GB/T 21838.3-2022 "Metallic materials—Instrumented indentation test for hardness and materials parameters—Part 3: Calibration of reference blocks" specifies the calibration method for reference blocks used in the indirect verification of instrumented indentation testing machines. It clarifies that reference machines require regular inspection and calibration, covering aspects such as test force, indenter, displacement measurement device, and test cycle. Different ranges of test forces have specific tolerance requirements, the indenter must meet particular dimensional and accuracy standards, and the displacement measurement device must comply with corresponding resolution and tolerance criteria. Calibration must be conducted under specified temperature conditions, with an appropriate number of indentations made according to different range requirements, ensuring the accuracy of the reference block's values and providing a reliable basis for the verification of testing machines.