Hardness refers to how well a material can resist being indented or scratched. In other words, it describes how difficult it is for another object to press into its surface.
Hardness is not measured directly. Instead, it is calculated from other measurements—such as how deep an indentation goes or how much force is applied during a test.

For more information, visit: https://www.struers.com/knowledge/hardness-testing

There are several common methods used to measure the hardness of steel, each suited to different part sizes and accuracy needs:

Rockwell Hardness Testing (HR):
One of the quickest and easiest testing methods. The HRC scale is the most widely used for steels because it provides fast, consistent results with minimal preparation.

Brinell Hardness Testing (HB):
Often used for larger forgings or castings. This method presses a carbide ball into the material using high loads—sometimes up to 3000 kgf—making it well‑suited for coarse‑grained or uneven surfaces.

Vickers Hardness Testing (HV):
Ideal for small parts, situations requiring high accuracy, or when multiple indents must be made in a limited area. It uses a diamond pyramid and can measure very small and very hard materials precisely.
Each method has its own strengths, so the best choice depends on the size of the component, the surface condition, and the level of detail needed.

Each hardness method has its own strengths, depending on speed, part size, surface condition, and accuracy needs:

Rockwell testing (HRC, HRB, etc.) is the fastest testing method, taking roughly 10 seconds because it measures indentation depth, not indent size. Since it doesn’t require optical measurement, Rockwell https://www.struers.com/knowledge/hardness-testing/rockwell) testing machines are typically the most affordable among bench testers. Downside: You cannot see the indent, so surface imperfections or unusual indent shapes may go unnoticed.

Brinell testing (HB) is commonly used for large forgings, castings, or coarse‑grained materials. Brinell testing uses a carbide ball and high test loads—typicall between 62.5 kgf and 3000 kgf—making it ideal for uneven or rough surfaces. The large indent helps provide a more representative reading on heterogeneous materials.

Vickers testing (HV) is suited for small parts or coatings, areas where high accuracy is needed, or when performing many indents in a limited space. Vickers uses a diamond pyramid to create a precise indent that can be measured very accurately.
A major advantage of the Vickers scale is that HV values are comparable across different test loads, making results easy to interpret and compare.

Brinell — Best for large, coarse, or inhomogeneous materials (like cast iron or forgings) because the large ball indenter "averages" out the surface.

Rockwell — as it is a fast and easy to use hardness method.  Because it provides a direct reading of hardness, it eliminates one potential source of operator influence. For steels, the most commonly used Rockwell scales are HRC and HRB.

Vickers/micro-Vickers — precise, wide load range.

Rockwell hardness testing measures how deep an indenter penetrates into a material under a specific set of forces. In simple terms:
The deeper the indentation, the softer the material. A smaller indentation depth means a higher hardness value. For regular Rockwell scales, each Rockwell point (HR) corresponds to 0.002 mm of indentation depth.

Here’s how the process works:

• Step 1 — Apply a preload:
  The hardness test begins by applying a small initial force (the preload). This seats the indenter and sets the depth-measuring system to zero.
• Step 2 — Apply the major load:
  Next, the hardness tester machine applies the major load (for example, 150 kgf for HRC). This load is held for about 3–5 seconds to allow the material to settle.
  
  
• Step 3 — Return to preload:
  The load is then reduced back to the preload level. At this point, the tester measures the permanent indentation depth.
  
  
• Step 4 — Calculate hardness value:
  The Rockwell value is calculated automatically from the remaining depth.
  Today, Rockwell hardness testers handle the full test cycle automatically, providing fast and reliable results in a matter of seconds.
  There are multiple Rockwell scales (e.g., HRC, HRB, HR15N, etc.), each designed for different hardness ranges and material types, from very hard steels to softer metals and plastics.

Brinell testing measures hardness by pressing a hardened steel or carbide ball—typically 10 mm, 5 mm or 2,5 mm in diameter—into the material using a high test load (up to 3000 kgf). After the load is removed, the diameter of the resulting indentation is measured optically.
The Brinell hardness value (HB) is then calculated from the test force and the measured indent diameter, giving a reliable hardness number for coarse or heterogeneous materials such as castings and forgings.

Vickers testing uses a diamond pyramid‑shaped indenter with a square base and an apex angle of 136°. The indenter is pressed into the material under a defined test load, which can range from very low (micro‑Vickers) to high (macro‑Vickers).
After the load is removed, the two diagonal lengths of the diamond‑shaped indentation are measured optically. These measurements are then used to calculate the Vickers hardness value (HV).
Because the geometry of the indenter is constant, the Vickers scale is continuous across all test loads, making it suitable for both micro‑ and macro‑hardness testing.

Rockwell hardness is mainly defined by two standard families: ASTM E18 and the ISO 6508 series.

ASTM E18 outlines how Rockwell and superficial Rockwell tests should be performed, including basic test principles, how the machine should operate, and how equipment like indenters and reference blocks must be checked.
ISO 6508‑1 covers the internationally agreed test methods for both regular and superficial Rockwell scales.
ISO 6508‑2 describes how to verify and calibrate Rockwell machines and indenters to ensure they perform correctly.
ISO 6508‑3 sets the rules for calibrating the reference blocks used to check machine accuracy.
Together, these standards ensure Rockwell testing is consistent, reliable, and comparable worldwide.

Brinell hardness testing is mainly defined by ASTM E10 and the ISO 6506 series. Together, these standards describe how Brinell tests must be performed and how the equipment is checked to ensure accurate, reliable results.

ASTM E10
This standard explains how Brinell hardness tests should be carried out on metallic materials. It covers the test method itself, the requirements for Brinell testing machines, and the procedures for using and verifying machines, indenters, and reference blocks.

ISO 6506‑1
Defines the general Brinell test method, including how the ball indenter is used, test forces, and how the indentation is measured. It applies to both fixed and portable Brinell testers.

ISO 6506‑2
Specifies how to perform direct and indirect verification of Brinell hardness testing machines and indenters, ensuring the equipment remains accurate.

ISO 6506‑3
Provides the requirements for the calibration of Brinell reference blocks, which are used for routine accuracy checks.

Both ISO and ASTM hardness testing standards require that the material thickness be 7 to 10 times the indentation depth, depending on the specific test method. This ensures that the measurement is not influenced by the material beneath the indentation.

Because of this requirement:
Softer materials (such as brass or aluminium) produce deeper indents and therefore must be thicker than harder materials like steel or hardened alloys.
Lower test loads create shallower indents, which means thinner samples can be tested reliably.

As practical guidance:

• For Rockwell HRC, samples thicker than 2.0 mm are generally safe to test.
• For Vickers HV1, a thickness of 0.5 mm is typically sufficient for reliable results.

Surface preparation requirements vary significantly between hardness test methods:

Vickers:
Vickers testing requires a high‑quality, mirror‑like finish—especially in micro‑Vickers testing (below 1 kgf)—because the indentation is very small and measured optically. Any surface scratches, roughness, or contamination can significantly affect the accuracy of the diagonal measurement.

Rockwell:
Rockwell testing is less demanding in terms of polishing, but surface cleanliness is critical. Even a small amount of oil, dirt, or scale on the anvil (the support surface) can introduce errors of several Rockwell points, because it affects how the sample seats during testing.

Brinell:
Brinell testing has the lowest surface preparation requirements. A fine polish is not necessary; the key requirement is having enough contrast between the indentation and the surrounding material so that the indent can be measured clearly.

If you have already confirmed that your hardness tester is functioning correctly, you may still observe variation in readings when testing the same component. This is usually caused by natural variations in the material itself.

Hardness testing cannot be performed on exactly the same spot twice, so each indent is made in an area with a slightly different microstructure. Because real materials are never perfectly homogeneous, these microstructural differences lead to normal, unavoidable scatter in the results.

Some variation is therefore expected—even with a perfectly calibrated instrument—especially in materials with coarse grains, mixed phases, or heat‑affected zones. 
Both ISO and ASTM hardness testing standards explicitly recognize that materials exhibit natural variability. Because of this, the standards include permitted tolerance bands for repeatability and reproducibility.

If your hardness tester appears to be reading incorrectly, follow these steps to identify the cause:

Verify the issue using a certified reference block
Test a hardness reference block (test block) to confirm that the readings are actually off.
Make sure the block has a valid, unexpired calibration certificate.

Check measurement consistency
Make a series of indents and observe whether the readings are stable or fluctuating.

Consistent but incorrect readings point to calibration or setup issues
Inconsistent readings may indicate mechanical, surface, or environmental problems.

Compare deviation to ISO/ASTM tolerances
Especially at low test loads or with very hard materials, ISO and ASTM standards allow a relatively wide acceptable deviation. Ensure the difference you see is actually outside the permitted tolerance band.

Inspect and clean the test setup
Ensure the anvil surface, its seating, and contact surfaces are clean and free of contamination. Any dirt, oil, or wear can create significant measurement errors.

Examine the indenter
Check the indenter for chips, wear, or deformation. If possible, swap in a spare indenter and see if the results improve.

Validate the optical system (for optical methods)
On Vickers or Brinell optical systems, use an object micrometer to confirm the optical calibration is still accurate.

Have the test loads verified (specialized equipment required)
As a final step, have the applied test forces checked. They must be within the 1%–1.5% tolerance specified in the standards. Incorrect test loads will directly cause incorrect hardness values.