GB/T 4340.3-2025 "Metallic materials — Vickers hardness test — Part 3: Calibration of reference blocks" specifies the calibration method for reference blocks used in the indirect verification of Vickers hardness testing machines. It applies to the calibration of indentations with diagonal lengths not less than specified dimensions. The standard stipulates that the calibration environment temperature must be controlled at 23±5°C, with temperature fluctuations not exceeding 1°C. The reference blocks must meet requirements for thickness, flatness, and parallelism. The standard also regulates the equipment used for calibration, operational procedures, and requirements for indentation inspection, adhering to the principle of traceability to ensure the accuracy and uniformity of the reference block values. This provides a reliable benchmark for the verification and calibration of Vickers hardness testing machines, ensuring the accuracy of hardness test results.