Metal Hardness Testing: Methods and Scales

Introduction:

Metal Hardness testing is a useful technique for evaluating the mechanical characteristics of metals, but several obstacles and difficulties may arise throughout the procedure. Metal hardness testing is crucial in various industries to assess a material’s resistance to deformation, wear, and scratching.

Several methods and scales are employed to measure hardness, each with its own advantages and applications.

Metal Hardness Testing: Common Methods

1. Rockwell Hardness Test:

Principle: Measures the depth of penetration of an indenter under a large load compared to the penetration under a smaller load.

Scales: Depending on the indenter and load used, different scales, such as Rockwell A, B, and C, are used.

Application: Suitable for various materials, from soft to very hard.

2. Brinell Hardness Test:

Principle: Measures the diameter of the indentation made by a spherical indenter under a specific load.

Formula: Hardness number (HB) = Load (kg) / (π * D2/4), where D is the diameter of the indention.

Application: Suitable for softer materials.

3. Vickers Hardness Test:

Principle: A square-based pyramid-shaped diamond indenter is used to make an indentation.

Formula: Hardness number (HV) = Load (kg) / Surface area of the indentation.

Application: Ideal for a variety of materials and frequently utilized with extremely hard materials

4. Knoop Hardness Test:

Principle: Similar to the Vickers test but uses a rhombic-based pyramidal diamond indenter with a different geometry.

Formula: Hardness number (HK) = Load (kg) / Projected area of the indentation.

Application: Mainly used for brittle materials.

5. Shore Hardness Test:

Principle: Measures the resistance of a material to indentation using a durometer with a spring-loaded indenter.

Scales: Shore A (soft rubbers and elastomers) and Shore D (hard rubbers and plastics) are commonly used.

Application: Mainly used for polymers and elastomers.

6. Mohs Hardness Scale:

Principle: Qualitative scale based on the scratch resistance of minerals.

Scale: Ranks minerals from 1 (softest – talc) to 10 (hardest – diamond).

Application: Primarily used in geology and mineralogy.

7. Superficial Hardness Tests:

Principle: Similar to Rockwell or Brinell tests but with lighter loads, suitable for thin and hard surfaces.

Scales: Superficial Rockwell and Superficial Brinell scales.

Application: Used for thin materials or surfaces that cannot withstand heavy loads.

The Following Are Some Typical Challenges Encountered While Testing Metals Hardness Testing Challenges:

• The roughness of the surface: The precision of indentation measurements may be impacted by uneven or rough surfaces.
• Dimensions and Form of the Sample: Accurate and significant hardness measurements may be difficult to achieve from small or irregularly shaped samples.
• The homogeneity of samples: Inconsistent hardness readings might result from variations in the composition or microstructure of the material.
• Depth of the Indentation: Variations in indentation depth can occur in several hardness tests, such as the Rockwell, impacting accuracy.
• Anisotropy: Certain materials exhibit anisotropy or the variation in hardness levels in different crystallographic directions.
• Flexible Recuperation: Certain materials may undergo elastic recovery after the stress is removed, which could result in erroneous hardness values.
• Impact of Microstructure: Variations in the material’s microstructure, such as grain size or phase distribution, may affect hardness readings.
• Choose Your Scale: Inaccurate results can arise from using the wrong hardness scale for the material.
• Temperature sensitivity: Extremely high or low temperatures can affect the hardness of certain materials.
• Operator Proficiency: The operator’s expertise and experience might affect the reproducibility and dependability of hardness readings.

Video 01: TYPES OF HARDNESS TEST / HARDNESS