Testing of material is one of the important and essential steps for judging suitability for engineering application after heat treatment. Most of the properties of interest are mechanical properties since heat treatment essentially alters mechanical properties. Many testes and testing materials have also be included now a days.

Hardness has been defined in several ways, based on the principle and the manner in which test is conducted. There are 30 types of methods used for measuring different types of hardness. There are generally three types -

1. Scratch hardness :

Scratch hardness can be defined as that property of the material by virtue of which it resists wear or abrasion.

Moh's Scale of Hardness :

● Moh's scale of hardness consists of 10 standard minerals. It is most commonly used for scratch hardness. Each one of the minerals has been assigned as a hardness number. Though Moh's scale finds wide application in the field of minerology, it is hardly used for metal and alloys which may have hardness value lying between two consecutive scratch hardness according to Moh's scale. Thus comparative hardness value of metal and alloys, which are of great significance can not be estimated.

Standard moh's scale of hardness
Standard Moh's scale of Hardness

2. Indentation hardness :

● Indentation hardness is a measure of resistance offered by a material to plastic deformation. It is measure by estimating the size of indentation. Indentation hardness measurement involve pressing an indenter of known material and well defined geometry in to the surface of the sample or work piece. The size or depth of indentation is used as hardness measuring parameter.

● Indentation hardness test is very common and finds varied applications in the field of metallurgy. It is most widely used for metals and alloys. Three most commonly used are :
1. Rockwell hardness test
2. Brinell hardness test
3. Vickers hardness test

3. Rebound or dynamic hardness :

Rebound hardness is a measure of resistance of the materials to strike and rebound. For determining rebound hardness, an indentor is dropped on the surface of the material under specific set of conditions. The energy of the impact or height of rebound of the indentor forms the basic of measurement of rebound or dynamic hardness. Shore seleroscope is the most commonly used rebound hardness tester.

References :

1. Heat treatment principle and techniques  by : T.V. Rajan,  C.P. Sharma,  Ashok Sharma.
2. Physical Metallurgy Principle and Practice by : Raghavan V.,
3. Lectures of IITs & BPUT (ODISHA).

Author :
Subir Kumar Sahu.

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