The French/AFNOR designations 16MND5, 18MND5, and 20MND5 refer to different grades of low alloy steels used in construction and pressure vessel applications, particularly in the nuclear industry. While these grades may have similar chemical compositions, they typically differ in terms of their mechanical properties, heat treatment requirements, or intended applications. Let's explore potential differences between these grades:

1. **Carbon Content**: While the chemical compositions may appear similar, there could be slight differences in the carbon content among these grades. Even small variations in carbon content can affect the material's strength, toughness, and weldability.

2. **Alloying Elements**: Although the primary alloying elements may be the same in all three grades (such as manganese and nickel), there could be differences in the exact composition or trace elements present. These variations can influence the material's properties and performance under specific conditions.

3. **Mechanical Properties**: The grades 16MND5, 18MND5, and 20MND5 may have different specified mechanical properties, such as yield strength, tensile strength, elongation, and impact resistance. These properties are critical for determining the suitability of the material for specific applications and service conditions.

4. **Heat Treatment Requirements**: Each grade may have specific heat treatment requirements to achieve the desired combination of mechanical properties. Differences in heat treatment processes, such as quenching and tempering, can result in variations in the microstructure and properties of the steel.

5. **Intended Applications**: The choice of grade may also depend on the intended application and service conditions. Certain grades may be better suited for high-temperature environments, corrosive conditions, or applications requiring high strength and toughness.

6. **Standards and Specifications**: It's essential to refer to relevant standards, specifications, or technical documents provided by organizations such as AFNOR or the nuclear industry regulatory bodies for detailed information on the differences between these grades. These documents typically outline the requirements, properties, and permissible variations for each grade.

To obtain a more detailed understanding of the differences between 16MND5, 18MND5, and 20MND5 according to the French/AFNOR steel grading system, it may be necessary to consult authoritative sources, such as technical standards, material data sheets, or industry experts familiar with these materials and their applications in specific contexts, such as nuclear construction and pressure vessel fabrication.