performed for very soft materials to very hard materials. This test can also be performed for ceramics materials.
• This method is preferred because of its only diamond indenter. Diamond indenter is very much hard itself so it can be used for measurement of hardness of a range of materials.
• Area measurement of a square based impression is easier as compared to circular impression. Lesser chances of error present.
• Test is easier to perform as it is independent of indenter size.
• Extremely accurate readings and measurements.

For studying further about Vicker hardness test, follow the post on Wikipedia.

Vicker hardness test Limitations

• Care must be taken during surface preparation because extremely smooth surfaced specimen is required to carry out the test.
• Test includes microscopic measurements. Care must be taken during handling.
• Time consuming process.

How to perform Vicker hardness test

For performing Vicker hardness test, following equipment and material is needed:

• Vickers hardness testing Machine (Filar Microscope 100X)
• Diamond indenter
• 3 Plain carbon steel specimens
• AISI 1040 (Quenched)
• AISI 1040 (Normalized)
• AISI 1040 (Annealed)

To study about effect of temperatures on steel hardness, follow austenitizing temperature effect on steel.

Procedure

• Suitable steel specimens were selected as mentioned.
• These samples were ground coarsely in order to remove grease or any foreign material.
• Then the sample was subjected to fine grinding, polishing and subsequent etching (to view phases).
• The sample was then placed on the anvil under the indenter of testing machine.
• Using microscope, a suitable area was selected for indentation.
• The indenter was then swung over the work piece.
• Then the machine was adjusted. Appropriate load and time were selected for the test.
•  ‘START’ button was then pressed, to carry out the test.
• Then for observations, the microscope was again swung over the work piece.
• Two vertical lines were made coincident first to avoid zero error.
• By placing one line over a corner, the second line was moved diagonally over the other corner, the button was then pressed to record reading.
• Then the eyepiece was rotated at 90o to observe second reading with the same procedure.
• The diagonal lengths and the hardness number were automatically displayed on the screen.

Correction factor

For cylindrical / spherical specimens, a correction factor is considered for an accurate value. The correction factor is given as

d/D = n

Where

d = diagonal length of indent D = Diameter of specimen

Standard charts are available in ASM handbook and mention standards, from which accurate reading can be found. The value of n is compared by the manual and the hardness value corresponding to n is the original hardness value of the specimen.

Discussion on Result

• The Vickers hardness value for annealed sample is the lowest because by slow cooling more ferrite is formed which is a relatively softer phase.
• Normalized steel specimen has more Vickers hardness value as compared to that of annealed sample because by fast cooling in air, less ferrite is formed and more pearlite is present which is a harder phase.
• In quenched specimen, martensite is formed due to very fast cooling in water. Martensite is the saturated solution of Fe3C which is a very harder phase. That’s why this specimen has maximum hardness value than that of annealed and normalized specimens.