What is Measured?

The Rockwell hardness test measures the permanent depth of indentation produced after applying a major load while maintaining a minor preload.

The machine directly displays the Rockwell hardness number corresponding to the selected Rockwell scale.

Typical observations include:

• Minor load
• Major load
• Rockwell scale used
• Rockwell hardness number (HR)

Why are the Calculations Required?

Although the Rockwell machine directly displays hardness, the observations help to:

• Compare hardness values of different materials.
• Verify repeatability of measurements.
• Calculate the average hardness from multiple trials.
• Check consistency of the testing process.

Observation Table

Sequential Calculations

Average Rockwell hardness:

$$HR_{\text{avg}} = \frac{HR_1+HR_2+HR_3}{3}$$

Solved Numerical Example

Observed hardness values:

• Trial 1 = 58 HRC
• Trial 2 = 59 HRC
• Trial 3 = 57 HRC

Average hardness:

$$HR_{\text{avg}} = \frac{58+59+57}{3} = 58$$

Therefore,

$$HR_{\text{avg}}=58\ \text{HRC}$$

Interpretation of Results

• Higher Rockwell hardness indicates greater resistance to permanent indentation.
• Smaller indentation depth corresponds to a higher hardness value.
• Consistent readings indicate proper specimen preparation and correct machine operation.
• Significant variation between trials may indicate improper surface finish, incorrect loading, or specimen misalignment.

Result

The Rockwell hardness number of the given specimen is determined from the test, and the average hardness value is reported using the selected Rockwell scale (such as HRA, HRB, or HRC).