1、Core Functions
（1）、This is the core function of the K value.
For the same material, Brinell hardness values measured with the same K value and corresponding standard combination shall remain consistent, regardless of different laboratories or different models of Brinell hardness testers. This ensures cross‑equipment and cross‑scene comparability of test results, providing a unified basis for material quality control and product acceptance.
（2）、Control Indentation Size to Guarantee Test Accuracy:
National standards clearly require that the indentation diameter (d) formed after the indenter penetrates the specimen must be controlled within 0.24D to 0.6D. The fixed value of the K value is exactly to meet this requirement:
An excessively high K value means the test force is too large relative to the indenter diameter, resulting in an overly small indentation (d < 0.24D), increased measurement error of the indentation diameter, and ultimately a higher hardness value.
An excessively low K value means the test force is too small relative to the indenter diameter, resulting in an overly large indentation (d > 0.6D), or even piercing thin specimens, leading to a lower hardness value. Meanwhile, the indentation edge is prone to deformation, impairing measurement precision.
（3）、Adapt to Different Materials and Specimens to Expand Test Range:
The six standard K values cover various metal specimens from extremely soft to extremely hard and from thick to thin. Selecting an appropriate K value avoids specimen damage caused by improper test conditions and ensures the test results truly reflect the material hardness characteristics.
2、Application Principles
（1）、Select K Value According to Material Hardness:
The basic principle is the harder the material, the larger the K value.
For medium-to-high hardness materials such as high-strength steel and cast iron, prioritize K =30.
For medium-hardness non-ferrous metals such as aluminum alloy and copper alloy, select K = 10 or K = 5.
For extremely soft metals such as lead and tin, select K = 2.5 or K = 1.（2）、Adjust K Value Based on Specimen Thickness:
For thin specimens, an excessively high K value may cause deformation or piercing due to excessive test force. In this case, appropriately reduce the K value and use a smaller test force with the corresponding indenter diameter to ensure the specimen is intact and the indentation size complies with standards.
（3）、Strictly Follow Standard Combinations and Prohibit Non-Standard Matching:
In any test scenario, match the corresponding indenter diameter and test force according to the standard K value. Random matching of test parameters (e.g., non-standard combination 5/31.25) is strictly prohibited to avoid invalid test results.
