The ISO 18203 standard specifies the method for determining the thickness of the hardened layer on the surface of steel. The standard applies to steel treated by different surface hardening techniques (e.g. carburizing, nitriding or quenching, etc.) and aims to provide a reliable test method for measuring the thickness of the hardened layer. The thickness of the hardened layer is a key factor affecting the surface performance of steel (e.g. abrasion resistance, corrosion resistance and fatigue strength), so accurate measurement of this thickness is essential to ensure material mass and performance. The standard details the use of appropriate equipment and techniques, such as Metallurgical Microscopy or ultrasonic methods, to obtain accurate hardened layer thickness data to ensure test accuracy and repeatability.