Different Types of Hardness Tests

Definition

Hardness is defined as the resistance of a material to localized plastic deformation, such as an indentation, scratching, or abrasion. In materials science, a hardness test is a mechanical evaluation used to determine how well a material can withstand surface penetration under a specified load and indenter geometry.

Main Content

1. Brinell Hardness Test

• Uses a hardened steel or tungsten carbide ball indenter to create a spherical impression on the material surface.
• It is primarily used for materials with coarse structures or those that are too rough for other sensitive testing methods, such as large castings.

2. Rockwell Hardness Test

• Measures the depth of penetration of an indenter (diamond cone or steel ball) under a large load compared to the penetration made by a preload.
• It is highly popular in industrial quality control because it provides a direct reading without the need for optical measurements.

3. Vickers Hardness Test

• Utilizes a square-based diamond pyramid indenter with a 136-degree angle between faces.
• It is known as the "universal" hardness test because it can be used for both very soft and very hard materials across a wide range of test loads.

Indenter Geometry Comparison:
     Brinell           Rockwell          Vickers
      (Ball)         (Diamond Cone)    (Pyramid)
       ___                / \             / \
      /   \              /   \           /   \
     |     |            /     \         /_____\
      \___/            /_______\

Working / Process

1. Specimen Preparation

• The surface of the material must be cleaned and polished to ensure the indenter makes proper contact without interference from debris or roughness.
• The specimen must be supported on a stable anvil to prevent movement during the application of the force.

2. Application of Force

• The machine applies a predetermined minor load to set the indenter, followed by a major load that is held for a specific dwell time (usually 10–15 seconds).
• The force application ensures that the material reaches a state of plastic equilibrium, allowing for consistent measurement.

3. Measurement of Indentation

• In Brinell and Vickers tests, an optical microscope is used to measure the diameter or diagonal lengths of the permanent mark left on the surface.
• In the Rockwell test, the machine automatically calculates the hardness value based on the difference in penetration depth between the preload and the major load.

Advantages / Applications

• Quality Control: Essential for checking the uniformity of heat treatment processes in steel and alloys.
• Material Selection: Helps engineers determine if a material is suitable for high-wear applications, such as gears, bearings, and cutting tools.
• Non-Destructive Tendency: While technically an indentation test, the damage is often minimal, allowing the finished part to be used after testing (depending on the precision required).

Summary

Hardness testing is a critical mechanical evaluation method used to measure a material's resistance to permanent surface indentation. By applying controlled forces through standardized indenters, engineers can determine the durability and suitability of metals for manufacturing. Key terms to remember include Indenter, Major/Minor Load, Dwell Time, and Plastic Deformation.